Precipitation Mechanism of Mesoscale Cloud System in a Cold Vortex over Northeast China. II: Numerical Simulation
东北冷涡中尺度云系降水机制研究 II: 数值模拟

Precipitation resulting from a hybrid of cumulus and stratus in a cold vortex in Northeast China on July 8 2003 was studied. A hybrid cloud simulation using the ARPS (Advanced Regional Prediction System) model was conducted. On the basis of the observation and simulation, the hybrid cloud microphysical structure, hydrometeor formation process, and precipitation mechanism were investigated. The results are as follows: (1) The cumulus in the hybrid cloud featured multi-level microphysical structure. The ice content maximum was located at the highest level among five hydrometeors, followed by, from high to low, snow, cloud water, graupel, and rain water appeared in the warm cloud area. Rain water content is hightest in all hydrometeors, graupel was the second most abundant type of content. With a longer lifetime, the cumulus exhibited essentially stable microphysical structure. (2) The hydrometeors had different microphysical growth processes. The ice mixing ratio increased via sublimation. The snow resulted mainly from ice crystals, growth through accretion, collection of ice, and sublimation. The accretion of supercooled cloud water contributed greatly to snow growth. Both of their production rates reached maxima at the same height. The rich supercooled cloud water was favorable to the accretion growth of snow. The high-, medium-, and low-level snows had different formation mechanisms. The upper-level snow grew by collecting ice crystals. Then, snow fell to the low level and grew through accretion of supercooled cloud water. Graupel was produced by raindrop freezing and auto conversion of snow. Thus, graupel production was associated with supercooled raindrops. The source of the graupel was cloud water accretion and collection of snow and ice crystals; accretion was the most important growth process. Rain water was produced by melting of graupel. At the supercooled level, the accretion growth of graupel was dominant. The upper-level ice and snow seeded the middle cloud. The rich supercooled water promoted the accretion growth of ice crystals. It was shown that the cloud water was mainly consumed by collection by raindrops, accretion by snow and graupel. (3) Regarding the precipitation mechanism, rain water came mainly from melting of graupel and had little to do with the warm cloud process. Therefore, precipitation was dominated by the cold cloud process, which was consistent with observations.