Simulations of an isolated two-dimensional thunderstorm : sensitivity to cloud droplet size and the presence of graupel

摘要

Cloud Resolving Models (CRMs) which are used increasinglyto make operational forecasts, employ Bulk MicrophysicsSchemes (BMSs) to describe cloud microphysical processes. In thisstudy two BMSs are employed in a new Nonhydrostatic σ-coordinateModel to perform two hour simulations of convection initiated by awarm bubble, using a horizontal grid resolution of 500 m. Differentconfigurations of the two BMSs are applied, to test the effects of thepresence of graupel with one scheme (2-configurations) and ofchanging the cloud droplet sizes in the second scheme (4-configurations),on the simulation of idealised thunderstorms. Maximum updraftsin all the simulations are similar over the first 40 minutes, butstart to differ beyond this point. The first scheme simulates thedevelopment of a second convective cell that is triggered by the coldpool that develops from the outflow of the first storm. The cold poolis more intense in the simulation with graupel because of melting ofgraupel particles, which results in relatively large raindrops, decreasesthe temperature through latent heat absorption, causing strongerdowndrafts, which all contribute to the formation of a more intensecold pool. The second scheme simulates the development of a secondcell in two of its configurations, while two other configurations donot simulate the redevelopment. Two configurations that simulate thesecondary redevelopment produce a slightly stronger cold pool justbefore redevelopment. Our results show that small differences in themicrophysics formulations result in simulations of storm dynamicsthat diverge, possibly due nonlinearities in the model.