Sensitivity study of vertical resolution in WRF numerical simulation for sea fog over the Yellow Sea

The performance and sensitivity of the Weather Research and Forecasting (WRF) model to different vertical resolutions for simulating the Yellow Sea fog are studied. Numerical experiments with combinations of three vertical resolutions (35η, 44η and 63η) and two planetary boundary layer schemes (YSU, MYNN) are designed and conducted for the simulation of 10 sea fog cases. The sensitivity of fog-area and fog-top height to vertical resolution is statistically analyzed, and a typical case is investigated in detail to reveal the effects of fog-top long-wave radiation cooling and turbulence inside the fog. The statistical results show that:(1) the fog-area simulation is significantly improved with increases in vertical resolution, and the model also performs better in fog-area simulations for those cases that have large fog height differences between the experiments with different vertical resolutions; (2) the YSU scheme is more sensitive than the MYNN scheme. Compared to the 35η-experiments, the averaged probability of detection (POD) and equitable threat score (ETS) in the 44η-experiments have improved by 13.29% and 10.22%, respectively. Detailed analysis of the typical case indicates that a reliable modeling of fog-top long-wave radiation cooling and turbulence inside the fog strongly depends on vertical resolution. It can be outlined as:(1) a coarse vertical resolution with a weak turbulence intensity leads to the failure of simulation; (2) a positive feedback——"increased cloud liquid water→enhancing long-wave radiation→strengthening cooling→cloud liquid water increased"——forms near the fog top, and fine vertical resolution is more helpful to maintain and strengthen this feedback loop than the coarse resolution; (3) only the simulations with fine vertical resolution can capture the downward buoyancy turbulence that is produced by fog-top long-wave radiation cooling. The intensity of the buoyancy turbulence is comparable to that of the shear turbulence near the sea surface, and it results in the phenomenon that sea surface temperature is higher than air temperature, which is often observed during sea fog occurrence.