| The governing dynamics of storm cold outflows was studied using a hydrostatic cold outflow model and the results verified against observations. Results indicate that asymmetries in wind pattern inside the cold outflow and in spreading are produced by the constant modification of the pressure field inside the cold outflow caused by the movement of the downdraft. The cold outflow can maintain its maximum depth up to a threshold travel speed of the downdraft. For downdrafts travelling faster than the threshold speed, which depends on the downdraft's rate of discharge, that maximum depth decreases. The rate of spread of a cold outflow is also profoundly influenced by the downdraft's travel speed.; Analyses of pressure and surface winds beneath strong convection during the arrival of two downbursts at the ground showed that they satisfy the Bernoulli equation. This is interpreted to indicate that pressure forces play an important role in the occurrence of surface windburst caused by the arrival of a downburst. At the time of arrival of the downbursts at the surface, thermodynamic analyses suggested that the air had descended from a dry layer at 700-760 hPa. A case of a downburst produced by a convective cell generated by a cold outflow collision was studied. The changes in the near surface flow brought about the collision of cold outflows were analyzed. During the sudden demise of a group of strong convective cells, due to low level mesoscale air interception by a cold outflow, no increase in rainfall production was detected that would suggest downburst occurrence despite that downbursts were occurring in another location of the mesonet.; The components in the equation for the rate of change of relative vertical surface vorticity were computed for two days during the VIN (Virginia-Illinois-NOAA) experiment. The contribution of the combined action of the vertical advection and tilting terms were found to be more important to the rate of change of relative surface vorticity than the convergence term contribution in the area of the mesonet. However, the latter contribution cannot be neglected when strong downdrafts are present inside the mesonet. |
