Numerical Simulations of Spatial Distributions and Diurnal Variations of Low-level Jets in China during Early Summer

The detailed spatial distributions and diurnal variations of low-level jets (LLJs) during early summer (May, June, and July) in China are documented using 2006-2011 hourly model data from the Weather Research and Forecasting Nonhydrostatic Mesoscale Model (WRF) with a 9-km horizontal resolution. We found that LLJs frequently occur in the following regions of China: Tarim Basin; northeastern China; Tibetan Plateau; and southern China. The LLJs are classified into two types, boundary layer jets (BLJs, below 1 km) and synoptic-system-related LLJs (SLLJs, within 1-4 km). The LLJs in the Tarim Basin and Tibetan Plateau are mainly BLJs. The SLLJs over southern China and northeastern China are associated with Meiyu front and Northeast Cold Vortex (NECV), respectively. BLJs in all regions show pronounced diurnal variations with maximum occurrences at nighttime or in the early morning, whereas diurnal variations of SLLJs vary, depending on the location. Over northeastern China, SLLJs occurrences show little diurnal variations. Over southern China, the timing of maximum diurnal SLLJs occurrence propagates eastward to 112°E (May), 109°E (June), and 105°E (July), which is consistent with diurnal variations of precipitation. From the analysis of momentum balance, the diurnal variation of BLJs is mainly caused by inertial oscillation at nighttime and vertical mixing in the boundary layer during the daytime. The horizontal advection term in the momentum equation is the dominant term for the diurnal variations of SLLJs over southern China. Horizontal distributions of occurring frequencies (Shading, unit: %) of SLLJs (a, b and c) and BLJs (d, e, and f) in China for May (a and d), June (b and e) and July (c and f) averaged from 2006 to 2011