Impact of water vapor transfer on a Circum-Bohai-Sea heavy fog: Observation and numerical simulation

Abstract

a wide-range winter frontal fog that happened over Circum-Bohai-Sea (CBS) on December 17–192,016 was observed using satellite cloud image inversion, surface, ocean buoy, and radiosonde data. The evolution of fog processes was investigated with numerical simulations using the Weather Research and Forecasting (WRF) Model together with the FNL (Final) reanalysis data. The results show that the WRF model can reproduce the features of the heavy fog generation and development. The observed results show following characteristics: (1) Before fog appearance, the southwest low-level jet (LLJ) of which the wind speed was over 16 m/s at 925 hPa developed. The LLJ carrying warm and wet air provided a stable inversion layer about 1200 m thick and continuous vapor transport to the CBS area, which is favorable for the formation of heavy fog. With the weak northwestern cold air moving towards the CBS area, a typical frontal fog process was triggered; (2) the weak cold air did not destroy the structure of low-level thermal inversion, and the heavy fog maintained. With the weak front moving towards the east, the foggy coverage continually extended over the sea surface, finally covering the Bohai Sea and the North Yellow Sea. A water-vapor sensitivity test suggests that with the LLJ continually conveying water vapor to the area, the accumulative effect of an increase in water vapor content took place in the low-level atmosphere, which changed the temperature and humidity structure of the boundary layer. The liquid water content, coverage area, and thickness of fog changed drastically.