Radar Observations of Mesoconvective Vortices over the Central United States

Composite studies have indicated that midtropospheric vortices (MCVs) of 50-200-km radial extent are a common structural characteristic of many large mesoscale convective systems (MCSs). In some cases, MCVs are found to persist beyond the cessation of the MCSs from which they are spawned. These MCVs are of particular interest because of their frequent association with the subsequent regeneration of deep convection. Since MCVs are quasi-balanced circulations that are relatively nondispersive, it is likely that deep convection that is regenerated within their circulation is more predictable than many other forms of deep convection. The current study examines the frequency and characteristics of MCVs that occurred during the warm season (May-September) over the central United States for a three-year period (1998-2000). Although not widely documented, since the vertical profiles of diabatic heating are favorable for vorticity production within the stratiform precipitation regions of MCSs, it is anticipated that MCVs may also occur in other midlatitude or subtropical regions that support large MCSs.

The MCVs described in this study are detected through animations of operational radar reflectivity data and other supporting observations (e.g., satellite, wind profiler) in the United States National Weather Service's operational data stream. Results indicate that long-lived MCVs, which persist beyond the dissipation of their parent MCSs, may be up to an order of magnitude more common (occurring 15-30 times per warm season) than documented in earlier studies that relied solely on still satellite imagery for MCV identification. Such long-lived MCVs are most commonly observed in environments of weak ambient vertical shear. The regeneration of deep convection within MCVs occurred in approximately half of the observed MCV cases, and was often associated with an environmental condition of high relative humidity above the boundary layer. The longest live cases that persisted beyond a single diurnal cycle were invariably associated with the redevelopment of deep convection. Here, the deep convection reinvigorates the vortex, which would otherwise weaken due to differential advection even in weak ambient shear. Examples of precipitation patterns within the observed spectrum of MCVs will be presented at the conference.