The turbulent structure and transport in fog layers observed over the Tianjin area

This paper investigates the vertical structure and turbulence activities in fog events. Three fog cases that occurred in the winter of 2010 over Tianjin, China were selected, including two advection-radiation fog events and an advection fog event. Field observations collected at a 255-m tall meteorological tower in Tianjin were analyzed, including turbulence measurements using the eddy covariance systems installed at three levels, measurements of temperature, horizontal wind and humidity collected at 15 levels, surface radiation fluxes and horizontal visibility. The results suggest that the advection fog was more enduring and thicker than the advection-radiation fog. The fog events were characterized by low wind speed throughout the fog layer. A temperature inversion and low-level jet were observed above the advection-radiation fog layer. The surface net radiation reflected some differences among the fog events. The collapse of turbulence was a necessity for the formation fog, and moderate turbulence was favorable to the development and maintenance. The heat and water vapor fluxes in the advection-radiation fog were weaker than those in the advection fog, in which the stratification was slightly unstable. The relationships among the turbulent transport efficiencies of water vapor, temperature and momentum were examined. The results suggest the applicability of local similarity in the fog layer for the momentum transport efficiency.