资源环境科技发展态势分析平台

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 (z(1)) in simulating the continental PBL processes. The impact of z(1) 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 z(1) for an advection fog event occurred on 27 March 2012 over the Yellow Sea. Seven experiments with various z(1) (28, 22, 14, 8, 4, 1 and 0.4 m) are conducted.

Evaluations for the continental PBL indicate that z(1) below 8 m is irrational in simulating surface temperature and PBL height over land. However, the model with z(1) = 8 m gives the best performance in terms of reproducing sea fog. When z(1) gets below 8 m, the sea fog occurs too early and the fog area is too small. As z(1) exceeds 8 m, the fog forms too late and the fog area becomes underestimated. These model sensitivities can be explained by the impact of z(1) on virtual potential temperature at z(1) [theta(v)(z(1))]. Since the heat capacity of the air in the lowest model layer is proportional to z(1), a lower (higher) z(1) causes a quicker (slower) response of theta(v)(z(1)) to surface cooling, thus leading to an earlier (later) sea fog formation. After the fog onset, especially for a lower z(1), the variation of theta(v)(z(1)) 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.