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.
summerdrought and flood sharpsoutheast of Northwest China
NETL
NSTL
万方数据
维普
