一次层状云系水分收支和降水机制的数值研究

A numerical study of the moisture budget and the mechanism for precipitation for a stratiform cloud system

中文关键词:  层状云系， 水分收支， 降水机制， 数值研究

英文关键词:Stratiform cloud system, Moisture budget, Precipitation mechanism, Numerical study.

中文摘要:

      对2002年10月18—20日河南省层状云系的水分收支和降水机制用MM5模式模拟的结果表明，河南省域以外的水物质主要通过西和南边界输送到区域内，19日降水主要时段总水物质通量在水平方向上为净流入。对河南省域水汽、水凝物和总水物质的水分平衡等式中各项的估算表明该区域水物质基本达到收支平衡。估算的河南省域总水物质降水效率、凝结率、凝华率和水凝物降水效率及水汽降水效率分别约33.1％、27.7％、13.1％、69.7%和31.1％，总水物质降水效率与水汽降水效率接近是由于参与的水物质总量中水汽占绝大部分。约58.2％以上的冰晶转化为雪，超过82.1％的雪融化，不到 11.1％的雪转化为霰，霰粒子几乎完全融化。冰晶通过凝华过程增长。雪主要由冰晶转化产生，凝华增长率比撞冻增长率高得多。雨水由暖云和冷云过程产生和增长，雨水碰并云水量和冰粒子融化量对雨水的贡献相近，云雨自动转化量小。可见，在主要降水时段，降水是由冷云和暖云过程共同产生的。冰粒子凝华增长对雨水的贡献最大超过35％，撞冻增长的贡献最高不足12％，可见水汽对降水粒子增长重要。催化层、冰水混合层和液水层对降水的贡献分别约为15％—27％、45％—50％和23％—38％，表明此“催化-供给”云中冰粒子在冰水混合层的增长对降水的贡献相当大。

英文摘要:

      The moisture budget and precipitation mechanism for a stratiform cloud system are investigated based on simulated results of the MM5 model. The light medium rainfall caused by the stratiform cloud system occurred in the Henan province during 18-20 October 2002. Both water vapor and hydrometeors are transported through the western and southern boundaries into the Henan region. There is a net inflow at the horizontal direction during the main precipitation period. Every term in the balance equations of the water vapor, hydrometeors or all water substances throughout the Henan region was calculated. The results show that the budgets of water vapor, hydrometeors and all water substances are approximately balanced. The precipitation efficiency of all water substance, condensation efficiency, deposition efficiency, precipitation efficiency of hydrometeors, and precipitation efficiency of water vapor for the Henan region are generally about 33.1%, 27.7%, 13.1%, 69.7% and 31.1%, respectively. The rainfall efficiency of all water substance is similar to that of water vapor because water vapor is predominant in all water substances. The conversion efficiencies among different precipitable particles are evaluated. with the result that more than 58.2% of cloud ice is converted into snow, above 82.1% of snow particles melt, less than 11.1% of snow is converted into graupel, and almost all of graupel melts. Ice water increase is mainly caused by deposition. Most of snow particles are produced by the conversion from ice crystals. Snow increase is due to both deposition and accretion of cloud water with the growth rate of deposition is greater than that of accretion. Rain are formed and increased through the warm cloud and cold cloud processes. The amounts of accretion of cloud water by rain are nearly equal to those of melting of ice-phase particles. The amounts of cloud water auto-converting into rain are relatively small. Therefore, during the main precipitation period, the precipitation is jointly produced by the warm cloud and the cold cloud processes. The proportion of rain amounts finally produced by deposition to the amounts of all of source terms of rain is more than 35%. The ratio of rain amounts ultimately produced by accretion of cloud water by ice phase particles to the amounts of all of source terms of rain is less than 12%. Water vapor plays an important part in production processes of precipitation particles. The proportions of precipitation from the seeded cloud-layer, mixed-phase cloud-layer, and liquid phase cloud-layer to the surface rainfall are about 15%-27%, 45%-50%, and 23%-38%, respectively. The precipitation contribution ratio of the mixed phase cloud-layer to the surface rainfall is the largest because of the increment of ice-phase particles in the cloud-layer.