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

Abstract

The lowest model level is the interface of energy and mass exchanging between the surface and planetary boundary layer (PBL). Previous studies mostly examined the role of the lowest model level height (z1) in simulating the continental PBL processes. The impact of z1 on simulating marine processes (e.g., sea fog), however, remains unclear. The present study explores the sensitivity of the Weather Research and Forecasting (WRF) model with the Yonsei University (YSU) PBL scheme to z1 for an advection fog event occurred on 27 March 2012 over the Yellow Sea. Seven experiments with various z1 (28, 22, 14, 8, 4, 1 and 0.4 m) are conducted.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.