The Sensitivity of Two-Dimensional Simulations of Tropical Squall Lines to Environmental Profiles

Abstract

Two dimensional experiments are carried out to determine the effect of various wind and thermodynamic structure on squall line characteristics. Two ideas concerning the effect of shear are found useful in explaining many of the outcomes of the numerical experiments. First, in two dimensions, shear in the absence of vorticity sources and sinks is detrimental to convection (Kuo, Asai). Second, there is a specific value of low-level slim interacting with a cold pool which produces deep uplift and hence strong forcing of convection (Rotunno et al.). Results suggest that moist midlevel air tends to be favorable for squall lines. Increasing the total buoyancy or altering the distribution of buoyancy with height, such that it is increased at low levels, produces stronger systems with updrafts more tilted from the vertical. The formula by von Kárm´n for the speed of a gravity current gives qualitative agreement with the speed of most of the simulated systems. However, at least two additional factors need to be considered to accurately determine propagation speed. First, the wind speed ahead of the system can be modified from environmental values. Second, the propagation speed depends on the surface pressure jump across the gust front and this is not always accurately determined by the temperature anomaly in the shallow cold pool. A diagnosis of the contributions to the surface pressure jump shows that the warming external to the cold pool and waterloading can be significant. It is found that for vertically oriented or downshear tilted updrafts, the positive contribution to the pressure jump due to waterloading can sometimes exceed the negative contribution due to warming. This results in the system moving faster than predicted by the gravity current model whereas, for an upshear tilted updraft, the effect of warming can outweigh waterloading, causing the system to move slower. An examination of the vorticity balance that occurs for a case having a vertically oriented updraft suggests that it cannot be regarded as purely one between the environmental shear and the cold pool.