Boundary Layer Structure Features and Diagnostic Analysis of One Successive Heavy Fog Event
Heavy fog occurred in most areas of middle and south of Hebei Province and Tianjin for 3 days successively from 30 November to 2 December 2010, during which Shijiazhuang experienced a 34 h dense fog, including a 7 h strong dense fog. The diagnostic analysis on structure characteristics of successive heavy fog boundary layer and the formation, development and maintenance, and dissipation of the fog was made based on intensive automatic weather station data and Tianjin 250 m meteorological tower gradient observation data as well as conventional observation data and NCEP/NCAR reanalysis data. The result shows that prior to the foggy weather, the continuous east wind on surface is beneficial to the accumulation of water vapor. When the east wind turns to north wind, the vapor begins to condense, and eventually, becomes heavy fog. After the formation of heavy fog, the direction of wind turns to east again, helping maintain and strengthen the fog density. The long time SW warm and wet flow under 850 hPa and the inversion on the near surface layer are the main causes for the durative heavy fog. The water transfer of the low level three branches of vapor and reconstruction of southwest jet at 850 hPa are responsible for the formation of the heavy fog directly. During the strengthening period, the wind speed on boundary layer is 1-2 m·s-1, especially during the strong dense fog, the wind speed is only 1 m·s-1. When the wind with speed greater than 4 m·s-1 travels down to the ground surface, the heavy fog disperses affected by the destroyed inversion.