A numerical cloud model study of the Hallett-Mossop ice multiplication process in strong convection

Abstract

Numerical simulations including the Hallett-Mossop ice multiplication process have been conducted on strong maritime and continental convective cloud cases using a two-dimensional, time-dependent cloud model. Inclusion of the process induced rapid glaciation of the maritime cloud, but much less significant icing in the continental case. This is due in part to the smaller water contents and narrower droplet distribution in the model continental cloud, but is also limited by the fact that graupel does not appear in the region conductive to ice multiplication until late in the life cycle of the modeled cloud for the continental case. In the maritime case, the ice multiplication process is initiated by the probabilistic freezing of rain and thereafter maintained by the downward flux of graupel through the generation zone and, hence, is governed by the gross cloud dynamics. Low updraft speeds are not required for ice multiplication to occur, and the cloud need not be in a dissipation stage. Slight dynamic effects result from the early glaciation of the cloud cell caused by the released latent heat of fusion. Earlier precipitation fallout occurs in the maritime case with ice multiplication active but does not result in much change of total precipitation in this one case.