Meteorological conditions for severe foggy haze episodes over north China in 2016-2017 winter

This paper aims to identify the meteorological conditions of severe foggy haze events that frequently occurred over North China. We analyzed data collected at 162 ground observation stations operated by China Meteorological Administration (CMA), as well as data from National Centers Environmental Prediction (NCEP) over North China from December 1, 2016 to January 9, 2017. During this period, more than 72% of the regional mean atmospheric visibility was less than 10 km, with a minimum of 1.15 km. The analysis on atmospheric background fields revealed that during the pollution development-maintenance period there were southerlies and lower wind speed in the lower troposphere compared to that during the pollution dissipation period. Slow southerlies transported the southern pollutants to North China, while high pressure system at the 500 hPa level and increasing temperature (caused by air pollutant absorbed radiation) at 850 hPa suppressed the convection and led to pollutants accumulation over the ground. During the pollution dissipation period, there were northerlies and higher wind speed, and the fast northerlies quickly transported the pollutants. The analysis on the dynamic and thermodynamic effect suggests that the smaller horizontal wind vertical shear is attributed to 500 hPa decreased wind speed. The air pollutant warming effect on 850 hPa from absorbed solar radiation and cooling effect on near surface from reduced radiation near surface could lead to a larger correlation between atmospheric visibility and thermodynamic conditions for more than 76%. This coupling structure between air pollutant and thermodynamic situation provide favorable conditions for foggy haze events under air pollutant transport and weak vertical exchange conditions. Therefore, in order to predict foggy haze episodes in North China, we need to better understand its dynamics, especially for decreased middle level wind speed and lower level south flow.