Variability of Heavy Precipitation of Long-Lived Mesoscale Convective Vortices Found in the Southern Plains

The purpose of this research was to analyze favorable atmospheric conditions for long-lived Mesoscale Convective Vortices (MCVs) that produced heavy precipitation. An MCV is a low pressure center found within a Mesoscale Convective System (MCS). The structure of an MCV is comparable to that of a Tropical Cyclone, as they tend to form in weak moderate vertical shear and are denoted by their cyclonic flow at low to mid-levels and anticyclonic outflow aloft. Many times after the antecedent MCS dissipates, the MCV can assume its own identity and become the source for convection initiation the following day. When environmental conditions are favorable, an MCV can persist several days through multiple MCS cycles. Such a case is known as an MCV series.

Mid-level relative vorticity and surface precipitation were analyzed using the NCEP North American Regional Reanalysis from 1979-2011. Hovmӧller diagrams were examined to identify time periods with long-lasting, slow-moving vorticity maxima. Six cases were selected for further analysis composite analyses for heavy precipitation cases and dry cases were created for multiple variables of the atmosphere including 500-mb geopotential height anomalies, 925-mb meridional wind anomalies, precipitable water anomalies, and mean integrated vapor flux.

A ridge was found to have been displaced further south for heavy precipitation cases in the 500-mb geopotential height composite anomalies along with uniform distribution of higher heights over the United States. Dry cases displayed a tighter ridge further north and larger gradients between lower and higher heights. Strong southerly flow was evident in the 925-mb meridional wind composite anomalies for the heavy precipitation cases. This flow helped transport moisture from the Gulf of Mexico for three MCV series' that went on to produce heavy floods in the Southern Plains. Sounding data confirms southerly flow and a moist atmosphere for heavy rain events.