Atmospheric turbulence and the planetary boundary layer (PBL) can significantly modulate the fine particulate matter (PM_2.5) concentration near the ground surface. However, the mechanism by which they affect the variation in PM_2.5 is poorly understood. Using surface‐based meteorological and PM_2.5 observations, turbulence measurements, and high‐resolution soundings, this study analyzed the properties of turbulence and PBL stratification during a severe regional pollution transport event in Xiangyang, a city in the middle reaches of the Yangtze River, from January 3 to January 9, 2019. During the growth stage, PM_2.5 was observed to increase continuously under a northerly wind of 2–4 m s⁻¹ that brought a polluted air mass to central China. During the entire pollution event, the turbulent kinetic energy and sensible heat flux tended to be lower at high PM_2.5 concentrations. This correlation varied greatly depending on the stage of the event. A negative correlation existed in the initial and maintenance stages, whereas the relationship was positive during the growth stage, when regional transport dominated the variation in PM_2.5. There was no obvious inversion layer below 500 m during the entire pollution episode, and a weak inversion layer appeared at higher altitudes in the initial and maintenance stages. Vertically homogeneous northerly winds favored the transboundary transport of pollutants, causing a significant increase in ground‐level PM_2.5 concentration. By contrast, the presence of wind shear in the prevailing northerly winds suppressed pollutant transport to the ground, resulting in a low‐value gap in PM_2.5 concentration between high altitudes and the ground surface.

Both the similar and different effects of freezing drizzle, sleet and snow on microphysical properties of supercooled fog were analyzed for fourteen events during a comprehensive wire icing, fog, and precipitation observation experiment conducted at Enshi radar station (30°17′N, 109°16′E; 1722 m a.s.l.) on a hilltop in Shibanling, Hubei, China. Liquid precipitation is in a relatively stable form in mountainous areas. Short-term precipitation (1–3 h) is dominant with temperature below 0 °C. The wet scavenging effect of freezing drizzle on small fog droplets with a size range less than 6–12 μm is weak but is stronger for fog droplets with a larger size, which is opposite to the effects of solid precipitation, broadening the fog droplet spectra significantly. As the fog droplet diameter increases, the distributions of droplet spectra change from leptokurtosis to platykurtosis and from positive skewness to negative skewness. Occurrence of freezing drizzle would improve the positive correlation of N-r in dissipation and oscillation periods, resulting in the N-r relationship having a weak negative correlation in the maturity period, and resulting in the transition of the N-L and N-r relationships into positive correlations in the development period. Meanwhile, the emergence of solid precipitation particles would result in negative values for the correlation coefficients of N-L and N-r. The change in relationships among the microphysical properties was caused by the occurrence of different phase precipitation, showing the influence on the main microphysical mechanisms of supercooled fog.

Research on three types of wire icing evolution mechanism is of great significance for ice disaster recognition and prediction. Comprehensive field observations of wire icing were conducted in the winters of 2015/2016 and 2016/2017 at the Lushan Mountain Meteorological Bureau Observatory (elevation 1164.5 m), Jiangxi Province, China, and the ice thickness, weather phenomena, meteorological elements, and fog spectrum were measured. Four icing cases were recorded, in which high ice-producing conditions, such as freezing rain, snow and supercooled fog, occurred. The icing growth and shedding mechanisms in three types of weather (freezing rain, snow, and supercooled fog) were analyzed and compared. Considering mixed ice-producing conditions, the ice thickness was simulated by integrating freezing rain, snow, and supercooled fog icing models, with inputs such as the precipitation rate and wind speed. The average measured icing growth rates in freezing rain, snow, and supercooled fog were 0.4, 0.3, and 0.2 mm h⁻¹, respectively. The correlations between the icing growth rate and both the temperature and the wind speed were stronger in the snow and supercooled fog than in freezing rain. With the decreasing temperature, the icing growth rate increased faster with snow, while that in supercooled fog increased faster as the wind speed increased. In freezing rain, snow, and supercooled fog, the icing growth rates were all positively correlated with the ice thickness, with correlation coefficients of 0.55, 0.67, and 0.79, respectively. Ice was shed when the temperature remained below 0 °C, the wind speed fell to 2.7 m s⁻¹, and the fog liquid water content fell to 0.036 g m⁻³ in supercooled fog, and when the solar radiation increased and the temperature exceeded 0 °C in freezing rain. The average sticking efficiency of the wire to snow was 0.03; its variation range was 0.01–0.10. The integrated model can simulate the changes in actual ice thickness well.