A Modeling Study of Rainfall Processes in Tropical Convective Systems

Cloud microphysics plays an important role in atmospheric water and energy cycles through the following processes: 1) governing the development of convective system from convective elements into stratiform clouds; 2) determining precipitation efficiency; 3) forming diabatic heating through phase changes and hydrometeors-radiation interaction. The above processes are investigated with the analysis of cloud microphysics budgets based on a 2D cloud resolving simulation. The cloud model is forced by the vertical velocity derived from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Experiment (TOGA COARE). Results on the first two issues will be discussed in the meeting. The analysis shows that the collection of cloud water by rain and the melting of graupel are the main microphysical processes responsible for development of convective and stratiform rainfall respectively. The vapor condensation and deposition are main source for growth of water and ice clouds in convective and stratiform rainfall respectively. From the microphysics perspective, the ratio of the ice water path to the liquid water path is proposed to be a better criterion for classifying clouds into convective and stratiform types. The budgets suggest that the threshold values of cloud ratio can be used to better define convective, mixed, and stratiform rainfall (e.g. when the corresponding cloud ratios are smaller than 0.4, 0.4-1.0, and greater than 1, respectively). The budget further shows that total moisture source (surface evaporation and moisture convergence) is converted into hydrometeors through vapor condensation and deposition rates such that the large-scale and cloud-microphysics precipitation efficiencies are statistically equal. This relation is valid regardless of the area size where the average is taken. The precipitation efficiency can be larger than 100% as a result of hydrometeor convergence from the neighboring atmospheric columns. This is most evident in light-rain conditions (<5 mm/h for 96- and 48-km domains and <10 mm/h for 24-km domain). Whereas the precipitation efficiency is normally smaller than 100% in heavy-rain conditions (>5 mm/h) associated with hydrometeor divergence.