| Convective clouds are the main precipitation cloud system in South China. The merger of convective clouds play an important role in cloud’s developing and precipitation and severe storm weather are phenomenon after clouds merged. Many researchers have studied the mechanism of cloud merging based on observation data and numerical model, and got a lots of achievements in domestic and overseas. But in our country, especially in Yangtze and Huaihe River basin, the statical feature and mechanism of clouds merger are not clearly. For this reason, the geographical and synoticl condition, physical process, interaction of the convective clouds merger in summer are studied in this project based on multi-years observation data. The major contents of the project include observation technique, statistical analysis and mechanism research of clouds merger.In the observation technique part, there are three methods for data pretreatments, convective cloud detecting and Tracking. First, a new restoration is proposed for radar data pretreatment based on mathematical morphology including erosion, dilation, open and close operations. The results show that this method can remove the noise region, at the same time retain effective information to a certain extent. Second, for the convective cloud detection, a method based on multi-thresholds is introduced with statistic results of cloud imager. The convective clouds could been detected by three thresholds and been classified into 18 kinds by cloud factors. Third, the correlative matching method and overlaps technique of convective cloud tracking are combined to increase the speed of tracking.As the results show of statistical analysis, convective cloud merger has a typical spatial and temporal distribution characteristic, coinciding very well with the thunderstorm day’s distribution. And more than 80% merged clouds have apparent increase with enlarging of intensity and area, and lengthening of lifetime. For the 35 heavy rain and 43 hailstorm events in Huaihe and Yangze River Basin from 2001 to 2009, the occurrence probabilities of cloud merger were 94% and 65%, respectively. The average cloud merging times were 11.6 and 1.9. Based on the combination of Tmin (minimum of two cloud top bright temperature clouds) and dTmin (difference of two cloud top lowest bright temperatures), the cloud top temperature conditions and the statistic probabilities of occurrence with different Ar (ratio of two cloud areas) were analyzed. And three indicators for severe weather forecasting were revealed with Dis (distance of two clouds) and Ar factors.Based on the observational study of cloud merger, a concept model has been created. There are three dynamic factors caused clouds to merge. First, the horizontal asymmetry of vertical movement was the environmental factors. Second, surface pressure gradient force of atmosphere was one of dominant factors for cloud bridge generate and clouds merging. Thirdly, the main cause of cloud cores merging is the coupling of vertical motion between one core’s downdrafts and another’s updrafts. For the micro-physical charaters of clouds merger, there are rain drops fields at the joint region during the period of echoes merging. And only in the merger process, which is initial form the low layer, the quantity of ice partial could be increased quantity.In a case of MCS, there are five stages of cumulus mergers in FY2D/E satellite imageries. First of all, the mesoscale convective system is came from the merging of multi-cells. Second, during the developing and maintaining period of mesoscale convective system,3 stages of the cumulus mergers are occurred as following listed. First stage is a developing system is merged with several nearby new generation cells. Following stage is a systems merging between a mature new generation system and the old convective system. The third stage is the combination and interaction progress of the new and old systems. Finally, two centers of cloud with cold cloud top temperature are merged as the dissipating of convective system. As the centers have been merged, the system is developed again and the cold cloud areas are increased. As results of the merging effect, not only a large and complex convective system is produced and became more intensity, but also the lifetime of convective system is greatly enhanced with more convective energy. In the other hands, the mechanisms of merging in five stages could be included two kinds:change of inner dynamical structure and collision with outer force. At the building stage of mesoscale convective system, collision with outer force was the main causation of cumulus merging. But with the development and maintain of system, the inner change of dynamical structure, such as pressure gradient force, convergence and up-down circumfluence, was the primary mechanism for merger. |
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