Mesoscale Structure of Precipitation Bands in a North Atlantic Winter Storm

Abstract

The present study discusses the meso- and microscale structures of Precipitation regions within a midlatitude winter storm over the North Atlantic, observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic. Two wide regions of precipitation separated by a narrow band were observed at low levels by airborne radar. These regions were aligned parallel to the cold front and were sampled by aircraft at three different levels. The calculated mesoscale frontogenetical forcing is dominated at low levels by confluence and at mid-levels by the tilting term. The absolute magnitudes are smaller than those reported by Shapiro, and Bond and Fleagle, and are consistent with the broader and less intense front in this study. The frontogenetical forcing due to melting of ice crystals was estimated from observations of precipitation particles. The analysis indicates that the cooling due to melting of ice particles is not a dominant frontogenetical forcing at the observed stage in storm evolution. Precipitation rates larger than those observed (by a factor of 3) behind the cold front are needed before the thermal impact of melting could contribute to frontogenesis as much as confluence at the same level. The region of precipitation ahead of the cold front appears to be linked to convective instability observed in the warm sector. The observed precipitation region to the west of the cold front is consistent with the trajectories of failing particles carried by the relative wind flowing toward the back of the system. The decrease in precipitation rate observed right behind the front can be interpreted as ice particles failing through a deep region in which temperatures are close to 0°C. The presence of such a region leads to a nonuniform precipitation distribution, with areas that would appear as precipitation bands in radar images, and others in which precipitation is reduced.