Formation of Mesolows or Pressure Troughs in Advance of Cumulonimbus Clouds

Abstract

Mesoscale lows or pressure troughs have been observed downwind of many mid-latitude cumulonimbus cloud systems, especially those that subsequently produce severe weather such as tornadoes, large hail, or damaging wind gusts. Case studies are presented which link the formation and existence of mesolows or troughs to subsidence warming in the upper troposphere and lower stratosphere. This subsidence warming, which is believed to result from the interaction of the convective cloud with a sheared environment, leads to an instability between the meso-? and meso-? scales by hydrostatically reducing surface pressures, thereby organizing and increasing the low-level convergence ahead of existing convection.