Characteristics of Low-level jets during 2015–2016 and the effect on fog in Tianjin

Abstract

Characteristics of wind direction, wind speed, wind shear, and height of low-level jets (LLJs) are obtained and studied using the wind profile data collected by a wind-profile radar located at Xiqing station in Tianjin from 2015 to 2016. Results on the general characteristics of LLJs show that the dominate wind direction of LLJs ranges from 180° to 270° (47%), which is also the prevailing wind direction of strong LLJs (wind speed larger than 14 m s–1). The wind speeds of LLJs are mostly lower than 14 m s−1, and the occurrence proportion of strong LLJs is only 22.8%. The occurrence height distributions of LLJs are relatively uniform throughout the year except the peak at the lower heights (500–600 m). Moreover, the seasonal and diurnal variations of LLJs are investigated, respectively, and the results show that the occurrence frequency of LLJs during warm season is larger than that during cold season, which may be due to the increasing amplitude of inertial oscillations in the boundary layer. The diurnal variation of all categories of LLJs shows a pronounced a unimodal pattern, with the peak of occurrence number appearing at around midnight. In addition, the relationship between LLJs and fog events in Tianjin is investigated based on the data obtained from the surface automatic weather station (AWS) and a 255-m atmospheric boundary layer meteorological tower, since LLJs frequently occur in foggy days (almost 50%) in Tianjin. Eleven fog events accompanied by LLJs are analyzed and statistical results show LLJ plays a critical role in fog life cycle. The effect of northerly LLJs on fog is to enhance turbulent mixing in the fog layer, which is induced by wind shear associated with northerly LLJs and propagates downward to fog layer. There is a threshold relationship between the northerly LLJs and fog development, strong LLJs lead to inversion layer collapse and fog dissipation, while weak LLJs weaken the strength of inversion and atmospheric stability, which are favorable for fog development. Therefore, for fog events accompanied by northerly LLJs, there are negative correlations between fog durations and the strength of LLJs. However, the main contribution of northerly LJJs to fog is to result in fog dissipation in Tianjin, since the duration of fog with northerly LLJs is shorter than that without LLJs. The main contribution of the southwesterly LLJs are to transport water vapor to fog area and increase the humidity in fog layer. Therefore, the southwesterly LLJs are favorable for the fog formation, development and prolong the duration of fog.