A Severe Frontal Rainband. Part IV: Precipitation Mechanisms, Diabatic Processes and Rainband Maintenance

Abstract

A three-dimensional kinematic cloud model has been used to study the precipitation processes within an intense, narrow cold-frontal rainband (NCFR). A triple-Doppler radar analysis has provided the necessary kinematic flow field. The leading edge of the advancing cold air was viewed as a density current, which contained a well-defined and intense rotor circulation. Observed and predicted local precipitation rates were in excess of 200 mm h?1. The model indicated that heavy precipitation formed through riming, associated with the development of graupel. Coalescence growth at temperatures above 0°C was also important. A parameterization of the Hallett-Mossop ice multiplication process was included in the model. Copious amounts of small ice crystals were produced by this mechanism, but the model results were insensitive to their presence. The rather high temperatures associated with the region splinters formed (?3°to ?8°C), and the circulation pattern, prevented their growth to hydrometeor sizes. The model output was used to diagnose the two-dimensional frontogenesis equation for the cross-front potential temperature gradient. Diabatic processes were found to be important to the maintenance of the cross-front temperature gradient despite strong frontolysis associated with tilting. Heating associated with condensation immediately ahead of the density current and cooling from evaporation immediately behind were found to be important in maintaining the density contrast across the front, and therefore the rainband itself. Subsidence warming in the descending branch of the rotor effectively displaced the cold air to a position behind the wind shift line. This particular distribution of diabatic heating processes, including melting, is considered essential to the maintenance of the intense circulations pattern in this NCFR when viewed in light of the recent theoretical studies discussed by Moncrieff.