Modeling Study of a Tropical Squall-Type Convective Line

Abstract

A multidimensional and time-dependent cloud scale model is used to investigate the dynamic and micro-physical processes associated with convective and stratiform regions within a tropical squall-type convective line. The evolution of the total convective and stratiform portions of rainfall is also estimated by using model output. A three-dimensional version of the model covers a horizontal domain about 96 ? 96 km2. Frequently, the horizontal extent of an observed stratiform region is over a few hundred kilometers. Therefore, a two-dimensional version of the model with a 512 km horizontal length is also used to incorporate a complete stratiform region. Two-dimensional model result recapture many interesting features as observed. In particular, the fractional portion of stratiform rain as well as its fractional area coverage are in good agreement with observations. A significant amount of ice particles melted to rain near the freezing level in the trailing part of the modeled squall system during its mature and dissipating stages. The mesoscale circulations above and beneath the freezing level in the stratiform region are also well simulated. Three-dimensional model results could not recapture these features associated with the stratiform region. But explosive growth and a convex-leading edge associated with the convective region are well simulated. The orientation of the three-dimensional simulated convective line is perpendicular to the environmental wind shear as observed. Both of the modeled propagation speeds for the squall systems are in fair agreement with observational case studies. Sensitivity tests on ice-phase microphysical processes and mesoscale middle and upper level ascent are made to investigate their roles on the formation and structure of tropical squall-type convective lines. Parcel trajectory analyses are also performed to understand the dynamics of simulated squall-type convective lines. Specifically, the origins of air circulation in the convective and stratiform region are investigated using the model generated wind fields. The heat budgets and their associated microphysical processes within the convective and stratiform region are also examined using the model results.