Research achievements in cloud and precipitation physics of stratiform clouds are summarized in this paper. Several problems associated with artificial precipitation for stratiform clouds are discussed, and a method of evaluating artificial precipitation conditions by using conventional observation data is offered. Specific results are listed below:
The structure of a stratiform cloud is not uniform and includes layered structures in the vertical direction. “seeding-feeding” cloud is typical structure of stratiform cloud precipitous, and interaction of the seeding cloud and feeding is the main process in precipitation. The stratiform cloud precipitous may be divided into the following three layers according to its microphysical structure: An ice phase layer, a mixed layer with ice and liquid water, and the liquid water layer. The ice phase layer is the seeding cloud, and the mixed and liquid water layers are feeding clouds.
A study of stratiform cloud precipitation processes corresponding to the three-layer structure of the cloud revealed differences in microphysical processes, particle formation, and growth processes. Sublimation is the main growth mode of ice and snow crystals in the ice phase layer, followed by the polymerization process of these crystals. Snow, or polymers, falling into the mixing layer continue to grow by sublimation or the Bergeron process, in addition to accretion, and some of the ice crystals and snow are converted into graupel. In liquid water, snow, or polymers, begins to melt and to collect cloud water. The accretion and sublimation growth processes are also important for ice particles. Each layer in the cloud provides a different contribution to precipitation. The contribution rate of the seeding cloud is generally less than 30%, and that of the feeding cloud is more than 70%.
On the basis of the above study results, the following conclusions are drawn: (1) The seeding-feeding cloud benefits from the transformation of cloud water to precipitation, and mutual cooperation among the ice phase, mixing, and liquid water layers results in the formation of effective precipitation. Therefore, the seeding-feeding cloud should act as a seeding condition for artificial precipitation. (2) For a stratiform cloud to be seeded, the cold cloud process is important. For effective precipitation, a super-saturated water vapor amount with respect to ice, in addition to the content of super- cooled water, should be high. (3) The artificial precipitation principle for stratiform clouds should be considered. Although the Bergeron-Findeisen process is critical to precipitation formation, aggregation and accretion in the mixing layer, super-cooled water and super-saturated water vapor are also important.
Finally, combined with achievements in stratiform cloud research, conventional detection data is used for the discriminant method on artificial precipitation conditions of stratiform clouds to determine that the seeding-feeding cloud structure is distinguished by satellite cloud pictures and radar and that the precipitation mechanism and the liquid water layer can be discriminated by using radar range height indicator (RHI) echo.