Characteristics of Low-level Jets in Shanghai during the 2008-2009 Warm Seasons as Inferred from Wind Profiler Radar Data

摘要

In order to understand the characteristics of low-level jets (LLJs) over coastal areas of China Continent and its linkages with rainfall during the warm seasons, half-hourly data from a wind profiler radar (WPR) at the Qingpu site during the Meiyu periods of 2008 and 2009 in addition to data from one month prior to and after (non-Meiyu periods) were used to develop a climatology of the LLJs over Shanghai, China. Two peaks in LLJ incidence were revealed at 500-800 m and 2100-2200 m altitudes. Thus, we classified the observed LLJs into two types: (1) boundary-layer jets (BLJs, below 1 km) and (2) synoptic-system-related LLJs (SLLJs, within 1-3 km). BLJs and SLJs showed different behavior in their temporal variation of the occurrence frequency, wind direction, and relation to rainfall. The BLJs displayed a more evident diurnal cycle than the SLLJs, with maximum incidence occurring in nighttime and early morning. The SLLJs occurred more frequently during Meiyu periods than non-Meiyu periods, whereas the occurrence frequency of BLJs increased throughout the warm seasons with no peak during Meiyu periods. BLJs are mostly southerly winds that might be induced by inertial oscillation with a strong background southerly geostrophic wind due to the west-east land-ocean thermal difference. SLLJs most frequently appeared as southwesterly and westerly winds embedded in the East Asian monsoon circulation. The relation between LLJs and precipitation was examined by comparing the frequency of LLJs occurrence on rainy days (cases), with daily (4-hourly) accumulated rainfall within 1 to 10 mm, and non-rainy days (cases) because the missing rate of WPR data is high during heavy rainfall events. Both BLJs and SLLJs occur more often on rainy days than on non-rainy days. In shorter time scales (4 hours), both BLJs and SLLJs tend to occur frequently during, before and after rainy cases, except SLLJs before rainy cases.