A Numerical Simulation of the 22 July 1979 HIPLEX-1 Case

Abstract

A HIPLEX- A cloud case has been used to describe the effects of ice-phase cloud seeding on a cold base cloud. The cast was used at the First International Cloud Modeling Workshop/Conference held in 1985 in Irsee, Federal Republic of Germany, to provide modelers with a well-observed cloud-seeded case. Solid dry ice pellets (CO2) were dropped into the cloud at about the ?10°C level. Aircraft and radar observations of the cloud were obtained over approximately 40 min of its life history. Seeding produced clear ice seeding signatures in this continental type cloud. Graupel formed within 10 min and rain was observed at the +10°C level 13 min after seeding. About 9.4 kT of rain was estimated to fall from the cloud. The modeling results also show enhanced ice in the seeded cloud and can be compared with an unseeded model cloud (not available in nature). The precipitation path from cloud ice to snow to graupel/hail to rain from the melted graupel is evident. The Bergeron process was important for snow initiation, while snow accretion of cloud water was dominant for further growth. Light rain amounting to about 13.1 kT resulted in the seeded model cloud, and 9.6 kT in the unseeded model cloud?a 36% increase. The observers considered aggregation of snow to be important for the initiation of graupel and to account for the early appearance and rapid growth of graupel. The model results suggest that the early formation of snow near the 0°C level leads to melt into rain and the transport of rain into cloud ice and snow regions. Rapid freezing of the rain into graupel then occurs as the rain collects snow.