Observation and Simulation of a Heavy Fog Event in Qiongzhou Strait

From February 14th to February 25th, 2018, the Qiongzhou Strait, China, experienced a persistent heavy fog that was unprecedented in the past 67 years. Observations and simulation results are utilized to diagnose causes of the heavy fog event. This persistent heavy fog was closely related to the continuous occurrence of fog in various types. More fog was observed along the northern coast than the southern coast. The fog over sea lasted longer than that over land. Fog along the southern coast was more sensitive to humidity, whereas fog on the northern coast was more sensitive to wind. The wind speed on the northern coast is beneficial to the generation of advection fog. Contrastingly, the southern coast was in either a weak wind or static wind state, which was in favor of the generation of radiation fog. A low altitude inversion layer appeared before the heavy fog occurred, controlled by the warm and humid airflow in the south. When the heavy fog dissipated, the wind velocity gradually increased, controlled by the dry and cold airflow in the northeast. The variation characteristics of meteorological conditions were successfully reproduced using the WRF model. Simulated minimum visibility and sea fog area were consistent with observations. Air-sea temperature difference is of vital importance to the formation of the heav fog. The stability of atmosphere boundary layer below 150 m was sensitive to the change in sea surface temperature (SST). The higher SST led to higher boundary layer and lower stability. which further resulted in a shorter duration and smaller area of sea fog. The decrease in the SST led to the lower height of boundary layer and stronger stability, which further resulted in a longer duration and expanding area of the sea fog.