Abstract
In this study, relationships between low-level jet (LLJ) and low visibility associated with precipitation, air pollution, and fog in Tianjin are investigated based on observational data from January to December, 2016. Statistical results show 55% of precipitation is accompanied by LLJ, and two causes responsible for the relatively high percentage are presented. The result of case analysis shows that some southwesterly LLJs are favorable for the formation of precipitation by transporting water vapor when the water vapor channel from the South China Sea or Bengal Bay to Bohai Rim region is established. Statistical results show 55% of pollution episodes (PEs) are accompanied by LLJs. When pollutions are observed in the southern industrial regions, nocturnal southwesterly LLJ, which can carry polluted air masses from polluted regions to Tianjin and induce turbulent mixing, can enhance surface PM2.5 concentration and is favorable for the formation of surface pollution at night. Nocturnal northerly or southeasterly LLJ leads to clear air masses mixing with polluted air masses and is favorable for increasing visibility. Contributions of southwesterly LLJs to the formation of fog and precipitation are similar, which both rely on establishing the water vapor channel despite occurrence heights of LLJs being different.
Highlights
Low-level jet (LLJ) is an intense, narrow, quasi-horizontal current of wind associated with strong vertical shear in the lowest few kilometers of the troposphere and has been frequently observed in many continents including America [1,2], Africa [3,4], and Asia [5,6].Characteristics of LLJs have attracted widespread attention and been widely studied in observational studies [7], numerical simulations [8], and theoretical analyses [9]
Precipitation usually forms in association with the regional updraft and convective instability, while air pollution and fog usually occur under low wind speed, high relative humidity, and strong stable stratification
LLJs observed during precipitation episodes are out of our scopes due to the high missing rate of wind-profile data [21] and effect of disturbances induced by precipitation on the detection of wind speed [39]
Summary
Low-level jet (LLJ) is an intense, narrow, quasi-horizontal current of wind associated with strong vertical shear in the lowest few kilometers of the troposphere (usually under 3000 m) and has been frequently observed in many continents including America [1,2], Africa [3,4], and Asia [5,6]. Atmosphere 2020, 11, 1197 in the transport of heat [14], moisture [15], and pollutants [16,17], LLJ is closely related to low-visibility events including severe rainfall events, fog, and air pollution. Among all low-visibility events mentioned above, the relationship between LLJ and precipitation, which usually forms in association with the regional updraft and convective instability, attracts the most attention from researchers. Studies about the effects of LLJ on fog and air pollution, which usually occur under low wind speed, high relative humidity, and strong stable stratification, are rare. Due to important roles of LLJs in the transport of heat, moisture, and pollutants, relationships between LLJ and precipitation, air pollution, and fog are investigated in this study to improve our understanding about the role of LLJ in low visibility.
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