Comparisons of spatiotemporal distribution characteristics and surface meteorological conditions for fog and heavy haze events of different levels in Anhui province

Fog and haze are low visibility weather with similar formation conditions. Based on hourly observations collected at 78 surface stations in Anhui province and physical conditions favorable for fog and haze formation, a set of methods are established to diagnose fog days of different levels and heavy haze days. The time series data of fog days of different levels and heavy haze days are reconstructed. According to the synchronization of extremely dense fog events at each station, Anhui province was divided into five regions with different distribution characteristics of fog and haze, and the similarities and differences in surface meteorological conditions for different levels of fog and heavy haze in each region are discussed. The results show that: (1) Extremely dense fog in Anhui mainly belongs to radiation fog. The spatial distributions of extremely dense fog, dense fog and fog are basically the same. The region to the north of the Huaihe River and the region to the south of the Yangtze River are high incidence areas of extremely dense fog, but the spatial and temporal distribution characteristics and influence weather systems of extremely dense fog are different between the regions. The distribution characteristics of heavy haze show high occurrence in the north, low occurrence in the South and the least in mountainous areas. (2) Annual variation of extremely dense fog shows a bimodal pattern with peaks occurring in January and April; daily variation is unimodal with the peak at 06:00 BT; the annual variation of heavy haze is unimodal with the peak in January, and the daily variation is bimodal. (3) During high incidence period of extremely dense fog (02:00—08:00 BT), the meteorological condition is characterized by large temperature decrease range (TDR) with the average TDR 1℃ higher than that during heavy haze period, significantly low wind speed that is weaker than 1.5 m/s without obvious dominant wind direction for more than 75% of the samples. During heavy haze events, the wind speed exceeds 1.5 m/s for more than 75% of the samples in some areas, which is significantly higher than that during fog events. Dominant wind directions are northwesterly, northerly and northeasterly. It indicates that wind speed, wind direction and temperature decrease range are the key surface meteorological factors for heavy haze to evolve into extremely dense fog.