Numerical Simulation of a Subtropical Squall Line over the Taiwan Strait

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Abstract
A two-dimensional, time-dependent, and nonhydrostatic numerical cloud model is used to study the development and structure of a subtropical squall line that occurred during TAMEX (Taiwan Area Mesoscale Experiment). The model includes a parameterized ice-phase microphysical scheme and long- and shortwave radiative transfer processes, as well as heat and moisture fluxes from the ocean surface. It was found that dynamic and kinematic structures of this simulated subtropical squall line are quite similar to its counterparts observed in the tropics and midlatitudes. For example, the squall line has a quasi-steady structure with a successive generation of cells at the gust front that propagate rearward relative to the front, the precipitation, and an evaporatively cooled downdraft at low and midlevels. This particular subtropical squall line is also shown to have a distinct midtropospheric rear inflow and a moderate anvil component of the total precipitation. The vertical transport of horizontal momentum, as well as latent heat release by the simulated subtropical squall system and by squall systems that occur in other geographic locations (both simulated and observed), are compared and presented. We also investigate the roles of 1) heat and moisture fluxes from the ocean, 2) longwave radiative cooling, 3) microphysical processes, and 4) presumed mesoscale convergence lifting on the structure and propagation of this subtropical squall line. Among the seven two-dimensional simulations considered, the general structure of the squall system, such as its propagation speed and its “weak-evolution”-type multicell characteristics, do not change significantly in most of the cases. It was found that each process has a different impact on the total surface precipitation over an 8-h simulation time. The order of importance of each process to the total surface precipitation, beginning with the most important, is microphysics, longwave radiative transfer, heat and moisture input from the ocean, and prestorm mesoscale convergence lifting.

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