【作者】 郝立生；

【导师】 闵锦忠； 丁一汇；

【作者基本信息】 南京信息工程大学 ， 气象学， 2011， 博士

【摘要】 本论文利用国家气象信息中心整理的全国752站1951-2008年逐日逐月降水资料、NCEP/NCAR再分析的1951-2008年月平均环流资料、NOAA重构的1951-2008年月平均海温等资料,采用多种统计方法、环流合成分析等方法,综合分析华北降水时空变化及关键影响因子、降水季节演变主模态、印度洋偶极子、东亚季风环流变化与华北夏季降水减少的关系。主要结论如下：1)华北各区域降水变化不一样,如果研究华北降水整体变化趋势,可能选择山西、河北(含京津)作为代表区更为合理。华北近50年降水减少主要是由于夏季雨量减少造成的,其中夏季雨日减少,尤其暴雨雨日减少造成的影响最为重要；华北近50年雨季峰值雨量显著减小和雨季变长是华北夏季降水量减少的又一个内在特征。2)华北夏季降水在1965年发生了气候突变,之后降水显著减少,这是由于在1965年之后亚洲中高纬夏季环流发生明显改变造成的。第一,500 hPa高度场环流由突变前的经向环流突出转变为纬向环流突出,贝加尔湖高空槽活动减少,使得华北上升运动过程减少；第二,蒙古地区高空出现降温,一方面造成对流层中上层位势高度降低,导致高空急流位置南移,引起东亚夏季风减弱,使得水汽很难越过长江到达华北；另一方面降温还造成对流层底层气压升高,使得地面低压天气过程减少。这几个方面共同影响,结果造成华北夏季降水出现减少趋势。3)华北降水季节演变存在两个主要模态,第一模态主要受ENSO和IOD演变过程控制,具体是前期受ENSO、IOD共同影响,IOD的作用更大一些,而在当年,ENSO的影响更加明显；第二模态主要受IOD演变过程控制。近50年,第一模态逐渐减弱,而第二模态逐渐增强,两者的共同作用使得近50年华北夏季降水出现减少趋势。4)近50年印度洋海温升高、正IOD指数加强,通过改变东亚冬季、春季降水分布,影响夏季海陆热力对比,造成夏季西太平洋副热带高压加强、位置偏西偏北,华北被副高控制,同时,孟加拉湾西南风水汽输送减弱,华北水汽来源不足,结果造成近50年华北夏季降水出现减少趋势。5)随着近50年印度洋、太平洋海温升高和正IOD指数增强,东亚季风环流发生明显改变,夏季西南风显著减小,蒙古气旋环流转为反气旋环流,河套由辐合转为辐散环流,500hPa高度场上华北夏季“东高西低”的环流形势转为“西高东低”形势。因此,水汽来源不足和缺乏有利的动力条件,使得华北夏季降水出现减少趋势。更多还原

【Abstract】 This paper was based on 752 stations daily and monthly precipitation datasets supplied by National Meteorological Information Center, NCEP/NCAR reanalysis data, and NOAA_ERSST_V3 data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA (http://www.cdc.noaa.gov). The aim was to present the spatial-temporal variation of the precipitation in North China and indicate the relationships between variation of key influencing factors, dominating modes of seasonal evolution in North China precipitation, Indian Ocean Dipole (IOD), the East Asian monsoon and the precipitation reduction in North China by using statistical methods, synthesis analysis and other mathematic methods. The main conclusions are as follow:1) The precipitation trend in North China is different significantly. In order to analysis the overall trend of North China, it is more reasonable to select Shanxi, Hebei (including Beijing and Tianjin) as representative region. The reduction of annual precipitation in recent 50 years is due to the decrease of summer rainfall which is manifesting that the reduction of rainy days, especially the reduction of rainstorm days. Both the peak value of summer precipitation decrease and the rainy season becoming longer are the internal characteristics of summer precipitation reduction in North China.2) The summer precipitation in North China had an abrupt change in 1965, followed by a significant reduction. This change is in accordance with the variation of mid-high latitude circulation in Asia. First of all, at 500 hPa height field, the circulation converting into zonal circulation from the previous meridional circulation and Lake Baikal upper trough reducing activities, which leaded to decreasing the upward movement over North China. Secondly, at 500 hPa layer, the temperature in Mongolia obviously dropped. For one thing, the lower temperature would lower potential height at upper troposphere, and lead to the high-altitude jet stream turning toward south, which caused the East Asian summer monsoon to weaken. So water vapor will not reach in North China crossing Yangtze River. For another, the lower temperature caused air pressure of the lower troposphere in this region to increase and the low pressure weather process activities to reduce at the ground.3) There are two dominating modes of seasonal evolution in North China precipitation. The first mode mainly is controlled by both ENSO and IOD evolution, the second mode is mainly affected by IOD evolution. In the past 50 years, the first mode gradually weakened, while the second mode gradually increased, the combined effect of both makes summer precipitation in North China to decrease.4) During the recent 50 years, increased Indian Ocean SST and enhanced IOD index affected summer sea-land thermodynamic contrast by changing the East Asian winter and spring precipitation, resulting in strengthening the West Pacific subtropical high in summer, and its location moving towards north and west. So North China was controlled by the subtropical high. Meanwhile, water vapor carried by southwest monsoon from Bay of Bengal was less. Thus, lack of moisture sources in North China results in summer precipitation in North China appearing a decreased trend.5) With the Indian Ocean and the Pacific Ocean SST increasing and the IOD index enhancing in the past 50 years, significant changes occurred in East Asian monsoon. Southwesterly winds in summer decreased significantly, Mongolia cyclone circulation converted into anticyclone circulation, the circulation convergence converted into divergence near Yellow River Loop, the circulation from "east high pressure with west low pressure" to "east low pressure with west high pressure" at high field over North China. Insufficient moisture sources and lack of favorable dynamic conditions result in a decreasing trend in North China summer rainfall.更多还原