Modeling study of the effects of entrainment-mixing on fog simulation in the chemistry–weather coupling model GRAPES_Meso5.1/CUACE CW

Entrainment-mixing processes of fog with the surrounding ambient air are extremely intricate and impose significant effects on the microphysical and radiative properties of fog. However, it is difficult to utilize the default Thompson scheme of the atmospheric chemistry model GRAPES_Meso5.1/CUACE to examine the effects of different entrainment-mixing mechanisms on the microphysical and radiative properties of fog. To address this issue, this scheme is modified to include homogeneous mixing degree to investigate the effects of various entrainment-mixing processes on typical regional fog simultaneously occurring in the Northeast China and Yangtze River Delta regions from December 31, 2016, to January 2, 2017, and from January 6 to 8, 2017. It is revealed that inhomogeneous entrainment-mixing processes can result in smaller fog droplet number concentration and lower liquid water path, and larger fog droplet size. These phenomena, in turn, can lead to a decreased fog optical thickness and increased visibility. Furthermore, the effects of inhomogeneous entrainment-mixing processes depend on fog thickness, i.e., the effects in thin fog in the Northeast China region are more significant than those in thick fog in the Yangtze River Delta region. This primarily occurs because the proportion of evaporated grids in thin fog is higher than that in thick fog by 16% and 6%, respectively. These findings enhance the theoretical understanding of entrainment-mixing processes and lay the foundation for improving model parameterization.