Relationships between Low-Level Jet and Low Visibility Associated with Precipitation, Air Pollution, and Fog in Tianjin

: 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 PM 2.5 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 di ﬀ erent.


Introduction
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]. Characteristics of LLJs have attracted widespread attention and been widely studied in observational studies [7], numerical simulations [8], and theoretical analyses [9]. Moreover, several mechanisms including baroclinicity, inertial oscillations, terrain effect, and gusts have been put forward from past studies as the responsible process for the development and intensification of LLJs [9,10].
Visibility is the farthest distance at which the human eye can distinguish a target against a background, which can significantly impact all types of transportation including shipping, aviation, rail, and road transportation [11][12][13]. Previous studies pointed out that, due to important roles of LLJ   The wind-profile data in the boundary layer is obtained at Xiqing station (54527), using Radian CFL-06 (Beijing Institute of Radio Measurement, Beijing, China). Detailed information of the wind-profile radar is presented in Table 2 and can be found in Wu et al. [26]. In theory, the wind-profile data in the range of 150-9080 m can be obtained. However, due to large quantities of missing data in the lower cover range and high elevations of the wind-profile radar, only data between 270-3080 m (270,390,510,630,750,870,990,1110,1230,1350,1470,1590,1710,1830,1950, 2120, 2360, 2600, and 2840 m) are used in this study. Sai and Miao [34] pointed out that LLJs mostly occurred at high elevations (700 h Pa and 850 h Pa) while there were few LLJs at lower elevations in Tianjin. Therefore, effects of missing data under 270 m and above 3000 m on underestimation of LLJs can be neglected in this study. In addition, strict quality control is applied to the wind-profile data [30], and the block time average method is used to avoid the effect of fluctuating data with an averaging time interval of 1 h. Profile data of wind, relative humidity, temperature, and turbulence data within 250 m are obtained from a 255-m meteorological tower, which is situated in the atmospheric boundary layer at the Meteorological Observation Station (54517) (red star in Figure 1b). Detailed information about observations in the 255-m meteorological tower is presented in Table 3. Observations of meteorological parameters (wind, relative humidity, and temperature) are recorded automatically and converted to a 30-min moving average. Strict quality control is applied to the turbulence data [23,35]. Preprocessing including spike removal, double coordinate rotation, and trend removal are performed using Eddy Pro software (Advanced 4.2.1, LI-COR Biosciences Inc., Lincoln, NE, USA), and the 30-min average is calculated. Wind-profile data obtained from the tower is not continuous at some time periods. Therefore, wind-profile data under 270 m cannot be made up using tower data. The observation of the boundary layer was also done using an MP-3000A microwave radiometer (Radiometrics, Boulder, CO, USA), which has 35 channels with 21 water-vapor channels and 14 temperature channels. The microwave radiometer detects temperature and liquid water vapor profiles from the surface to 10 km. The temporal resolution of the microwave radiometer is 1 minute, while the vertical resolutions in the range of 0-0.5 km, 0.5-2 km, and 2-10 km, are 50 m, 100 m, and 250 m, respectively.

Criteria of LLJ, Precipitation, Pollution, and Fog
Criteria of LLJs based on the vertical profile of horizontal wind speed [36] varied in previous literatures and are crucial for identifying LLJs. Based on two parameters: the maximum wind speed (V max ) and difference (∆V) between V max and the minimum wind speed (V min ) above the location of V max , criteria of LLJs in this study are V max ≥ 10 m s −1 and ∆V ≥ 5 m s −1 . The identification of precipitation in this study is based on the data of rainfall obtained from 13 stations. When precipitation is simultaneously observed over at least four stations and the average accumulative rainfall within 24 h is larger than 1 mm, it is considered a precipitation event in Tianjin. According to the environmental air quality standards issued by the Ministry of Environmental Protection of China, the pollution conditions can be categorized into three levels: (1) clean days, (2) light pollution days (mild pollution and moderate pollution), and (3) heavy pollution days with the daily mean PM 2.5 mass concentration being 0-75 µg m −3 , 75-150 µg m −3 , and over 150 µg m −3 , respectively. In this study, a pollution episode (PE) is referred to as an episode during which the daily mean PM 2.5 exceeds 75 µg m −3 for three consecutive days. Moreover, pollution episodes with peak PM 2.5 values less than 300 µg m −3 are termed light-pollution episodes (LPEs), and, otherwise, heavy-pollution episodes (HPEs) [37]. The identification of fog is based on RH and visibility (vis). In this study, if RH ≥ 90% and vis < 1 km, with no precipitation being observed, it is a fog event [35,38].

Results and Discussion
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. Due to different formation conditions of precipitation and pollution as well as fog, relationships between LLJ and precipitation, and pollution as well as fog are investigated, respectively.

Relationship between LLJ and Precipitation
Based on criteria in Section 2.2, monthly occurrence numbers of all precipitation events and precipitation events accompanied by LLJs in 2016 are counted and presented in Figure 2a. Statistical results show that the average visibility during precipitation episodes in Tianjin in 2016 was approximately 10 km with the lowest visibility being 1.3 km. 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]. Precipitation event accompanied by LLJ in this study manifested that the LLJ was observed within 48 h prior to the onset of precipitation. Thirty-eight precipitation events were observed and the annual rainfall was 594 mm in Tianjin in 2016. Figure 2a shows that 79% of precipitation occurred from May to October (rainy months in Tianjin) and 55% of precipitation was accompanied by LLJ. The relatively high occurrence frequency of LLJ before the onset of precipitation induces two points that we focused on. One is why LLJ often occurred before precipitation. The other is whether the LLJ, which occurs prior to the onset of precipitation, influences the formation of precipitation. northerly LLJ was induced by baroclinicity associated with a cold front, which confirmed the first reason for precipitation accompanied by LLJ. The northerly LLJ brought cold airflow to the warm and moist air masses, which was favorable for triggering convective systems [20,42]. Therefore, southwesterly LLJ, in this case, is favorable for the formation of precipitation by transporting the water vapor, while northerly LLJ is favorable for the formation of precipitation by triggering convective systems. Another light rain (case 2) with the duration being 11 h and accumulative rainfall being 3.5 mm, was observed from 1400 UTC 25 December 2016. Figure 3b   A previous study [30] has revealed that LLJs in Tianjin are always generated due to combined effects of inertial oscillation and strong baroclinicity. Statistical results in this study show that there are two main causes responsible for the relatively high occurrence frequency of LLJ before the onset of precipitation in Tianjin. Synoptic systems including frontal activity and cyclone play important roles in inducing baroclinicity [10], which is favorable for the generation of LLJ. Meanwhile, the frontal activity or cyclone is an important ingredient for the formation of precipitation. Therefore, one reason for the occurrence of LLJ before the onset of precipitation is the occurrence of synoptic system in Tianjin. LLJ was observed before the onset of precipitation on 11 May 2016 ( Figure S1 in the supplement), which verified the effect of the front on the formation of LLJ and precipitation. The other reason is that LLJ just occasionally occurred before precipitation due to inertial oscillation at night or due to baroclinicity, while precipitation occurred due to convection or other mechanisms. The examples of two causes will be shown thereinafter.
The statistical result shows that prevailing winds of LLJs in Tianjin in 2016 are southwesterlies ( Figure S2 in the Supplement), though some northerly LLJs are also observed in the summer and autumn. Moreover, previous literature [40] shows that one of the important effects of LLJ on precipitation is to transport water vapor. The result of climatological trajectories in Jiang et al. [41] indicates that moisture in North China originates, respectively, from Eurasia (14.4%), Eastern China (10.2%), the Bay of Bengal-South China Sea (33.8%), the Indian Ocean (10.7%), and the Pacific (30.9%). The Bay of Bengal and the South China Sea are major sources for rainfall in North China. To investigate whether LLJs in Tianjin influence general precipitation, case analysis is performed. It is worthwhile to note that occurrence heights of southwesterly LLJs, which occurred before the onset of precipitation, were usually in the range of 1350-1830 m. Two cases (general precipitation events accompanied by southwesterly LLJs) are investigated and will be shown hereinafter.
A moderate rain (case 1), with the duration being 9 h and accumulative rainfall being 23.3 mm, was observed from 0500 Universal Time Coordinated (UTC) 2, May in 2016. Figure 2c presents the profile of water vapor density from 0000 UTC 1 May to 0400 UTC 2 May, and Figure 2d presents the contour chart and observations of meteorological parameters at 850 hPa at 2000 Local Standard Time (LST) 1 May obtained from CMA. Persistent southwesterly LLJs located at 1470 m with the maximum wind speed being 25.8 m s −1 , which was observed before the onset of precipitation ( Figure 2b). The southwesterly LLJ was induced by baroclinicity associated with land-ocean thermal differences (Figure 2d) [5] and the occurrence of LLJ verified the second reason for precipitation accompanied by LLJ mentioned above. The southwesterly LLJ transported warm and moist airflow from lower latitudes toward Tianjin, which was conducive to the increase in water vapor density at 1470 m ( Figure 2c). Moreover, turbulent mixing induced by vertical wind shear associated with the LLJ increased the water vapor density under 1470 m ( Figure 2c). Figure 2d shows that the water vapor beneficial to the moderate rain event in Tianjin originated from the South China Sea and was transported through the water vapor channel from the South China Sea to the Bohai Rim region. Therefore, southwesterly LLJ associated with the western of subtropical high over the northwestern Pacific, as the main channel of water vapor in Tianjin [15], is favorable for precipitation in this case. From five hours prior to the onset of precipitation, with the northwesterly cold air moving toward Tianjin, northerly LLJ was observed at 750 m, with the maximum wind speed being 17.5 m s −1 . The northerly LLJ was induced by baroclinicity associated with a cold front, which confirmed the first reason for precipitation accompanied by LLJ. The northerly LLJ brought cold airflow to the warm and moist air masses, which was favorable for triggering convective systems [20,42]. Therefore, southwesterly LLJ, in this case, is favorable for the formation of precipitation by transporting the water vapor, while northerly LLJ is favorable for the formation of precipitation by triggering convective systems.
Another light rain (case 2) with the duration being 11 h and accumulative rainfall being 3.5 mm, was observed from 1400 UTC 25 December 2016. Figure 3b presents the profile of water vapor density from 0000 UTC 24 December to 1200 UTC 25 December, while Figure 3c,d present the contour charts and observations of meteorological parameters at 850 hPa at 0800 LST 24 December and at 0800 LST 25 December, respectively. Persistent southwesterly LLJ located at 1710 m, with the maximum wind speed being 12.1 m s −1 , was observed before the onset of precipitation ( Figure 3a). The weak southwesterly LLJ was also induced by baroclinicity associated with weak land-ocean thermal differences ( Figure 3c). However, there was no pronounced increase in water vapor density (Figure 3b), which indicated that southwesterly LLJ in this case did not transport water vapor to Tianjin. Figure 3c shows that water vapor originating from the South China Sea can only transport to the Southeastern China, while water vapor channel from the South China Sea to the North China was not established. An increase in water vapor density was observed from about 0000 UTC to 1200 UTC on 25 December at heights between 600 m and 2200 m. Figure 3d shows that the water vapor channel from Bengal Bay to North China at 850 hPa was established and led to the increase in water vapor density in Tianjin. However, due to the lack of strong LLJ and winds, the increase in water vapor density and intensity of rainfall were weak. Therefore, the southwesterly LLJ in this case carried warm and dry airflow from Hebei Province to Tianjin, which indicated that there was no correlation between southwesterly LLJ and the formation of precipitation in this case.
Atmosphere 2020, 11, x FOR PEER REVIEW 9 of 21  Comparing the two cases mentioned above, there are two main discrepancies, which may be the main reasons responsible for different effects of southwesterly LLJs on precipitation in Tianjin. One is the strength of LLJ. Strong southwesterly LLJ in case1 can transport water vapor to Tianjin, while weak southwesterly LLJ cannot. Second, the establishment of the water vapor channel from the South China Sea or Bengal Bay to Bohai Rim region at 850 hPa. For case1, strong southwesterly LLJ with the establishment of the water vapor channel is favorable for the formation of precipitation by transporting water vapor. While for case2, weak southwesterly LLJ without the establishment of the water vapor channel has no contribution to precipitation.

Relationships between LLJ and Air Pollution as Well as Fog
Based on criteria in Section 2.2, monthly occurrence numbers of all pollution episodes (PEs) and PEs accompanied by LLJs in Tianjin in 2016 are counted and results are presented in Figure 4. PEs accompanied by LLJs in this study manifested that LLJs were observed within 12 h prior to the onset of PEs or during PEs. There were eleven PEs in 2016, which included five HPEs and six LPEs. Fog events were usually simultaneously observed during PEs (64%), especially during HPEs (100%). The high occurrence frequency of fog during PEs can be attributed to southwesterly LLJs [26] and the feedback mechanism of aerosols [43]. Figure 4a shows that 55% of PEs were accompanied by LLJs in 2016, which indicated that effects of LLJs on pollution should be focused on. It should be noted that LLJs associated with PEs all occurred at nighttime or early morning and occurrence heights of LLJs were always in the range of 510-870 m. Moreover, statistical results show that southwesterly LLJs usually occurred before the onset of PEs (Figure 4b), which indicated that southwesterly LLJs may be favorable for the formation of PEs. By contrast, southeasterly and northerly LLJs all occurred during the dispersion stage of PEs, which suggested that southeasterly and northerly LLJs may play important roles in scavenging pollutants. There were six PEs accompanied by LLJs (Figure 4a). However, there are eight LLJs in Figure 4b. The cause was that there were two PEs accompanied by two types of LLJs. Before the onset of PE, southwesterly LLJs were observed, while southeasterly or northerly LLJs occurred during the dispersion stage of PE.
accompanied by LLJs in this study manifested that LLJs were observed within 12 h prior to the onset of PEs or during PEs. There were eleven PEs in 2016, which included five HPEs and six LPEs. Fog events were usually simultaneously observed during PEs (64%), especially during HPEs (100%). The high occurrence frequency of fog during PEs can be attributed to southwesterly LLJs [26] and the feedback mechanism of aerosols [43]. Figure 4a shows that 55% of PEs were accompanied by LLJs in 2016, which indicated that effects of LLJs on pollution should be focused on. It should be noted that LLJs associated with PEs all occurred at nighttime or early morning and occurrence heights of LLJs were always in the range of 510-870 m. Moreover, statistical results show that southwesterly LLJs usually occurred before the onset of PEs (Figure 4b), which indicated that southwesterly LLJs may be favorable for the formation of PEs. By contrast, southeasterly and northerly LLJs all occurred during the dispersion stage of PEs, which suggested that southeasterly and northerly LLJs may play important roles in scavenging pollutants. There were six PEs accompanied by LLJs (Figure 4a). However, there are eight LLJs in Figure 4b. The cause was that there were two PEs accompanied by two types of LLJs. Before the onset of PE, southwesterly LLJs were observed, while southeasterly or northerly LLJs occurred during the dispersion stage of PE.

Relationship between Southwesterly LLJs and Air Pollution
To investigate the relationship between LLJs and PEs in detail, several PEs accompanied by southwesterly, northerly, and southeasterly LLJs were chosen to perform further investigation. One HPE was observed from 0000 LST 16 December 2016, and the pollution dissipated completely at 1000 LST 22 December with the duration being more than six days. The effect of the boundary layer structure on pollution during this HPE has been investigated in Han et al. [44] based on vertical profiles of PM 2.5 and meteorological parameters. Persistent strong southwesterly winds carried polluted air masses from polluted southern industrial regions to Tianjin, which led to high values of PM 2.5 at high elevations (above 500 m) in Tianjin ( Figure 6 in Han et al. [44], Figure 7a in this study). The vertical profile of light-extinction coefficient of aerosols (Figure 8a) showed that a polluted layer was present at the altitude of 500-800 m from 0000 LST to 0300 LST on 16 December while the air near the surface was relatively clear. From 0300 LST 16 December, aerosols were transmitted downward, which led to the increase in PM 2.5 concentration at lower elevations (Figure 8a). There are two main results obtained in Han et al. [44]. One is that pollution appeared at higher elevations first, while it was observed at surface a few hours later. The result suggested that pollution at surface resulted from downdraughts ( Figure S3 in the Supplement), which can transport pollutants from high elevations to the surface. The other is that rime was the main cleaning mechanism that cleared away pollutants during the fog episode while the effect of wind was weak. Results in Han et al. [44] pointed out that downdraughts transported pollutants from high elevations to the surface. However, the vertical wind at 40 m was always positive ( Figure S3 in the Supplement), which cannot explain the increase in surface PM 2.5 concentration. Therefore, there must be other mechanisms that can transport pollutants from high elevations to the surface rather than downward airflows. Strong nocturnal southwesterly LLJs (larger than 16 m s −1 ) located at approximately 500-800 m were observed from 0000 LST 16 December at Xiqing (Figure 5f). At the same time, an increase in turbulence kinetic energy (TKE) within 200 m was observed (Figure 5d). Previous research has pointed out that nocturnal LLJ is vital for the formation of the "upside down" boundary layer [45,46] and the generation of turbulence in the layer between the location of LLJ and the surface [47][48][49]. Therefore, turbulence mixing induced by wind shear associated with southwesterly LLJs led to pollutants mixing down from high elevations to surface, and was the main mechanism for nocturnal surface pollution. Previous research has shown that southwesterly winds can carry polluted air masses from polluted southern industrial regions to NCP [50,51]. Compared with southwesterly winds, southwesterly LLJs can not only transport polluted air masses horizontally, but also can transport vertically by increasing turbulent mixing. Moreover, southwesterly LLJs also transported water vapor to Tianjin, which increased the surface specific humidity and RH (Figure 5a,b). The high humidity facilitated aerosol secondary formation by heterogeneous reactions, which were also favorable for the increase in PM 2.5 concentration (Figure 5a) and decrease in visibility (Figure 5b).
At 2200 LST 18 December, the RH at surface exceeded 90% and surface visibility decreased to 74 m (Figure 5b). Dense fog formed due to radiative cooling ( Figure S4 in the Supplement) under weak wind ( Figure 5c) and high humidity, which can be attributed to the southwesterly LLJs and winds. The important role of southwesterly LLJ in transporting water vapor was also confirmed through wind field in Tian et al. [15], when the channel of water vapor from the East China Sea to the Bohai Rim region was established. Once fog formed under weak winds (<1 m s −1 ) (Figure 5c), PM 2.5 concentration decreased rapidly (Figure 5a), which confirmed that there was a scavenging effect of dense fog on aerosols in Tianjin [52]. The PM 2.5 concentration remained at a relatively low level during the dense fog episode, especially when RH was approximately 100% (Figure 5a,b). Moreover, vertical profiles of PM 2.5 ( Figure S5 in the Supplement) showed that an abrupt decrease in PM 2.5 concentration was observed under 140 m from 22 LST 19 December (fog episode), while there was a slight change in PM 2.5 concentration at higher elevations (above 140 m). The height of 140 m was exactly the fog-top height, which can be estimated using the profile of RH [53]. The result also verified the conclusion that there was a scavenging effect of fog on aerosols in Tianjin. Even though the formation of fog was favorable for removing pollutants, the capacity of wet deposition by fog was limited and the PM 2.5 concentration remained higher than 150 µg m −3 (Figure 5a,b).
top height, which can be estimated using the profile of RH [53]. The result also verified the conclusion that there was a scavenging effect of fog on aerosols in Tianjin. Even though the formation of fog was favorable for removing pollutants, the capacity of wet deposition by fog was limited and the PM2.5 concentration remained higher than 150 μg m −3 (Figure 5a,b).

Relationship between Northerly or Southeasterly LLJs and Air Pollution
From 1100 LST 21 December, wind directions at high elevations turned to be northerly ( Figure  S6 in the Supplement). However, there was a slight change in the surface PM2.5 concentration ( Figure  5a) due to the low wind speed near the surface (lower than 1 m s −1 at the surface and 3 m s −1 at 250 m). Results in Han et al. [44] pointed out that rime was the main cleaning mechanism that decreased PM2.5 concentration during the fog episode. However, the wet deposition of rime or fog was limited. The PM2.5 mass concentration remained higher than 150 μg m −3 during the dense fog episode. Therefore, there must be other mechanisms that can clear away pollutants, besides fog or rime.
With the weak northwesterly cold air moving toward Tianjin, a front formed [15]. Strong northerly winds led to the dispersion of pollution at high elevations (Figure 5e and Figure 9 in Han et al. [44]). While the wind direction near the surface was still southerly (Figure 5c (Figure 5d) increased due to vertical wind shear, which was favorable for clearing away pollutants. Therefore, besides the cleaning mechanism of rime or fog, strong turbulence induced by nocturnal northerly LLJ was crucial for the dispersion of pollution. Moreover, turbulent mixing led to cold and dry air above the fog layer gradually mixing with droplets, aiding the erosion of the inversion layer and fog bank [54].

Relationship between Northerly or Southeasterly LLJs and Air Pollution
From 1100 LST 21 December, wind directions at high elevations turned to be northerly ( Figure S6 in the Supplement). However, there was a slight change in the surface PM 2.5 concentration (Figure 5a) due to the low wind speed near the surface (lower than 1 m s −1 at the surface and 3 m s −1 at 250 m). Results in Han et al. [44] pointed out that rime was the main cleaning mechanism that decreased PM 2.5 concentration during the fog episode. However, the wet deposition of rime or fog was limited. The PM 2.5 mass concentration remained higher than 150 µg m −3 during the dense fog episode. Therefore, there must be other mechanisms that can clear away pollutants, besides fog or rime.
With the weak northwesterly cold air moving toward Tianjin, a front formed [15]. Strong northerly winds led to the dispersion of pollution at high elevations (Figure 5e and Figure 9 in Han et al. [44]). While the wind direction near the surface was still southerly (Figure 5c (Figure 5d) increased due to vertical wind shear, which was favorable for clearing away pollutants. Therefore, besides the cleaning mechanism of rime or fog, strong turbulence induced by nocturnal northerly LLJ was crucial for the dispersion of pollution. Moreover, turbulent mixing led to cold and dry air above the fog layer gradually mixing with droplets, aiding the erosion of the inversion layer and fog bank [54]. A similar cleaning mechanism of nocturnal southeasterly LLJ on pollution was also observed during two PEs occurring in March and November. However, variations of TKE during these two PEs were not obtained due to the missing turbulence data. Therefore, one LPE, which occurred from 0000 LST 13 February to 2300 LST 16 February 2015, accompanied by southeasterly LLJ was introduced to show the cleaning mechanism of nocturnal southeasterly LLJ on pollution in Tianjin. Figure 7 presents variations of PM2.5 mass concentration, relative humidity, visibility, and TKE from 0000 LST 13 February to 2300 LST 15 February 2015. Abrupt decrease in PM2.5 mass concentration as well as relative humidity and increase in visibility were observed at 0300 LST 15 February (Figure 7a and Figure 7b. Nocturnal southeasterly LLJ, with a maximum wind speed of 17.4 m s −1 , was observed at the height of 510 m from 0000 LST to 0900 LST 15 February (Figure 7d). The southeasterly LLJ induced a pronounced increase in TKE at nighttime (Figure 7c) and led to a decrease in PM2.5 mass concentration and relative humidity. The result manifests that the effect of southeasterly LLJ on pollution was similar to that of northerly LLJ in Tianjin, which was favorable for clearing away pollutants. Therefore, nocturnal northerly or southeasterly LLJ, as an important source of nocturnal turbulence, is favorable for the dispersion of pollution [55,56]. A similar cleaning mechanism of nocturnal southeasterly LLJ on pollution was also observed during two PEs occurring in March and November. However, variations of TKE during these two PEs were not obtained due to the missing turbulence data. Therefore, one LPE, which occurred from 0000 LST 13 February to 2300 LST 16 February 2015, accompanied by southeasterly LLJ was introduced to show the cleaning mechanism of nocturnal southeasterly LLJ on pollution in Tianjin. Figure 7 presents variations of PM 2.5 mass concentration, relative humidity, visibility, and TKE from 0000 LST 13 February to 2300 LST 15 February 2015. Abrupt decrease in PM 2.5 mass concentration as well as relative humidity and increase in visibility were observed at 0300 LST 15 February (Figure 7a,b). Nocturnal southeasterly LLJ, with a maximum wind speed of 17.4 m s −1 , was observed at the height of 510 m from 0000 LST to 0900 LST 15 February (Figure 7d). The southeasterly LLJ induced a pronounced increase in TKE at nighttime (Figure 7c) and led to a decrease in PM 2.5 mass concentration and relative humidity. The result manifests that the effect of southeasterly LLJ on pollution was similar to that of northerly LLJ in Tianjin, which was favorable for clearing away pollutants. Therefore, nocturnal northerly or southeasterly LLJ, as an important source of nocturnal turbulence, is favorable for the dispersion of pollution [55,56]. A similar cleaning mechanism of nocturnal southeasterly LLJ on pollution was also observed during two PEs occurring in March and November. However, variations of TKE during these two PEs were not obtained due to the missing turbulence data. Therefore, one LPE, which occurred from 0000 LST 13 February to 2300 LST 16 February 2015, accompanied by southeasterly LLJ was introduced to show the cleaning mechanism of nocturnal southeasterly LLJ on pollution in Tianjin. Figure 7 presents variations of PM2.5 mass concentration, relative humidity, visibility, and TKE from 0000 LST 13 February to 2300 LST 15 February 2015. Abrupt decrease in PM2.5 mass concentration as well as relative humidity and increase in visibility were observed at 0300 LST 15 February (Figure 7a and Figure 7b. Nocturnal southeasterly LLJ, with a maximum wind speed of 17.4 m s −1 , was observed at the height of 510 m from 0000 LST to 0900 LST 15 February (Figure 7d). The southeasterly LLJ induced a pronounced increase in TKE at nighttime (Figure 7c) and led to a decrease in PM2.5 mass concentration and relative humidity. The result manifests that the effect of southeasterly LLJ on pollution was similar to that of northerly LLJ in Tianjin, which was favorable for clearing away pollutants. Therefore, nocturnal northerly or southeasterly LLJ, as an important source of nocturnal turbulence, is favorable for the dispersion of pollution [55,56]. In conclusion, nocturnal LLJ plays an important role in the formation and dispersion of PE in Tianjin. The mechanism of LLJ influencing the PM2.5 mass concentration is summarized in a schematic figure (Figure 8). Nocturnal southwesterly LLJ, which occurs before the onset of PE, is favorable for the formation of PE, especially surface pollution at nighttime. The southwesterly LLJ can carry polluted air masses from polluted southern industrial regions to Tianjin and results in the formation of PEs at high elevations in Tianjin (Figure 8b). Turbulent mixing induced by nocturnal southwesterly In conclusion, nocturnal LLJ plays an important role in the formation and dispersion of PE in Tianjin. The mechanism of LLJ influencing the PM 2.5 mass concentration is summarized in a schematic figure (Figure 8). Nocturnal southwesterly LLJ, which occurs before the onset of PE, is favorable for the formation of PE, especially surface pollution at nighttime. The southwesterly LLJ can carry polluted air masses from polluted southern industrial regions to Tianjin and results in the formation of PEs at high elevations in Tianjin (Figure 8b). Turbulent mixing induced by nocturnal southwesterly LLJ can transport pollutants from high elevations to the surface, which leads to the occurrence of surface PEs at nighttime (Figure 8c). Nocturnal northerly or southeasterly LLJ, which occurs during the dispersion stage of PE, can carry dry and clean airflow to Tianjin. Strong turbulent mixing induced by wind shear associated with the northerly or southeasterly LLJ leads to dry and clean airflow mixing with polluted or saturated air masses, which results in the dispersion of PE (Figure 8e) or fog [26].
Atmosphere 2020, 11, x FOR PEER REVIEW 16 of 21 LLJ can transport pollutants from high elevations to the surface, which leads to the occurrence of surface PEs at nighttime (Figure 8c). Nocturnal northerly or southeasterly LLJ, which occurs during the dispersion stage of PE, can carry dry and clean airflow to Tianjin. Strong turbulent mixing induced by wind shear associated with the northerly or southeasterly LLJ leads to dry and clean airflow mixing with polluted or saturated air masses, which results in the dispersion of PE ( Figure  8e) or fog [26].

Relationship between Southeasterly LLJ and Fog
In addition, previous research pointed out that southwesterly LLJ was also favorable for the formation of fog in Tianjin by transporting water vapor [15,26], especially under high aerosol loadings during PE. However, fog was not always observed during PE, even though southwesterly LLJ occurred before the onset of fog and PE. To investigate and verify the effect of southwesterly LLJ observed and variations of RH mainly relied on variations of temperature (Figure 9a). Figure 9c confirmed that southwesterly LLJ, in this case, cannot transport water vapor, due to being without a water vapor channel at 925 hPa. The result indicated that southwesterly LLJs are always favorable for the formation of PEs, when pollutions are observed in the southern industrial regions. However, southwesterly LLJs, only accompanied by the existence of a water vapor channel at 925 hPa, can transport water vapor to Tianjin and support the formation of fog.

Relationship between Southeasterly LLJ and Fog
In addition, previous research pointed out that southwesterly LLJ was also favorable for the formation of fog in Tianjin by transporting water vapor [15,26], especially under high aerosol loadings during PE. However, fog was not always observed during PE, even though southwesterly LLJ occurred before the onset of fog and PE. To investigate and verify the effect of southwesterly LLJ on fog, an LPE occurred from 15 March to 18 March 2016 (Figure 9a) was chosen to perform case analysis. Persistent southwesterly LLJ located at 750 m, with the maximum wind speed being 17.4 m s −1 , was observed from 2100 LST 14 March, 2016. RH was always lower than 90% and visibility was always higher than 1 km during the LPE (Figure 9b). Strong southwesterly LLJ was favorable for the formation of the LPE by transporting pollutants horizontally and vertically. However, fog was not observed and variations of RH mainly relied on variations of temperature (Figure 9a). Figure 9c confirmed that southwesterly LLJ, in this case, cannot transport water vapor, due to being without a water vapor channel at 925 hPa. The result indicated that southwesterly LLJs are always favorable for the formation of PEs, when pollutions are observed in the southern industrial regions. However, southwesterly LLJs, only accompanied by the existence of a water vapor channel at 925 hPa, can transport water vapor to Tianjin and support the formation of fog.
In addition, a common feature where heights of LLJs provided favorable conditions for PEs or fog were much lower than for the precipitations, which was found in Tianjin. Statistical results show that southwesterly LLJs associated with precipitations are usually located at 1300-1800 m in Tianjin, while heights of southwesterly LLJs, which provided favorable conditions for PEs or fog, are usually lower than 1000 m. Precipitation events can usually be classified into two types including the stratiform precipitation associated with relative stable stratification and convective precipitation related to vigorous overturning [57]. As we all know, two types of rain clouds form at high elevations and cloud base heights are usually 1-2 km or so, which manifest that only LLJs located at high altitudes are favorable for the formation of precipitations by transporting water vapor, when the water vapor channel from the South China Sea or Bengal Bay to Bohai Rim region at 850 hPa is established. At the same time, PE and fog, which usually form under low wind speed and strong stable stratification, always occur within the boundary layer. Therefore, only southwesterly LLJs occurring within the boundary layer or near the top of the boundary layer are favorable for the formation of PEs in Tianjin by transport pollutants from polluted regions to Tianjin. Moreover, when the water vapor channel from the East China Sea or the Yellow Sea to the Bohai Rim region at 925 hPa is established, southwesterly LLJs located at approximately 925 hPa support the formation of fog. In conclusion, heights of LLJs, which provided favorable conditions for PEs or fog, were much lower than for the precipitations in Tianjin.

Conclusions
In this study, relationships between LLJs and low-visibility events including precipitation, air pollution, and fog in Tianjin are investigated based on observations from wind-profile radar, thirteen AWSs, a 255-m meteorological tower, and a microwave radiometer from January to December in 2016.
Statistical results show that 79% of precipitation in Tianjin occurred from May to October and 55% of precipitation was accompanied by LLJ. Two main causes responsible for the relatively high percentage of precipitation accompanied by LLJ in Tianjin are presented. One is the occurrence of frontal activity or cyclone, which can foster the formation of LLJ and precipitation simultaneously. The other is that LLJ occasionally occurred before precipitation. The result of case analysis also verified that some LLJs occurring before precipitation were induced by baroclinicity associated with land-ocean thermal differences, while some were induced by baroclinicity associated with the front. Moreover, the result of case analysis shows that some strong southwesterly LLJs are favorable for the formation of precipitation by transporting water vapor to Tianjin, when the channel of water vapor from the South China Sea or Bengal Bay to Bohai Rim region at 850 hPa is established. Concurrently, some weak southwesterly LLJs without the establishment of the water vapor channel have no contribution to precipitation in Tianjin.
Statistical results show that 55% of PEs were accompanied by LLJs in 2016, and fog events were usually simultaneously observed (64%) during PEs. The result of case analysis shows that nocturnal southwesterly LLJ usually occurring before the onset of PE is favorable for the formation of surface pollution at nighttime. The southwesterly LLJ can carry polluted air masses from polluted southern industrial regions to Tianjin and enhance turbulent mixing, which leads to the occurrence of surface PEs at nighttime. Nocturnal northerly or southeasterly LLJ occurring at the dispersion stage of PE can carry clean air masses to Tianjin and induce strong turbulent mixing, which results in the dispersion of PE. In addition, southwesterly LLJs, which are only accompanied by the existence of a water vapor channel at 925 hPa, are favorable for the formation of fog by transporting water vapor. The effects of southwesterly LLJ on fog and precipitation are similar in Tianjin and strongly rely on establishing a water vapor channel. Yet, occurrence heights of LLJ and the water vapor channel are different.
In conclusion, LLJ plays an important role in the formation or dispersion of low-visibility events in Tianjin. southwesterly LLJs are usually associated with low visibility in Tianjin by transporting pollutants or water vapor and enhancing turbulent mixing when pollution is observed in the southern industrial regions or the channel of water vapor from the South China Sea or Bengal Bay at 850 hPa or the East China Sea or the Yellow Sea at 925 hPa is established. The occurrence of northerly or southeasterly LLJ during fog or pollution episodes always leads to the dispersion of fog or pollution, and can be treated as an indicator of high visibility.