Characteristics of Jet Stream in Low-altitude Airspace and Impact to Fog in Tianjin Urban Areas

This study analyzed the temporal and spatial distribution characteristics of jet stream in low airspace (1500 meters) and the impact to fog in Tianjin urban areas, based on the data of the boundary layer wind profiler and the 255 m Meteorological Tower during 2016-2018.

The statistical results show that: (1) As for the distribution height of wind jet stream, there are two peak heights within 1500 m, one peak is within 500-600 m, about 23% of the wind stream jet is located at the height; The other peak is located in the 200-300 m range, and about 19% of the jet stream is located at the height; and wind jet streams are more evenly distributed in other low-altitudes airspace. (2) As for the height and frequency seasonal distribution characteristics of the jet stream, the average height of the jet stream is generally located at 700-1000 m, and the seasonal variation of the jet height is not obvious, but the height of the cold season is higher than that of the warm season. The frequency of stream jet in 300-1500 m layer is the lowest in December and the highest in May. In contrast, the low-level jet within the 300 m layer occurs more frequently during the transition from warm to cold season, accounting for about 70% in autumn and winter, with the lowest frequency in June and the highest in October. (3) As for the diurnal distribution characteristics and wind direction of the jet stream, the frequency of wind stream jet at 300-1500 m layer increased at sunset, and the high frequency lasted until dawn of the next day, the character is the same for the low-altitude jet below 300 m, and the frequency during nighttime is more than 70%, far more than the frequency of wind jet at the upper layer. The prevailing jet direction is southwest, accounting for 47%; Followed by northerly wind, accounting for 17%; The southeast (90-180°) jet direction is less than 20%, while the northwest low-level jet stream least frequent. (4) The southerly jet before the fog, which was conducive to the formation of fog, transported water vapor to the fog area; while the northerly jet have bidirectional effect of leading to fog burst or dissipation, which afford to the fog area with cold air and kinetic energy.

In conclusion, Due to sparse vertical layer data, such as the FNL reanalysis data, the features of low-altitude airspace jet are rough, especially those below 300 meters. This study supplemented the understanding of the distribution characteristics of the low-altitude jet under 300 m and the effect to fog in Tianjin urban area, which is important for fog forecasting and low-altitude flight meteorological service .