Evaporation of Melting Ice Particles Within a Melting Layer Model

Most melting layer analyses in the literature assume that melting occurs within highly saturated environments where the relative humidity is no less than 90%, and that melting begins as soon as the temperature is 0 °C. Previous studies of melting ice particles in these conditions have assumed that mass loss due to evaporation is negligible. However, not all melting occurs within such highly saturated environments. Snowflakes do fall and melt in subsaturated conditions such as beneath mesoscale convective system (MCS) anvils, below the outflow ice cloud base of MCSs, and in elevated precipitation systems advecting over a dry lower layer (such as the leading edge of warm-frontal precipitation). In such instances, precipitation estimates obtained using remote sensing instrumentation (e.g. ground-based and satellite-based radar) may overestimate the amount of rain reaching the surface due to evaporative losses. This is especially true of the United States (U.S.) National Weather Service Doppler radar network in the Upper Midwest, southwestern Texas, and Intermountain West, where radar oftentimes cannot sample below 2 km above ground level. These are also the areas that historically possess lower relative humidity environments and evaporation occurs at a greater degree than in the more humid eastern regions of the U.S.

The first part of this study uses a one-dimensional melting layer model with constant lapse rates and relative humidity profiles throughout the simulation. These simulations will be used to evaluate how subsaturated conditions modify the melting process by calculating the fractional mass change per particle size and the corresponding changes in radar reflectivity and rainfall rate profiles. The second part of this study incorporates observed subsaturated profiles into the melting layer model. These simulations will help to determine when and where significant mass loss is occurring on the melting ice particles and if the mass removal due to evaporation is significant within observed subsaturated profiles.