GOES WMSI—progress and developments

A downburst index has been developed to assess the magnitude of convective downbursts associated with heavy precipitation-producing (“wet” type) convective systems. The Wet Microburst Severity Index (WMSI), designed for use during the warm season, is composed of relevant parameters that represent the simultaneous physical processes of convective updraft development and downburst generation, incorporating convective available potential energy (CAPE) and the vertical equivalent potential temperature gradient between the surface and the mid-troposphere. Since convective storm updrafts require buoyant energy, CAPE, computed from real-time Geostationary Operational Environmental Satellite (GOES) sounding data, is a very important parameter used in mesoscale analysis. Since updraft strength is proportional to CAPE, large CAPE would result in strong updrafts that could lift a precipitation core within a convective storm to a dry air layer in the mid-troposphere. In addition, the amount of mid-level water vapor relative to the low-level water vapor, represented by vertical equivalent potential temperature gradient, is important in the determination of downdraft strength due to evaporational cooling as dry air is entrained into the convective system. In contrast to deep convective storms that develop during the warm season, forcing by a vigorous upper-level troughs and rapidly moving cold-frontal systems as well as strong and diffluent winds aloft have been associated with intense downburst-producing convective systems that develop during the cold season. A major factor that has been determined to affect downburst strength associated with synoptically-forced convective systems is the downward transport of higher momentum possessed by winds in the mid-troposphere. This paper will review the development of the WMSI algorithm and the GOES WMSI product. Product validation efforts will be discussed. Validation data for the 2003 and 2004 convective seasons indicated favorable results: a statistically significant correlation between GOES WMSI and the magnitude of observed surface wind gusts for both daytime (r = 0.66) and nighttime (r = 0.64) events. This paper will also investigate the role of downward momentum transport in the magnitude of convective winds during the cold season. Case studies, contrasting downburst-producing, warm and cold-season convective systems, will be presented. Finally, a modification to the use of the GOES WMSI will be explored to apply to cold-season forecasting situations.