【作者】 蒋品平；

【导师】 赵平；

【作者基本信息】 中国气象科学研究院 ， 气象学， 2009， 硕士

【摘要】 利用1960到2008年的NCEP-NCAR再分析资料、中国740站点的逐日降水资料,首先分析了中国东部副热带各区域的雨季进退情况,接着对江南和江淮地区春季降水异常的环流背景和外强迫影响因子进行了研究,重点讨论海陆热力差异对降水异常和降水区域的影响;最后利用MM5v3中尺度区域模式、NOAA的CMAP降水资料和再分析资料,研究了东亚及周边海域表面温度差异和地形对春季我国东部西南风降水的影响。得出以下主要结论:(1)江南是中国雨季最早出现的地区,14侯进入雨季,4月上旬达到季风雨标准,属于深对流降水。接着华南、江淮、华北地区先后进入雨季。江南春季(3～5月)降水量占全年的比重最大,达到38%。中国南方降水增加的同时,南风开始出现并持续加强。从“风”和“雨”的角度上看,东亚副热带季风爆发。随着纬向海陆热力差异在中国东部副热带地区较早地产生反转,副热带季风发展和降水加强。(2)春季江南和江淮多、少雨年的大气环流背景有很大的不同:江南多雨时副高偏强,海陆气压梯度偏大、江南有异常偏南气流和水汽通量输送,低层辐合增加,上升气流明显增强。江淮多雨的主要原因是由于高原和其北部的脊明显偏弱并形成东高西低的异常场,脊前西北气流减小而导致使副高范围明显偏大;副高西部的西南风可以影响到更北的区域,西南风的向北推进可以带来大气环流变化如:低层水汽辐合和上升运动异常区明显偏北,降水正异常中心移到江淮地区。西南风和大气环流的变化所造成的江南和江淮降水异常与局地的纬向海陆热力差异有较大的关系,由于纬向海陆热力差异提早向夏季型转变,使同纬度地区的经向风和风的去向区域的西南风加强,进而使降水增加。当纬向热力梯度增加纬度偏南时,长江以南西南风增强,江南降水增加;当纬向热力梯度增加偏北时西南风可以输送到更北的地方,江淮降水增加。(3)数值模拟结果印证了分析结果:改变春季东亚表面温度差异对江南西南风降水发生以及向北推进有重要影响;当东亚大陆温度增加和其周边海洋表面温度减小时,冬季型温度梯度更早地向夏季型转换,青藏高原东侧低压以及西太平洋副热带高压都加强,伴随着东部的低层西南风加强;于是,江淮地区的上升运动增强,而江南地区的上升运动减弱,导致春季江淮降水增加,而江南降水减少,使东部雨带出现在江淮,而不是在江南;当热力差异减弱时,我国东部大陆没有雨带出现;在春季热力差异显著增强的情况下,即使没有南海热带季风爆发,春季位于江南的雨带也可以向北推进到江淮地区,形成类似于夏季江淮梅雨期的雨带。(4)东亚地形可以加强中国东南部春季的西南风和降水,青藏高原的地形作用使春季降水聚集在江南地区;但没有东亚地形春季副热带季风降水和夏季热带季风仍然能够出现。更多还原

【Abstract】 We analyzed the rainband moving north and south in subtropical East Asian and then investigated the circulation and the other forcing factors of the spring precipitation anomaly over Southeastern China and Yangtze-Huai River (YHR) area and mainly discussed the influence of land-sea thermal contrast on precipitation area and precipitation anomaly, by use of NCEP–NCAR reanalysis dataset and 740 stations daily rainfall datasets in China from 1960 to 2008. Finally, we using the NOAA’s Climate Prediction Center’s merged analysis of precipitation, studied the impacts of surface temperature differences between East Asian and seas around on the rainfalls for southwesterly winds in spring over Eastern China, by the fifth-generation PSU/NCAR Meso-scale Model version 3 (MM5v3). The major conclusions are as follows:(1)The Rainy season first appears at the 14th pentad over Southeastern China, where the precipitation which accounts the largest part ,about 38%, for the rainfall in whole year becomes monsoonal and deep convective in the first third of April, and then over South China and YHR area, North China. As the precipitation in Southeastern China increase, the southwest winds appear and strengthen, which means the outbreak of the East Asian sub-tropical monsoon. Sub-tropical monsoon and precipitation develops and strengthens, with the earlier reversing of the zonal land-sea thermal contrast over sub-tropical Eastern China.(2) There is a great difference between the spring circulation in more and less precipitation year over Southeastern China and that over YHR area. In spring rainy years over the Southeastern China, the sub-tropical high is stronger and pressure gradient is shaper, which causes the southerly winds and water vapor flux transmission anomaly and low-level convergence, obvious increase of updraft. More spring rainfall in YHR area is mainly due to the ridge is obviously weak over the Qinghai-Tibet Plateau and its northern area, which reduces the northwest wind in front of the ridge and forms anomaly field that the east higher than the west, that is obvious larger subtropical high. Southwest winds of the western subtropical high can affect the region further north,which can change the circulation that the area of low-level moisture convergence and upward motion is more north than usual, which results in positive anomaly center of rainfall moving to YHR region.Rainfall anomalies over Southeastern China and YHR area caused changes southwest winds and atmospheric circulation in by have a lot of thing to do with local anomalies of zonal land-sea thermal contrast. Precipitation increases caused by the southwest winds strengthening in the region of the same and higher latitude, which caused from zonal land-sea thermal contrast varying to the summer feature earlier. When the zonal thermal gradient increase more south than usual, it lead to precipitation enhance in Southeastern China that the south-west wind and water vapor transmission and low-level flow convergence, updraft increasing in Southeastern China. When the zonal thermal gradient increase more north than usual, it results in increased precipitation YHR area that the southwest winds can affect the region further north and flow convergence zone and the center of the upflow are also located in YHR region.(3)Numerical simulation result confirms diagnosis that: the surface temperature differences in spring exert strong influence on the occurrence of the southwesterly winds and rainfall over southern China and their northward advances. When surface temperature increases over East Asia and decreases over the oceans around, the temperature gradient with a winter feature earlier changes to that with a summer feature, with the strengthening of the east of the Tibetan Plateau low and the subtropical high over the western Pacific, the lower-tropospheric southwesterly winds over Eastern China. Accordingly, it leads to an increase of spring rainfall over the YHR valleys and a decrease over southern China that the upward motion increases over the YHR valleys and decreases over southern China. Thus, the rain belt over eastern China appears over the YHR valleys but not over southern China. Under a weaker condition of the thermal contrast, the rain belt does not occur over eastern China. When the spring thermal contrast pronouncedly strengthens, the rain belt over southern China may advance northward to the YHR valleys during spring although there is no onset of the tropical monsoon over the South China Sea. This forms a rain belt similar to that of the YHR valleys during the Meiyu period.(4)The topography of East Asia can enhance southwest wind and precipitation in spring over Southeastern China. The topography of the Qinghai-Tibet Plateau make spring rainfall more concentrated in the Southeastern China. However, sub-tropical and tropical monsoon rainfall can still appear, without East Asia topography.更多还原