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 t...