The effect of parameterized ice microphysics on the simulation of vortex circulation with a mesoscale hydrostatic model

It has been proposed that ice microphysics, particularly the melting effect, can play an important rôle in the generation of mesoscale structure and evolution of convective weather systems and associated stratiform rainfall. In this paper, parameterized cloud ice and snow crystals are incorporated into an explicit (grid-resolved) convective scheme as prognostic variables and tested using an observed mesovortex on a grid resolution of 25 km.

With the inclusion of ice microphysics parameterization, the resolvable-scale precipitation begins to develop nearly 1 h earlier and undergoes a more rapid acceleration. Meanwhile, the resulting maximum upward motion and locally accumulated rainfall are significantly larger than that without ice microphysics. However, the model produced a relatively weak mesovortex circulation with the maximum cyclonic vorticity located more than 50 mb higher when the ice microphysics is incorporated. It is found that the freezing and sublimation provide a positive forcing for the rapid development of the mid-tropospheric warm-core vortex circulation, while the melting tends to destroy the concentration of cyclonic vorticity in the lower levels. In particular, intercomparisons among all sensitivity experiments so far performed reveal that the melting effect can be of equal importance to those of the hydrostatic water loading and evaporative cooling in retarding the development of the CISK-like instability and in reducing the intensity of the mesovortex. The results indicate that the vertical distribution of diabatic heating may be more important than the total heating in determining the strength of mesovortices, when the melting effect is considered.