The Second GEWEX Cloud System Study Science and Implementation Plan

The purpose of the GEWEX Cloud System Study (GCSS; GEWEX is the Global Energy and Water Cycle Experiment) is to develop better parameterizations of cloud systems within climate and numerical weather prediction models, with an emphasis on the climate applications. The strategy of GCSS is to use two distinct kinds of models to analyze and understand observations of the behavior of several different types of clouds systems. Cloud-system models are models with sufficient spatial and temporal resolution to represent individual cloud elements, but covering a wide enough range of space and time scales to permit statistical analysis of simulated cloud systems. These models are compared with detailed observations, representing several specific cases taken from field experiments, and with statistical composites obtained from satellite and meteorological analyses. Single-column models represent the column physics of an atmospheric general circulation model and are used to test cloud parameterizations in an un-coupled mode by comparison with field data and statistical composites. GCSS has five Working Groups, which deal with boundary-layer clouds, cirrus clouds, extratropical layer cloud systems, precipitating deep convective cloud systems, and polar clouds, respectively. The many accomplishments of the working groups are summarized in this document. In addition, GCSS organized a major international Workshop which was held during 1998. In the original GCSS strategy, as articulated in the first GCSS science plan, data is collected in various field programs and provided to the CSM Community, which uses the data to " certify " the CSMs as reliable tools for the simulation of particular cloud regimes, and then uses the CSMs to develop parameterizations, which are provided to the GCM Community. We report here the results of a rethinking of the scientific strategy of GCSS. The main elements of the proposed new strategy are a more active role for the large-scale modeling community, and an explicit recognition of the importance of data integration. In addition, we discuss ways in which GCSS can approach the key climate issue of cloud feedback.