Parameterizing Ice Particle Shape Evolution During Riming: Impacts on Convective and Orographic Precipitation.

Description

Microphysical models traditionally subdivide ice into categories based on observed growth mechanisms, such as pristine vapor grown ice, snow, and rimed ice (graupel). Categorizing ice in such a manner requires specifying thresholds and conversion rates that can lead to systematic biases in model results. A new paradigm in microphysical modeling is emerging that predicts particle properties (size, shape, density, and mass) instead of using pre-defined categories. We have developed a bulk microphysical model that evolves particle shape during both vapor diffusional growth and riming. The vapor diffusional growth model allows particle shapes and density to develop naturally depending on the changing environmental conditions. We have recently developed a method to include shapedependent riming in the bulk model. The model predicts the natural tendency for isometric ice particles to rime earlier, and more rapidly, than particles with more extreme shapes such as dendrites and columns or needles. Because rime accumulates more naturally on the modeled particles, shape and particle density evolve based on the environmental temperature and the state of the liquid drop size distribution. Intermediate rimed states are possible with the model providing a better transition to graupel than with traditional schemes. Simulations of orographic precipitating cases and squall lines indicates that the model may improve the prediction of stratiform precipitation especially along the leeward side of mountains. Predicting habit-dependent riming produces a "habit-sorting" effect on precipitation production whereby weakly-riming particle shapes are advected farther downstream allowing such particles to amplify precipitation production in different regions of the cloud system.