Potential vorticity patterns and their relationship to heavy precipitation in Mesoscale Convective Systems (MCSs)

Convective systems occurring on four successive days (27 June to 1 July 1999) deposited an impressive amount of precipitation over Kansas, Oklahoma, Arkansas, and neighboring states. During the four-day period, many stations received more than 5 in of rain, in many cases a significant portion of their growing season rainfall. A feature common to all these events is a persistent elongated potential vorticity (PV) maximum, or PV slot upstream from the area of heavy rain. This PV maximum appears also as a dry slot in the GOES-8 water vapor imagery. The connection between a persistent dry slot in the water vapor satellite imagery and a succession of organized convective events has been pointed out in the past by a number of authors; however, the important relation between the related PV slots and the dynamic intensification of mesoscale convective systems (MCSs) is investigated here.

The PV slots were narrow, but amenable to analysis using model output and observational data. Although the main track of large-scale PV flow was located near the US-Canadian border, a small, weaker branch of the PV current formed well south of this track in northwest flow directed across Colorado toward Arkansas. An old frontal boundary at the surface lifted prefrontal air containing high values of convective potential energy initializing convection in the path of the PV slot aloft. Thunderstorms that formed in the path of the PV slot quickly grew into organized convective systems resulting in a succession of heavy rain events.

We will present several diagnostic computations (using observations as well as high resolution model output) in an attempt to describe the development of these systems. Because they developed within the profiler array, there is an opportunity to use hourly wind data to describe a vorticity budget in good temporal detail and, if other thermodynamic analyses are adequate, a potential vorticity budget in lesser detail.