• Convection initiation
• Convective evolution in comple mesoscale environments
• Orographic effects
• Long time scale dynamics of mesoscale convective systems
• Cloud microphysics and precipitation

Stanley Trier and Ahijevych continued research to determine how information from the Rapid Update Cycle-II (RUC-II) analyses and forecasts can be used to assess the potential for development, growth or dissipation of organized deep convection over 0-2 h. An objective algorithm was developed that determines the depth of thermodynamically unstable conditions over locations in the eastern two-thirds of the United States, using a combination of 1-h RUC forecasts and extrapolated analyses. Fuzzy logic is used to incorporate information into the algorithm on other RUC-derived environmental thermodynamic and kinematic parameters that are important to convective initiation and evolution including vertical wind shear, relative humidity, and differential advections. In particular, these additional parameters guide the modification of the threshold values of CAPE (convective available potential energy) and CIN (convective inhibition) favorable to support convection in differing meteorological situations.

Trier and Ahijevych collaborated with Cindy Mueller, Dan Megenhardt, and Nancy Rehak (each of NCAR/RAP) to implement the experimental algorithm in real time (August 2002). Preliminary evaluation of algorithm performance has indicated that the "depth of instability" diagnostic is particularly effective in nowcasting growth or decay of mesoscale convective systems that often occur from after dark, until slightly after sunrise, over the central United States. This has important practical implications for routing of commercial air traffic.

Related website: http://www.mmm.ucar.edu/bamex/science.html

Tuttle and Carbone completed a study of a long-lived convective episode (duration of 50 hours and span of over 2800 km) that occurred over the central U.S. in mid-July 1998. The event consisted of two mesoscale convective systems (MCSs). The initial weaker MCS traveled eastward across the upper plains states (Fig. 2) into northern Minnesota, where it decayed. Interactions between the cold pool of the decaying system, and a strong moist southerly flow, led to the generation of an intense MCS that traveled south. The investigators found that favorable cold pool, low-level wind shear interactions were largely responsible for the longevity of these systems. Figure 3 depicts the radar reflectivity evolution in a time-longitude plot (Hovmoller) where data have been averaged in the latitudinal direction. Figures 4 and 5 repeat the contours of reflectivity superimposed on Hovmollers of the surface moisture (mixing ratio) and the low-level wind shear (over a 2.5 km depth). The important thing to note is that the second MCS traveled mostly to the east side of the moisture plume (Fig. 4), but followed the core of strong southerly shear almost perfectly. Even though the air was very unstable on the west side of the moist plume (CAPE as high as 4000), the shear vector was weak and of the wrong orientation to initiate or maintain convection. High moisture and instability, alone, were not enough to ensure the longevity of this system.

Figure 2. Swath of radar reflectivity for July 14-15, 1998. Swath is created by plotting reflectivity data at one-hour intervals. The first MCS initiated over southwestern Montana around 21:00 Z on July 13 and traveled eastward across Montana, North Dakota and into northern Minnesota. Second MCS formed from the cold pool of the first system and traveled southward through Minnesota, Iowa and Kansas.

Figure 3. Hovmoller depiction of radar reflectivity where data have been average in the latitudinal direction. The first MCS enters the domain at 0:00 Z on July 14 at -110 deg longitude and dissipates at 2:00 Z on July 15 at -87 deg. Second MCS can be seen forming at 18:00 Z on July 14 at -95 deg. and decaying at 20:00 Z on July 15 at -98 deg.

Figure 4. Hovmoller depiction of reflectivity (contours) superimposed on low-level mixing ratio taken from 3-hourly RUC analyses. Note plume of moisture centered on -97 deg (between 12:00 July 14 and 12:00 July 15) and that the second MCS stayed mostly on the east side of the plume.

Figure 5. Hovmoller depiction of reflectivity (color contours) superimposed on low-level wind shear vectors (shear calculated over the lowest 2.5 km of the RUC analyses). North is taken toward the top of the page in the usual sense. Note that the second MCS followed the core of strong southerly shear almost perfectly.

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 principal investigators at NCAR, NSSL, NWS and several universities (e.g., University of California Los Angeles, University of Oklahoma, St. Louis University, Texas A&M University, Pennsylvania State University, Colorado State University and University 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 mesoscale convective vortices (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 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, a wind profiler, an acoustic sounder radiometer, a mesonet, and soundings.

Rotunno and Rosella Ferretti (University of L'Aquila, Italy) analyzed data from the Mesoscale Alpine Programme (MAP) and conducted companion numerical simulations of two MAP cases that behaved very differently, in spite of the fact that their respective forecasts were similar. Their analysis suggests that the most important difference between the two cases was the presence of a cold stable air mass in the Po Valley in Intensive Observation Period 8 (IOP8). This persisted through the period in which the large-scale moist tongue was progressing eastward and prevented the most humid air from reaching the Lago Maggiore-Toce Area (LMTA). They also found that the greater rainfall in the LMTA in IOP2B was augmented by the development of conditional instability (with associated convective rain), due to the effect of the Alps on the eastward passage of the moist tongue; in IOP8 the atmosphere remained locally stable throughout the period.

Powers and Davis completed a publication summarizing the analysis of a cloud-resolving, large-domain simulation of the development of Tropical Storm Diana (1984), given only synoptic-scale features in the initial condition. The simulation was performed using the MM5 model on a single domain of 1.2-km grid spacing, and dimensions of 1000x1060x37, run on 552 processors of SCD's IBM SP. The simulation revealed that the mesoscale vortex responsible for the incipient tropical storm formed within a large MCS that developed where synoptic-scale ascent in the lower troposphere maximally destabilized the thermodynamic environment. The remnant vortex organized convection on a time-scale fast enough to compensate for the deleterious effects of the vertical wind shear in which it was embedded. As the convection became more widespread, the shear over the center weakened. As the tropospheric shear decreased to only 2-3 m s-1, a period of rapid intensification and scale contraction ensued as the simulated vortex became a strong tropical storm.

Davis and Lance Bosart (State University of New York, Albany) examined the formation of several tropical cyclones during the 2000 and 2001 Atlantic hurricane seasons, concentrating on late season storms forming poleward of 20oN. All ten such storms featured a baroclinic precursor, generally a sub-synoptic-scale trough that had extruded deep into the subtropics and initiated cyclogenesis, albeit very weak in some cases, along a remnant frontal boundary. In all developing cases, the vertical shear was initially near, or larger than, the empirical threshold for tropical cyclogenesis; in all cases, the shear decreased markedly prior to the formation of a tropical storm. Davis and Bosart hypothesized that in weakly baroclinic cases, the pre-existing disturbance focused convection that subsequently produced a mesoscale vortex capable of self-amplification, similar to Diana (1984). In stronger cases, frontal cyclogenesis provided the initial disturbance capable of self-amplification, through air-sea interaction. Through a detailed modeling study of Hurricane Michael (2000), it was concluded that diabatic transport and redistribution of potential vorticity in the early convection was responsible for the rapid decrease of vertical wind shear over the nascent storm center (Fig. 6).

Figure 6. Infra-red satellite images of the frontal cyclone preceding Michael (left, 1200 UTC 15 October, 2000) and Michael itself (1200 UTC 17 October, 2000).

During the past year, Melvyn Shapiro, long-term visitor from NOAA/Environmental Technology Laboratory, studied initial condition sensitivity and error growth in forecasts of the 25 January 2000 East Coast snowstorm. This is one of the first studies to demonstrate that regions of forecast sensitivity to initial- condition (analysis) errors propagate with the group velocity of expanding Rossby-wave packets. Work also included the assimilation of Total Ozone Mapping Spectrometer (TOMS) total ozone for improved prediction of extratropical weather systems. This collaborative effort with Kun-il Jang, Xiaolei Zou and Qiang Zhao (all Florida State University) and Arlin Kruger (NOAA/Goddard Space Flight Center) presents a method and results of incorporating TOMS satellite observations of total columnar ozone into the data assimilation and prediction of extratropical cyclones. Results suggest that the assimilation of satellite measurements of ozone can improve the skill of operational weather forecasts.

Shapiro also conducted studies on large-amplitude gravity-waves breaking over the Greenland lee, and the subsequent formation of downstream synoptic-scale tropopause folding and stratospheric-tropospheric exchange. The most important findings in this study are: 1) the major influence of large-amplitude topographic gravity waves in the development of downstream tropopause basal jet streams and subsequent explosive lee cyclone development; 2) the topographic excitation of Rossby-wave packets by Greenland and their effect on forecast skill over Europe and North Africa on 24-72-h time scales; and, 3) the role of topographic gravity waves in the exchange of air and trace constituents between the stratosphere and troposphere.

Some of the most severe weather events nationwide are commonly associated with individual storms, whether isolated or within much larger convective systems. The incomplete representation of precipitation physics, especially the ice phase, remains a significant impediment to improving the quantitative forecast of warm-season precipitation. Critical components of these forecast problems include a limited understanding of the many processes that control the precipitation output, as well as the inability of current microphysical parameterization schemes used in numerical models to reproduce the observed range of precipitation characteristics and important details about the lifecycles of convection.

Morris Weisman and Jay Miller continued their comparison of WRF model simulations of storms with those observed during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) field campaign, conducted near Goodland Kansas in May-July 2000. Two storms in particular, July 5 (Fig. 7) and June 29 (Fig. 8), cover the broad spectrum of Low-Classic-High Precipitation supercell storms and present the opportunity to improve the understanding of precipitation processes and their impact on storm lifecycle. The most basic observed features such as overall storm orientation and movement, as well as rotating updrafts, are reasonably well modeled. However, there are deficiencies apparent in the WRF model representation of precipitation processes that prevent it from replicating the simplest of observed features, such as bounded weak echo regions (BWERs).

Figure 7. Horizontal sections of (top) radar reflectivity in dBZ, (bottom) updraft in m/s with overlaid horizontal wind vectors at mid-levels for the STEPS-2000, June 29 case. The observed and WRF-simulated structures are shown on the left and right sides, respectively.

Figure 8. As Fig. 7, except June 29, STEPS-2000 case.

Charles Knight commenced a systematic analysis of the STEPS radar data, examining the first echo development in the cumulus clouds with the special purpose of finding what the multiparameter data from S-band Dual Polarization Doppler Radar (S-Pol) might reveal about the first formation of precipitation in this continental area. (This kind of investigation had only previously been conducted in Florida, with a much more prominent warm rain process.) The analysis is not yet complete, but several instances of positive ZDR columns have been found, briefly accompanying the very first development of radar echo from precipitation. These exist only briefly, presumably because they are composed of supercooled raindrops in very low concentrations, that freeze soon and cease producing the positive ZDR signal.

Knight is analyzing the case of 23 June, 2000 from STEPS with William Hall, Miller, and Andy Detwiler (South Dakota School of Mines and Technology). Penetrative convection develops within preexisting anvil precipitation (which might also be described as a stratiform region of cloud that starts as anvils). The two interests are the unusual microphysics of precipitation growth (within a convective cloud that develops within a preexisting field of ice crystals), and the origin of the instability. One hypothesis for the instability is mid-level cooling caused by evaporation of the anvil precipitation. The multi-parameter radar data clearly show that most of the precipitation growth in the new convective elements is by riming, presumably of the preexisting ice crystals; but the ZDR data also suggest that water drops are raised from below the melting level and grow by coalescence, while ascending. Figure 9 shows dBZ, radial velocity, ZDR, and LDR in a vertical slice, through a very early stage of one of the convective elements.

Figure 9. A vertical slice, from SPol radar data, of reflectivity factor (dBZ, upper left), radial velocity (m s-1, upper right), ZDR (dB, lower left) and LDR (dB, lower right) through a convective element in an early stage, forming within anvil precipitation. The crosses show a 5 km grid, with the origin at the SPol radar (1.1km MSL). The melting level is at about 3km MSL but is not uniform, responding to the convective motions, also shown in the radial velocity field. Note the enhanced radar echo at 20-25 km range, about 4-6 km height: and the slightly negative ZDR associated with it, both of which are interpreted as a result of riming, probably on preexisting ice in the anvil precipitation. The ZDR within the anvil precipitation is about 0 dB (equidimensional ice) above the melting level and about 0.5 dB (drizzle with drops a little less than 1 mm in diameter) below.

NCAR investigators Andrew Heymsfield and Aaron Bansemer worked together with investigators Michael Poellot (University of North Dakota), Cynthia Twohy (Oregon State University), and Hermann Gerber (Gerber Scientific Corp.) in the Cirrus Regional Study of Tropical Anvils and Cirrus Layers Florida Area Cirrus Experiment (CRYSTAL FACE) project during July 2002. Their goal was to study the properties of ice cloud layers in Florida using aircraft measurements. Direct measurements of the condensed (liquid + ice) water content and extinction by instruments developed by these investigators represent some of the first direct measurements of these properties in clouds formed in association with deep convection.

On July 26, the UND Citation aircraft spiraled downwards from above to below the melting layer of a deep cloud layer, measuring the condensed water content directly (Fig. 10). These observations will be used to develop more reliable models of the complex microphysical processes operative within the melting layer, to evaluate how passive microwave remote sensors are influenced by the ice particle melting, and to develop algorithms to retrieve microphysical properties in the melting layer from radars. It will also be possible, for the first time, to directly calculate ice particle melting rates and the associated cooling of the air within the melting layer.

Figure 10. On July 26, the UND Citation aircraft spiraled downwards from above to below the melting layer of a deep cloud layer, measuring the condensed water content directly.

Heymsfield and Bansemer, together with Twohy (Oregon State University) and Kevin Noone (Stockholm University, Sweden) have been investigating cloud particle data collected by airborne probes during the 4th Convection and Moisture Experiment (CAMEX-4) during the summer of 2001. Their research focused on a 3-day series of flights through Hurricane Humberto, one of the very few times that microphysical data has been collected in the upper regions of a hurricane. Horizontal flights through the center of the hurricane (Fig. 11) reveal many small particles (high concentrations in red, sizes along y axis) in the eyewalls, and larger, lower concentrations of particles farther from the storm center. They also show regions of enhanced ice particle aggregation in the outer rainbands. Heymsfield and collaborators are using this data to better understand the aggregation process at colder temperatures, and to document the microphysical characteristics of such storms in order to increase the use of detailed microphysics in hurricane models.

Figure 11. Andy Heymsfield and Aaron Bansemer, together with Cynthia Twohy of Oregon State University and Kevin Noone of Stockholm University have been investigating cloud particle data collected by airborne probes during the 4th Convection and Moisture Experiment (CAMEX-4) in the summer of 2001. Their research focuses on a 3-day series of flights through Hurricane Humberto, one of the very few times that microphysical data has been collected in the upper regions of a hurricane. Horizontal flights through the center of the hurricane (see figure) reveal many small particles (high concentrations in red, sizes along y axis) in the eyewalls, larger, lower concentrations of particles farther from the storm center, and regions of enhanced ice particle aggregation in the outer rainbands. They are using this data to better understand the aggregation process at colder temperatures and to document the microphysical characteristics of such storms to increase the use of detailed microphysics in hurricane models.

Knight analyzed data on the time-dependence of nucleation of ice from water by AgI as a function of supercooling, published by Vonnegut and Baldwin (1984, J. Climate and App. Met. 486-490) in terms of the classical theory of heterogeneous nucleation. The dependence of nucleation rate upon supercooling appears to be much too small to be explained by the theory, however, further examination of the possibilities will be carried out.

Knight has constructed a new ice single-crystal hemisphere-growing device, to be used in the laboratory for studies of ice growth from pure water and from solutions, particularly of the biological antifreezes. The ice is grown from a single-crystal seed at the end of a cold finger, from water or solution, at a controlled temperature. With the solution temperature controlled at a few tenths of a degree C above freezing, and the cold finger a few tenths below, the ice hemisphere (several cm in diameter) grows with a very shallow temperature gradient across its interface, and its shape is very sensitive to differences in interfacial structure and growth mechanism. Large basal facets have been produced from pure water in this way, but the technique has yet to be applied to the effect of solutes on ice growth.