Impact of Improved Surface Flux Parameterization on Simulation of Radiation Fog Formation in the Yangtze River Delta, China

Abstract

Meteorological conditions within the boundary layer play significant roles in radiation fog formation, which typically occur under stable conditions. The stratification conditions in the surface layer are represented by the stability parameter (ζ), calculated as the ratio of the reference height z to the Monin-Obukhov length L (i.e., ζ = z/L). Current surface layer schemes exhibit uncertainties under strong stable conditions (ζ > 1). The Grachev2007 scheme for ζ > 1 and the Li2014 and Li2015 schemes for calculating ζ are implemented into the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). Two successive radiation fog events in the Yangtze River Delta are simulated to compare the improved scheme with the default scheme. Both fog events occur under high-pressure conditions characterized by clear sky and light wind during the nighttime. The results indicate that strong stable conditions dominate before fog formation, and the improved scheme improves threat scores for fog formation. Regarding surface flux, due to reduced surface thermal resistance in parameterization, increased surface heat exchange in the improved scheme enhances cooling from sensible heat flux for ζ > 1, which is conducive to fog formation. Regarding turbulent thermal mixing, the increased surface dynamic exchange in the improved scheme enhances surface drag and reduces wind speed for ζ > 1. This weakens the contribution of wind shear to turbulent kinetic energy, ultimately promoting fog formation. The findings of this paper are applicable to radiation fog simulations in other regions, such as plain areas covered with grassland, cropland, or other vegetation, providing support for improving fog simulation.