The impacts of intermittent turbulence on a dense radiation fog in Tianjin

Intermittent turbulence in general refers to a brief turbulent burst, which is the main mechanism of scalar diffusion in the stable boundary layer (SBL). The impacts of intermittent turbulence on a radiation fog were investigated based on the measurements at a 255-m meteorological tower and the Weather Research and Forecasting model. Observational results showed that intermittent turbulence inhibited fog formation. As intermittent turbulence weakened, radiation fog formed in the SBL. During the fog development and maturity stage, intermittent turbulence at the high levels promoted the vertical development of fog. However, the downward propagation of intermittent turbulence did not reach the surface. Low intermittent strength of turbulence and weak turbulent mixing at 40 m indicated that there was a barrier layer hindering the transmission up and down. The barrier effect led to explosively reinforced fog at the surface. Intermittent turbulence is not considered in the original Yonsei University (YSU) scheme, leading to the underestimation of the simulated turbulent diffusion coefficient (k_m). The average ratio of observed k_m to simulated k_m was 4.30 during the fog episode. Thus, three sensitivity experiments – a double k_m, a quadruple k_m from the original YSU scheme, and an updated YSU scheme – were designed to study the contributions of the increase in k_m to fog evolution. The results showed that the increase in k_m can improve the simulation of fog-top height and correct the onset timing of fog. Thus, an improvement in the original YSU scheme is necessary for a reasonable description of intermittent turbulence.