Kinematic, Dynamic, and Thermodynamic Analysis of a Weakly Sheared Severe Thunderstorm over Northern Alabama

Abstract

A kinematic, dynamic, and thermodynamic analysis of a weakly sheared, airmass thunderstorm observed over northern Alabama is presented. Most notable is the fact that the dominant cell in this storm closely resembles the Byers and Braham model for warm-based, airmass storms. Several phenomena never documented for this storm-type are discussed. One of these is a strong and deep downdraft observed at midlevels with an associated “weak-echo” trench. Its origin appears to be related to a wake entrainment process. A midlevel inflow which causes a visible constriction in the storm cloud is also observed. This inflow results in a division of the thermal buoyancy into two maxima in the vertical: one associated with the precipitation core and the other with strong positive vertical motions in the growing cumulus turret. In addition, a downdraft separate from that seen at midlevels develops at low levels and causes a microburst outflow at the surface. This downdraft appears to be initiated by precipitation loading and intensified by negative thermal buoyancy. The Byers and Braham model of the cumulus, mature and dissipating stages, is discussed in light of these new features.