层状云系人工增雨潜力评估研究

The Study of Evaluation of Potential of Artificial Precipitation Enhancement in Stratiform Cloud System

层状云系是中国人工增雨催化作业的主要对象。云系人工增雨潜力的大小是判断是否作业的最为重要的科学依据。但是,目前中国还没有一套比较完整的综合判断云系人工增雨潜力的评估方法。自然降水形成过程是个复杂的科学问题,因此判断人工增雨潜力也是一个非常复杂的科学问题。作者利用中尺度MM5数值模式模拟的层状云系研究与人工增雨潜力有关的要素,从理论上研究了不同要素与降水的关系。在分析云体的垂直结构、降水机制、水汽厚度、冰面过饱和水汽量、云水厚度、过冷水含量、冰晶浓度、降水效率(凝结水降水效率和凝华水降水效率)等潜力要素基础上,获得了新的潜力要素,例如“催化供给”云结构、降水机制、冰面过饱和水汽量,并提出了定性综合判断云系人工增雨潜力思路。

Stratiform cloud system is major object for artificial precipitation enhancement in China, The question whether cloud systems have seeding potential attracts much attention of the investigators. It is very necessary to explore seeding potential of stratiform cloud systems, in order to strengthen positive seeding effects for precipitation enhancement. At present, there is no the integrated technique to assess seeding potential in China. The formation processes of nature precipitation is a complicated science problem, so that the evaluation of the artificial precipitation enhancement is also a complicated science problem. In this paper, a statiform cloud system, which affected Henan province on 18 - 20 October 2002, is simulated by MM5 numerical model to study the relationship of the potential factors affacting rainfall. （ha the basis of analysis of the cloud vertical structure, the precipitation mechanism, amount of super-saturated water vapor with respect to ice, thickness of cloud water, supercooling water content, ice concentration and precipitation efficiency （precipitation efficiency of condensation water and sublimation water）, some new potential factors are obtained and the way of qualitative integrated assessment for seeding potential is suggested. （1） The vertical structure of cloud. The conditions of conversion of cloud water have a relationship with the distribution of cloud water resources and cloud structures. It is very difficult to produce the availability precipitation by auto conversion of cloud water and collision between the particles in warm stratiform cloud. But if ice particles or raindrops descend from upper cloud into the cloud, the precipitation efficiency may be increased. Therefore, ＂seeding-feeder＂ cloud is beneficial to precipitation formation and also one of the artificial precipitation enhancement factors. （2） The supercooled water content. In the ＂seeding-feeder＂ cloud, there is high rainfall intensity in the area where supercooled water content is larger. Considering the important role of ice phase particles in precipitation formatiom, the supercooled water content can be taken as the potential factor. （3） The precipitation mechanism. In generally, the rainfall in stratiform cloud system is increased by releasing cool cloud seeding agent, such as silver iodide, to effect the microphysical processes of ice phase, and it is shown that the cool cloud processes have different contributions on precipitation in the different position and the different developing stage of the cloud system. Therefore, if only in the nature cloud, which in the cool cloud processes have a lager influence on precipitation, the artificial seeding plays an important role in precipitation enhancement. （4） The cloud water content in the warm zone of the cloud. After the ice phase particles are grown by the subli- mation and the accretion in the cool region of the cloud, they descent into the ware region and grow continually by collecting liquid water. If thickness of the feeder cloud is large and water content in it is also high, increment of ice particles to be melting is large, so that it is of great advantage for precipitation enhancement. （5） Supersaturated water vapor amount with respect to ice. In stratiform cloud system, if the cold cloud processes or ice phase microphysical processes play an important role in precipitation formation, the change of direction of supersaturated water vapor amount with respect to ice accord with rainfall intensity, while supersaturated water vapor amount with respect to ice is large, the rainfall intensity is large, too. It is shown that there is a good relationship between them. Under the condition of the cold cloud processes having a larger contribution for precipitation, supersaturated water vapor amount with respect to ice is an important potential factor. （6） Precipitation efficiency. Precipitation efficiency of condensation water and sublimation water shows the ability of condensation water and sublimation water to be converted into precipitation particles and surface rainfall. They are indispensability important factors in study of the potential of artificial precipitation enhancement. The analysis indicates that distribution of the precipitation efficiency is inhomogeneous in the stratiform cloud. （7） On the basis of study of the potential factors for artificial precipitation enhancement, the method used to assess qualitatively the seeding potential is presented： （i） There is ＂seeding-feeder＂ cloud structure in the cloud system; （ii） for ＂seeder-feeder＂ cloud system, the contribution of cold cloud process to precipitation needs to be investigated. The more contribution the cold cloud process makes to rainfall, the higher seeding potential the cloud system has; （iii） supercooled water content is high, and ice concentration in supercooled area is low, and cloud water content in warm cloud sector is high and the thickness of warm cloud sector is big, and feeder cloud layer is thick, then seeding potential would be large; （iv） vapor conditions, including vapor flux and supersaturated vapor content over ice, etc. , need to be analyzed. If there are strong updraft, strong horizontal convergence of vapor, large upward vapor vertical flux, and sufficient supersaturated vapor content over ice in thick cloud levels, then seeding potential should be remarkable. It is not necessary that all of these factors of seeding potential need to be analyzed, however, several, factors should be chosen and comprehensively studied, when selecting the opportunity and the location for one seeding operation.