Effect of both sea-land thermal difference and low-level circulation on finely location of Bohai coastal fog

Based on mesoscale automatic weather stations, NCEP FNL reanalysis data and satellite retrivaled fog from 2015 to 2020, the distinction of the land fog and sea fog over the Bohai coastal zone were analyzed. It shows although Bohai coastal fog often occur, they frequently divided into land fog or sea fog by western coastline. In order to explore the causing of this phenomenon, the Bohai coastal fog were selected as the research object in this work. Four types of fog events were taken into account according the following situations, (1) only land fog, (2) Land fog drifting to over marine areas, (3) only Sea fog, (4) Sea fog drifting to over inlands.

According to the statistics of the above four fog types, the characteristics and diurnal variation difference of fog show that, although the sea fog and land fog have the same inter-monthly distribution trend and all the most in winter, the difference was still significant. Land fog was more than sea fog in the autumn and winter, while on the contrary in the spring and summer. The event of sea fog and land fog clearly separated by the western coastline mostly occurs in the spring and winter night and lasted less than 12 h.

In order to further understand the reason that the coastal fog do or not cross the coastline, Firstly, we compared the two land fog events. One land fog occurred while not crossing the coastline, the other spread to the sea. The results show that, under the condition of a weaker weather systematic low-level circulation, the sea-land breeze thermal circulation humidify the inland air, which is favorable to land fog formation. The eastward inland fog moving into the marine area and dissipate due to the higher sea temperature. The stronger offshore wind is favorable for land fog drifting to the sea, and the fog over the sea can develop with the lower sea temperature. The sensitivity simulation experiment using WRF further made sure, when the SST increased by 5%, as the sea-land breeze strengthened, the fog area that is slightly offshore in the control experiment would retreat to the west of the coastline, and the land fog also showed more dense; while when the SST decreased by 5%, the sea-land wind is suppressed, resulting in the fog cases that originally stopped at the west coastline will spread and cover the Bohai area and maintain for a long time.

Secondly, by comparing the two marine fog events, which did or not diffuse westerly across the coastline, the results explore that, under the condition of obvious weather systematic low-level circulation, sea wind is favorable for sea fog diffusing to inland. Without systematic low-level circulation transportation, intensity of thermal differences between sea and land and suitable land temperature will determine whether sea fog can cross the western coastline. In conclusion, the thermal differences and its intensity between sea and land jointly to the favorable systematic low-level atmospheric circulation determine the finely location of fog in the coastal zone area.