Characteristics of turbulence and dissipation mechanism in a polluted radiation–advection fog life cycle in Tianjin

Abstract

Characteristics of turbulence and dissipation mechanism during a polluted advection–radiation fog life cycle occurring on 26–27 November, 2018 are investigated, using the measurements at a 255-m meteorological tower located in Tianjin. The increase in relative humidity during fog formation phase is attributed to the radiative cooling and advection, indicating that the fog type is advection–radiation fog. The dispersion of PM2.5 at all heights can be attributed to the strong winds, while dispersion of PM2.5 at surface is inhibited and delayed by the thick fog. The four phases during fog life cycle are all linked to the turbulence intensity. Weak turbulence is crucial for fog formation, and the threshold σw2 range is between 0.01 and 0.10 m2 s−2. The fog dissipation at high elevations is due to strong cold and dry advection. Downdraughts at high elevations lead dry air to sink down into the lower fog layer and aid the erosion of the fog layer at medium elevations. However, the collapse of fog bank at surface is mainly due to the strong turbulence. The theory that there is a threshold relationship between turbulence and fog dissipation is verified through case analysis. However, there are two main application conditions: one is that only radiative cooling, turbulent mixing, and gravitational settling are involved in the dissipation phase of fog. The second is that the dissipation time should be at night or dawn (radiative cooling should be positive), and the fog layer collapses due to strong turbulent mixing induced by nocturnal LLJ or synoptic system.