CCP is an interdivisional program, established 1 October 1994, to improve the basic knowledge of cloud-related processes and their parameterization in climate models. The objectives of CCP were established by an executive committee advisory to the NCAR Director and consisting of William Cooper (joint appointment with ATD), Donald Lenschow (joint appointment with ATD), and Mitchell Moncrieff; Jeffrey Kiehl and Kevin Trenberth (CGD); and Brian Ridley (ACD). Initially, CCP will focus on two types of cloud systems-precipitating tropical convection and marine stratocumulus. The main reason for these choices are: 1) they are important in the climate system; 2) improved parameterizations of deep and shallow convection are required for general circulation models (GCMs); 3) cloud resolving models that explicitly treat motions of scale ranging from 1 km to more than 1000 km have advanced to a point where good progress is anticipated in spanning the gap between GCMs and small-scale processes; and 4) the NCAR WB-57F and C-130 aircraft provide new observational opportunities.
While still a fledgling program, CCP has progressed in three areas: 1) modeling of cloud systems, on time scales up to a few weeks of convection over the ocean with particular attention to Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) and Global Atmospheric Research Programme, Atlantic Tropical Experiment (GATE) data. This is being undertaken by Wojciech Grabowski and Xiaoqing Wu (visitor, University of California, Los Angeles, UCLA) in conjunction with the TOGA simulations reported below; 2) new capabilities for analysis of satellite data by David Johnson (joint appointment with CGD); 3) workshop sponsorship and participation, namely Chin-Hoh Moeng (GEWEX Cloud Systems Study (GCSS) Workshop in KNMI, De Bilt, Netherlands on boundary-layer model intercomparisons) and Moncrieff and Kiehl (Kananaski, Alberta, Canada on microphysical processes in GCMs); and 4) measurement of entrainment of tropospheric air into the boundary layer.
Grabowski, Wu, and Moncrieff studied interactions of cloud dynamics and cloud microphysics with radiation and large-scale forcing on a time scale of one week. Idealized experiments involving steady large-scale forcing and environmental conditions, as well as more realistic experiments driven by observed large-scale conditions, were performed. Simulations using observed conditions simulated evolution of cloud systems during the period of 1 September through 7 September 1974 during Phase III of GATE. This seven-day period was characterized by the evolution of cloud systems as a response to changing large-scale advective tendencies and low-level wind shear associated with easterly wave activity. Nonsquall cloud clusters, squall cloud clusters, and scattered convection were all observed and simulated by the model. The graphic shows snapshots of the total liquid and ice fields from two-dimensional simulations of GATE Phase III convection forced by an easterly wave. As the forcing changes, the cloud systems evolve from disorganized clusters, through a squall line, to scattered convection. These results compare well with observations, and will be used to develop convective parameterization schemes for numerical weather prediction models and climate GCMs.
Moncrieff, Jean Luc Redelsperger (Centre National de Recherches Meteorologiques, Meteo-France, France), David Gregory (Hadley Centre, England), Steven Krueger (University of Utah), and Wei-Kuo Tao (NASA, Goddard) are establishing a cloud resolving model intercomparison project for Working Group 4: Precipitating Convective Cloud Systems of the GCSS that involves scientists from several countries. This is a two-part project focusing on convection in the December 1992 westerly wind burst observed during TOGA COARE. The first part is a three-dimensional simulation of a squall-line-type mesoscale convective system. The initial conditions are from LeMone et al. (1994). This will be evaluated against in situ observations obtained in COARE. The second is a two-dimensional simulation of longer-term behavior on cloud systems using time-dependent forcing averaged over the Intensive Flux Array. The data are being provided by Xin Lin and Richard Johnson (Colorado State University).
Johnson developed a new facility for the analysis of data from satellites, and with Kiehl and others he used that system in support of data analyses in Central Equatorial Pacific Experiment (CEPEX), Winter Icing and Storms Project (WISP), and other field experiments. The facility uses commercial software to display satellite data, and supports analyses of multispectral datasets, merging of data from aircraft, and other joint uses of the satellite data with data from other observing systems.
The CEPEX analysis involved high-resolution (1 km) multi-spectral studies of tropical cloud systems, with a particular emphasis on the structure and characteristics of the high albedo regions in large tropical convective complexes. The high-resolution satellite study attempts to bridge the gap between highly localized in situ measurements and larger scale radiation budget observations with the hope of improving the description of clouds in climate models. The regions of highest albedo have been identified with the overshooting tops of the convective cores, with a systematic reduction in albedo with increasing distances away from the core, suggesting that the unusually high albedos sometimes observed in tropical clouds are the result of a combination of dynamical and microphysical processes.
In conjunction with the development of the satellite data analysis facility at NCAR, MMM has been collaborating with Lei Shi (Sea Space Corporation, San Diego) on the development of techniques to use temperature and water vapor retrievals from satellite microwave sounding systems for initialization of mesoscale models, such as the NCAR/Pennsylvania State University Mesoscale Model (MM5).
MMM contributes to the NCAR GTCP, led by ACD, by providing methods and tools for understanding and modeling chemical transformations and transport in the troposphere. Efforts in FY 95 concentrated on evaluation of techniques for flux measurement, development of new modeling techniques, incorporation of sulfur into a global climate model, and the development of some new techniques for the measurement of aerosols from aircraft.
The intermittent sampler, designed and built by Cooper and Stephen Shertz (ACD), was operated on board the Electra during the Boreal Ecosystem-Atmosphere Study (BOREAS) by Cooper, Shertz, and Kenneth Davis (visitor, University of Colorado). This system can measure fluxes using sensors that are too slow to provide flux measurements by conventional eddy-correlation techniques. Data from different operating modes of the intermittent sampler have been compared to each other and to standard methods for measuring fluxes, using the BOREAS data. Intermittently sampled fluxes of water vapor and carbon dioxide show good agreement with traditional eddy-correlation fluxes. Carbon dioxide fluxes observed using eddy accumulation were also evaluated, and were of the proper magnitude, but uncertainties in analysis techniques led to uncertainties too large to permit a good evaluation of the technique. The eddy accumulation mode allowed fluxes of isoprene to be measured on several occasions, using measurements of isoprene mixing ratios by James Greenberg (ACD). The observed isoprene fluxes are consistent with estimates from a global emissions model created by Alex Guenther (ACD).
To understand the global distribution of aerosol sulfate in the troposphere and learn what impact aerosol sulfate has on climate, joint work involving Mary Barth (joint appointment with ACD and CGD), and Philip Rasch and Kiehl (CGD) was initiated that will lead to a global sulfur model. Emissions of sulfur dioxide, aerosol sulfate, and dimethyl sulfide, gas and aqueous chemistry conversion from sulfur dioxide to aerosol sulfate, wet deposition, and dry deposition have been incorporated as parameterized processes in a version of the NCAR Community Climate Model (CCM). Analysis of the results will show how sulfur emissions from certain regions of the world are influencing aerosol sulfate loadings in the rest of the world, and how gas and aqueous chemistry affect the amount of sulfate in tropospheric aerosols.
In preparation for future field observations, some new aerosol detection capabilities were acquired and tested. A TSI Scanning Mobility Particle Spectrometer was operated during the Small Cumulus Microphysics Study (SCMS) field experiment, partly to learn about operational problems with this instrument. On the basis of that experience, a new flow-control system has been designed to support routine airborne use of the instrument, and the pressure dependence of its operation was evaluated. This instrument, which covers the aerosol size range from about 0.01 to 0.5 micrometers, should be a valuable component of future studies of aerosols in the troposphere.
Using the Clark model, Grabowski, Wu, and Moncrieff studied interactions among cloud dynamics, cloud microphysics, radiation and large-scale forcing on time scales ranging from several days to several weeks. Idealized experiments involving steady large-scale forcing and environmental conditions, as well as more realistic experiments driven by observed conditions, were performed. A simple technique was applied to force the anelastic cloud-resolving model with the time-evolving large-scale horizontal wind field and time-evolving large-scale advective tendencies of temperature and moisture derived from observational budget-type studies.
The approach was used to study the effects of convection during a westerly-burst period (1 December 1992 through 10 January 1993) during TOGA COARE. This 40-day period selected for the TOGA COARE case was characterized by dramatic variability of cloud systems in response to changes of the large-scale forcing, horizontal winds and ocean temperature. Preliminary comparisons between model-generated cloudiness and satellite visible and infrared radiation (IR) images show very encouraging agreement. Sensitivity experiments using both idealized and observed conditions highlighted the role of the upper-tropospheric moisture in the overall "climate" of simulated convection. These two-dimensional experiments are now being extended to three dimensions.
A study by Moncrieff and Ernst Klinker (European Centre for Medium-Range Weather Forecasts, ECMWF) of organized convection in the TOGA COARE region as represented (parameterized) in the ECMWF operational model has been completed and submitted for publication. The main finding was that as a result of the dynamical scale of the cloud systems ("superclusters") exceeding the mesh size, the model partly resolved the circulations. This led to specific errors in the momentum transport, explained in terms of a dynamical model. This caused a model wind bias over the western Pacific which suggests that the representation of fluxes by organized convection is an uncertainty to be considered in numerical weather prediction (NWP) models.
The USWRP concept has been around for at least 15 years, and is now undergoing a renewal and redefinition under the leadership of William Hooke (NOAA) and Richard Carbone. Carbone now serves as the lead scientist of the program, and has organized science teams to reexamine issues and opportunities in light of recent research progress, the continuing NOAA National Weather Service (NWS) modernization, and the NWS' need to define an improved North American observing and forecast system of the future. Three USWRP Prospectus Development Team (PDT) meetings were held in FY 95: 9 October 1994, 15-17 May 1995, and 18-20 September 1995. There is not yet a crisp definition of the focused program, but one is beginning to emerge from the findings of the science teams (e.g., Emanuel et al., 1995) and agency considerations. A likely initial focus will concern the fundamental and applied research necessary to objectively determine the best practicable mix of model attributes, data assimilation schemes, and observations in a future forecast system.
Science Team 1 made ten specific recommendations concerning various data infrastructure issues, including optimal use of existing and emerging operational data sources, the use of programmable observing systems, mitigating the oceanic data void, land surface properties and processes, water vapor measurements, improved measurement of the upper ocean, and global rawinsonde coverage. Team 1 also identified 17 emerging basic research opportunities, including the fundamental physics of land-air interaction, targeted observing strategies, dynamics of landfalling tropical cyclones, mesoscale convective system (MCS) dynamics, orographic and other influences on potential vorticity, mesoscale frontal cyclones, and ensemble forecasting and data assimilation techniques.
Team 2 delved further into data issues, focusing specifically on the eastern Pacific data void, downstream effects of the Rocky Mountains, the quantitative precipitation forecast as related to the life cycle of organized convection, and coastal cyclone issues for both tropical and extratropical cyclones. Team 3 examined special issues in the coastal zone with oceanographers and identified distinct regional components. These include the landfall of tropical cyclones in the southeast, west coast topographic/marine boundary-layer interactions, coastal water influences on extratropical cyclone evolution, and the importance of hydrology to forecasts of lowland and coastal flooding events.
Early in FY 96, the Science Advisory Committee (SAC) will formally advise the agencies as to near-term program emphases. It is expected that a highly focused joint agency research program will begin to fund investigators in FY 96. It is likely that the initial USWRP focus will explore issues associated with the "best practicable mix" of observations, data assimilation schemes, and models in the future forecast system. NCAR and MMM are prepared to fully participate in the USWRP and to shape and integrate our projects given the broader scientific guidance provided by the SAC and the sponsoring agencies. It is expected that Carbone will serve as lead scientist for the first three years of the program.