Occurrence of Fogover Surface Cold Patches in theYellow Sea

Abstract

The west coast of South Korea in the Yellow Sea is one of the foggiestplaces on Earth in the summer. However, marine sea fog remainsdifficult to predict. To this end, the Fog and Turbulence Interactions inthe Marine Atmosphere (FATIMA) Project launched a field campaigncoupled with on-shore modelling efforts between 20 June to 09 July2023 in the Yellow Sea off the coast of South Korea. The research vesselONNURI from the Korea Institute of Ocean Science and Technology(KIOST) was utilized, with a suite of instrumentation that probed overmultiple decades of scales of both atmospheric and oceanographicprocesses. A Vaisala FD70, that measures visibility as well as droplet type, wasimplemented to identify a fog episode that occurred 03 - 04 July. At00:00 July 3 UTC, visibility dropped to 160 meters (m) with a steadydecreasing trend to 73 m at 13:35 UTC. Visibility did not increase abovethe 1 km threshold until 03:37 UTC on July 4 . This event marked thelongest fog duration during the campaign and is considered to beIntense Operational Period (IOP) 5. In order to observe the behavior of the atmosphere and ocean duringthat time, the ship stayed stationary during the fog event. Measurementcycles were executed every 2 hours that included deploying a standardCTD (Conductivity, Temperature and Depth), a VMP (VerticalMicrostructure Profiler) and C-CAMS (an atmospheric measurementsuite from the Naval Postgraduate School). A Sea Snake from theNational Oceanic and Atmospheric Administration (NOAA) whichmeasures the sea surface temperature (SST) 24/7 was deployed off theside of the R/V ONNURI. With regard to oceanic conditions, the SST drastically dropped prior tothe visibility decreasing below 1 km. The SST dropped from 22.2°C at 20:55 UTC to 18.3°C at 21:09 UTC on 02 July. The temperature thenincreased for a time before decreasing again. A direct relationshipbetween the SST and visibility could be identified from the data, andthere was also evidence of turbulent mixing at the 70 to 80 m depthrange. Overall, there were indications of tidal mixing that resulted in coldpatches. Vaisala CL61 backscatter plots showed evidence of air advectionover the surface cold patch with could have caused the cold sea fogevent.