Cluster Analysis of the Circulation Situation Occurring during Fog and Haze Weather in North China

40 in situ measurements of fog and haze over North China from 2000 through 2013 were used and cluster analysis is applied in the geopotential height field from the same period of NCEP daily reanalysis data. Results show that surface circulation situations can be respectively divided into four and five clusters when fog or haze weather occurs. Clustering results reveal that the atmospheric circulation has different characteristics when fog and haze occurs, weak high pressure is the only common type for both. The transportation of the humidity field and cold (warm) advection are also significantly different in fog and haze weather. In addition, the analysis results show that fog and haze conversion exists on the same day and fog coexists with haze at the same moment. The authors also analyze the circulation characteristics of these two scenarios, and provide explanations for the phenomena.