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From 1961 to 2008, the overall frequency of dust storms in northern China has shown an unquestionable reduction. However, the Hunshdak Sandy Lands of northern China display an increasing frequency in dust storm activities, especially during the period 2001 to 2008. In an attempt to explore the cause of this increase, a comprehensive investigation was conducted by examining the climate variables, the average normalized difference vegetation index (NDVI) and the local inhabitant migrations. The climate variables include local precipitation, temperature, aridity, evaporation, relative humidity, soil moisture and wind speed. Moreover, by analyzing the 2001-2008 average anomaly charts (relative to the 30 year climatology of 1971-2000) of the upper air and surface conditions, an advantageous atmospheric circulation background for drought development over the Hunshdak was confirmed. Meanwhile, a multivariable step-regression model was employed to distinguish the significant variables of the climate elements mentioned before. The model output suggests that aridity is the leading factor impacting the Hunshdak dust storm frequency. During 2001 to 2008, the lack of local precipitation, higher temperature and strong evaporation deteriorated the local surface condition to below that before 2000, which is verified by the reduction of vegetation cover (NDVI), soil moisture and relative humidity. Furthermore, compared to the 30 year climatology of the wind speed observed during dust storm occurrence time, the mean velocity of 2001-2008 was reduced by 3.0 m s −1 , indicating that even with relatively weaker winds, dust storms still occurred primarily due to the degeneration of surface conditions around the Hunshdak.

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ABSTRACT:The evolutionary characteristics of dust storms in spring in northern China springs are determined by synthesizing the previous patterns and analyzing the geopotential height at 500 hPa. Some potential seasonal predictors are found through detecting the atmospheric circulation factors in both qualitative and quantitative analyses. Based on the frequency series obtained through comprehensive investigations of dust storm events which occurred from 1970 to 2005, two sample sets, each containing 6 years, are selected to represent the most frequent and infrequent occurrences of spring dust storms. For the first set, most of the West Pacific subtropical highs in the previous summer are relatively small and weak compared to those in normal years. In the previous winter, the mid-latitude region of the Eurasian continent is strongly controlled by a large and active low system, simultaneously with a strong and westerly positioned trough over East Asia. The northern hemisphere polar vortex, in general, changes from weak to strong with a large area from the previous winter to the current spring. The geopotential height over the Tibetan Plateau keeps a lower than normal value with a relatively strong India-Burma trough from the preceding summer in to the spring. For the second set, the evolution displays opposite features. Furthermore, by using a bootstrapping technique, indices of the atmospheric circulation elements are detected quantitatively. The forecast skill score of the 36 year hindcast indicates that those indices can provide forecast signals in certain situations for spring dust storm seasonal predictions in northern China.