A Mesoscale Vortex Couplet Observed in the Trailing Anvil of a Multicellular Convective Complex

Johannes Verlinde Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

Search for other papers by Johannes Verlinde in
Current site
Google Scholar
PubMed
Close
and
William R. Cotton Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

Search for other papers by William R. Cotton in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

Observations collected during the Oklahoma–Kansas PRE-STORM experiment are used to document the evolution and structure of a mesoscale vortex couplet that developed in the mesoscale convective system that occurred on 16–17 June 1985. The evolution of the circulations was captured by dual-Doppler radar observations for 1.4 hours. This allowed an evaluation of the various terms of the vertical vorticity equation, which give insight into the mechanisms that are important in the generation of the circulations. The primary mechanism responsible for the formation of the observed vortices was the interaction of the larger-scale flow with low level momentum transported to higher levels by multiple convective updrafts. As a consequence vertical shear of the horizontal wind was important to initial vorticity production. The vorticity generated in this manner was subsequently increased in strength due to middle level convergence. When the convection weakened and dissipated, the primary source of vorticity was removed, and because this was an unbalanced circulation on a scale less than the Rossby radius of deformation, the vortex broke up and spun down. Comparisons are made with other documented cases, and differences and similarities are pointed out. It is hypothesized that this circulation is a common kind in precipitating mesoscale systems, which has hitherto largely been undetected because its size is too large to be easily observed in a Doppler radar network set up to study thunderstorms, yet too small to be detected by standard sounding networks or most research sounding networks.

Abstract

Observations collected during the Oklahoma–Kansas PRE-STORM experiment are used to document the evolution and structure of a mesoscale vortex couplet that developed in the mesoscale convective system that occurred on 16–17 June 1985. The evolution of the circulations was captured by dual-Doppler radar observations for 1.4 hours. This allowed an evaluation of the various terms of the vertical vorticity equation, which give insight into the mechanisms that are important in the generation of the circulations. The primary mechanism responsible for the formation of the observed vortices was the interaction of the larger-scale flow with low level momentum transported to higher levels by multiple convective updrafts. As a consequence vertical shear of the horizontal wind was important to initial vorticity production. The vorticity generated in this manner was subsequently increased in strength due to middle level convergence. When the convection weakened and dissipated, the primary source of vorticity was removed, and because this was an unbalanced circulation on a scale less than the Rossby radius of deformation, the vortex broke up and spun down. Comparisons are made with other documented cases, and differences and similarities are pointed out. It is hypothesized that this circulation is a common kind in precipitating mesoscale systems, which has hitherto largely been undetected because its size is too large to be easily observed in a Doppler radar network set up to study thunderstorms, yet too small to be detected by standard sounding networks or most research sounding networks.

Save