Mesoscale Processes over the Baltic Sea

Abstract: The objective of this thesis is to study mesoscale processes above the Baltic Sea, which is a small, semi-enclosed sea where land-sea interaction may have a large impact on the offshore conditions. It is only the last tens of years that offshore research has become more popular, and one reason for this is the increasing offshore wind energy, which poses the need for accurate estimates of wind speed and turbulence conditions in the marine environment. In this thesis a range of mesoscale processes over the Baltic Sea are studied using the mesoscale model WRF (Weather Research and Forecasting) and different types of measurements.It is found that mesoscale effects are largest during spring and summer, when stable conditions dominate. The whole Baltic sea surface is affected by warm-air advection and low-level jets. There is very little spatial variation in seasonally averaged fields, which shows that the extent of mesoscale effects is several hundred kilometres. Wind speed and temperature profiles can thus not be described solely by the local conditions even far out over sea surface. Sea breezes are relatively unimportant for modyfing the seasonally averaged wind fields, but other types of low-level jets have a large influence. Results show that most of the low-level jets are likely created by inertial oscillations initiated when air flows across a coastline from the convective land surface to stable sea surface.Evaluation of the model shows that the discrepancies are largest during very stable conditions, but are also quite large during very unstable. The reasons for this are discussed.Several cases of boundary layer rolls are investigated using measurements and simulations and it is found that the rolls are likely created over the convective mainland and advected out over the stable sea surface, which may be a new finding that has not been reported before.