E1.3 Three-Moment Representation of Rain in a Cloud Microphysics Model

Abstract

Two-moment microphysics schemes have been commonly used for cloud simulation in models across different scales – from large-eddy simulations to global climate models. These schemes have yielded valuable insights into cloud and precipitation processes. However, the size distributions are limited to two degrees of freedom, and thus the shape parameter is typically fixed or diagnosed. We have developed a three-moment approach for the rain category to predict the rain size distribution and thereby improve cloud microphysics representations for more accurate weather and climate simulations. The approach is applied to a new microphysics scheme with advanced ice microphysics parameterizations, i.e., the Predicted Particle Properties (P3) scheme.

Idealized rainshaft model tests show that the new rain scheme yields substantially improved simulated rain properties compared with the original two-moment scheme, using the Spectral Bin Model (SBM) as a reference. Only 4% of the original two-moment P3 simulations have mean drop sizes and rain rates within ±20% of the SBM results, but this increases to more than 95% agreement in our three-moment version of P3. We also apply the new scheme to 3-D real-case simulations of both a drizzling stratocumulus case and a squall line mesoscale convective system (MCS). The impacts on simulated rain properties will be discussed and compared to observed properties.