EFFICIENCY OF CLOUD AND PRECIPITATION MODIFICATION WITH HYGROSCOPIC AEROSOLS

S. M. Shmeter and G. P. Beryulev

A review of foreign publications on hygroscopic cloud modification aimed at changing characteristics of the precipitation formation process is given. Major results of the South African, Thailand, and Mexican Cu—Cb cloud seeding experiments carried out in 1990—1998 are discussed. The results of the numerical modeling of convective-cloud hygroscopic seeding, obtained by American and Israeli scientists, are analyzed. Using the field and numerical experiments, it is shown that precipitation from continental-origin warm convective clouds can be increased by several tens of percent. The seeding of marine convective clouds, on the contrary, does not produce a noticeable precipitation enhancement. From the field experiments it is found that in seeded convective clouds, due to intensive coagulation, the concentration of small particles decreases, while large particles, on the contrary, grow in number. Moreover, hygroscopic seeding increases the lifetime of cloud and the precipitation from it. There are indications that the seeding-induced precipitation enhancement is caused by both microphysical and dynamic factors, in particular, by forced convection in front of the subshear boundary of a convective cloud. The influence of the seeding procedure on modification efficiency is discussed. A critical review is given of the available estimates of the prospects of cold cloud modification with hygroscopic agents as well as with hygroscopic agents combined with crystal-forming substances.