Thursday, August 20, 2009

Life cycle of summertime diurnal thunderstorms

On Tuesday evening (8-18-09), thunderstorms erupted over the greater Memphis area, dropping flooding rains (due to very slow movement of the cells) and creating copious cloud-to-ground lightning. These storms provide the opporunity to describe the classic life cycle of diurnal ("heating of the day") thunderstorms.

The typical life cycle of single cell thunderstorms, or small clusters of storms, on a hot summer day involves initial growth of a few cells which then produce an "outflow boundary," or gust front, as the storm falls apart. A downrush of air from a collapsing thunderstorm reaches the ground and spreads out, generally in all directions. The leading edge of this cooler airmass (or outflow) is the outflow boundary or gust front. See the life cycle diagram below and note the downrush of air in the dissipating stage (image courtesy: Wikipedia).

The outflow boundary itself serves to ignite new storms (or intensify existing cells) as it acts much like a cold front with converging air on its leading edge and cooler air displacing warmer air, which then rises. As the warmer air rises, clouds form and with enough lift, new showers or storms can form. The new storms mature, then collapse, creating their own outflow, and the cycle continues. See the diagram from Wikipedia below.

This is exactly what happened on Tuesday evening. The radar imagery captured that evening shows just how these types of storms have their own life cycle, using energy from nearby collapsing storms to invigorate existing storms or create new growth. The gust fronts in this movie loop are seen as thin blue lines eminating out from the larger cells. Notice that as they reach younger cells, those cells intensify (notice in particular the new storms that fire up just north of Millington and showers in central Shelby County that explode as the southward moving outflow boundary passes through those areas, causing rapid growth just east of Bartlett along I-40 [the red line]).

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