Characteristics of Tower-Level Low-Level Jets and Their Impacts on the Urban Heat Island in Tianjin

Abstract

To date, the characteristics of the low-level jets (LLJs) that appear below 300 m, referred to in this study as tower-level LLJs (T-LLJs), have remained unidentified. The results in this study show that approximately 22% of LLJs in Tianjin appear below 300 m, indicating that greater attention should be given to T-LLJs. Thus, the characteristics of T-LLJs in Tianjin are investigated using data obtained from a wind-profile radar and a 255-m high meteorological tower. The results show that T-LLJs frequently occur during the transition from the warm season to the cold season and prefer to appear at night. Compared to the LLJs that appear between 300 and 1000 m, T-LLJs exhibit distinct monthly and diurnal variations, likely attributable to specific underlying causes. The case study suggests that the generation of T-LLJs can be partly attributed to inertial oscillation. Moreover, sensitivity tests indicate that the land‒sea thermal contrast is one of the main causes of T-LLJs, and that urban heat islands (UHIs) exert nonnegligible influence on T-LLJs in Tianjin. In addition, since UHIs are mainly nocturnal phenomena, the impacts of nocturnal LLJs on UHIs are investigated. The results show that nocturnal LLJs contribute to enhance turbulent mixing and heat transport, which can weaken atmospheric stability near the surface. Consequently, a nocturnal UHI is always weaker when it occurs concurrently with a LLJ, as opposed to occurring without a LLJ.