Distribution and Features of Low-level Jets in China during the Warm Seasons Using WRF Model

The detailed distribution and features of Low-level jets (LLJs) in China are documented using hourly output from 9-km horizontal resolution Weather Research and Forecasting mesoscale model (WRF) for warm season (May, June and July) from 2006 to 2011. The Low-level jets are classified into two types based on height: Boundary Layer Jets (BLJs, about below 1 km) and synoptic-system-related LLJs (SLLJs, within 1-4 km). We found that there are four rich LLJ regions in China: Tarim Basin, Southeast China, North China and Tibetan Plateau. LLJs in Tarim Basin and Tibetan Plateau mainly include BLJs. SLLJs in Southeast China and North China are affected by Meiyu Front and Northeast Cold Vortex (NECV) respectively, thus the frequency of SLLJs varies from May to July: rich region of SLLJs in Southeast shifts toward north from May to July in consistence with moving of Meiyu Front in Southeast China, while SLLJs occur more frequently in May and June than July in consistence with varying of NECV. The vertical structures of LLJs in North China and Southeast China both exhibit double peaks, which suggests there are two types of LLJs in North China and Southeast China. BLJs in the four rich regions all show evident diurnal cycle with maximum in the nighttime and early morning, while diurnal variation of SLLJs varies, depending on their locations. SLLJs in North China peak in the early evening, and the diurnal SLLJs maximum moves eastward in South China, which is consistent with diurnal cycle of precipitation. We found that inertial oscillation mainly account for diurnal cycle and formation mechanism of BLJs through momentum decomposition and sensitive experiment (eliminating flux at surface).