A Diagnostic Modeling Study of the Stratiform Region Associated with a Tropical Squall Line

Abstract

Wind and thermodynamic data analyzed in the startiform region of a tropical squall line have been combined with a kinematic, three-dimensional cloud model to study the precipitation processes in this region. The flux of condensate into the stratiform region from the convective region has been parameterized by specifying vertical profiles of cloud water, cloud ice, snow and graupel at the boundary between these two regions, through the use of a one-dimensional time-dependent cumulus model. A standard case, in which all four forms of condensate stream into the stratiform (anvil) is studied in detail. The graupel entering this region rapidly removes snow advected into the anvil from the cells as well as snow produced by the mesoscale updraft. The snow produced by the mesoscale updraft actively contributes to this precipitation by providing mass for the graupel particles to feed upon. A series of sensitivity studies are discussed which reveal two distinct regions: a precipitation region due to fallout from the convective cells followed by a horizontally homogeneous region where the precipitation is produced by the mesoscale updraft. The anvil region is nearly entirely below water saturation which eliminates mixed-phase growth processes in this region and hence allows ice particles to grow only by deposition and collection. We conclude that the condensate produced by the mesoscale updraft is an important source of precipitation and is largely responsible for the extensive region of stratiform precipitation to the rear of the convective line.