两种干旱指数在重庆极端干旱事件中的应用

利用重庆34个国家站1981-2015年地表湿润度和MCI_GB干旱过程指数序列对比分析重庆极端干旱事件的气候特征,并采用五类格点日数据诊断了重庆近年来极端干旱年旱情集中发生时段的环流特征.结果表明:上世纪60年代以来,地表湿润度所表征的旱情在除重庆西部和主城部分地区变湿外,其余大部地区都为变干趋势,其时间序列存在2～8年的显著周期.而上世纪80年代以来,两种不同干旱指数表征的重庆持续和极端干旱事件分别都存在显著的2～4年和2～8年周期. 1981-2015年期间, MCI_GB极端累计旱情呈"东北部偏重"分布型,地表湿润度则为"长江沿线偏重"分布型,两种干旱标准化指数在重庆东北、西南和东南部地区的累计强度有一些差异;对照重庆干旱地方标准,地表湿润度与其分型更类似,强度也更为接近. 2001年、 2006年和2011年极端干旱特定时段大气环流所表现的前兆信号为:7月开始至结束期间, 500 hPa欧亚中高纬由"负—正—负"转为"负—正"型变压场分布,在中亚中纬度地区均为"南高北低"型气压场分布,均不利于西北路径冷空气南下影响我国西南地区;综合整月OLR场、 Walker环流和水汽场来看,重庆地区对流活动偏弱,仅为-15～0 w/m~2,垂直高低层以700 hPa为界,水平南北方向上以长江为界,气流升降运动呈反向分布,加之水汽输送不充分、整层水汽辐合较弱,这都不利于降水的持续发生,易造成干旱事件的集中发生和后续维持.

Application of Two Drought Indexes to Chongqing Extreme Drought Events

Two different drought indexes, surface wettability and MCI_GB, were used to compare and analyze the climate features of extreme drought events based on the traditional meteorological data and MCI (meteorological drought composite index) drought data in 1981-2015 recorded at 34 national meteorological stations in Chongqing, and to diagnose the circulation characteristics over Chongqing during the extreme drought events with compositing 5 types of global grid re-analysis data. The results showed that the drought symbolized by surface wettability index became drying in most areas of Chongqing from 1961 to 2015 except a few places in its western and central urban parts, and there existed a distinct periodic oscillation of 2~8 years. The persistent extreme drought events symbolized by the two different drought indexes had significant periodic oscillation of 2~4 and 2~8 years since 1980s, respectively. The accumulative extreme drought intensity of MCI_GB index from 1981 to 2015 displayed a distribution pattern of "north-eastern Chongqing serious" while that of surface wetness index displayed a distribution pattern of "Yangtze River coastal serious". The standardized drought distribution of the two indices had some differences in cumulative intensity in northeastern, southwestern and southeastern Chongqing. The index of surface wettability was more similar to the Chongqing local drought standard on intensity and distribution. In the specific period in July of 2001, 2006 and 2011, when extreme drought occurred, the middle latitude area of Central Asia displayed a "South higher, North lower" pressure phase, which was not conducive to the northwestern cold-air coming south to influence the southwestern China on anomaly field of air pressure level of 500 hPa. Considering the effects of sea surface temperature, OLR, Walker circle and water vapour condition of July in combination, it was concluded that the convection of Chongqing area was rather weak while the rising-falling airflow on the vertical and horizontal boundary were opposite, and the water vapor supply was also not sufficient. As a result, the persistent precipitation probability would decrease sharply, and the drought event was more likely to remain.