On the Mechanisms of Rain Formation in a Simulation of an Idealized Supercell

In deep convective clouds, rain is produced by growth from cloud droplets via the so-called “warm rain process”, melting of ice particles, and shedding of liquid water from hailstones undergoing wet growth or melting. Rain produced by each of these mechanisms may be characterized by different drop size distributions (DSDs). Ultimately, these drops sediment and advect, reaching the surface in some combination, which can lead to unusual DSDs that are not well represented by simple microphysics parameterization schemes. Disdrometer and radar measurements in superell storms have demonstrated such “exotic” DSDs.

The two-moment Morrison microphysics scheme is modified to include three rain classes representing raindrops formed by warm processes, melting, and shedding. A high-resolution simulation of an idealized supercell storm is performed with the standard Morrison scheme and is compared to one using the modified “RainClass” scheme. Results show that rain originating from melted ice dominates the mass of the rain field at low levels. However, preferred regions of warm rain are found along the rear flank, and preferred regions of shed rain are found in the left forward flank. DSDs from the individual rain categories at a given location may be substantially different, leading to a total combined DSD not well modeled by an inverse exponential function. Despite the increased complexity of the rain field, however, the simulated DSDs still have smaller variability than observed. This suggests that the model is still too rigid in its treatment of rain DSDs. These and other results will be discussed.