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GERD RAGETTE

Abstract

Wind measurements were made near Alberta, Canada, hailstorms during the summers of 1966–68. Pilot balloons were released from a network of ground stations before, during, and after the passage of storms. Out of some 20 cases, five hailstorms were analyzed in detail. The main objective was to study the mesoscale wind pattern associated with the storms from the surface to the highest possible altitude. Patterns of divergence, vorticity, and vertical motion were derived and examined in relation to the radar echoes.

Downward air motions were found in the lower levels throughout most of the horizontal extent of the echoes. In three cases, downdrafts extended downwind of the precipitation; the mesoscale downdraft core was located in the rear portion of the echo with the descending air flowing opposite to the direction of storm motion. In the one case in which downdrafts were confined to the precipitation area, the downdraft core was located in the central part of the echo and the air descended toward the right of the direction of storm motion. Divergence accompanying the downdrafts extended from the surface up to about 4,000 or 5,000 ft and to a higher altitude in the case of a rapidly dissipating storm. A divergence maximum was found in all cases at about 1,000 ft. In general, divergence was associated with anticyclonic vorticity.

Mesoscale updraft areas were observed in some of the cases. A region of ascending air was found on the right flank of a supercell storm. The velocities in this updraft reached 1 m/s at 2,000 ft. The horizontal motion of this ascending region was directed toward an indentation in the echo. In another case, a mesoscale updraft was observed ahead of a squall line. The convergence patterns accompanying the updrafts indicated a low-level convergence maximum at about 1,000 ft in all cases.

In three cases, gust fronts were observed ahead of the storms. They were zones of cyclonic shear closely identified with maxima of cyclonic vorticity and convergence in the updraft region, the effects of which were evident a considerable distance in advance of the gust front. Two different gust front organizations were observed. One seems to be associated with storms that have passed their most active stage. In these cases, the gust front was located several kilometers ahead of the precipitation, and the air pushed ahead of the echo. The other appears to represent an early stage of gust formation, and was associated with a storm still in full vigor. Here, precipitation was encountered ahead of the gust front and the airflow was not directed ahead of the echo, but toward its right-hand side.

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