a Numerical Investigation of Tropical Squall Lines.

The structure and dynamics of tropical squall lines are investigated using two-dimensional and three -dimensional numerical models. Much of the structure found in squall lines is similar to the symmetrical response that might be expected of a non-sheared atmosphere to mid -level warming and low-level cooling. Two dimensional experiments underline the importance of shear in determining squall line characteristics. Two ideas concerning the effect of shear are found useful in explaining many of the outcomes of the numerical experiments. First, is that in two dimensions, shear in the absence of vorticity sources and sinks is detrimental to convection. Second, that there is a specific value of low-level shear interacting with a cold pool which produces deep uplift and hence strong forcing of convection. Experiments are also carried out to determine the effect various thermodynamic structures have on squall line properties. Results suggest that moist mid-level air tends to be favorable for squall lines. Increasing the total buoyancy or altering the distribution of buoyancy with height produces stronger systems with updrafts more tilted from the vertical. Von Karmans formula for the speed of a gravity current is tested against the propagation speed of the simulated systems. Although qualitative agreement is obtained, 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 of the cold pool. A diagnosis of the contributions to the surface pressure jump shows that the warming external to the cold pool and water loading can be significant. It is found that for vertically-oriented or downshear tilted updrafts, the positive contribution to the pressure jump due to water loading can sometimes exceed the negative contribution due to warming. This can result in the system moving faster than predicted by the gravity current formula. On the other hand, for an upshear tilted updraft the effect of warming outweighs waterloading and the squall line can move significantly slower than predicted. (Abstract shortened with permission of author.).