Microphysical Characteristics through the Melting Region of a Midlatitude Winter Storm

The microphysical characteristics of a precipitation type transition region within a midlatitude winter storm are discussed in relation to the background thermodynamic and kinematic fields. A deep region in which the temperature was close to 0-degrees-C (the transition region) was observed along the Atlantic coastline of Nova Scotia. This transition region was approximately 30 km wide and about 2 km deep. At 80 kPa, a large horizontal temperature gradient marked the boundary between the transition region and the colder air. The observed thermal structure is linked to diabatic processes, and in particular, to the freezing of small droplets, the refreezing of semi-melted particles and the melting of precipitation. Large, partially melted aggregates were located just downwind of the deep transition region. Particle trajectories near the transition region are very sensitive to the background temperature and wind fields and may lead to regions of reduced and enhanced concentrations at the surface and aloft. A conceptual model of the flow fields suggests that this case resembles warm and cold conveyor belts similar to those found in synoptic systems, but on a smaller scale. The transition region in this case is located at the boundary between the warm and cold conveyor belts.