A Composite Study of Comma Clouds and their Association with Severe Weather over the Great Plains

Abstract

Sixty-eight comma-cloud systems over the Great Plains during two spring seasons were examined using satellite imagery and rawinsonde data. Composite soundings were produced for each of ten distinct parts of the comma cloud in order to describe quantitatively the atmospheric structure associated with wave cyclones that produced 585 severe weather events. Composite sectionals and soundings document the different kinematic and thermodynamic environment of each part of the comma-cloud system. Relative-wind, isentropic analyses show air flow relative to the storm and provide additional evidence that the clear region intruding northward east of the cyclone center is an area of strong ascent of dry air with a previous history of subsidence. Stability computations from the mean soundings suggest that the most likely location for severe weather is near the central part of the comma tail, in agreement with the tabulated severe weather reports. A case study from 21–22 March 1981 is conducted to investigate the generation of a secondary line of convection which formed in the center of a dry intrusion after the main area of convection passed to the east. This “dry-slot convection” is hypothesized to occur if the upper cloud edge of the comma tail moves ahead of the associated surface boundary during the day, leaving a region of moist boundary-layer air exposed to solar heating. With rising motion, adiabatic cooling and drying aloft, and increasing warming and moisture convergence near the surface, a rapid destabilization can occur, resulting in development of secondary lines of convection vigorous enough to produce severe weather.