The Structure and Thermodynamics of an Intense Mesoscale Convective Storm in Oklahoma

Abstract

On 14 May 1970 a cold front passed through the mesonetwork of the National Severe Storms Laboratory in central Oklahoma. As it did so, intense convection developed and thunderstorms produced more than 2 inches of rain at some points within the network. On this date a total of 58 rawinsonde observations were made at nine stations within the network, 42 of them during the period from about 1 h prior to frontal passage to 2 h afterward. We have analyzed these as well as other data to arrive at a picture of the structure of the mesoscale system and of the thermodynamical processes operating in it. The front first encountered potentially unstable rnoist air as it passed through the network. As this air was lifted frontally the instability was released, with remarkable results. A mesoscale downdraft?updraft doublet developed in the warm air aloft, with peak speeds at 400 mb of 2?3 m sminus;1 over 10 km widths transverse to the front, the descent being above the surface frontal position and the ascent (which produced almost all the precipitation) being about 30 km toward the colder air to the northwest. The downdraft appears to he driven by intense cooling due to evaporation of the initial deep cumulus clouds into the very dry air aloft. The updraft is due to condensational heating on the mesoscale, in saturated air of nearly neutral stability, with convective activity superimposed. We conjecture that these diabatic effects permit the Mesoscale vertical motions to proceed for several hours without large perturbation of the isentropic surfaces. The character of the convection, and of the mesoscale circulation, is not accounted for by a simple model of an entraining convective plume. We present evidence that the balloons tended on the average to be drawn into the convective?scale updrafts and to avoid the downdrafts, thus yielding spurious indications of the percentage of volume occupied by active convective updrafts and downdrafts. We find, on the other hand, that deviations of balloon ascent rate and equivalent?potential temperature of individual soundings from the means of their neighbors can be used to estimate convective transports. The virtual source of equivalent potential temperature, thus determined, is in reasonable agreement with the apparent source independently obtained as a residual in the mesoscale budget.