Distribution characteristics of the fog derived from the MTSAT satellite data in Zhejiang Province and its adjacent sea area

Fog is a common weather phenomenon, and is also a hazardous weather which is one of main causes of sea and air traffic accidents. So far, the climatological distribution of fog was still a lack of detailed understanding in Zhejiang and its adjacent sea regions. The spatial and temporal distribution of fog acquired from the long time series, is beneficial to monitor the fog formation, evolution and dissipation, and it is significant to the fog distribution and forecast. The fog information of Zhejiang Province and its adjacent sea regions was acquired by using the MTSAT geostationary satellite and weather stations whose data covered the period ranging from 2008 to 2012. Moreover, the fog products on hour scales with 0.05 °× 0.05 ° grid resolution were worked out. The results showed that the method of fog identification which classified the threshold of solar altitude with the Normalized Fog Index (NDFI) of the MTSAT data is well suitable for the fog monitoring in the study area. The accuracy of fog identification was nearly 75% that could meet the operational using demand. In the Zhejiang land, the average annual cumulative hour of fog occurrence were about 411.7 h which accounts for 4.7% of the whole year time. The spatial distribution of fog in the Zhejiang land shows that fog occurring frequency in the south area was more than that in the north area, and the frequency in the mountainous area was higher than that in the plain. The south and west mountains, some islands of Zhoushan and Wenzhou were the high-risk areas of the Zhejiang land. In time scale, the fog of the Zhejiang land was more in autumn and winter and less in spring and summer, and the time ranging from 22:00 BT to 09:00 BT was the peak of fog occurring. The spatial distribution of fog in the sea surrounding Zhejiang indicated that sea fog mainly occurs in the coastal area, presenting the pattern in which the fog occurring frequency was decreasing from coastal to offshore. The average annual cumulative hour of fog occurrence in the East China Sea (ECS) were about 311.7 h, and fog occurring frequency was the highest in the southwest of the area. Sea fog of Zhejiang more often occurs in the Wenzhou coastal zone and the Qiantang River estuary, and it had a distinct seasonal characteristic that more fogs in spring and summer, less fogs in autumn and winter. Comparing to land fog, sea fog was not easy to dissipate and can last a longer time because it is the advection fog. Arranging the sea zones by fog occurring frequency, the descending sequence was the southeastern part of ECS, the oceans east of Taiwan, Mideastern part of ECS, southwestern part of Yellow Sea, the midwestern part of ECS, and Taiwan, northwestern part of ECS, southwestern part of Yellow Sea and northeastern part of ECS.