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-July) in China are documented using 2006-11 hourly model data from the Weather Research and Forecasting (WRF) Model with a 9-km horizontal resolution. It was found that LLJs frequently occur in the following regions of China: the Tarim basin, northeastern China, the Tibetan Plateau (TP), and southern China. The LLJs over China 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 the TP are mainly BLJs. The SLLJs over southern China and northeastern China are associated with the mei-yu front and northeast cold vortex (NECV), respectively. The 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. From the analysis of model data, the diurnal variation of BLJs is mainly caused by inertial oscillation at nighttime and vertical mixing in the boundary layer during daytime. Over northeastern China, SLLJ occurrences show little diurnal variation. Over southern China, two diurnal modes of SLLJs, propagation and stationary, exist and have seasonal variations, which is generally consistent with diurnal variations of precipitation.