GEWEX Cloud System Study (GCSS) Working Group 4: Precipitating Convective Cloud Systems

Abstract

The authors present the objectives of the working group on precipitating convective cloud systems. These center on developing physically based parameterizations for global models in which basic research into the large-scale role of cloud systems is an important part. The approach calls on a range of expertise: cloud-resolving modeling and contributing research, observational evaluation of the model results, and tests of parameterizations in single-column models. Ongoing studies focus on oceanic cloud systems in Tropical Oceans Global Atmosphere Coupled Ocean?Atmosphere Research Experiment (TOGA COARE). First, cloud-resolving modeling of organized convection on a timescale of a few hours concentrates on momentum transport and surface fluxes. Results are evaluated against data obtained during the 22 February 1993 Intensive Observation Period, which include airborne Doppler radar measurements of a squall line. Second, multiday simulations focus on the environmental effects of cloud systems as they respond to changes in specified (observed) large-scale tendencies and sea surface temperature. In this case, observational 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 in the intercomparison, are summarized. In the longer term, cloud system research in Working Group 4 will contribute to the Climate Variability and Predictability?Global Ocean?Atmosphere?Land System program, as regards the large-scale effects of cloud systems up to intraseasonal timescales. Another contribution will be to space-borne measurements; for example, cloud-profiling capability will provide data critical to the comprehensive evaluation of upper-tropospheric moisture distribution in cloud-resolving models. Besides additional studies in tropical cloud systems, convection in cold air outbreaks and convection over continents have a high priority.