The Climatology of Low-Level Jet in Beijing and Guangzhou, China

The important roles of low-level jet (LLJ) in transport of heat, moisture, and pollutants have long been recognized. However, partly due to the lack of long-term observations, the characteristics of LLJs in Beijing and Guangzhou have not been well understood. Using long-term wind profiler observations and numerical simulations, the basic climatological characteristics of LLJ in Beijing and Guangzhou were documented, and their relationships with large- and local-scale forcings were investigated. The occurrence frequencies of LLJ in both megacities approximately exhibit a bimodal distribution during an annual cycle. In terms of the seasonality, the LLJs in Beijing occur more often in spring and winter, and those in Guangzhou appear more frequently from October to December and from February to April. Pronounced diurnal variations of LLJ are found in Beijing, where most LLJs occurred during the nighttime, and the diurnal variations of LLJ in Guangzhou are less prominent. Favorable synoptic conditions for LLJ formations in Beijing are those having a southwest-northeast or southeast-northwest pressure gradient across Beijing. Dominant synoptic patterns associated with LLJs in Guangzhou are those having a low-pressure or high-pressure system to the northwest. Overall, the LLJs are primarily induced by synoptic forcings and modulated by the diurnal evolution of planetary boundary layer. The key findings from this study provide us a better understanding of the LLJs in these highly populated regions.