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

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(-1)) 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.