Study Of Daytime Sea Fog And Atmospheric Visibility Over The Yellow Sea Based On GOCI Satellite Data

Sea fog detection is of great scientific significance and practical value for marine development,shipping safety and meteorological research.With the increasingly frequent sea activities,there is an urgent need for high spatiotemporal resolution remote sensing sea fog products.Sea fog is most frequently observed over the Yellow Sea in the adjacent sea of China.Based on GOCI(Geostationary Ocean Color Imager)satellite data,coastal sounding data and buoy meteorological data,a new daytime sea fog detection algorithm has been developed over the Yellow Sea in this paper.Applying this algorithm for long time series satellite remote sensing data(2012-2016),the temporal and spatial distribution and its changing characteristics of sea fog over the Yellow Sea was analyzed.After having a certain understanding of sea fog,the atmospheric horizontal visibility was discussed and a remote sensing algorithm based on GOCI was established subsequently.Lastly,the temporal and spatial distribution of atmospheric horizontal visibility was analyzed.Some significant results are revealed as follows:(1)Based on spectral analysis,differences in spectral characteristics were found over different underlying surfaces,which includes land,clean-sky sea surfaces,middle/high level clouds,low stratus clouds and sea fog.Due to the theory that the middle/high level clouds have very high reflectivity in the visible bands,the middle/high level cloud deduction index was established,which can effectively separate stratus/sea fog from middle/high clouds to some extent.Then differences was found based on the statistical results of water vapor content between sea fog and low stratus,and the water vapor absorption ability of sea fog was different from low stratus at band 745nm and 680nm.Based on this,the separation index of sea fog and low stratus was established,which can realize the separation of sea fog and low status to a certain degree.The development of high turbidity water identification index can reduce the false judgement from high turbidity water as sea fog.The remote sensing algorithm was verified using coastal sounding data,buoys data and monitoring reports released by the National Meteorological Centre,which demonstrated the algorithm had the ability to detect sea fog,and the POD of this algorithm was 72.2%.(2)Based on results from the sea fog retrieval algorithm,a coupling remote sensing atmospheric horizontal visibility algorithm was established.According to sea fog pixel,fog albedo and solar zenith angle was used to calculate the visibility of fog area;meanwhile,the clean-sky pixel visibility was retrieved based on GOCI aerosol optical depth data.The algorithm was tested by buoys visibility data,the overall accuracy is 76.7%and the Kappa index was calculated as 68%,which showed the algorithm had the ability to retrieve visibility to a certain degree.(3)Due to the high spatiotemporal resolution,a new definition of sea fog frequency(SFF)was developed.After analyzing the sea fog results from 2012-2016,the interannual/seasonal/daily variability of sea fog frequency over the Yellow Sea was found.During the study period.2012 and 2014 was high SFF year,2013 and 2015 was low SFF year.Besides,influenced by the strong El Nino evert,the SFF of 2016 was significant decreased.The fog season is from April to July and the high SFF areas is distributed along the west of the Gulf of Vietnam to Baengnyeongdo Island,southeast coast of Shandong Peninsula,Jiangsu coastal area,the mid-east region of the Yellow Sea and east of Yangtze River estuary.The highest appealed in June over the east of Yangzte River estuary.According to the diurnal variation,the highest SFF during the day in fog season is from 08 to 10(Beijing Time),the lowest is from 13 to 14 and gradually increase from 15 again.The factors affecting the sea fog include the monsoon circulation,a certain sea air temperature difference,a specific wind direction and the appropriate wind speed and ocean current.(4)The temporal and spatial distribution of atmospheric horizontal visibility over the Yellow Sea was analyzed.The visibility value of offshore is larger than nearshore,and the western part of the Yellow is higher than the eastern part.In addition,the central part is smaller than the northern and southern part of the Yellow Sea.The seasonal average of visibility in the summer reached a maximum of 13.6km and a minimum of 5.7km in winter,and spring's was about 7.8km and 8.8km in autumn.Besides,the atmospheric visibility's highest month appeared in Auguest(17.5km)and lowest in February(5.2km).The diurnal variation has no significantly tendency,most area showed the lowest visibility appeared from 08 to 09 and the peak was at 15(Beijing Time).