On Precipitation Mechanisms and their Artificial Modification

Abstract

Natural precipitation processes are re-examined on the basis of accumulated knowledge of the microphysical aspects and field observations, with particular attention to the implications for cloud seeding. It appears that the active lifetime of a convective cell is much the same as the time required to grow precipitation particles and, therefore, that artificial nucleants should be inserted during the inception of the cell. The accretion process appears to be dominant in convective precipitation and there is evidence that the effectiveness of the sweeping action could be enhanced irr many cases by adding more precipitation particles. The dominant precipitation mechanism in the stratiform systems characteristic of extratropical cyclones is the ice crystal process. It is proposed that the uniformly high precipitation efficiencies of such systems, in the face of the great variability of the concentration of natural ice nuclei, result from the exponential increase in active ice nuclei with decreasing temperature. It is suggested that opportunities exist for redistributing some of the precipitation by seeding if the nucleants are released into the proper cloud layer at the correct time. Orographic precipitation often involves convective clouds as well as forced uplifting and the characteristic synoptic features and topography of the particular region are of major importance. It is believed that opportunities exist for reducing the loss of condensate in the downslope flow by seeding upwind of the barrier. It is concluded that opportunities exist for the modification of each of the three general types of precipitation considered, but that their realization depends on much more complete observations and a more quantitative approach than have been typical of past cloud seeding experiments.