Sensitivity of WRF simulations with the YSU PBL scheme to the lowest model level height for a sea fog event over the Yellow Sea

Evaluations for the continental PBL indicate that z1 below 8 m is irrational in simulating surface temperature and PBL height over land. However, the model with z1= 8 m gives the best performance in terms of reproducing sea fog. When z1 gets below 8 m, the sea fog occurs too early and the fog area is too small. As z1 exceeds 8 m, the fog forms too late and the fog area becomes underestimated. These model sensitivities can be explained by the impact of z1 on virtual potential temperature at z1 [θv(z1)]. Since the heat capacity of the air in the lowest model layer is proportional to z1, a lower (higher) z1 causes a quicker (slower) response of θv(z1) to surface cooling, thus leading to an earlier (later) sea fog formation. After the fog onset, especially for a lower z1, the variation of θv(z1) is dominated by turbulent heating that transports warmer air above to the very shallow lowest model layer, resulting in a lower vertical growth and even earlier dissipation of the sea fog.