Abstract

This paper describes analyses of data collected from four seeded storms during the 1978 summer program of Grossversuch IV in Switzerland. The storms all met the Soviet criteria for hail-forming potential and were seeded with Soviet-type OBLAKO rockets.

A seeding “quality” was estimated in each of the cases by observing the internal structure of the storm cells with C, S and X-band radar, and the trajectory, time of residence and dispersion of the AgI aerosols in the seeded clouds with radar and chemical techniques. The notion of “seeding coverage” is presented as the ratio of the surface area of precipitation in which the seeding chemical is found at the ground to the surface area of the rainfall (using the 40 dBZ radar reflectivity contour near the ground).

The study of two cells on 30 June 1978 shows that the seeding coverages were small (7% and 25%) and that estimated residence times for AgI in those portions of the cloud colder than −5°C were too short to allow for significant ice phase modification. The other two cells, seeded 11 and 14 July 1978 had seeding coverages of 100% and AgI residence times, in cloud colder than −5°C of 500–700 seconds, which should be adequate for modification of the water–ice balance in these clouds.

Positive correlations exist between precipitation intensity and seeding chemical concentration when the seeding aerosol has a long residence time in cloud colder than −5°C (11 July case). This is not so when the AgI aerosols are scavenged in a short time interval as occurred in the two case studies of 30 June.

The hail suppression plan for the Grossversuch IV experiment was shaped in accordance with the Moldavian hail suppression organization. The seeding criteria attempts to guarantee that the seeding time always occurs at the same stage of development of a growing storm. This criterion is based on six 3-cm radar parameters in the RHI mode and by a radiosonde. These parameters are cloud top height, height of maximum reflectivity, the temperatures at these heights, the ratio of cold to warm parts of cloud and the maximum reflectivity. If the hail probability is determined to be greater than 50%, the maximum reflectivity Zm is ≥45 dBZ, and Zmax has a height above or at the freezing level, then seeding is carried out.

Although the Soviet criteria were met for seeding purposes in all cases described here, the results show that the reflectivity structure of the storms is also very important and should be allowed to play a prominent role in assessing where and when the seeding agent should be injected, if at all, in attempting to suppress hail growth.

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