西北东南部夏季旱涝急转异常分析及预测研究

利用NCAR/NCEP月平均再分析资料、中国气象局国家气象信息中心提供的中国西北东部地区(陕西、甘肃、青海、宁夏及内蒙古西部地区)1961—2012 年夏季(5—8 月)156 个台站逐日降水量以及国家气候中心提供的74 个环流指数,通过定义夏季旱涝急转指数,对西北东南部夏季旱涝急转事件的环流特征进行分析,并建立了预测模型,结果表明:近50 a 来西北东南部夏季旱涝急转现象年际差异较小,相比较而言,1992 年之前旱涝急转事件频发,而之后少发。在旱转涝年的旱期北极极涡偏东偏弱,乌拉尔山脊偏强,东亚大槽偏西偏深,另外中高纬西风带偏强,阻止了冷空气南下,也没有异常的水汽输送,在垂直场上主要表现为下沉运动,因此降水偏少。而涝期乌拉尔山阻高偏强,蒙古低压加深,西太平洋副热带高压偏西偏强,西北东南部正好位于西太平洋副热带高压西侧和蒙古低压底部,有充足的西南暖湿气流,另外中高纬度西风带偏弱,有利于冷空气南下,而且西太平洋有异常的水汽输送带,能到达西北东南部,在垂直场上主要表现为上升运动,因此降水偏多;涝转旱年正好相反。另外,利用前期大气环流指数对旱涝急转指数建立的集合预报模型具有一定的预测能力,从而为西北东南部夏季旱涝急转现象的短期气候预测提供参考依据。

Analysis and Predication of the Drought-Flood Sharp Turn in Summer over the Southeast Part of Northwestern China

Using the re-analyzed monthly mean NCAR/NCEP data,daily summer (from May to August) precipitation data of 156 stations from 1961 to 2012 in the east part of Northwestern China and 74 Circulation Indices from CMA, and using precipitation anomaly percentage of summer, the evolution features of the drought and flood sharp turns in recent half century are studied by defining the drought-flood sharp turn index, and the circulation characteristics of drought-flood sharp turn phenomenon over the southeast part of Northwestern China are analyzed. Additionally, a prediction model of drought-flood sharp turn phenomenon is established. The result showed that the annual differences of drought and flood sharp turns in summer are small over the southeast part of Northwestern China in recent half century. By comparison, the frequencies of drought-flood sharp turn events were more before 1992, and it were less after 1992. In drought period of drought-turn-flood years, Arctic polar vortex leans to east and is weak, the Ural ridge leans to be strong, East Asian trough leans to west and be deep, also the westerlies at the middle and high latitudes leans to be strong and prevents cold air to move southward. In addition, there is no unusual moisture transport belt, which cause that there is no abnormal cold air and warm-moist airstream to affect the southeast part of Northwestern China. And the vertical field displays downdraft. So, there is less rainfall in drought periods. In flood period of drought-turn-flood years, the Ural blocking leans to be strong, the Mongolia low pressure is deeper, the Subtropical high pressure leans to west and be strong, the southeast part of Northwestern China is located at the periphery of the subtropical high pressure and the bottom of Mongolia low pressure. There is enough warm and moisture air flow from southwest, so the rainfall tend to be more. Also, the westerlies at the middle and high latitudes tends weak which helps the cold air move southward. In addition, the anomaly moisture conveyor belt coming from the Pacific Ocean may reach this region, and the vertical field displays updraft, so the rainfall leans to be more. In flood-turn-drought years things are opposite. Ensemble forecasting model of drought-flood sharp turn is established with the pre-atmospheric circulation index which is significantly associated with the drought-flood sharp turn index. Additionally, ensemble forecasting model is examined by using of the recent ten years of data, and we found that the model has the ability to predicate the drought-flood sharp turn phenomena in summer in the southeast region of Northwest China, thus it provides a reference for short-term climate prediction.