Influence of fog on stratification and turbulent fluxes over the ocean

Sammanfattning: In this thesis a case of advection fog over the Baltic Sea is studied. The period examined is from June 5th to 7th 1995. Data is taken from the instrumented mast, situated on the island Östergarnsholm, a small and flat island without trees outside of Gotland. From the measurements among others the heat flux, relative humidity and temperature are analyzed. In the evening June 5th 1995 the fog is advected in over Östergarnsholm. This can both be seen from the increasing relative humidity and the decreasing temperature. Before the fog arrived the boundary layer was stably stratified. This stratification quickly changed to neutral as the fog reaches Östergarnsholm. After careful evaluation the neutral stratification is shown not to be neutral at all. The stratification closest to the ground up to 15 meters is unstable and above the stratification is stable. From this the conclusion is made that the fog is low only 15 meters high during this period. At noon June 6th the air temperature decreases dramatically below the sea surface temperature at the same time as the relative humidity increases up to 100%. The fog is now thick enough to have most of the outgoing radiation coming from the top, which decreases the temperature a few degrees. As the stability is investigated it shows unstable stratification up to the highest level (28 meters). The assumption is made that the fog is at least 30 meters deep. Also the normalized standard deviations for temperature and vertical velocity are examined to find out if they behave as the variation in the undisturbed boundary layer.2