| In this paper, Based on WRF meso-scale model which couples CAMS cloud scheme with cloud microphysics process, a low trough cold front precipitation case occurred on September 25th, 2012 in Shanxi Province was simulated. According to the simulation results, the macro and micro cloud structure and supercooled water distribution was analyzed combining with other data, such as aircraft detection, satellite, radar and ground rainfall observation. The potential of precipitation enhancement in that cloud was studied preliminarily. At the same time, the CAMS cloud scheme coupled in WRF V3.2 is ported to WRF V3.5 in this article. Variables, such as conversion efficiency, relative supersaturation, condensation, deposition, evaporation, sublimation, are increased to study conversation efficiency between cloud and precipitation. The main results are as follows.1. Simulation results and measurements indicate that, this case was mainly influenced by a low trough cold front system. The frontal is tilted backwards. The horizontal structure of cloud is uniform. The great value area of water content corresponding to the low value area of cloud top temperature in cloud field, the distribution of precipitation field corresponding to the distribution of the great value area of cloud water content. Before 700hPa windshear, southwest airflow corresponding to the great value area of cloud water content and optical thickness. Stratiform Cloud Precipitation mostly in northern Shanxi, ground rainfall intensity is between 1～3mm/h, the max radar echo intensity is about 30dBZ. In general, the weather situation, precipitation, cloud top temperature, radar echo, hydrometeors evolution in simulation are basically consistent with which in measurements.2. Before frontal, high and cold cloud was formed with ice particles. The cloud top temperature was about -40 ℃, and no precipitation occurred on ground. Near the front, clouds are mixed by warm and cold particles. The cold cloud region was formed by little supercooled water and much ice particles, with biggest precipitation on ground. Behind the front, high and cold cloud was formed with no supercooled water but a few ice particles. The cloud top temperature was about -30℃ and the precipitation on ground is also very light.3. Supercooled water of cloud is fewer in this cold front precipitation case. The supercooled water was mainly distributed in -5℃ layer before the front, which was maintained by water vapor through the vertical updraft on one hand, and consumed by ice particles deposition growth process on the other hand. The updraft condition, which in and under supercooled water layer was weak and over it was strong, is not benefit to keep large amount of liquid water, but promote ice particles development quickly. In this region, 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, and supercooled water is not easy to exit, which leads to only little supercooled water in cloud. In this case, with the IN concentration increasing, only ice concentration increased, the snow and graupel content and super cooled water during 0～-5℃ layer was not affected.4. In this precipitation case, water vapor in Shanxi Province is mainly from southwest airflow. Water vapor delivered northeastern to Shanxi region via Chongqing, Hunan, Hubei, Shaanxi, Henan. The main transportation boundaries of water vapor and hydrometeors in Shanxi region were western and southern boundaries. Southwestern warm and wet airflow with westerly component and southerly component is main water transportation region in cloud. Total hydrometeors and water vapor were mainly convergencing before 18 pm,25th. After 18 pm, various type of water substance show a net exportation horizontally. The estimation of each item, which in water balance equation of water vapor, total hydrometeors and total water substance, indicate that water substance is basically reach balance. In simulation time, each conversation efficiency in cloud precipitation is calculated, each value are listed as follows. Water vapor condensation rate is 1.0%, water vapor deposition rate is 1.2%, precipitation efficiency of total water substance is 1.3%, precipitation efficiency of total hydrometeors is 26.7%, precipitation efficiency of total water vapor is 1.3%.5. In conclusion, the natural precipitation efficiency of this low trough cold front stratiform Cloud is lower. Since less supercooled water and quickly development of ice particles, the potential condition of cold cloud seeding is weak. This case is not suitable for cold cloud precipitation enhancement. |
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