On Pre-Frontal Precipitation Zones in the United States

Abstract

Results of computations of vertical velocity and precipitation amounts for two winter time pre-frontal precipitation zones are presented. Since each extensive precipitation zone seemed to be consisted of a series of several meso-scale precipitation centers, computations are made to obtain averaged conditions for such meso-systems. It is found that there is a tendency that the level of maximum upward motion rises with time for such meso-scale precipitation systems of pre-frontal type. In one of the cases examined, the weakening of pre-frontal precipitation occurred as the system approached to the quasi-stationary anticyclone where large-scale low level subsidence was evident. In case of well developed precipitation systems, contribution of condensation above the 6 km (MSL), due to upward flux of water vapor through this level, may reach 1/4 of total precipitation amount. Some computations are made for a cold- and warm-frontal precipitation. The former showed similar characteristics with the pre-frontal type, but the result for the latter indicates the classical model of warm frontal rain is satisfactory for the interpretation of the computation.