Understanding Precipitating-Cloud Structures: A New Convective-Stratiform Rainfall Separation Scheme

Convective and stratiform rainfalls are different in dynamic, thermodynamic, and cloud microphysical aspects. Their separations have been largely based on the magnitudes of signals such as reflectivity and rainfall intensity. Recent studies show that convective rainfall separated by such schemes contains considerable amount of rainfall associated water vapor divergence. In this talk, a new rainfall separation scheme based on surface rainfall budget is compared to existing convective-stratiform rainfall separation schemes based on the rain intensity. The rainfall is partitioned into eight types, depending on atmospheric drying/moistening, water vapor convergence/divergence, and hydrometeor convergence/divergence. The rainfall types are combined into convective and stratiform rainfall categories. The analyses are conducted using the grid-scale data from a two-dimensional cloud-resolving model simulation during Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). The analysis of cloud microphysical budgets reveals that the new separation scheme is consistent with convective-stratiform rainfall separation schemes, but the new separation scheme provides more detailed thermodynamic and cloud microphysical structures of tropical precipitation systems than the existing schemes do. The new separation scheme shows that over convective rainfall regions, maximum water vapor convergence may not lead to maximum rainfall, but it causes large atmospheric moistening and hydrometeor divergence. The two rain types associated with water vapor convergence over convective regions have qualitatively similar cloud microphysical budgets in which the collection of cloud water by rain is a key rainfall source. The rain type associated with cloud hydrometeor convergence could have the rain rate three times higher than the rain type associated with cloud hydrometeor divergence does. Thus, the cloud hydrometeor convergence is a key process for making the maximum rainfall with the precipitation efficiency of 100%. All three rain types correspond to the upward motions throughout the troposphere with their maxima in the lower troposphere. The cloud hydrometeor convergence also is a major rainfall source for the rain types associated with the water vapor divergence over stratiform rainfall regions. The new convective-stratiform rainfall separation scheme provides dynamic, thermodynamic, and cloud microphysical framework for quantitatively studying rainfall processes, in particular, maximum rainfall processes.