Thermally driven mesoscale flows — simulations and measurements

The wind climate shows often large local variations in coastal areas. The present study was based on measurements at several sites on the island of Gotland in the Baltic Sea and on simulations with a numerical meso-gamma-scale atmosphere model (the MIUU model). It is common for thermally driven flows, e.g. sea breezes and low level jets, to evolve resulting in supergeostrophic winds. The most important parameters affecting these flows were found to be temperature and roughness differences between the land and sea. Although the wind speed increases climatologically over a see, a case where the wind speed decreased when advected out over a cold sea was investigated. This happens if the stable boundary layer (SBL) over the sea is deep enough. A deep SBL is favoured by strong thermal winds in combination with large temperature differences between the land and sea. With a shallow SBL the wind speed increases over the sea.