Comparison of low‐level jets along the north coast of China in summer

Abstract

A data set obtained from wind-profile radars located along the north coast of China in summer was used to compare the characteristics of the low-level jets (LLJs) at two different sites, Tianjin and Shanghai. In this paper, the characteristics and evolutions of the low-level jets were investigated, and the inertial oscillations were analyzed by the Hilbert-Huang transform technique. The results showed that LLJs (≤14 m s À1 ) dominate at both Tianjin and Shanghai, with percentages of 76.5% and 84.8%, respectively. In terms of LLJ strength and frequency of appearance, evident diurnal changes were detected at the two sites, and nocturnal LLJs overwhelmed daytime LLJs in both strength and frequency. Influenced by the local topography and synoptic forcing, the LLJs were characterized by totally different distributions of wind directions and heights. The preferred direction at Shanghai was from the southwest and east, whereas northeast and south directions dominated at Tianjin. In the matter of jet heights, LLJs at Shanghai tended to occur at more consistent levels (56.3% of observed LLJs below 600 m) than at Tianjin. The LLJ wind speed shear at Shanghai was also stronger than that at Tianjin. The Hilbert-Huang transform technique was applied to reveal the intrinsic mode functions with cycles of local inertial oscillation (19 h for Tianjin and 23 h for Shanghai), and the patterns of inertial oscillation power agreed with the time-height distribution of the observed LLJs.

Low‐Level Jets Over the Bohai Sea and Yellow Sea: Climatology, Variability, and the Relationship With Regional Atmospheric Circulations

“…This basic detection method defines the LLJs by examining the horizontal wind maximum vertically, without considering the associated generation mechanism. These criteria were widely used or adopted in later literature (Doubler et al, 2015;Du et al, 2014;Miao et al, 2018;Pham et al, 2008;Wei et al, 2014;Whiteman et al, 1997;Wu & Raman, 1998). The threshold values vary due to the strength, distribution, and background circulation of LLJs.…”

Intermittent turbulence contributes to vertical diffusion of PM<sub>2.5</sub> in the North China Plain

“…6 indicate that the wind shear associated with the LLJ 'nose' may play an important part in the generation and transport of turbulence in the ABL. Previous study (Wei et al, 2014) has revealed that the LLJ is a common phenomenon in Tianjin region, due to the combined effects of plain terrain for inertial oscillation (Lundquist, 2003) and strong baroclinicity related to the land-sea temperature contrast offshore (Parish, 2000). In addition, the reduced convection in winter is helpful to maintain the LLJs even during the daytime.…”

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

“…The quality of wind profiler measurements was strictly controlled following the procedures proposed by Wei et al (). In particular, the wind speed measurements of every two continuous levels of every day have been checked as a whole, and the wind speed measurements deviated from the mean value by more than 2 standard deviations were discarded.…”

“…The wind profilers deployed in Beijing and Guangzhou are the CFL-16 profiler, produced by the China Aerospace Science and Industry Corporation (Table 1), which provide 25 levels of wind speed and direction below~3 km AGL with a vertical resolution of 120 m, which observe the atmosphere at 6 min intervals. The wind profiler measurements collected in The quality of wind profiler measurements was strictly controlled following the procedures proposed by Wei et al (2014). In particular, the wind speed measurements of every two continuous levels of every day have been checked as a whole, and the wind speed measurements deviated from the mean value by more than 2 standard deviations were discarded.…”

“…In contrast, the studies on the LLJ climatology over East Asia (Du et al, 2012;Liu et al, 2012;Wei et al, 2013;Wei, Zhang, & Ye, 2014) have been rather limited compared to those in the Great Plains of North America, probably due to the different geographical conditions in East Asia and the lack of sufficient suitable observations. Fortunately, since 2013 several wind profilers have been deployed by China Meteorological Administration (CMA) in several large cities of eastern China, including Beijing and Guangzhou.…”