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One of the two primary scientific programs in the division is the Prediction of Precipitating Weather Systems (PPWS) program. Its goal is to advance the understanding and prediction of significant precipitation events in order to substantially reduce forecast errors toward the limits of predictability. The accurate prediction of precipitating weather systems is an important topic in the U. S. Weather Research Program (USWRP). There is good potential for advancements in this area because of emerging operational observing systems, high-resolution non-hydrostatic forecast and data assimilation systems, and the continued rapid growth in computer power. Within MMM, there is broad interest and expertise in the observation, analysis, and prediction of precipitation systems, along with opportunities to leverage division resources through collaboration with other NCAR divisions, government laboratories and the university community. The research within MMM focuses on specific areas where MMM's expertise is best suited to advance the science. These areas include mesoscale dynamics and predictability, the life cycle of mesoscale precipitating weather systems, mesoscale data assimilation and high-resolution numerical weather prediction (NWP). These topics are highly interrelated, and research contributing to the advancement of mesoscale assimilation and forecast systems is focused toward development of a new multi-agency Weather Research and Forecasting (WRF) Model.

The skill of precipitation forecasts is limited by both fundamental and practical constraints. The practical constraints include the accuracy of the forecast model and the accuracy of its initial conditions. This is in turn determined by the available observations and their quality and by the scheme used to assimilate those observations. The fundamental constraint is the finite limit of predictability, which arises even for an accurate model and initial conditions from the influence of unresolved scales.

Understanding the relation between meso- and synoptic-scale flows involves, in part, understanding how atmospheric dynamics change as the Rossby number increases. At small Rossby number, virtually all dynamical theories rest upon the foundation of quasigeostrophy (QG), which is the leading-order theory in Rossby number. David Muraki (Simon Fraser University, Canada), Chris Snyder and Richard Rotunno have introduced a convenient technique for extending QG to an additional order in Rossby number; they call this extended theory "QG+1."

Greg Hakim (University of Washington), Snyder and Muraki have applied QG+1 to simulations of quasi-two-dimensional, balanced, decaying turbulence to understand the observed preference for cyclonic vortices on the tropopause at subsynoptic scales. These simulations produce numerous small cyclonic vortices with sharp edges, while the anticyclones are infrequent, relatively large scale and diffuse. These asymmetries appear to arise not from corrections to the basic geostrophic balance (e.g., gradient wind balance) but through the action of the horizontally divergent component of velocity during frontogenesis; in essence, the asymmetries are tied to the fact that on average warm air rises and cold air sinks during frontogenesis, thus leading to a mean cooling at the surface.

Fuqing Zhang (USWRP postdoc, now Texas A&M University), Snyder and Rotunno have explored the limits of predictability for precipitation within the context of the "surprise" snowstorm that paralyzed Washington, D.C. on 25 January, 2000. Their work began by analyzing a number of practical influences on the skill of the 36-h forecast of this storm, such as the model resolution and the initial conditions. They found that reducing the horizontal resolution from 10 km to 30 km, or using another, equally plausible initial analysis, can significantly degrade the precipitation forecast. In both cases, the degradation of the forecast is intimately tied to moist processes, which result in the growth of forecast differences at horizontal scales of a few hundred to a few tens of kilometers. These experiments have led Zhang et al. to consider more explicitly how initial errors of small scale and small amplitude can alter the subsequent forecast. Using an embedded, 3-km grid, they have shown that initial differences with scales of less than 100 km and amplitudes of less than 1 K, grow rapidly by altering the position and timing of individual convective elements (in this case, in a region of negative lifted index over Louisiana). The differences then contaminate larger scales, altering the mature cyclone and the precipitation over the East coast 36 h later. It is clear that this growth from small to large scales places an upper bound of a few tens of hours on skillful, deterministic precipitation forecasts.

Thomas Hamill (NOAA-CIRES Climate Diagnostics Center), Snyder and Rebecca Morss have also used a quasi-geostrophic model, along with a three-dimensional variational assimilation (3DVAR) scheme, to explore the characteristics of forecast and analysis errors at synoptic scales. They find that both forecast and analysis errors reflect the influence of the dynamics: The errors have significant projection on the subspace of leading Lyapunov vectors. Their time-mean vertical distribution in both energy and potential enstrophy is similar to that in the "true" state, and error variance in potential vorticity is typically confined to regions in which the true state has large gradients of potential vorticity. A consequence of this dynamical influence is that the spectrum of the covariance matrix for both forecast and analysis errors is steep and small samples (or ensembles, of a few 10's of members) can provide much information about the errors.

The fact that a small ensemble can provide useful information about forecast errors provides potential for improving data assimilation schemes, thereby decreasing the practical limitations on forecast skill. The resulting assimilation schemes are typically referred to as ensemble Kalman filters (EnKF). Hamill, Snyder and Jeff Whitaker (NOAA-CIRES Climate Diagnostics Center) have examined the use of distance-dependent truncation of the ensemble information in the EnKF. Snyder and Zhang have applied the EnKF to the analysis and prediction of convective scale motions using the simple cloud model developed by Juanzhen Sun. They have shown that a 50-member EnKF is able to estimate tangential and vertical velocity and temperature, given simulated Doppler-radar observations of radial velocity alone (extracted from a reference simulation of a supercell thunderstorm). Typically, about 4 volume scans (or 20 minutes) of observations are required to produce a good estimate of the unobserved variables. These results hold substantial promise for the application of the EnKF to meso- and convective scales, where more traditional assimilation schemes such as 3DVar can be problematic.

Given the location and uncertainty of an observation, the EnKF can provide a quantitative estimate of the impact of that observation on the analysis uncertainty. This provides a basis for adaptive observational strategies, which seek improved forecasts by reallocating observational resources to improve the analysis. Hamill and Snyder completed a study within the quasi-geostrophic model that tests the use of the EnKF in the adaptive design of observing networks. They have developed a simple and efficient algorithm to choose the locations of observations: using the ensemble Kalman filter, they find the location at which the impact of an observation is estimated to be largest. They then find the next best location given the first observation and continue the process up to the desired number of observations. In these tests, they find that as few as half as many adaptive observations are required to produce analyses with the same uncertainty as those obtained for a given fixed observation network.

Expected fractional reduction of analysis error variance from application of adaptive observation algorithm based on an ensemble Kalman filter. Results are shown for Day 14 of the 90-day test in a quasigeostrophic model (see Figure 1).

In response to the need for improved forecasting capabilities to support the United States Antarctic Program at McMurdo Station MMM has developed and implemented an experimental, MM5-based NWP system for Antarctica. The system, known as AMPS (Antarctic Mesoscale Prediction System), has operated since the 2000-2001 field season. AMPS employs the Polar MM5, a version of the model containing parameterizations and features aimed to better capture polar conditions. These features encompass packages such as modified radiation schemes and the inclusion of sea ice. AMPS provides higher resolution over the regions of key forecast concern than other available Antarctic guidance, with 10-km horizontal grids over the Western Ross Sea/McMurdo Station and the South Pole areas. AMPS has served to assist the daily forecasting for McMurdo and the South Pole performed by the Space and Naval Warfare Systems Center (SPAWAR) for NSF, and was also employed in the successful medical rescue of Dr. Ronald Shemenski from the South Pole in April, 2001. An AMPS forecast archive is maintained to support the research of modelers, polar meteorologists and grad students.

Figure 1 presents an example of an AMPS forecast of surface temperatures (shaded) and winds (barbs) in the immediate Ross Island area (hr 24, 29 Nov 0000 UTC initialization). Information from such products is used for planning flight operations and scientific activities on the ice, and the AMPS output is distributed via the web at http://www.mmm.ucar.edu/rt/mm5/amps. The MMM scientists behind the AMPS Project are Bill Kuo, Jordan Powers, Jim Bresch and Kevin Manning.

Increasing our understanding of how precipitation systems initiate, mature and decay is a fundamental problem in atmospheric science. This understanding is central to quantifying the intrinsic predictability of such systems and improving methods to forecast them. The principal type of system considered is that in which deep moist convection is organized, long-lived and exhibits upscale growth. A second major topic involves the dynamics of systems in which precipitation is strongly localized by frontal or orographic circulations and may involve frozen precipitation. These two subsets of precipitating systems probably represent the greatest challenge for PPWS.

Stanley Trier and Christopher Davis collaborated with investigators within NCAR/RAP (Cynthia Mueller, Daniel Megenhardt, and James Wilson) to examine the utility of mesoscale kinematic and thermodynamic information from RUC analyses and forecasts for short-range (0-3 h) forecasts of convective initiation and evolution. Testing on five widespread convective outbreaks, each of which caused significant disruption to United States air traffic operations, indicated that both absolute values and hourly trends of derived thermodynamic parameters, including convective available potential energy (CAPE) and convective inhibition (CIN) in multiple lower-tropospheric layers, were useful in determining when the local onset of deep convection would occur. They were also useful in identifying where, within preexisting areas of widespread convection, convection might subsequently weaken or decay. Future work is planned to examine these parameters, along with other thermodynamic and kinematic parameters (e.g., the vertical wind shear) as potential predictors for local initiation, areal growth, decay and movement of deep convection over a wider range of cases. Algorithms, based on the statistics from such studies, will be developed to aid in the automated short-range forecasting of these aforementioned aspects of deep convection.

Studies on warm season precipitation "episodes" by Richard Carbone, John Tuttle, David Ahijevych, Christopher Davis, Stanley Trier and L. Jay Miller have progressed from their initial efforts to characterize the two-dimensional climatology. "Episodes" are defined as time/space clusters of heavy precipitation that often result from sequences of organized convection such as squall lines, mesoscale convective systems and mesoscale convective complexes. Understanding the dynamics of convective episodes is crucial for developing improved, non-local representations of convection in numerical models. The climatologies also provide the basis for statistical/dynamical prediction of warm season rainfall, perhaps leading to realistic probabilistic representations of precipitation in forecast models.

Three avenues of research and related service activities were undertaken during the past year:

John Tuttle and Richard Carbone are nearing completion of an investigation of a long-lived convective system that persisted for two days over the central U.S. on 14-15 July, 1998. The event featured an abrupt change in its orientation and propagation vector that occurred about midway through its life (see Fig 2). The convection initiated over the higher terrain of southwestern Montana (Figure 2-left image) and became loosely organized into a N-S line perpendicular to the W-E oriented, low level shear vector (Figure 2-right image). A cold pool formed and raced ahead of the storm resulting in a succession of discrete propagation events across Montana and North Dakota. Upon entering Minnesota the convection intensified and assumed an E-W orientation in response to the increased moisture, the strong southerly flow and the N-S oriented shear vector. There were no indications of any strong fronts that could have accounted for the abrupt changes or the longevity of the system. Following its reorientation, the line moved slowly southward initially, but in the highly unstable and favorably sheared environment, a strong rear-inflow jet developed and the system bowed southward into southern Minnesota and Iowa. The storm then decayed rapidly as it moved into dryer, more stable air due to subsidence aloft. It was concluded that favorable cold pool - low level wind shear interactions and changes in the lower tropospheric shear vector orientation can explain the life cycle transformation.

L. Jay Miller is studying a Mesoscale Convective System (MCS) that persisted for more than 2 days, 21-23 June 1998. A sequence of intensification, decay, and regeneration led to this relatively long-lived mesoscale convective event.

Figure 3 shows the radar reflectivity swath associated with this compound event. Early afternoon convection on 21 June along the front range of the Rocky Mountains in northern Colorado organized into an MCS as it moved eastward across Kansas. The MCS reached its most intense rainout phase in central Kansas as it encountered moist, southeasterly flow from the Gulf. New organized convection that developed in eastern Kansas ahead of the older MCS continued eastward as a new MCS. Eventually it dove southeastward as it encountered northward-streaming, gulf-coast moist air west of the Appalachian Mountains.

Trier and Davis completed an observational study of a serial mesoscale convective system (MCS) on 27-29 May 1998, that possessed a persistent mesoscale convectively generated vortex (MCV). Through novel trajectory diagnostics applied to Rapid Update Cycle (RUC) analysis output, they demonstrated that balanced lifting, resulting from the interaction of the MCV with the ambient vertical shear, contributed in large part to the thermodynamic destabilization that allowed the redevelopment of deep convection within the multi-day MCS/MCV event. The portion of the vortex located within the lower troposphere intensified during nocturnal episodes of organized MCS activity. This appeared to aid in the horizontal transport of conditionally unstable air toward the location deep convection. In these ways, the MCV was documented to be a crucial link between relatively quiescent periods characterized by balanced flow and intermittent periods of organized deep convection that produced flooding rains.

Using the MM5 model, Davis and Trier simulated the first full diurnal cycle of the May 1998 MCV/MCS. The simulation, initialized with a RUC analysis and nested to 1.5-km horizontal grid spacing over the area of convection, correctly reoriented convection from a north-south band to and east-west band overnight (see Figure 4) in response to northward transport of warm, conditionally unstable air within the nocturnal low-level jet. As in the RUC analyses, balanced vertical motion was found to contribute substantially to mesoscale lifting and thermodynamic destabilization, which localized the convection. Horizontal transport of moist, unstable air into the nocturnal convection was significantly modulated by the MCV. In contrast to other studies of MCVs, Davis and Trier found that the re-intensification of the MCV at night began in the lower troposphere with the formation of a line-end vortex on the northern end of the north-south oriented convective line. Intensification of the mid-tropospheric vortex followed in response to the development of a stratiform precipitation region (see Figure 5). Melting of hydrometeors appeared to contribute substantially to the development of the mid-level circulation.

David Ahijevych constructed Hovmoller diagrams for four warm seasons. He also computed radar-derived rainfall as a function of universal time and longitude for the 1997-2000 warm seasons and compared this to estimates obtained using different data sets.

Initial work on precipitation echo frequency was led by John Tuttle. Figure 6 is an animation of the July 1998 diurnal cycle of precipitation radar echo frequency. It reveals the monthly averaged genesis of convective systems over the western cordillera, propagation and regeneration of convection eastward and southward, and interaction with the Gulf of Mexico sea breeze initiated convection over the interior of the southeastern U.S.

In earlier work, Richard Rotunno, Morris Weisman, and Joseph Klemp formulated a theory suggesting that squall line structure, strength and longevity was most sensitive to the magnitude of the component of low-level (0-3 km AGL) vertical wind shear perpendicular to squall line orientation. An "optimal" state was proposed whereby the deepest leading edge lifting and most effective convective re-triggering occurred when these circulations were in near balance. This state was based on the relative strength of the circulation associated with the storm-generated cold pool and the circulation associated with the ambient shear. Following this work, many subsequent studies have brought into question the relevance of such an optimal state to observed squall lines. They note the existence of strong, long-lived systems in sub-optimal conditions and they raise the question of the potential role of deeper-layer shears in promoting system strength and longevity in such situations. In an attempt to clarify these issues, Weisman and Rotunno have completed and analyzed an extensive set of simulations. They used both a simplified two-dimensional stream-function model and a full two-dimensional and three-dimensional cloud model, and they have been able to re-confirm the primary role of the low-level shear in controlling squall line structure and strength. They further clarify that a wider range of environments other than strictly "optimal" support significant squall lines in the simulations. This is also evident from observations.

Collaborations have continued with Jeff Trapp (visitor, NSSL) and Nolan Atkins (Lyndon State College) on the observation and simulation of tornadic circulations within quasi-linear convective systems such as squall lines and bow echoes. A set of idealized simulations have been completed that reproduce many of the characteristics of such systems, including the tendency for surface mesocyclones to develop north of the apex of the bow for environments of moderate to strong low-level environmental vertical wind shear. Analyses show that the initial source of these low-level circulations is the downward tilting of the horizontal vorticity associated with the cold pool-updraft interface, forced by the downdrafts of strong, short-lived convective cells near the leading edge of the systems. The actions of Coriolis forcing then promotes the strengthening and upscale growth of the cyclonic member of the tilted vortex couplet, producing a significant mesocyclone at the surface that could support the development of a tornado. This process appears quite distinct from that associated with supercell tornadoes, whereby a deep, quasi-steady, dynamically forced rotating updraft usually precedes the development of the tornado. Attempts are also underway to simulate observed systems to compare with the idealized cases.

Davis continued to lead the coordination of the Bow Echo and MCV Experiment (BAMEX), now scheduled for May 20 - July 6, 2003. BAMEX is a collaboration among PIs at NCAR, NSSL, NWS and several universities (UCLA, Texas A&M, Penn State, CSU and U of Alabama). The goals of this experiment are: (1) to obtain kinematic and thermodynamic documentation of the development of system-scale circulation features behind the leading convective line in maturing and decaying MCSs; (2) to understand mechanisms of convective regeneration near MCVs and the dynamics of MCV intensification that appear critical for multi-day events; (3) to understand the cause of damaging surface winds in bow echoes; and (4) to assess the predictability of long-lived MCSs and their effects on weather. The planned observing facilities include two Doppler P-3s, a dropsonde aircraft and a movable ground-based observing system consisting of two Doppler radars, wind profiler, acoustic sounder radiometer, mesonet and soundings. Please refer to http://www.mmm.ucar.edu/bamex/science.html for more information.

Christopher Davis and Lance Bosart (University at Albany, SUNY) continued their study of the formation of Hurricane Diana (1984) by examining the behavior of numerous sensitivity simulations. Development was dependent on a pre-existing upper-tropospheric trough-ridge couplet that focussed vertical motion, grid-resolved condensational heating, and lower- tropospheric potential vorticity anomalies that merged to form a tropical storm. Simulations with cumulus schemes that allowed more grid-scale precipitation on the 9-km grid exhibit unrealistic grid-scale overturning and slower intensification, primarily due to production of cyclonic vorticity anomalies at large radii. Use of an innermost nest with 3-km grid spacing, without a cumulus scheme, generally improved the intensity prediction. Storm track depended primarily on synoptic-scale structure at upper levels. Cumulus schemes that allowed more grid-scale overturning enhanced the anticyclonic outflow aloft. The outflow deformed the tropopause, building an anticyclone poleward of the storm and facilitating cut-off low formation equatorward of the storm. Using PV attribution, it was shown that these upper-level changes were responsible for an enhanced easterly steering flow and more westward storm track.

Jordan Powers and Davis extended the analysis and simulation of Diana using regional, cloud-resolving simulations from the MM5 model. The simulation consisted of a single domain of 1.2 km grid spacing and dimensions of 1000 x 1060 x 37, run on 552 processors of SCD’s IBM SP. For comparison purposes, 3-km and 9-km grid simulations have been performed. Simulations using 1.2 km and 3 km grid spacing showed markedly similar overall storm evolution, whereas the 9-km grid produced a storm with an unrealistically extensive circulation and no tight inner core. The higher-resolution simulations show Diana to develop, consistent with observation, in three distinct phases: initial MCS activity, quiescence and persistence of the incipient vortex, and convective regeneration and tropical cyclone formation (see Figure 7)

Ying-Hwa (Bill) Kuo, Wei Wang and Qinghong Zhang (ASP postdoctoral fellow) have performed a high-resolution numerical simulation of Hurricane Danny (1997) over a four-day period, from its genesis stage to its landfall. The simulation began at 0000 UTC 16 July 1997 when only a weak surface low was present over northern Gulf of Mexico. The PSU/NCAR MM5 model with triply-nested (81/27/9 km) grids was able to successfully simulate the development of a small tropical cyclone 72 h into the simulation, and its subsequent landfall over the Gulf coast. Subsequent numerical experiments at 3-km and 1-km grid resolution successfully captured interesting mesoscale structures of the storm, including the concentric eyewall, the eyewall replacement cycle and the trochoidal oscillations in storm track, as observed by the ground-based Doppler radars. Additional numerical experiments with 3-km and 1-km MM5 indicated that the simulation of the genesis of Danny was very sensitive to the choice of precipitation physics and planetary boundary parameterizations, and to the initial condition. The use of explicit cloud parameterization at cloud-resolving resolution (at least 3 km) is essential in simulating a realistic storm structure. Simulations with 9-km grid resolution using convective parameterization cannot properly reproduce the detailed storm structure as observed by the radars.

Richard Rotunno and R. Ferretti (University of L'Aquila, Italy) continued their analysis and simulation of Mesoscale Alpine Programme (MAP) cases. Although the large-scale flow was similar, important differences in mesoscale atmospheric structure made the difference between moderately intense rain in IOP2B, and relatively light rain in IOP8 of MAP. Rotunno and Ferretti have done a side-by-side analysis of these two cases with respect to precipitation, thermodynamic structure and wind. MM5 simulations of these cases agree well with the available data and, hence, provide a valuable interpretive tool. Analysis of the large-scale dynamics show that there was, in both cases, a moist tongue of southerly flow moving from west to east of the MAP area (northwestn Alps). The most important difference between the cases was the presence of a cold stable air mass in the Po Valley in IOP8, which persisted through the period in which the large-scale moist tongue was progressing eastward. The latter cold air mass prevented the most humid air from reaching the MAP area. Another important difference between the two cases occurred during the eastward passage of the cold front (the western boundary of the moist tongue) which is generally retarded at lower levels with respect to higher levels. In IOP2B this orographically induced differential advection of cold air produced strong conditional instability, and consequently, an additional episode of convective rain in the MAP area. In IOP8 the prefrontal air in the Po Valley was so cold that differential advection only reduced the already large static stability and, subsequently, there was no additional period of convective rain.

The Severe Thunderstorm Electrification and Precipitation Experiment (STEPS) was held in Eastern Colorado and Western Kansas in May through July 2000 with the goal of better explaining the relationship between kinematics, precipitation production and electrical characteristics of convective storms on the High Plains. Morris Weisman and L. Jay Miller focused on the analysis and simulation of supercell storms observed on 29 June and 5 July, which exhibited differing precipitation characteristics. Miller and Sarah Tessendorf (SOARS and CSU graduate student) have completed preliminary dual-Doppler analyses of the high-precipitation tornadic storm observed on 29 June. These analyses reveal many of the characteristic kinematic features associated with supercell storms, including a strong, quasi-steady rotating updraft and associated bounded weak echo regions (BWER) during the storm's mature phase. But they also reveal a more complicated multiple updraft configuration during other periods in the storms lifetime. Miller's preliminary dual-Doppler analyses of a more nearly low-precipitation supercell storm observed on 5 July reveal a more unicellular rotating updraft, reaching magnitudes of over 60 m/s. Weisman's initial simulations of the 5 July storm have been successful at replicating many of the observed storm characteristics, offering much hope that simulations in conjunction with the observations will provide useful insights into the precipitation mechanisms associated with the STEPS storms. Such analyses will then be used to improve the microphysical representations within cloud scale simulations of such events.

In collaboration with Andrzej Wierzbicki (University of South Alabama, Mobile) and Richard Laursen (Boston University), Charles Knight completed the development and application of a new etching technique that reveals the crystal surface orientations at which biological adsorb to ice, and whether or not the adsorbate is engulfed within the ice during growth. (These "antifreeze" molecules are peptides that adsorb to ice and prevent ice growth from supercooled water by a kinetic mechanism that is, as yet, poorly understood.) A major new finding of the first application of this technique has been that there is a lot of such adsorption to ice such that the adsorbate is not engulfed in the ice as it grows, but presumably is pushed ahead, along with the moving interface. Many synthesized peptides have been characterized in terms of their effectiveness as antifreezes, but the results have been ambiguous because there are several potential reasons for the variability. This new method can be used to remove some of that ambiguity.

Knight, in collaboration with K. Rider (Colorado School of Mines) completed a study of the crystallization of a clathrate hydrate from its pure melt. The interest in and motivation of the study has been to explain the high variability of crystal habit, which appears to be neither an impurity effect nor a result of crystal imperfections, though it is difficult to prove the latter with certainty. It has thus been a fundamental difficulty in crystal growth theory, because there are no other known reasons for such a variability. The hydrate is a cubic, completely faceted material, with only (111) faces, which, at small sizes and over a substantial range of supercooling, may grow either slowly as octahedra, or much faster as thin plates, or thin needles. The growth manifestations and the ways in which the different forms are initiated are complicated, but it is argued that the growth mechanism is probably dominated by layer nucleation at the face corners. When the faces are small, asymmetry of the corners may lead to very different growth rates on adjacent but crystallographically equivalent faces, causing the plate and needle habits. The early portion of this work was done in collaboration with a group researching clathrate hydrates (especially methane hydrate) for the petroleum industry at the Colorado School of Mines.

The primary goal of mesoscale data assimilation research is to develop and support state-of-the-art data assimilation systems for application in high-resolution mesoscale models. These data assimilation systems can be used for a variety of purposes including the assimilation of data from new observing systems, the optimal use of observations, and understanding the observational requirements for accurate precipitation forecasts and optimal strategies for obtaining targeted observations.

Dale Barker, Yong-Run Guo, Wei Huang and Qingnong Xiao have continued to add additional capabilities to the MM5 3DVAR system. In collaboration with Francois Vandenberghe (NCAR/RAP) and Shu-Hua Chen (post-doc visitor, now University of California, Davis), the direct assimilation of SSM/I brightness temperature has been coded and initial tests have been performed. In order to run 3DVAR in any particular area of the globe, regional background error statistics are required. A system is under development that will interpolate background errors (calculated via the NMC-method of averaged forecast differences) to a chosen domain. The system will then rescale them based on regional/resolution dependent observation/forecast difference values. Al Bourgeois has been working on portability and initial parallelization of the MM5 3DVAR system using the MPP framework of the WRF model. The code now successfully runs on a variety of platforms including the DEC Alpha, SGI, PC/Linux, IBM-SP, Fujitsu and NEC-SX5. An initial subset of 3DVAR has been parallelized and tested. Multiple/single processor results have been shown to be bit-reproducible.

The MM5 3DVAR system has been run in real-time at NCAR since July 2001 for the 135/45 km domains of the AOAWS MM5 model for the Taiwanese Civil Aviation Administration (CAA). More recently, the system has been ported to the CAA Fujitsu VPP5000 and run in real-time. Real-time applications provide an opportunity to test the accuracy and robustness of the 3DVAR system prior to operational implementation in 2002. A project between MMM and the Korean Meteorological Administration (KMA) was initiated in 2001. The goal is the eventual implementation of 3DVAR in the KMA's MM5-based operational regional data assimilation and prediction system (RDAPS). Collaboration among Yong-Run Guo, Dale Barker and Dong-Hyun Shin (KMA) has so far resulted in the porting of 3DVAR to the NEC platform, calculation of background error statistics for the Korean domains (via the NMC-method) and initial case-study assimilation of South Korea's high-density automatic weather station (AWS) surface observation network.

The deployment of radar and satellite remote sensing systems offers great potential to improve numerical simulations of the weather by improving the initial state. There is a significant obstacle to the use of radar and satellite data because the quantities that can be measured by these instruments are not directly usable by the models and tend to be irregularly distributed in time and space. Four-dimensional variational (4DVAR) data assimilation allows these observations to be assimilated directly into the forecast model. However, a principle disadvantage of 4DVAR is its large computational requirement, due to the iterative nature of method and its heavy use of CPU and memory. This limitation has all but prohibited its use at operational weather forecasting centers, and previous experiments have been limited to very small domains or relatively coarse spatial resolutions. John Michalakes has begun a project, in collaboration with scientists at AER Inc., to produce a complete 4DVAR system optimized to run on highly-scalable distributed-memory parallel computers. The current MM5v3.4 forecast model will be the non-linear model in the new 4DVAR system. It is already coded to run efficiently on distributed memory multiprocessor machines. Developing the tangent-linear and adjoint models is accomplished with help from the automated Tangent-linear and Adjoint Compiler (TAMC) and by hand checking the results with visual comparisons the MM5v1 versions. The tangent-linear and adjoint models are then modified to run on distributed-memory multi-processor machines, using the parallelization techniques employed with the basic MM5 forward model. The project, funded by the DoD High Performance Computing Modernization Office, targets completion in January of 2003. At this time the new 4DVAR system will be made available to the MM5 user community and may also be implemented operationally at the Air Force Weather Agency. MMM expects follow-on work with AFRL to focus on WRF 4DVAR.

Yong-Run Guo and So-Young Ha (NCAR/ASP graduate student) have implemented several improvements to the current MM5 4DVAR system. These include: (1) compatibility with MM5 Version 3 for both input and output, (2) addition of a penalty term in the cost-function to control high-frequency oscillation, (3) portability of MM5 4DVAR system to Linux PC computing platform; and (4) compatibility of the new land-use category of the latest version of MM5 release. In addition, several bug-fixes have been released. These new improvements allow the existing MM5 4DVAR system to be compatible with the latest release of MM5, and also allow the MM5 4DVAR system to be operated on high-end linux PC systems. These improvements have been released to the MM5 user community.

Dale Barker and Qinghong Zhang (NCAR/ASP postdoc) have begun to contribute to a NOAA-funded project to determine the potential benefits of a space-based wind-finding lidar for regional NWP. This work is a collaboration among FSL, ETL, NCEP and NCAR/MMM. Initial MMM work has been in the calibration of the 11-day trajectory of the MM5 forecast reference run (defined as truth) in the OSSE. Results indicate that this forecast is a reasonable source for the calculation of simulated observations for later assimilation.

Fei Chen (NCAR/RAP) and Kevin Manning implemented a high-resolution land data assimilation system for the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model, version 5 (MM5). This system is based on a land surface model currently used in MM5 but is run separately from the atmospheric model itself. The purpose of this "offline" implementation of a land surface model is to assimilate, over a significant period of time (several weeks or several months), observed quantities that drive soil moisture and temperature fields, especially observed fields of radiation and rainfall. The final fields of soil temperature and moisture, representing the assimilation of several weeks or months of data, may then be used as initial lower boundary conditions for the atmospheric model.

Juanzhen Sun and Andrew Crook have investigated the sensitivity of storm forecasts with respect to initial conditions obtained through the 4DVAR data assimilation technique. The goal of the sensitivity study is to investigate the features that must be retrieved in order to produce a good forecast and the data required to obtain such features. Both simulated and observed radar data have been used in this study. In the idealized study, a 4-hour simulation of a supercellular convective storm was performed using a sounding obtained during the CASES-97 experiment and a warm bubble initiation technique. This control simulation was used to generate single-Doppler radial velocity and reflectivity data at a frequency of five minutes, similar to WSR-88D data. These data are degraded with regard to coverage and quality and assimilated by the 4DVAR system to provide initial conditions for the subsequent forecast. The environmental wind and moisture were also varied to test the sensitivity of the storm forecast with respect to the environmental conditions. The sensitivity study was performed at the early-growth stage and the mature stage of the storm. The accuracy of the subsequent forecast was then evaluated by its correlation with the control simulation. There are three major findings in this study: 1) the forecast is very sensitive to low-level moisture at the early growth stage, but not as sensitive during the mature stage; 2) the radial velocity observations play a more important role than reflectivity in retrieving the low-level convergence, which is one of the key features in determining the forecast skill; and 3) the lack of low-level radar observations is more detrimental during the mature stage than during the growth phase. Figure 8 shows the rain-water correlation with respect to forecast time for five experiments during the growth phase.

The simulated data experiments have shown that a successful short-term forecast of a supercell can be performed from initial conditions retrieved from simulated radar observations. To determine if this success carries over to real Doppler observations Sun and Crook have performed a number of tests using radar observations of a severe tornadic thunderstorm observed during the CASES-97 experiment. Reflectivity and radial velocity from successive five-minute scans were assimilated into a cloud model using the 4DVAR adjoint method. A number of assimilation and forecast experiments have been performed with varying large-scale conditions (shear and CAPE), assimilation length and microphysical parameters (rainwater fallspeed and evaporation rate). Their initial results suggest that the storm forecast is very sensitive to the large scale conditions as well as the parameterized evaporation rate. The parameterized evaporation allows for the retrieval of a cold pool at low levels.

Figure 9 shows a low level reflectivity observed by the Wichita WSR-88D radar over a two hour time period. Figure 10 shows a numerical forecast of the storm that successfully replicates the longevity and direction of propagation of the storm.

Rebecca Morss continued studying how adding observations to improve atmospheric analyses can influence errors in analyses and numerical forecasts. In collaboration with Kerry Emanuel (MIT), she used experiments with idealized data assimilation systems and forecast models to demonstrate why adding observations can degrade some analyses and forecasts, even with accurate observations and a perfect forecast model. She also identified several circumstances in which current data assimilation systems are more likely to use observational information to improve atmospheric analyses and forecasts.

Ying-Hwa Kuo in collaboration with Tae-Kwon Wee (COSMIC postdoctoral fellow) has performed a set of observing system simulation experiments to assess the potential impact of GPS radio occultation soundings from a COSMIC-like constellation on the regional analysis and prediction over the Antarctic. They first performed a 30-km natural run over a 72-h period. The natural run was then used to generate potential GPS radio occultation soundings from the proposed COSMIC constellation, with realistic orbit parameters. This allowed the simulated soundings to have a realistic distribution in time and space. The simulated COSMIC soundings were then assimilated into a 120-km MM5 model. They showed that the COSMIC GPS radio occultation soundings could provide major improvement in the quality of regional meteorological analysis of the Antarctic. The improvement in the regional analysis would have a significant impact on the quality of regional weather prediction. Compared with earlier studies over the mid-latitudes, the impact over the Antarctic region is appreciably more significant. The relatively large impact of GPS radio occultation soundings is attributed to two major factors: (1) the Antarctic is a data sparse region of the world, and (2) the mass field (which is measured well by the GPS system) dominates the geostrophic adjustment process over high-latitudes.

The overall goal of the WRF Model project is to develop a next generation mesoscale forecast model and assimilation system that will advance both the understanding and prediction of important mesoscale weather, and promote closer ties between the research and operational forecasting communities. The model is being developed as a collaborative effort among the NCAR Mesoscale and Microscale Meteorology Division (MMM), NCEP’s Environmental Modeling Center (EMC), FSL’s Forecast Research Division (FRD), the DoD Air Force Weather Agency (AFWA), the Center for the Analysis and Prediction of Storms (CAPS) at the University of Oklahoma, and the Federal Aviation Administration (FAA), along with the participation of a number of university scientists. Primary funding for MMM participation in WRF is provided by the NSF/USWRP, AFWA, FAA and the DoD High Performance Modernization Office. With this model, we seek to improve the forecast accuracy of significant weather features across scales ranging from cloud to synoptic, with priority emphasis on horizontal grids of 1-10 kilometers. The model will incorporate advanced numerics and data assimilation techniques, multiple relocateable nesting capability and improved physics, particularly for treatment of convection and mesoscale precipitation systems. It will be well suited for a range of applications, from idealized research to operational forecasting, and have flexibility to accommodate future enhancements.

The WRF model has these desirable characteristics: It is designed to be highly modular, and a single source code will be maintained that can be configured for both research and operations. It will be state-of-the-art, transportable, and efficient in a massively parallel computing environment (accommodating vector environments as well). Data assimilation systems and adjoint and tangent linear forms (for 3DVAR analysis and 4DVAR assimilation) will be developed in tandem with the model itself. Numerous physics options will be allowed, thus tapping into the experience of the full modeling community. It will be maintained and supported as a community mesoscale model to facilitate broad use in research, particularly in the university community. Research advances will have a direct path to operations. With these hallmarks, the WRF model is unique in the history of numerical weather prediction in the U.S.

During the past year, the WRF system has advanced substantially, facilitated by real-time experimental forecasting and the community release of WRF for further evaluation and testing. When the WRF model becomes sufficiently mature to be used operationally, it is expected to (1) replace the Meso-Eta model for the operational Threats forecasts at NCEP, (2) replace the MM5 model for operational use by AFWA and (3) take on the function of rapid updating, now served by the RUC model.

Recognizing the research focus within the WRF effort, alternative numerical techniques continue to be explored and adapted to the WRF framework to facilitate the comparative evaluation of their relative accuracy and efficiency in a controlled computational environment. Work has been progressing on three candidate prototype solvers; two of these prototypes are split-explicit Eulerian models based on mass and height vertical coordinates, respectively, while the third is a semi-implicit semi-Lagrangian formulation. Both the mass and height coordinate Eulerian are now available as run-time selectable cores within the WRF model framework.

During the past year, William Skamarock has implemented the Eulerian, split-explicit, flux-form, terrain-following mass coordinate prototype within the WRF model computational framework. The implementation includes 3rd order Runge Kutta (RK3) time integration methods that allow the use of high-order upwind advection operators and do not suffer from the large dispersion errors found in leapfrog schemes. The RK3 scheme, developed by Lou Wicker (NOAA/NSSL) and Skamarock, allows the use of both upwind (odd ordered) and centered (even ordered) high-order flux-divergence operators. It also exhibits low dispersion errors and allows a larger time step when used with either odd or even higher order advection operators. The RK3 scheme has been demonstrated to be robust and, as anticipated, the higher order schemes advection schemes are producing superior solutions at marginal resolution.

Both prototypes have been tested using idealized simulations of a variety of test cases covering a broad range of scales, including simulations of synoptic-scale baroclinic waves in a periodic channel with a 100-km horizontal grid and supercell thunderstorm evolution with a 1-km grid. These simulations and others are providing benchmarks for the WRF prototypes with published solutions from other models, and they demonstrate the robustness and accuracy of the new approaches used in the WRF prototypes. In order to provide an early capability to initialize the mass-coordinate version with real data, David Gill and Jimy Dudhia developed a converter program to interpolate the initial fields from the height coordinate provided by the Standard Initialization Package to the mass-coordinate grid.

Evaluation of idealized simulations has contributed to further improvements in the dynamic-model solver. In conducting mountain wave simulations Oliver Fuhrer (Swiss Federal Institute of Technology, Zurich), encountered artificial disturbances over small-scale terrain in the community release of WRF, as well as in other models. Joseph Klemp and Skamarock demonstrated that these errors are contained in the linear system of equations, and explained their behavior through analytic solutions to the steady-state, finite-difference equations. Their analysis documents that these errors arise if the order of accuracy in computing the metric terms associated with the terrain following coordinates was not the same as the accuracy used for the horizontal advection. The numerics for the metric terms in the WRF code were modified to insure consistency in these calculations. Klemp and Skamarock also used idealized mountain-wave simulation to design and implement a new filter that selectively removes small scale external modes that can arise during startup in the mass-coordinate prototype over regions of significant terrain.

Development of the semi-implicit semi-Lagrangian prototype is being led by Jim Purser (NOAA/NCEP). Purser has developed a package of efficient compact or Pade schemes, and implemented them within the WRF framework. These methods attain a high formal order of accuracy for the spatial operations of differentiation and quadrature, and form an integral part of the high-order, conserving, cascade interpolations used in the grid-to-grid interpolations needed for the semi-Lagrangian calculations. Purser is also developing high order Runge-Kutta time integration methods for semi-implicit solvers, as well as a hybrid vertical coordinate, for the semi-Lagrangian prototype. The solver for this prototype is presently under development and will be evaluated in comparison with the Eulerian prototypes.

Also within the context of the WRF model solvers, Skamarock, Stan Benjamin (NOAA/Forecast Systems Laboratory), Riener Bleck (Los Alamos Laboratory) and Zuwen He (University of Miami) have been developing hybrid coordinate model formulations for the nonhydrostatic compressible equations. The hybrid coordinate takes the form of a terrain-following sigma-like coordinate near the surface and relaxes to an isentropic coordinate (or any other specified coordinate) aloft. Zuwen has developed an initial hybrid approach based on an explicit solution technique that splits the integration of the acoustic modes from the coordinate-surface movement. Successful simulations of baroclinic waves and mountain waves indicate that further testing with convection and nonhydrostatic phenomena is warranted.

The WRF project continues to play a role in the evolution of frameworks and component architectures for high-performance computing in the atmospheric sciences. The WRF-developed Advanced Scientific Framework, in addition to supporting rapid development and deployment of the WRF model itself, has been adapted to WRF and MM5 3DVAR by Al Bourgeois and to other non-WRF models, such as the NOAA/NCEP's non-hydrostatic Eta model (Tom Black, NOAA/NCEP). The WRF software architecture consists of three distinct model layers: a solver layer that is usually written by scientists, a driver layer that is responsible for allocating and deallocating space and controlling the integration sequence and I/O, and a mediation layer that glues these pieces together. It supports a multi-level approach to parallelism adaptable, without change to the source code, to single-processor, shared-memory, distributed-memory and hybrid-parallel systems. The WRF software framework also provides performance portability across micro- and vector-processors. A novel aspect of this modeling system is its use of a data registry. The data registry, designed and implemented by John Michalakes, is the single place where developers list model variables and their characteristics. The WRF project is represented on the NASA-funded Earth System Modeling Framework (ESMF) project to foster reuse and interoperability of software in the geosciences. Work is also underway to leverage developments at DOE, DoD and other institutions to extend WRF software for inter-model coupling.

In order to streamline the handling of I/O throughout the many components of the overall system, Michalakes, Leslie Hart and Jacques Middlecoff (both NOAA/FSL) and Dan McCormick (AFWA) have designed and implemented an I/O Application Program Interface (API) that provides a standard way of specifying and accessing data within the model that is independent of any particular I/O package. For the initial version of WRF, they are using the API to implement the model I/O based on the NetCDF format. Other data formats, such as HDF and GRIB, will be coupled to the I/O API as the code matures. At present, work is continuing to integrate the new I/O interface within the WRF software framework and refine the formats used for the NetCDF output files. Michalakes and Jim Tuccillo (IBM) have developed and implemented the NCEP asynchronous I/O capability called "quilting" into the WRF framework layer responsible for I/O. Quilting designates a number of additional I/O server processes to collect and write model output so that model integration can proceed with minimal interruption due to I/O. Quilting will be part of the WRF 1.2 community release.

The rapid pace of WRF-model development has been greatly facilitated by the modular, hierarchical WRF design. Demonstrating the effectiveness of the plug-compatible WRF model-layer interface and the WRF data registry, Shu-Hua Chen (visitor, AFWA), Jimy Dudhia, Wei Wang, and David Gill, have been able to incorporate numerous physics packages into WRF in remarkably little time. By adhering to the WRF interface specification and coding conventions, the physics packages are automatically interoperable over shared- and distributed-memory parallel computers. The WRF software framework also supports multiple dynamical cores, selectable at run-time. Current options are the two Eulerian dynamic-core prototypes: one a height-based and the other a mass-based vertical-coordinate formulation. The semi-implicit semi-Lagrangian core under development at NOAA/NCEP will be another option when it becomes available.

In addition to interoperability, the WRF software aims at high-performance over a range of computing architectures using a single maintainable source code. WRF is currently ported to and supported on IBM, Compaq, SGI, Sun and Fujitsu systems as well as Linux-clusters (both Intel- and Alpha-based). Evaluation of performance and optimization testing was presented to HPC Asia 2001, an international conference on high-performance computing in September, and this work is continuing. WRF was one of the performance benchmark applications in the recent acquisition of the NCAR Advanced Research Computing System. The WRF real-time forecast system shows good performance and scaling efficiency, running at up to 90 billion floating-point operations per second on the large NSF Terascale Computing System (a 6 Teraflop/second Compaq supercomputer installed late in 2001 at the Pittsburgh Supercomputing Center). The test problem (see Figure 11) is a 12 km resolution 48-hour forecast over the Continental U.S. that captures the development of a strong baroclinic cyclone and a frontal boundary that extends from north to south across the entire U. S. This forecast executes in 10 minutes on 512 of the 3000 processors of the TCS, at a rate of almost 90 Gigaflop/second (not counting I/O time) (see Figure 12).

Jimy Dudhia and Shu-Hua Chen (visitor, AFWA) have continued to collaborate in incorporating a variety of physics options within WRF. The latest packages include a microphysical option, a boundary-layer option, a radiation option from the current Eta model physics packages and a land-surface model very similar to that in the Eta model. Collaborators for these schemes were Tom Black (NCEP/EMC), Fei Chen and Hsiao-Ming Hsu (both NCAR/RAP). S.-H. Chen also generalized the physics interface to work with both the mass and height coordinate versions of the WRF dynamics. These schemes are being tested prior to implementation in the next release of WRF.

The WRF Land Surface Model is being developed by F. Chen, together with scientists at NCEP and AFWA, as part of a project to unify and extend versions of the OSU LSM currently used by NCAR, AFWA and NCEP. David Gill has worked with Dudhia, in collaboration with Brent Shaw and John Smart (both NOAA/FSL), to bring land-surface data into the WRF model to support the LSM. Meanwhile, F. Chen and Hsu, with the help of S.-H. Chen, have implemented the land-surface model into WRF. The land-surface model parameterizes soil moisture, snow cover, skin temperature and vegetation processes. Workshops on land-surface modeling to coordinate this unification effort were held at NCEP in October, 2000, and NCAR in August, 2001.

The development of WRF physics is an ongoing research effort that must rely significantly on community participation. Therefore, a standard physics interface has been designed in order to streamline participation in developing and adapting physics to WRF. In this interface, the model solver calls a generic driver for each class of physics, which in turn calls the specific desired package. Thus, user-developed packages plug into the physics driver through the standard interface, and remain isolated from the model solver.

A major effort undertaken this past year has been the testing of both the height and mass coordinate WRF prototypes in real-time NWP applications. The real-time forecasting experiments are important in at least two aspects. First, they allow the new model to be evaluated under a large number of weather regimes, and synoptic conditions. These forecasts allow the development team to examine the model performance daily and detect any systematic problem. Second, they provide a test bed to test the new model's robustness under various weather conditions. Wei Wang began running the height coordinate model twice daily at NCAR since December, 2000. She began initially with a 30-km horizontal resolution CONUS (Continental U.S.) grid and later a 22-km CONUS grid (a grid similar to that in operational Eta model from NCEP before November, 2001). This prototype is also run daily on a 10-km grid over the Central U. S. The mass coordinate WRF model prototype has been run in real-time configuration since late August, 2001 on a 22-km CONUS grid. All model runs are initialized with Eta model data. Verification of the quantitative precipitation in these forecasts has been provided by Mike Baldwin (NOAA/NSSL/SPC) and compared to forecasts from the NCEP Eta model, an NSSL modified Eta model, the NCAR MM5 model and an NSSL WRF model (Jack Kain, NSSL). These comparisons are displayed on the NSSL Web site, http://www.nssl.noaa.gov/etakf/qpfplots/, and have been very useful for identifying and solving initial problems with the microphysics in real-data applications (see Figure 13).

They have also served as a valuable tool in validating the newer mass dynamical core, and allowed the development team to detect and correct some subtle problems in the mass-version dynamics. The height and mass versions are now producing very comparable forecasts (see Fig. 14). Mike McAtee (AFWA) has also been running real-time WRF forecasts for Air Force theatres and validating results against surface and sounding data. All the real-time WRF forecasts are linked from the WRF Web pages, http://wrf-model.org/REAL_TIME/real_time.html.

One of the primary objectives of the WRF developmental effort is to improve our ability to represent and forecast convective systems in the 6-12 hour time frame. The success of such an effort depends on many factors, including the use of sufficient resolution to represent the convective processes, accurately representing the mesoscale environment of the convective system, and appropriately forecasting the timing and location of significant convective triggering. As an important step in testing WRF’s abilities in this regard, an effort has begun to simulate and forecast significant convective outbreaks. An example is presented here from 11 June 2001. A severe convective system was spawned over western Minnesota early in the afternoon and organized into a large, bow-echo squall line with a dominant cyclonic mesoscale convective vortex in southeastern Wisconsin and northern Illinois that evening. This storm produced widespread wind damage and creating large disruptions in air travel. Real-time 30-km grid forecasts, however, merely indicated the potential for heavy rainfall over an area much broader than the observed event (see Figure 13). Morris Weisman and Wei Wang conducted a 16-hr, 4-km grid simulation with the WRF model initiated at 12 GMT, which forecast the structure, propagation and intensity of this system amazingly well, despite some errors in the timing and location of the initial convective triggering (see Figure 15). Such results offer hope that enhanced resolution can improve the short-term prediction of such significant convective events in meaningful ways.

Stanley Trier and Christopher Davis have begun to develop a suite of test cases over a broad range of meteorological regimes, which both WRF developers and the community can use to better understand the sensitivities of the model. The emphasis in this work has been on selecting observed cases that encompass a wide variety of meteorological regimes, and examining model sensitivities, such as resolution and physical parameterizations (e.g., PBL, cumulus and microphysical schemes) in these cases. These observed cases augment a preexisting set of idealized cases (e.g., supercell thunderstorms, squall lines, 2-D mountain-wave flow). The most detailed examination of model performance and sensitivities for these observed cases has been for simulations of a multi-day episode of organized convection (27-29 May 1998), and a shallow arctic cold front (10-12 December 2000) confined to the locations east of the continental divide. The WRF simulations captured the essence of these meteorologically diverse phenomena and compared favorably with similar simulations using other models (e.g., ETA, MM5). Despite the overall success of these simulations, there were significant errors in important details (in all of the models), including the magnitude of the precipitation in the 27-29 May case and the speed and intensity of the arctic front (particularly near the Continental Divide) on day two of that simulation. The magnitude of such errors were found to be sensitive to model configurations and parameters, such as cumulus schemes (27-29 May) and horizontal resolution (10-12 December). Ongoing and future work comprises an examination of WRF simulations of additional observed cases, including a rapidly deepening midlatitude cyclone, a tropical cyclone and orographically forced precipitation.

In May, 2001, the MM5 3DVAR system was chosen as the starting point for initial data assimilation capabilities of the WRF 3DVAR. Since that time, collaborators at NCEP (Derber, Wu), FSL (Devenyi), AFWA (McAtee) and CAPS (Xue, Gao) have begun work on the inclusion of additional capabilities. As part of these efforts, Wu has added the capability to read observational data files in BUFR format. Barker has modified the grid staggering in the 3DVAR system from the Arakawa B-grid of MM5 to an unstaggered grid, which has chosen for WRF 3DVAR for generality and simplicity. Bourgeois has modified the WRF software framework to accommodate the WRF 3DVAR and has extended the framework’s capabilities to provide parallelism for the 3DVAR code, both for WRF and MM5 applications. All MM5 3DVAR applications have now adopted the WRF 3DVAR system, allowing MMM 3DVAR efforts to concentrate on a single data-assimilation system. The first release of a basic version of the WRF 3DVAR, coupled to the WRF forecast model, is expected around the end of calendar year 2001.

Some of the priority objectives of the WRF project are to make the model and ancillary programs available to the broader research community, to facilitate use of the model in a wide variety of applications, and to solicit participation from the research community in the continuing evolution of the model. Specific tasks within MMM, therefore, include maintenance of up-to-date code, distribution of code, aiding in documentation of the modeling system, maintenance of WRF Web sites and mailing lists, provision of a user support e-mail address, distribution of WRF announcements and organization of WRF workshops and tutorials.

Joseph Klemp, William Skamarock, John Michalakes, Jimy Dudhia, Shu-Hua Chen, David Gill and Wei Wang, in cooperation with WRF developers at NOAA/FSL, released WRF Version 1.0 in December, 2000, followed by Version 1.1 in May, 2001. For these releases, a Web site for registering as a WRF user and downloading code has been provided. The primary supported programs are the model itself and the Standard Initialization. MMM also provides a converter from MM5 input to WRF input and some graphics capabilities with NCL and Vis5d. The user support e-mail address (wrfhelp@ucar.edu or wrfhelp@wrf-model.org) has been established for user questions.

This first release integrates the fully compressible nonhydrostatic equations in scalar-conserving flux form using a time-split small step for acoustic modes. Large time steps utilize the Runge-Kutta techniques discussed above and 2nd to 6th order advection operators can be specified. The vertical coordinate is a terrain following height coordinate that allows variable resolution with height. It is initially a single domain version and contains map-scale factors for conformal projections. The model code is written in standard Fortran 90 and is self-contained. It will run in parallel on both shared-memory and distributed memory platforms. The model can be configured to run either idealized or real data simulations. For idealized simulations, periodic, symmetric or open radiative lateral boundary conditions are available. For real-data cases, initial fields are interpolated from GRIB or MM5 files, and model physics can be selected from the above-mentioned options. Lateral boundary conditions are specified and merged to the interior with a relaxation zone.

During FY01, 375 users registered to download the WRF-Model code, distributed broadly among WRF principal partners, U. S. universities and government labs, the private sector and foreign institutions. In August, 2001, MMM organized and hosted the Second Annual WRF Users Workshop (115 participants) together with a WRF Users Tutorial (80 participants).

In the cooperative development of a complex forecast-model system, care must be taken to insure an orderly process in which the various major components are developed in a consistent fashion and integrated effectively into the overall design. Toward these ends, a Management Plan for the WRF Project was developed by the principal WRF partners and approved by the Interagency Working Group (IWG) of the U.S. Weather Research Program. This Plan established a WRF Oversight Board (Robert Gall as member) that is responsible for overall supervision of the WRF Project. The Plan also includes a WRF Science Board (Jimy Dudhia as member) that provides technical guidance to help WRF meet the needs of a broad user community, and WRF Development Teams that design and implement the various components of the overall WRF system. The WRF Coordinator (Klemp), together with the Development Team Leaders (including Joseph Klemp and Ying-Hwa Kuo), oversees the development efforts to ensure that the overall design goals are achieved and that milestones are accomplished on schedule. To provide specific focus on the major tasks within the development-team areas, approximately 15 WRF Working Groups have been created (including William Skamarock, John Michalakes, Dale Barker, Jimy Dudhia, and Tim Spangler of COMET as Group Leaders).

The WRF Oversight Board met in January to address funding commitments to WRF, and the WRF Science Board met in August, following the Users Workshop, to discuss priorities for development activities. The WRF development teams held planning workshops in February (Washington D. C.) and August (Boulder) to review the status of development efforts, and refine the plans and schedules for future work.

Further information on the WRF project, experimental real-time forecasts and the community-model release is available on the WRF web site, http://wrf-model.org/.

One of the two primary scientific programs in the MMM division is the Cloud and Surface Processes and Parameterizations (CaSSP) Program. The main goals of this program are to quantify the large-scale effects of mesoscale and microscale processes and to develop physically based methods to account for these effects in large-scale models. Until this is done, predictions from large-scale models - including both weather forecasts and forecasts of climate change - will be of limited accuracy. The emphasis within MMM is on understanding how the atmosphere, land and ocean surface and hydrological processes interact, and how these processes can be quantified. This effort includes five key research areas: deep convective cloud systems, microphysics, boundary-layer clouds, surface-atmosphere interactions and the physical chemistry of clouds. Two important components of this effort are nonhydrostatic fine-scale (large-eddy simulation, and cloud-resolving) models. These allow high-resolution definition of the mesoscale and microscale systems involved and are therefore useful for testing methods to quantify the effects of these processes on larger scales. Critical for the success of this program is the evaluation of these models against detailed observational studies of the underlying physical processes. The program contributes to the objectives of the WCRP Global Energy and Water-cycle Experiment (GEWEX) and the Cloud System Study (GCSS) component of GEWEX as well as to the objectives of USWRP.

Wojciech Grabowski investigated interactions between equatorially trapped disturbances and tropical convection. He is using the nonhydrostatic global model (developed by Piotr Smolarkiewicz and collaborators) and the cloud-resolving convection parameterization (CRCP), which represent subgrid scales by embedding a 2D cloud-resolving model in each column of the global model. The model setup in the "super-parameterization" is a constant SST aqua-planet integrated to radiative-convective equilibrium. The cloud-resolving models are aligned along the east-west direction and allow for coupling of thermodynamic variables and zonal momentum. Spontaneous formation of coherent structures with deep convection on the leading edge and strong surface westerly winds to the west (the westerly wind bursts) occur within the equatorial wave-guide (see Figure 16).

These coherent structures, which resemble the observed Madden-Julian Oscillations (MJO), occur in simulations with prescribed radiative cooling, interactive radiation, various orientations of CRCP domains (east-west versus north-south) and various horizontal resolutions of the global model. Interactive surface fluxes seem to be essential for the development, but not the maintenance, of strong MJO-like structures.

Mitchell Moncrieff began an analytic study of the time-averaged MJO based on nonlinear conservation properties of the steady-state Lagrangian equations of motion and thermodynamics. Two primary interacting scales have been identified, namely, a Rossby gyre circulation on an equatorial beta-plane and organized deep convection. Early results give a dynamical interpretation of the equatorial super-rotation that occurred in Grabowski's simulation, as well as the momentum transport properties of the gyre circulation and the organized convection.

b) Multi-scale organization of convection and intraseasonal tropical variability

Wojciech Grabowski and Mitchell Moncrieff investigated the large-scale organization of tropical convection in idealized two-dimensional (x-z) cloud-resolving simulations, using a periodic global-scale domain (20,000 km) and a horizontally homogeneous SST. Two sets of simulations were performed. One used prescribed radiative cooling, and the other included cloud-radiation interaction.

With prescribed radiation, simulated mesoscale organized convective systems several hundred kilometers in scale moved east-to-west at approximately the mean-wind speed (Figure 17). These systems are embedded within west-to-east propagating envelopes of convection several thousand kilometers in scale, whose propagation speeds resemble observed convectively coupled Kelvin waves. Convective momentum transport and the impact of convective systems on the temperature and moisture near the surface are key processes responsible for the large-scale organization of convection. With cloud-interactive radiation, an additional mechanism of organization occurs: large-scale, long-lived convective clusters occur within the ascending branches of weak overturning circulations that are steered by the mean wind. These circulations are a large-scale baroclinic response to horizontal gradients of radiative heating between the moist and dry regions. The robustness of these results is demonstrated by sensitivity tests using different radiation transfer models and microphysical parameterizations, as well as the effects of wind shear.

Wojciech Grabowski and Piotr Smolarkiewicz completed a basic nonhydrostatic anelastic numerical model for simulating moist atmospheric processes on small to planetary scales. The formulation of moist thermodynamics follows standard cloud models, i.e., it explicitly treats the formation of cloud condensate and the subsequent development and fallout of precipitation. In order to accommodate a broad range of temporal scales, the customized numerical algorithm merges the explicit scheme for the thermodynamics with the semi-implicit scheme for the dynamics, where the latter is essential for the computational efficiency of the global model. The coarse spatial resolutions used in present global models result in a disparity between the time scales of the fluid flow and the much shorter time scales associated with phase-change processes and precipitation fallout. To overcome this difficulty an approach based on the method of averages is used, namely, fast processes are evaluated using small time steps. This provides an accurate approximation to the large-time-step integral of fast forcing in a stiff system. This approach allows for stable integrations when cloud processes are poorly resolved and converges to the formulation standard in cloud models as the resolution increases. The theoretical developments were tested in simulations of small-, meso- and planetary-scale idealized moist atmospheric flows. Results from the small-scale simulations demonstrate that the approach compares favorably with traditional explicit techniques used in cloud models. Planetary simulations, on the other hand, illustrate an ability to capture moist processes in low-resolution large-scale flows.

Changhai Liu and Mitchell Moncrieff examined convective structures on an equatorial beta plane in a two-dimensional numerical model forced by constant sea surface temperature and by horizontally homogeneous radiative cooling. Two distinct patterns of spatial distribution of convective activities in the tropics were identified. The first is enhanced off-equator convection in the form of a double intertropical convergence zone (ITCZ). The second was characterized by a single ITCZ on the equator subsequent to quasi-equilibrium being established. Three new physical mechanisms for the ITCZ were identified. First, wind-induced surface flux variations played a vital role in the formation of the single ITCZ centered on the equator. Second, enhanced low-level convergence by planetary rotation, a response to convective heating, favored an ITCZ further from the equator. Third, Coriolis-induced trapping of convection-generated subsidence warming and drying preferred an ITCZ on the equator. The last two opposing dynamical processes compromised as a double ITCZ. This work may help in understanding why global models have difficulty in obtaining realistic ITCZs.

Changhai Liu and Mitchell Moncrieff conducted a two-dimensional numerical investigation into the role of planetary rotation in regulating the nonlinear response of a stratified fluid to steady slab-symmetric thermal forcing. Four kinds of vertical heating profiles mimicking various convectively generated diabatic heating were used. The Coriolis force traps the subsidence-induced adiabatic warming surrounding the heated region. Consequently, the neighborhood of the heat source (active convective area) in a moist atmosphere undergoes enhanced stabilization and drying. This region is thus unfavorable to convection initiation and development. This is a process demonstrated by two-dimensional cloud-resolving simulations of convective systems on f-planes maintained by a constant radiative cooling and surface fluxes of sensible heat and moisture. It is consistent with the observations that convection is more clumped and gregarious in the tropics than in higher latitudes.

Changhai Liu and Mitchell Moncrieff performed two-dimensional cloud-resolving numerical simulations of the evolution of convective cloud systems during the 12-day period from 30 May through 10 June, 1998 in SCSMEX. The analysis of the modeling results indicated that the observed evolving convective systems, evolving thermodynamic fields and evolving surface precipitation were reasonably reproduced. More evaluation of the simulation results against observations and detailed analysis of the model-generated cloud systems, such as radiative fields, surface fluxes, precipitation, thermodynamic budgets, latent heating profiles, condensate distribution, cloud amount, convective mass fluxes and convective organizations, are under way. Sounding data sets were obtained from Richard Johnson and Paul Cieleiski (CSU).

Changhai Liu and Mitchell Moncrieff simulated a continental tropical mesoscale convective band observed during TRMM-LBA. In contrast to widely studied mesoscale convective systems, this one occurred in a very shallow sheared environment and was short-lived and likely initiated by the thermal forcing in the planetary boundary layer. Many features of the observed system were simulated, such as the precipitation pattern, life cycles, convective line orientation and propagation behavior (see Figure 18). Sensitivity experiments indicated that the dynamical influence of ice-phase microphysics was minor for the generation of the convective band, but it was important in the late evolution stage. Sounding data sets were obtained from Steven Rutledge, Walt Peterson and Bob Cifelli (Colorado State University).

Xiaoqing Wu and Mitchell Moncrieff, collaborating with Guangjun Zhang (Scripps Institution of Oceanography) have begun a study of momentum transport by tropical convective systems simulated by a cloud-resolving model. Early results show that both linear and nonlinear contributions to the perturbation pressure field make significant contributions to momentum transport, especially in sheared conditions. Nonlinear terms are generally opposite in sign to the linear ones but of smaller magnitude. The thermodynamic forcing resulting from the buoyancy field within convective updrafts also contributes to the horizontal pressure gradient force across the updrafts. But compared to updrafts, the momentum transport by downdrafts is insignificant.

Wu and Fuqing Zhang (USWRP postdoc, now Texas A&M University) used three-dimensional (3D) cloud-resolving simulations of GATE cloud systems to evaluate two convective momentum parameterization schemes. Using the same large-scale conditions, the Wu and Yanai scheme and the Zhang and Cho scheme reproduced the apparent momentum source obtained from the CRM. The inclusion of cloud-scale pressure gradient in both schemes has a large impact on the in-cloud momentum and the parameterized apparent momentum source, especially in the upper troposphere. The agreement between the CRM-produced and parameterized cloud mass flux contributes to this success.

Wu, Moncrieff and Zhang, in collaboration with Xinzhong Liang (University of Illinois, Urbana-Champaign), incorporated the above convective momentum parameterization scheme in the NCAR Community Climate Model version 3 (CCM3). The 20-year simulation (1979 - 1998) shows a strong impact of convective momentum transport on the ITCZ. Also, the global precipitation distribution is closer to the observed distribution than the control CCM3 simulation.

Xiaoqing Wu and Mitchell Moncrieff previously demonstrated that the parameterization of cloud condensate and cloud fraction in the cloud scheme and the representation of cloud geometric association and inhomogeneity in the radiation scheme need to be improved in order to achieve an accurate energy budget. In a follow-up study, Enrica Bellone (NCAR/GSP), collaborating with Wu, Moncrieff, Doug Nychka (NCAR/GSP) and Bill Collins (NCAR/CGD), are using CRM data to study the impact of vertical cloud overlap assumptions on the radiation fluxes. A first step toward statistical representations of CRM output for sub-grid scale parameterizations processes is to treat clouds at different vertical levels as a Markov chain. The probability of cloud occurrence at a level immediately below is then estimated subject to additional hypotheses; for example, on wind profiles, rainfall intensity, etc.

Philippe Naveau (Institute Pierre Simon Laplace, Laboratorie de Meteorologie Dynamique, Ecole Polytechnique, Paris, France) and Moncrieff completed a statistically based formulation of convective mass fluxes in cloud-resolving simulations of squall lines and non-squall systems. They used extreme-value theory in their statistical formulation that distinguishes between mass fluxes in the convective and stratiform regions of cloud systems. The statistical approach was validated against the mass fluxes obtained explicitly from the cloud-resolving simulations.

Based on the existing EULAG mesoscale model and the adaptive-grid version of the global nonhydrostatic model EULAS, Piotr Smolarkiewicz has developed a new unified nonoscillatory-forward-in-time (NFT) adaptive-grid anelastic nonhydrostatic model code that covers all scales of motion from micro- to planetary. This new model is highly flexible, state-of-the-art in geophysical computational fluid dynamics, and capable of addressing a wide range of problems in geophysical research. The key advance that facilitated the unification of EULAG and EULAS, was treating the singularities at the Poles by posing Neuman conditions on a small circle around each Pole. This will dramatically improve communications in the massively parallel variant of the numerical code. The new unified model has been tested on multi-scale problems, such as micro-scale turbulence, Earth climate and Solar convection. This multi-scale, multi-purpose simulation system is becoming widely used.

Joseph Prusa (Iowa State University) and Piotr Smolarkiewicz continued development and testing of an adaptive grid-refinement approach for the nonoscillatory forward-in-time (NFT), anelastic nonhydrostatic, multiscale model for meteorological research (MMMR). The approach is based upon the use of generalized coordinates and their efficient numerical coding in a generic Eulerian/semi-Lagrangian NFT format. During the past year, several time adaptive stretching methods have been designed and tested. A divergence-free form of continuity - central to the efficiency of the conjugate residual elliptic solver in MMMR - was developed that is applicable to a wide range of problems. This development makes the use of MMMR possible in an equally wide range of applications. The time-adaptive transformations were tested in a mesoscale scenario and, especially, in a series of idealized climate simulations. Two versions, i) an equatorially enhancing meridional stretch, and ii) a bi-latitudinal, meridional stretch with maximum resolution at mid-latitudes, were tested, and the results compared against coarse- and fine-resolution, uniform grid results. The time adaptations were run over several specified periods, during which the coordinates changed from a uniform spherical system to a meridionally stretched one. Corresponding maximum transformed coordinate speeds (grid speeds) ranged from 1 to 15 m/s, the latter fast enough to adapt over 1000 km per day. Details of the climate simulations illustrated that the meridionally stretched cases showed some solution details with accuracy similar to that of the fine-resolution, uniform grid result. But enhanced results do not appear uniformly for all flow variables. This suggests that more sophisticated, self-adaptive coordinate transformations may be required.

Nils Wedi (ECMWF) and Piotr Smolarkiewicz developed an extension of the classical terrain-following coordinate transformation of Gal-Chen and Somerville (1975). This which accounts for a time-dependent curvilinear upper boundary in meteorological models formulated in a linear vertical coordinate. The derived mathematical framework has been implemented in the all-scale nonhydrostatic anelastic NFT model EULAG (see Figure 19). This development enables a thorough study of the impact of upper boundary conditions on the internal flows. For example, it allows for a direct comparison of Neumann and Dirichlet pressure boundary conditions, typically associated with rigid-lid and free-surface upper boundaries, within a single nonhydrostatic model code. The latter is essential as it eliminates uncertainties associated with differences in analytic/numerical formulations of various meteorological models with different upper boundary conditions. Aside from purely theoretical interests, this work appears to offer numerous benefits for practical applications. For example, it facilitates nesting tropospheric small-scale nonhydrostatic cloud models with large-scale hydrostatic isentropic/isobaric deep stratospheric models. Another example would be how it facilitates incorporating tidal cycle in ocean models with rigid upper boundary. Up to date, Wedi and Smolarkiewicz have performed a series of idealized Boussinesq-flow studies to address the impact of the upper boundary. One interesting finding is that accurate prediction of the shape of a material Neumann boundary may suffice to prevent/mitigate spurious reflections of gravity waves - notorious in atmospheric models.

Len Margolin (Los Alamos National Laboratory) and Piotr Smolarkiewicz continued their long-term study of implicit turbulence modeling properties of nonoscillatory solvers, which have demonstrated capability for simulating turbulent flows. An important result is that implicit turbulence modeling is free of unphysical parameters, which implies a significant increase in predictability. Another advantage important for applications lies in the simplicity and computational efficiency of the approach. By comparing with pseudo-spectral simulations of 3D periodic turbulence (accepted as the most accurate tool for direct numerical simulation of low Reynolds number flow), Margolin and Smolarkiewicz validated the accuracy and demonstrated the computational efficiency of the EULAG code (see Figure 20). They also analyzed large eddy simulations in the limit of very high Reynolds number, a regime in which the pseudospectral code cannot operate. They demonstrated convergence for energy spectra at two limits - as resolution is increased and as viscosity is decreased. They developed a theory that quantifies their resolution study and leads to the prediction of an asymptotic spectrum.

Dimitris Drikakis (Queen Mary, University of London), Margolin (Los Alamos National Laboratory) and Piotr Smolarkiewicz continued their investigation of the formation of spurious vortical structures in incompressible flow simulations. Recently several papers have appeared in the computational fluid dynamics literature, proposing an idealized instability problem as a benchmark for discriminating among numerical algorithms for two-dimensional Navier-Stokes flows. The problem is a double shear layer simulated at coarse resolution and with prescribed interface perturbation. A variety of second-order accurate schemes were tested. Results fell into one of two solution patterns - one with two eddies (the accepted correct solution) and the other with three eddies (see Figure 21). They draw the following conclusions. First, the third eddy is a product of truncation error details. Second, the three-eddy solution is the more physically realizable. And finally, this problem is a poor choice of benchmark to discriminate among numerical algorithms.

Andreas Dornbrack (DLR), Joseph Prusa (Iowa State University) and Piotr Smolarkiewicz continued their study of three-dimensional instabilities of counter-rotating vortices that occur in the wake of aircraft. This past year they performed a series of two- and three-dimensional numerical simulations of vortex decay. Effort was concentrated on the application of mesh refinement techniques in nonoscillatory forward-in-time schemes for the proper resolution of the vortex core. In particular, they validated the numerical model by comparing the two-dimensional simulation results with analytical solutions for the viscous decay of a single Rankine and Lamb-Oseen vortex. For a further examination of the model, they compared numerical simulations with recent experimental results of Leweke and Williamson (1998) who investigated the three-dimensional instability of a counter-rotating vortex pair to short waves. The present three-dimensional simulations of the vortex decay agree very well with the measurements. In addition to the model validation study, numerical simulations have been extended to establish the Reynolds number dependence of the simulated structures. Another series of runs tested the numerical scheme using dynamic grid adaptation that allows the computational grid to deform, to follow features of interest in an evolving solution (see Figure 22). Simulations with stationary grids for two-dimensional vortex decay indicate that an adequate resolution of wake vortex cores in the atmosphere with minimized overall computational effort is possible with this technique.

This study is collaborative among researchers from NRL (A. Warn-Varnas, S. Chin-Bing, D. King, E. Salusti, S. Piacsek and J. Hawkins) and Smolarkiewicz. Synthetic aparture radar (SAR) images, towed measurements, and conductivity-temperature-depth (CTD) casts from the JANE1984 cruise taken in the Gulf of Gioia around Cape Vaticano show the presence of an oceanic cold front. A hypothesis for its genesis is the breaking of internal tidal-generated solitons. This hypothesis was tested using the Boussinesq option of EULAG. An analysis of the energy distribution in presence of solitons among the acoustical normal modes and bottom loss was performed. Transmission loss patterns were established for diverse conditions. Simulations indicate that small spatial changes in the soliton field can have a pronounced effect on the acoustic propagation. The solitons produce acoustic mode coupling that can combine with the ocean-bottom acoustic mode to significantly affect the acoustic signal.

Piotr Smolarkiewicz, Igor Szczyrba (University of Northern Colorado), and Christopher Cotter (Cheshire Cat Computers, Inc.) further advanced their biomechanical modeling of brain injuries. Due to its elasticity, brain material can support shear (equivoluminal) waves. Earlier attempts to explain certain brain injuries via arguments of the classical theory of viscoelasticity exploited the Voigt model - a linear system of differential equations where the motion of the brain tissue depends merely on the balance between viscous and elastic forces. But Voigt model solutions have limited realism. For example, they evince strongly localized displacements of the brain tissue. The Voigt model was extended to a nonlinear viscoelastic fluid model. The resulting non-Newtonian fluid model permits nonlinear wave-front steepening, wave overturning and turbulent breaking. The numerical procedure was validated against small-perturbation linear theory and known Voigt solutions. The nonlinear numerical results suggest the existence of "brain turbulence," with relevance to highly localized brain injuries.

James Dye, Eric Defer (USRA), Sharon Lewis, Geoffrey Dix and Wiebke Deierling (University of Hannover, Germany), participated in the Airborne Field Mill (ABFM) project at Kennedy Space Center (KSC) in Florida during June 2000, February 2001, and June 2001. Lightning is a serious problem at KSC. (Strict Lightning Launch Commit Criteria were developed after the lightning strike to an ATLAS- Centaur launch in 1987.) The objective was to obtain simultaneous in-situ airborne measurements of the electric fields and microphysical content in anvils and thick clouds near KSC using the Univ. of North Dakota Citation jet aircraft. The aircraft was instrumented with 6 field mills designed and built by NASA Marshall Space Flight Center (MSFC) and an extensive array of microphysical probes which covered the range from a few microns to several millimeters including the new SPEC Cloud Particle Imager and the High Volume Particle Spectrometer. The aircraft measurements were made in coordination with radar measurements from the Patrick Air Force Base 74-C radar and the Melbourne NEXRAD radar. Measurements from the KSC LDAR and Cloud-to-Ground Lightning Sensing System, and surface electric field mill network provide information on lightning and surface electric fields.

In particular, the objective is to determine decay rates of electric fields in time and space within the anvils over KSC after lightning has occurred in the parent storm. These decay rates are to be compared with theoretically predicted rates, in a joint effort among NCAR, Hugh Christian, Monte Bateman and Doug Mach (both NASA MSFC), Tony Grainger (University of North Dakota), Phil Krider and Natalie Murray (both University of Arizona) and Paul Willis (NOAA Hurricane Research Division). Penetrations in a given storm initially were flown in the neighborhood of the convective cores of storms. Subsequent passes were made in the anvil at different distances downstream to examine the spatial and temporal decay of the electric field. In order to determine relationships between electrification and microphysics, spiral ascents and descents were made.

Analysis of these data sets is underway. Early results suggest that when the reflectivities in the anvils get low, approximately below 10 to 15 dBZ, the electric fields have decayed to a few kV/m or less. This and other comparisons with vertical reflectivity structure suggest that much of the charge in the anvils may be carried by precipitation-sized particles, which can sediment out of the anvil. But this hypothesis needs much more testing. Also, a radar-based rule might be useful for indicating when there is little hazard from natural or triggered lightning in anvils.

The properties of ice cloud layers sampled in Brazil and Kwajalein, Marshall Islands, during two TRMM field campaigns were studied by Andrew Heymsfield, Aaron Bansemer, James Dye and William Hall in MMM, Jeff Stith (NCAR/ATD), Paul Field (U.K. Meteorological Office), Tony Grainger (University of North Dakota) and Steve Durden (NASA/JPL). They studied the evolution of the particle size distributions and habits in the vertical during slow, Lagrangian-type spiral descents through ice cloud layers that were on average four kilometers deep. New instrumentation was used yielding better information on the concentrations of particles in the size range between 0.2 and 2 cm. The size distributions were found to have broadened from cloud top towards cloud base, with the largest particles increasing in size from several millimeters at cloud top to one centimeter or larger towards cloud base (Figure 23).

Also noted was that the concentrations of particles less than 1 mm in size decreased with decreasing height. The result was a consistent change in the PSDs in the vertical. Aggregation - as ascertained from both the changes in the PSDs and evolution of particle habits as observed in high detail with the Cloud Particle Imager (CPI) probe - was responsible for these trends. The size distributions were fitted to curves of gamma distribution form, and it was found that the gamma fit parameters vary in a systematic way in the vertical. A set of equations that can be used to derive bulk properties including the extinction, ice water content and precipitation rate were developed to facilitate the use of the size distributions and related moments in cloud-resolving and general-circulation models.

c) Parameterization of extinction coefficient and radiative properties in midlatitude ice clouds

A study of how ice crystal cross-sectional area varies with size and in the vertical was conducted by Andrew Heymsfield and Larry Miloshevich. They used data collected from aircraft and balloon borne ice crystal replicators during studies of synoptically-generated cirrus cloud layers in Wisconsin and Oklahoma. The area of ice particles was cast in terms of the "area ratio," which is the ratio of an ice crystal's projected cross-sectional area to the area of a circle having the crystal’s maximum diameter. It was found that the slope coefficient of the power-law area ratio versus particle diameter relationship was roughly constant in the lower half of the cirrus clouds studied, but becomes steeper with increasing height in the upper half of the cloud column (See Figure 24). The height dependence of this relationship was attributed to the processes of crystal growth, aggregation and sublimation and their impact on the crystal shape and other crystal characteristics. Profile measurements from 10 cirrus clouds were combined to produce a single parameterization for the mean trend in area ratio with diameter, and for the dependence of its coefficients on fractional height within the normalized cloud column.

Using the same data sets, Shaima Nasiri and Bryan Baum (both University of Wisconsin), Ping Yang (Texas A&M University), Andrew Heymsfield, Larry Miloshevich and Sharon Lewis studied the sensitivity of the scattering properties of ice cloud layers to the underlying assumptions of the particle size and habit distributions. They found that the near-infrared bands are sensitive not only to the discretization of the size distribution, but also to the assumed habit distribution. Additionally, the effective diameter calculated from a given size distribution tends to be sensitive to the number of size bins that are used to discretize the data, as well as to the ice crystal habit distribution. Using the results of this study, they developed new scattering models that were developed for a suite of wavelengths spanning visible, near infrared, and the infrared, for the retrieval of cloud properties from satellite radiometers.

Despite its tropical origin, the upper two-thirds of a typical hurricane is made up largely of ice. During the fourth Convective and Moisture Experiment (CAMEX) in August and September, 2001, Andrew Heymsfield and Aron Bansemer, together with Cindy Twohy (Oregon State University), Kevin Noone (Stockholm University), Paul Willis (NOAA Hurricane Research Division) and Paul Lawson (SPEC, Inc.), studied the microphysical properties of the ice regions of hurricanes. A suite of seven instruments that measured particle size distributions and ice water content was used to study the fraction of the total condensate in hurricanes that is lofted to the middle and upper troposphere (Figure 25). Sampling during three days when tropical storm Humberto developed into a hurricane, and then diminished back to tropical storm status, will be used by these researchers to study the changes in hurricane microphysical properties over the course of its life cycle.

A versatile software package to process microphysical data collected by a variety of imaging probes on a multitude of aircraft was developed by William Hall, Aron Bansemer, Andrew Heymsfield and James Dye. The software package is written in the IDL programming language and uses image analysis techniques to derive accurate particle dimensions and concentrations. The software is designed to ingest data from most data acquisition systems that are used in the community. This package has been used in the ice cloud studies cited above and has been shown to produce more accurate size distributions than have been obtained using previous software packages.

Chin-Hoh Moeng participated in two Large-eddy Simulation (LES) modeling studies. The first was of the shallow cumulus regime over the trade-wind region characterized in the western Atlantic Ocean by the Barbados Oceanographic and Meteorological Experiment (BOMEX). This study, led by Pier Siebesma (KNMI, Netherlands), compared results from ten LES groups to evaluate simple parameterization schemes currently used in GCMs. The second was a study of the diurnal cycle of shallow cumulus over land from a synthesis of observations at the Southern Great Plains Atmospheric Radiation Measurement (ARM) site. The simulated cloud field compared well among the eight LES groups worldwide and also agreed well with the observations. Mary Barth and Moeng are applying these cumulus simulations in an examination of the effects of cumulus clouds on chemical transport and reaction rates, while Ned Patton (visitor from PSU), Peter Sullivan and Moeng will examine cumulus effects on land-PBL interaction.

Two-dimensional (2D) models (e.g., most of the existing Cloud-Resolving Models) are often used to simulate atmospheric turbulence and cloud fields. Those results can then be used to calibrate or develop one-dimensional parameterization schemes for use in weather or climate models. PBL turbulence and clouds are inherently three dimensional (3D), and hence statistical properties from 2D and 3D simulations can be significantly different. To investigate statistics obtained between 2D and 3D simulations, Chin-Hoh Moeng, Peter Sullivan, and Jeff Weil (visitor, CU/CIRES) began a systematic study, which will cover both clear and cloudy PBLs. They built a 2D version of the NCAR 3D LES code, keeping all large-scale forcing and numerics the same between the 2D and 3D versions. Certain statistics, such as the total heat flux, are constrained by the forcing and hence are expected to be the same regardless of whether the turbulence is 2D or 3D. However, many statistics and turbulence features are expected to be sensitive to the dimensionality. The goal is to compare turbulence and cloud statistics of 2D and 3D flows and provide reasons for their similarities and differences.

Entrainment of air from the overlying free atmosphere across the top of marine stratocumulus cloud decks is central to their subsequent evolution. This has been extensively studied by LES, but confirming observational data have been difficult to obtain because of the difficulty in measuring the entrainment rate. Recent developments in aircraft instrumentation and observational techniques have led to the field study, Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II). This field campaign was carried out with the NCAR C-130 aircraft during July 2001 about 300 km off the coast of southern California by Bjorn Stevens (UCLA), Donald Lenschow, Gabor Vali (Univ. of Wyoming), Christopher Bretherton (Univ. of Washington), Alan Bandy (Drexel University) and Hermann Gerber (Gerber Scientific).

Noteworthy measurement capabilities included fast-response measurements of dimethyl sulfide, which has optimal tracer properties for measuring entrainment (See Figure 26); fast in-cloud measurements of temperature (S.P. Malinowski, Warsaw University, Poland), humidity and liquid water; the Wyoming Cloud Radar and the NCAR Scanning Aerosol Backscatter Lidar (SABL) which provide remote measurements of cloud and drizzle structure; and the NCAR GPS dropsondes for vertical profiles of temperature, humidity and winds. A total of nine research flights (about 82 hrs) were flown. All but one was at night, to avoid the complicat