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Department of Atmospheric Science

Thur., Apr. 20, 3:10 pm, EN6085

Predicting the evolution of ice particle shape in a microphysical model: Impacts on modeled precipitation

Dr. Anders Jensen

National Center for Atmospheric Research


A new microphysics parameterization is presented where the shapes of ice particles evolve, along with mass, density, maximum dimension, and fall speed. The framework uses spheroids to model ice, and therefore, two ice dimensions can evolve for each microphysical growth process. Parameterizing ice shape evolution in a microphysics model allows ice shape to directly impact both growth rates (through the capacitance for vapor growth and cross-sectional area for riming) and fall speed. Parameterizing ice shape evolution also allows ice to transition from vapor grown to rimed without needing to convert ice mass from a snow to a graupel category, which traditional bulk models do. For unrimed ice, modeling shape evolution produces mass and fall speed variations across a range of habits (growth temperatures). For rimed ice, the model captures various states of rime from lightly rimed to densely rimed particles. Predicting various states of rimed particles directly impacts spatial precipitation distribution because ice particle fall speeds depend critically on the amount of rime collected. A bulk parameterization of the framework is developed for use in the Weather Research and Forecasting (WRF) model and is called the Ice-Spheroids Habit Model with Aspect-ratio EvoLution (ISHMAEL). ISHMAEL is demonstrated in a quasi-idealized, 3D squall-line case. The development of the enhanced stratiform precipitation region in the squall line is discussed.

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