PRECIPITATION GENERATING CELLS

Raymond Wexler Allied Research Associates, Inc.

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David Atlas Air Force Cambridge Research Center

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Abstract

Advection of dry air aloft ahead of the upper-level trough forms a convectively unstable layer which transforms a relatively uniform echo to a cellular pattern with echo top at a lower elevation. A characteristic sequence of cell structure ensues: first, cells without trails, and lastly, trails without “heads.” Observation indicates that the cells are associated with cumuliform tufts imbedded in a stratiform deck. Reflectivities are maximum near the base of the cells. Updrafts required to produce equivalent precipitation intensities are 40 to 80 per cent of fall speeds and suggest growth of the ice crystals both during their ascent and descent through the updraft. Radar-deduced fall velocities are consistent with those of the observed crystals only if the cells are assumed to move with the winds at their bases ; this is the true generating level.

Abstract

Advection of dry air aloft ahead of the upper-level trough forms a convectively unstable layer which transforms a relatively uniform echo to a cellular pattern with echo top at a lower elevation. A characteristic sequence of cell structure ensues: first, cells without trails, and lastly, trails without “heads.” Observation indicates that the cells are associated with cumuliform tufts imbedded in a stratiform deck. Reflectivities are maximum near the base of the cells. Updrafts required to produce equivalent precipitation intensities are 40 to 80 per cent of fall speeds and suggest growth of the ice crystals both during their ascent and descent through the updraft. Radar-deduced fall velocities are consistent with those of the observed crystals only if the cells are assumed to move with the winds at their bases ; this is the true generating level.

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