The Mesoscale and Microscale Structure and organization of Clouds and precipitation in Midlatitude Cyclones. Part V: The Substructure of Narrow Cold-Frontal Rainbands

Abstract

The organization and structure of a narrow cold-frontal rainband (NCFR) on the small mesoscale and the microscale have been investigated through quantitative radar reflectivity, Doppler radar observations, airborne observations and surface measurements. The NCFR was composed of small mesoscale regions of heavy precipitation called “precipitation cores” (PCs) oriented at an angle to the synoptic-scale cold front; in horizontal cross section the PCs were roughly elliptical in shape. Areas of lighter precipitation called “gap regions” (GRs) separated the PCs. The PCs were so oriented that their loading edges were regions of strong low-level convergence. The weather associated with the passage of a PC resembled that of a squall-line gust front, with concurrent windshifts and pressure checks occurring ∼5 min before heavy precipitation and a fall in temperature. The changes in surface weather that accompanied the passage of a GR were more variable but tended to be less marked than for PCs. Thus, the sequence of weather experienced by a ground station can be markedly affected by its position with respect to the small mesoscale structure of the cold front. The loading edges of the PCs were generally marked by relatively strong updrafts and high liquid water contents. Ice particle concentrations were high, particularly in the upper regions of the updrafts and in the downdraft regions of the PCs. Considerable ice enhancement, probably due to ice splinter production during riming, was present in these two regions. Riming was the dominant mechanism for the growth of precipitation in the PCs. Several aspects of the small mesoscale structure of cold fronts are reminiscent of features seen with gravity currents. Also, the velocity of motion predicted by gravity current theory is in good agreement with the observed motion of the cold front. We now visualize a cold front on the small mesoscale as a series of parallel line segments, each passing through the long axes of a PC, connected by kinks in the GRs. In the vicinity of a kink, the circulation can form a meso-low which, in extreme cases, may be a preferred region for the development of tornadoes and downbursts.