Examination of the Physical Process of Severe Convective Windstorms

Abstract

Downbursts are strong downdrafts that induce an outburst of damaging winds at or near the ground, and a microburst is a very small downburst with an outflow diameter of less than 4 km and a lifetime of fewer than 5 minutes. The dangers posed by convective storm-generated downbursts have been extensively documented. Severe windstorms (i.e., widespread convective wind gusts > 25.7 m s?1 (50 kt)) resulting from mesoscale convective systems (MCS) cause significant disruption to society, including widespread power outages, tree and structural damage, and transportation accidents that affect multi-state regions and metropolitan areas along their track. A derecho, defined as a long-lived, widespread severe convective windstorm, is composed of numerous downbursts (intense localized storm downdrafts) organized into clusters or families of clusters. Derechos can produce winds above hurricane force along a track that may exceed several hundred kilometers. This dissertations shows how ground and satellite-based instrumentation can be combined to monitor windstorms over the American Midwest and Atlantic coastal region. Convective windstorm potential has been expressed as a grouping of stability parameters that are relevant for downburst generation. These include the lower-to-mid-tropospheric temperature and equivalent potential temperature (theta-e) lapse rates, vertical relative humidity differences, and the amount of convective available potential energy (CAPE) in the troposphere. Accordingly, this paper will provide background on convective windstorms and a three-step process that monitors the ambient environment that leads to the formation of severe storms and the specific characteristics of those that produce downburst winds. We found that downburst monitoring and subsequent prediction is essentially a three-step process with an objective to build a three-dimensional model of the thermodynamic structure of the ambient environment and conceptual model of downburst-producing convective storms in mid-latitude continental regions. Collection and exploitation of surface-based observations, ground-based microwave and radio profiler measurements, and satellite-based 2-D plan view images all serve as building blocks in the conceptual modelling process. Modification of sounding profiles with surface observations of temperature and humidity is an additional step that results in improved representation of the ambient environment.