The Dynamics of Circulations within the Trailing Stratiform Regions of Squall Lines. Part I: The 10–11 June PRE-STORM System

A dynamic version of the two-dimensional kinematic cloud model of Rutledge and Houze has been developed to investigate the effects of microphysics on circulations within the stratiform regions of mesoscale convective systems. The design of the model allows for specified inputs of hydrometeors, water vapor, and heat from the convective line. The stratiform region of the 10-11 June 1985 PRE-STORM squall line is simulated, with initialization based upon appropriate soundings, heat budgets, and 1D cumulonimbus model results. The model accurately simulates the evolution of the stratiform rain area. Significant ascent occurs in the stratiform region where in situ condensate production contributes increasingly to the surface rainfall, reaching 65% of the total in the mature stage and averaging 44% over the entire simulation. The ratio of condensate produced within the mesoscale updraft to that advected from the convective line generally agrees with water budgets from other studies. Simulated horizontal hows agree qualitatively with observations and include a sloping rear-inflow jet that develops with peak speeds approaching those observed. A transition zone, marked by a minimum in surface precipitation separating the convective and stratiform regions, broadens over time, especially late in the simulation after leading convective elements weaken. The model has several limitations: it is 2D and neglects radiation and large-scale baroclinicity. As in previous 2D models, surface rainfall is underestimated, implying the importance of 3D convergent forcing of strong ascent in the anvil cloud. Ascent is underestimated near stratiform cloud top, indicating that cloud-top radiative effects are possibly important in generating circulations there. Strong rear-inflow is restricted to within 100 km of the convective line, suggesting that rear inflow at higher levels far to the rear of the squall line (which occurred in the 10-11 June case) may require large scale baroclinicity.