Simulation Study on Stratiform Cloud Structure of Trough Cold Front and Characteristics of Supercooled Water Distribution
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Abstract:
Based on WRF mesoscale model which couples CAMS cloud scheme with cloud microphysics process, a low trough cold front precipitation process that occurred on 25 September 2012 in Shanxi Province was simulated. According to the simulation results, the macro and micro cloud structure and supercooled water distribution were analyzed combining with other data, such as aircraft detection, satellite, radar and ground rainfall observations. The structure and physical condition of supercooled water were studied so that seeding suggestions could be provided in this kind of cloud system. The simulated weather situation, precipitation, cloud top temperature, radar echo distribution, and hydrometeors evolution are basically consistent with the observations. The main results are as follows. This precipitation process was influenced by a low trough cold front system in front of which high and cold cloud was composed of ice particles. The cloud top temperature was about -40℃, and no precipitation on surface. Near the front, clouds were mixed by warm and cold particles. The cold cloud zone was made up of small amount of supercooled water and large quantity of ice particles, with heaviest precipitation on the surface. Behind the front, high and cold cloud had a small amount of ice particles, no supercooled water. The cloud top temperature was about -30℃ and the surface rain was light. The supercooled water was mainly distributed in the -5℃ layer in front of the front. Showing the characteristic of lower height and smaller content, the supercooled water was hold by water vapor through the vertical updraft on one hand, and consumed by ice particles sublimation growth process on the other hand. The updraft condition, which in and under supercooled water layer was weak and over it was strong, was not beneficial to the maintenance of large amount of liquid water, but promoted ice particles to develop quickly. In this area, water vapor is not saturated with respect to liquid water but saturated to ice. The water vapor was consumed by a lot of ice particles, not favorable for the existence of supercooled water, so only a small amount of supercooled water stay in cloud. During this process, along with the increase of concentration ice nuclei, the concentration of ice crystals got increased, but the snow and graupel content and supercooled water in the -5 to 0℃ layer was not affected.