Primary Study of Sea Fog Inland Penetration: Analysis of Two Sea Fog Cases over the Western Yellow Sea in Spring

Sea fog can penetrate coastal area and affect visibility inland, but the mechanism is not clear. We analyze two sea fog cases over the western Yellow Sea near Qingdao in spring 2008, by using various observation data, reanalysis data and Weather Research and Forecasting (WRF) model simulation. One of the fog cases occurred on 29-30 April and penetrated inland (penetration case), while another took place on 2-3 May and did not penetrate (no penetration case) (fig. 1). By comparing the two cases, our analysis suggests two factors that controlled the sea fog penetration. (1) The features of horizontal air advection. The warm moist air advection near the Yellow Sea surface was stronger in the penetration case than that in the no penetration case (fig. 2). More moisture helped to form a dense fog, which was favorable for sea fog penetration. Meanwhile, the dry air above the fog top was from land in the penetration case, which contributed to the formation of a dry air layer above fog top. But the air over the fog top was from sea in the no penetration case. (2) Vertical characteristics of atmospheric boundary layer. We find that the dry layer over the temperature inversion in the penetration case allowed strong longwave radiation cooling at fog top (fig. 3). This cooling enhanced the turbulent mixing and reduced air temperature near ground, which contribute to the development of the fog over sea and maintenance of the fog over land. In addition, both air advection and structures of atmospheric boundary layer were dominated by the synoptic conditions. Suitable pattern of the high and low pressure resulted in moist or dry air advection at different levels, which created the conditions of cooling and humidification for sea fog penetration. We hope the results are helpful to forecast the extent of fog penetration and the visibility in coastal area.