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P. Waldteufel and H. Corbin

Abstract

The procedures currently employed to retrieve vector wind information from single-Doppler radar observations are reviewed briefly. In particular, those procedures implying a linearity hypothesis for the wind field are shown to be particular cases of a method termed VVP (Volume Velocity Processing). This method, which makes full use of radar velocity data filling a volume, is first tested on simulated observations in order to assess its accuracy, then applied to actual data and shown to yield unbiased parameters of the horizontal vector wind field, as well as an estimate of the hydrometeor fall velocity and several control parameters.

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J-M. Brustet, B. Benech, and P. Waldteufel

Abstract

The possibility of applying infrared imagery to the study of a large, hot plume materialized by carbon particles resulting from the incomplete combustion of fuel oil is investigated.

In a specific case (the PROSERPINE experiment), due to the high carbon particle content, the lower part of the plume acts as a semi-opaque target. Using an infrared camera equipped with a detector sensitive in the 2–5.8 μm band, the thermal images are found to yield a plume geometry in good agreement with visible contours retrieved from visible photographs.

Thermal images provide access to the internal structure of a plume, down to scales which depend on the plume opacity. It appears that IR imagery is able to yield improved information concerning the turbulent fields of motion and temperature.

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D. Ramond, H. Corbin, M. Des̄bois, G. Szejwach, and P. Waldteufel

Abstract

A polar jet stream flowing over northern Europe is investigated using both the radiosounding network and the METEOSAT water vapor (WV) channel imagery. A maximum radiance band associated with the polar jet stream is shown to accurately delineate the tropopause break; a major difference in the subtropical jet stream radiative signature is noted, however. An analytic approximation of the radiative transfer equation is established for the METEOSAT WV channel and leads to the conclusion that the radiance field, away from the polar frontal zones, is representative of the temperature field on a water vapor isosteric surface.

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J. P. Lacaux, J. A. Warburton, J. Fournet-Fayard, and P. Waldteufel

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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