GEWEX CLOUD SYSTEM STUDY (GCSS) WORKING GROUP-4 - PRECIPITATING CONVECTIVE CLOUD SYSTEMS

The authors present the objectives of the working group on precipitati ng convective cloud systems. These center on developing physically bas ed parameterizations for global models in which basic research into th e large-scale role of cloud systems is an important part. The approach calls on a range of expertise: cloud-resolving modeling and contribut ing research, observational evaluation of the model results, and tests of parameterizations in single-column models. Ongoing studies focus o n oceanic cloud systems in Tropical Oceans Global Atmosphere Coupled O cean-Atmosphere Research Experiment (TOGA COARE). First, cloud-resolvi ng modeling of organized convection on a timescale of a few hours conc entrates on momentum transport and surface fluxes. Results are evaluat ed against data obtained during the 22 February 1993 Intensive Observa tion Period, which include airborne Doppler radar measurements of a sq uall line. Second, multiday simulations focus on the environmental eff ects of cloud systems as they respond to changes in specified (observe d) large-scale tendencies and sea surface temperature. In this case, o bservational data on the scale of the entire TOGA COARE Intensive Flux Array are used in model evaluations. Results and recommendations from the first model intercomparison workshop, as well as the next steps i n the intercomparison, are summarized. In the longer term, cloud syste m research in Working Group 4 will contribute to the Climate Variabili ty and Predictability-Global Ocean-Atmosphere-Land System program, as regards the large-scale effects of cloud systems up to intraseasonal t imescales. Another contribution will be to space-borne measurements; f or example, cloud-profiling capability will provide data critical to t he comprehensive evaluation of upper-tropospheric moisture distributio n in cloud-resolving models. Besides additional studies in tropical cl oud systems, convection in cold air outbreaks and convection over cont inents have a high priority.