Explicit prediction of ice crystal habits in a 3D Eulerian numerical weather prediction model—sensitivities to the ice nuclei distribution

This paper discusses the sensitivity of the distribution of solid hydrometeors predicted by the Spectral Habit Ice Prediction System (SHIPS) to the ice nuclei (IN) distribution entering a winter orographic storm. SHIPS was developed as an alternative to bulk microphysics parameterization for cloud resolving model application. SHIPS provides (1) flexibility of describing size distributions and (2) ability to evolve ice crystal habit (shape) and density of solid hydrometeors explicitly without the use of predefined categories in a 3D Eulerian framework.

Since SHIPS can grow axis lengths of ice crystals based on local temperature and humidity, there is sensitivity to where the ice crystals were initially nucleated. Efficiencies of subsequent aggregation and then riming processes depend on the ice crystal habit. Hence, the initial IN distribution can be critical to the prediction of the distribution of precipitation on the ground. Ice nucleation processes considered in SHIPS as the sink terms of IN budget are deposition-condensation nucleation, contact nucleation, and immersion freezing nucleation, while evaporation of hydrometeors with insoluble aerosols contributes as the source. Simulations were applied to the 2D WMO Cloud Modeling Workshop IMPROVE-2 case with several contrasting vertical distributions of IN as the inputs. The resultant distributions of solid hydrometeors are discussed with analysis of entrainment of IN from the tropopause and recycling of IN from evaporation.