Change Characteristic and Typical Circulation Type of Strong Cooling in Eastern Hexi Corridor
Using daily lowest temperature observation data of five meteorological stations in the east of Hexi Corridor during 1961-2010, the number of strong cooling ［the lowest temperature drop rate is ≥ 8 (10)℃ and the minimum temperature is ≤ 4℃ in 24(48 h) in recent 50 years］ was calculated. Climatic characteristics of spatiotemperal distribution and intensity of strong cooling were systematically analyzed with statistical methods. Then the ECMWF 500 hPa numerical forecast grid data (2.5°×2.5°) from 1991 to 2010 was used to analyze the strong cooling circulation features in this region. Finally, relations between strong cooling times and atmospheric circulation characteristics were studied. The results show that strong cooling times distribute differently in eastern Hexi Corridor and it is more obviously in highelevation mountains and northern desert edge than in oasis plain area due to the block of terrain, vegetation and mountains. Strong cool weather has obvious regional characteristics, and the times reduces with the increase of strong cooling station number. The decadal and annual times of strong cooling show a decreasing trend, and time series of strong cooling times has 4-6 year quasiperiodic variation, but no mutations. Such weather occurs mainly from January to May and from September to December, but most in April. Times of strong cooling with different intensities varies greatly reducing rapidly with the increase of cooling intensity. Intensity of 24 h strong cooling has a weak reducing tendency, while that of 48 h strong cooling shows a slightly increasing trend. Typical circulation of strong cooling weather in eastern Hexi Corridor is divided into two categories, including northwest airflow type and northern airflow type, and the number of northwest airflow type is more than that of northern airflow type. A significantly positive correlation exists between monthly strong cooling times and circulation characteristics including highaltitude cold air strength and cold air moving path, which means that a close relationship exists between strong cooling times and the highlevel cold air strength and cold air moving path. Intensity and moving path of highlevel cold air are strong signals for the forecasting of strong cooling weather.