Observational analysis of a fog event in the oyashio extension area

The northwest Pacific is the most foggy oceanic area in the world. Low visibility in fogaffects navigation heavily, yet the synoptic processes of sea fog formation over open oceans remain unclear due to the lack of in-situ observations so far. In September 12-14, 2019, a Chinese arctic research vessel, XiangYanghong 01, captured a fog event in the cold Oyashio Extension (OE) area. The mechanism of the fog formation and maintenance is investigated in the present study based on the in-situ observations. The results show that the fog formation was influenced by both a synoptic-scale warm front in a midlatitude cyclone system and a local SST front (sea surface temperature front). Warm humid air was transported from south to north along with the warm front, which resulted in the near surface air becoming warmer and wetter in the OE area. The fact that the moistening effect was greater than the heating one led to the relative humidity increasing and close to saturation. On the other hand, the northward warmer air climbed upward when met the colder air mass over the OE water, thus forming a large-scale frontal temperature inversion. The bottom of the inversion dropped further affected by a sinking branch from a vertical circulation in the marine atmospheric boundary layer, and this vertical circulation was very likely to be induced by the local SST front.  The lowered inversion was conducive to fog droplets, once formed, to be confined near the sea surface. The fog layer was formed in the warm moist air on the cold flank of the SST front. The contributions of the background atmospheric warm front and the local SST front to fog formation is clarified and a relevant conceptual model is proposed, which are helpful for understanding of the physical processes in the formation of marine fog and for fog forecast in open oceanic area.