The influence of predicted ice particle habit on the microphysical structure of squall lines and orographically-forced mixed-phase clouds

Description

A multi-moment bulk microphysical model that evolves particle shape and density during vapor growth, riming, melting and aggregation has been developed and used to simulate squall lines and orographic precipitation. Predicting ice allows for the representation of various states of ice from unrimed to lightly rimed to densely rimed without converting ice mass between predefined ice categories (e.g., snow and graupel). This leads to a different spatial precipitation distributions in both squall lines and orographic cloud systems compared to traditional microphysical schemes. These differences occur because ice is sorted in physical space based on the amount of rime, which controls particle thickness and therefore fall speed. Predicting these various states of rimed ice leads to a reduction in vapor growth rate. The transition zone of squall lines results from hydrometeor sorting; the production of small partially-rimed ice particles in the convective region leads to fall speeds that favor ice transport rearward of the transition zone, causing a local precipitation minimum. Sensitivity studies show that the fall speed of ice particles largely determines the location of the enhanced stratiform precipitation region and whether or not a transition zone forms. Simulations of an observed orographic case indicates that the habit model predicts planar and columnar particles at spatial locations that agree well with observations. The relatively large maxima in precipitation produced by traditional schemes does not often occur in the habit model since partially rimed ice and newly formed aggregates are appropriately advected downstream of the mountain crests.