The Spatial-temporal Structure Characteristics Of Low-Level Jet Over Bohai Rim

In this paper, the spatial and temporal characteristics of low-level jet (LLJ) over Bohai rim during the year2008is analyzed, using NOAA/ESRL radiosonde data of Beijing, Chifeng, Shenyang, Dalian, Zhangqiu and Qingdao, NCEP-FNL data (1°×1°) over Bohai rim and the daily precipitation data observed at743meteorological stations in China. The analyze in the basis of a new definition of LLJ angle from the vertical shear, the scale of LLJ over Bohai rim can be find by calculate the value of vertical shear of LLJ, and brief the correlation between LLJ and precipitation. Combined with the special topography of the Bohai rim, analyze the seasonal variation, diurnal variations and spatial distribution characteristics of the LLJ, and further analysis of the wind speed, wind direction, height and the strength of the LLJ. The main conclusions are summarized as follows:The Bohai rim is divided into three parts according to the characteristics of LLJ:located in the northeast Bohai rim (Liaodong Peninsula), in the northwest (Beijing and Chifeng) and in the south (Shandong Peninsula). The LLJ in Shenyang is the most in frequency and the strongest in intensity. So we can say Shenyang is the center of LLJ over the Bohai rim.The scale of LLJ over Bohai Pim is meso-scale, it is the most closely with the rainstorms and has large vertical wind shear. The LLJ and rain has a good correlation in summer at Shenyang Dalian and Zhangqiu and in autumn at Beijing Shenyang and Dalian. The highest number of LLJ is the northeast of Bohai rim (Liaodong Peninsula), the northwest at the second place, the least at the south (Shandong Peninsula). The high frequency of LLJ in this area is in lower isobaric surface as a whole, mainly in925hPa and850hPa level. While the isobaric surface of LLJ in northwest of Bohai rim is relatively high, but in northeast and south is relatively low. LLJ over Bohai rim exhibits evident seasonal variation. It occurs most in spring, followed by autumn and winter, at least in summer. It also has significant diurnal variation, at night (02:00BST) reaches the maximum, during the day (14:00BST) to a minimum, at other times (08:00and20:00BST) gradually increases. The wind speed of LLJ mainly around12-14m/s, the wind direction of LLJ is predominately from SW and NW, and the height of LLJ is around1500-1600m and700-950m. The LLJ with large (small) average speed corresponds to a low (high) average height in winter (summer). The strongest LLJ is the westerly LLJ over the northeast of the Bohai rim on925hPa from the point of view of duration. And the intensity of LLJ from the point of view of the size of the wind speed is stronger in the high-level. The annual average characteristics of LLJ shows that the LLJ’s mainly direction is NW in every layer, the strongest center of LLJ is located in the north of Bohai rim, and from the top layer to the lower layer the center location gradually eastward and the strength significantly reduced. The seasonal mean of LLJ characterized as the stronger intensity in autumn and winter at a high level (700hPa and825hPa) and in spring and autumn at a low level (925hPa). That is, in spring, from the high level to the low level, the wind direction of LLJ changes from the northwest wind to a southwest wind, with the intensity gradually increased. In autumn and winter, from the high level to the low level, the wind direction of LLJ is northwest, with the strength gradually decreases. From the top layer to the low layer the center location of LLJ over Bohai rim gradually eastward and the strength to achieve the strongest at night.