A Diagnostic Modelling Study of the Trailing Stratiform Region of a Midlatitude Squall Line

Abstract

A kinematic model with continuity equations for sensible heat, water vapor, cloud water, cloud ice, rain, snow, and graupel is used to determine the steady state thermodynamic and microphysical processes associated with the observed mesoscale air motion fields in the trailing stratiform region of the 22 May 1976 Oklahoma squall line. Two versions of the observed air motions are used: a low-resolution but areally extensive flow pattern derived from the composite sounding data of Ogura and Liou, and a high-resolution but less areally extensive flow field obtained from the dual-Doppler radar analysis of Smull and Houze. Model calculations based on the high-resolution Doppler-derived mesoscale air motions show that the location and horizontal scale of the region of most intense stratiform precipitation are determined by the pattern of horizontal transport and fallout of snow and low-density graupel particles advected into the stratiform region from the leading line of convective cells. The stratiform-region water budget implied by these calculations further indicates that the amount of stratiform rain reaching the surface is considerably enhanced by the passage of these particles through the region of mesoscale upward motion in the stratiform cloud behind the convective line. Vapor deposition onto existing ice particles and the collection of snow generated by the mesoscale ascent are the dominant growth processes in the stratiform region. Simulations using the low-resolution sounding-derived air motions as input to the model show that the mesoscale updraft accounts for the extensive nonraining stratiform cloud to the rear of the surface precipitation area.