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The Balance Level in Convective Storms

David AtlasAir Force Cambridge Research Laboratories, Sudbury, Mass.

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Abstract

Doppler radar observations of convective storms at vertical incidence show a height at which the mean reflectivity-weighted particle velocity relative to the ground is zero, i.e., the particles are just balanced by the updraft. This height is defined as the “balance level” or BL. While a transient BL may appear at any height in the storm, a steady or quasi-steady BL is found to exist only near maxima in the updraft profile. Such maxima provide a means of separating the particle size spectrum into rising and falling groups of comparable reflectivity at a discrete height. Two BL's have been identified, one at a primary updraft maximum near the storm top and the other at a secondary maximum lower down. The continued existence of the lower BL depends upon whether or not the group of rising particles, which grow further in the updraft above the secondary maximum, turn downward and fall back through the BL with fall speeds greatly in excess of the associated updraft speed there. Under conditions of strong shear aloft, such particles do not return within the storm core and so the lower BL may persist.

The layer including the BL is shown to be one of major particle growth. The increased precipitation content in that layer is also believed to be responsible for weakening of the updraft, although the precise relationship needs to be isolated. The region of the lower BL and associated minimum in updraft is shown to be a region of horizontal divergence which is reflected in the outward flow of particulates and a broadening of the reflectivity pattern. The BL is also a pseudo-generating level insofar as trails of precipitation appear to originate there. The discussion also treats: 1) the location of the low level roots of updrafts in a line of convective cells, 2) the nature of first echoes and their relation to the balance level, and 3) the region of trailing stratiform precipitation.

Abstract

Doppler radar observations of convective storms at vertical incidence show a height at which the mean reflectivity-weighted particle velocity relative to the ground is zero, i.e., the particles are just balanced by the updraft. This height is defined as the “balance level” or BL. While a transient BL may appear at any height in the storm, a steady or quasi-steady BL is found to exist only near maxima in the updraft profile. Such maxima provide a means of separating the particle size spectrum into rising and falling groups of comparable reflectivity at a discrete height. Two BL's have been identified, one at a primary updraft maximum near the storm top and the other at a secondary maximum lower down. The continued existence of the lower BL depends upon whether or not the group of rising particles, which grow further in the updraft above the secondary maximum, turn downward and fall back through the BL with fall speeds greatly in excess of the associated updraft speed there. Under conditions of strong shear aloft, such particles do not return within the storm core and so the lower BL may persist.

The layer including the BL is shown to be one of major particle growth. The increased precipitation content in that layer is also believed to be responsible for weakening of the updraft, although the precise relationship needs to be isolated. The region of the lower BL and associated minimum in updraft is shown to be a region of horizontal divergence which is reflected in the outward flow of particulates and a broadening of the reflectivity pattern. The BL is also a pseudo-generating level insofar as trails of precipitation appear to originate there. The discussion also treats: 1) the location of the low level roots of updrafts in a line of convective cells, 2) the nature of first echoes and their relation to the balance level, and 3) the region of trailing stratiform precipitation.

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