The relation between the spatial and temporal variations of the West Asian subtropical westerly jet (WASWJ) and the summer precipitation in northern Xinjiang has been explored using the NCEP/NCAR reanalysis data and the summer precipitation data at 43 stations in northern Xinjiang during 1961 to 2007. Results show that the position of the WASWJ is more important than its strength in influencing the summer precipitation in northern Xinjiang. When the jet position is further south, the anomalous southwesterly flow crossing the Indian subcontinent along the southern foothill of the Tibetan Plateau is favorable for the southwestward warm and wet air penetrating from low latitudes into Central Asia and northern Xinjiang and more rainfall formation. Further analysis shows that the interannual variations of the jet position are well correlated with the Arctic Oscillation (AO). In the weak AO years, the middle to upper troposphere becomes colder than normal and results in an anomalous cyclonic circulation at 200 hPa over Western and Central Asia, which enhances the westerly wind over middle and low latitudes and leads to the WASWJ located further south.

ABSTRACT:The Urumqi Institute of Desert Meteorology of China Meteorological Administration carried out a scientific experiment to detect the clear-sky atmospheric boundary layer by using a wind-profiling radar and L-band radar sounding system in the hinterland and southern margin of the Taklamakan Desert in the summer of 2010 and 2011. Based on wind profile and radiosonde data collected from this experiment, this paper analyses the vertical structure features of the clear-sky daytime convective boundary layer and night-time stable boundary layer in the hinterland and southern margin of the desert, and compared the result with the boundary layer features in the Tibetan Plateau and the Dunhuang region in Gansu. The results show that: (1) the summer clear-sky convective boundary layers in the hinterland and southern margin of the Taklamakan Desert can develop thickly. The mixing layer can reach a maxiumum thickness of 3700 m, above which a significant entrainment layer exists with average thickness about 500 m. The maximum thickness of the convective boundary layer can exceed 4000 m. (2) A temperature inversion phenomenon is found to exist remarkably over the summer clear-sky nights in the hinterland and southern margin of the desert. The thickness of the night stable boundary layer is about 400-600 m, but the residual layer above it generally can reach a thickness of more than 3000 m. (3) An atmospheric boundary layer with supernormal thickness under the clear summer sky is a phenomenon of arid regions in Northwest China.