贵州高原西北部冻雨的时空变化特征

Temporal and Spatial Characteristics of Freezing Rain Days over the Northwestern Guizhou Plateau

采用Mann-Kendall、Morlet小波分析、合成分析等方法,分析了近48a贵州西北部冻雨日数的时空分布及环流特征。结果表明:贵州西北部冻雨的空间分布差异显著,冻雨日数自东向西逐渐增多,冻雨日数出现频率最多的在威宁,年平均冻雨日数48.1d,冻雨日数出现频率最少的在六枝,年平均日数仅4.8d,贵州西北部冻雨主要出现在12月到翌年2月。近48a来贵州西北部冻雨日数呈下降趋势,年冻雨日数存在3～5a左右的周期,突变分析表明,贵州西北部冻雨日数突变特征明显,突变发生在1989年。冻雨日数偏多和偏少时期,其环流特征存在着显著的差异。

Freezing rains primarily occur in Guizhou Province in southwestern China, in particular its northwestern part suffering from the most freezing rains. Based on data of freezing rain days in the northwestern Guizhou plateau from 11 weather stations and NCEP/NCAR monthly mean reanalysis data for the period 1961-2008, the authors made an attempt to reveal the change characteristics of freezing rain days there using the Mann-Kendall test, Morlet analysis, and synthetic analysis. The temporal and spatial characteristics, period characteristics, and periodic characteristics of freezing rain days across northwestern Guizhou Province during recent 48 years were comprehensively analyzed. This would provide important reference for forecasting the occurrence of freezing rain days and preventing and reducing relevant disasters. Results indicate that the freezing rain days varied greatly over the study region. In general, it appeared more within the region between 26.5 °N and 27.5 °N, but less over the south of 26.5 ° N. The freezing rain days showed an increasing trend from the east to the west. The mean annual freezing rain days were found to be 4.848.1 d. As the altitude increased, the freezing rain days increased. Freezing rain disasters mainly emerged from December to February of the forthcoming year. As for the monthly variation, it occurred in January most. In general, the freezing rain days showed an obviously decreasing trend across recent 48 years. There were relatively more freezing rain days in the 1960s, 1970s, and 1980s, but fewer in the 1990’s and during the period 2001-2008. Using the Morlet analysis and power spectrum analysis, it was found that there exists a 35 year periodical variation. There were most significant changes in the mid-1970s and the early 1990s. In addition, there was a 2-year periodical variation during the period 1995-2000, and about a quasi 8-year periodical variation in the mid-1970s and 1990s. However, the cycle of the periodical variations was weak. Using the running-t test and Mann-Kendall test, an abrupt interdecadal change of freezing rain days was detected in the late 1980s, occurring in 1989. In years with more freezing rain days, the Ural Mountains High was essentially stronger than the normal years, exhibiting a relatively stable longitudinal circulation in Asia. This helps keep the cold air flowing along the northwest after trough. The southern branch of the trough was active. The southwest of warm air before trough was transported over Guizhou Province. The West Pacific Subtropic High was stronger than normal years, with its position being eastward. This conducted high-latitude cold air diffusing to southern China and subsequently influenced Guizhou. It was also found that there were obvious differences in the altitudinal distribution between years with more and less freezing rain days. There was a stronger positive anomaly region in New Earth Island. The polar vortex was weaker than normal years. Cold air activities were weak in high-latitude areas. There were strong and wide positive anomaly regions in high and middle latitudes in Eurasia areas. The warm air activities were stronger than normal years. The East Asian trough was weaker than normal years. The West Pacific Subtropic High was westward. It was difficult for the cold air to reach southern China, thereby not easy to form freezing rains.