In recent years there has been an increase in offshore wind energy, which poses the need for accurate wind speed estimates in the marine environment, especially in coastal areas where most wind turbines will be placed. This thesis is focused on the Baltic Sea, which is a small, semi-enclosed sea where land-sea interaction play an important role in explaining the wind patterns. Mesoscale model simulations can be used to study the marine environment, where observations are often scarce. In this thesis the Weather Research and Forecasting (WRF) model is used. In the first study simulations show that stable stratification over sea is very common in spring and summer and is associated with an increase in low-level jet occurrence and increased wind shear below 200 m, at heights where wind turbines are erected. The model performance in stable conditions is evaluated against aircraft measurements using several boundary layer parametrization schemes, and it is shown that the low-level jet height and strength is not accurately captured with any of the parametrizations. In the second study the advection of land features is investigated. From simulations, aircraft observations and satellite images it is shown that boundary layer rolls are created in the convective boundary layer over land, and advected several tens of kilometres out over sea surface, despite the stable stratification, where convective turbulence dissipates quickly. The occurrence of boundary layer rolls gives rise to horizontal wind speed variations of several meters per second over distances of kilometres, which can increase the uncertainty of short term wind speed forecasts in coastal areas with offshore flow. It is shown that mesoscale processes in and above the marine boundary layer are important in modifying the wind field in distances of at least 100 km from the coast and that models still need to be improved in order to capture these conditions. / Under de senaste decennierna har vindkraftsutbyggnaden ökat till havs, vilket innebär att det krävs tillförlitliga vindhastighetsuppskattningar över hav, särskilt i kustområden, där de flesta vindturbinerna kommer att placeras. Den här avhandlingen kommer att fokusera på Östersjön, vilket är ett relativt litet hav omgivet av landmassor, där land-hav-interaktion har en stor påverkan på vindmönstren över havet. Mesoskaliga modeller kan användas för att studera den lägre delen av den marina atmosfären. I den här avhandlingen används modellen "Weather Research and Forecasting" (WRF). I den första studien visar simuleringar att stabila förhållanden över havet är vanligt förekommande under sommar och vår, och sker i samband med en ökad förekomst av vindmaxima på låg höjd och ökad vindskjuvning under 200 m - alltså på höjder där vindkraftverk förekommer. Modellen, med flertalet gränskiktsparametriseringar, utvärderas för stabila fall mot flygplansmätningar, och resultaten visar att styrkan och höjden för vindmaxima är antingen över- eller underskattade oavsett parametrisering. I den andra studien undersöks advektiva fenomen. Simuleringar, flyplansobservationer och satellitbilder visar att avlånga rullvirvlar uppstår i det konvektiva gränsskiktet över land och advekteras flera tiotals kilometer över hav ut från kusten. Detta trots stabila förhållanden över havet, där den konvektiva turbulensen snabbt dör ut. Rullvirvlarna ger upphov till variationer i horisontell vindhastighet på flera meter per sekund över avstånd på några kilometer, vilket kan öka osäkerheten hos korttidsprognoser för vind när det är blåser från land. Sammanfattningsvis har det har visats att mesoskaliga processer i och ovanför det marina gränsskiktet har en stor inverkan på vindfältet åtminstone 100 km från kusten, och modeller behöver fortfarande förbättras för att kunna fånga dessa företeelser. / StandUp for Wind

Boundary layer rolls are a rather frequent phenomena, where regions of alternating up- and downdraft motion causes clouds to form in elongated, parallel rows oriented with the mean wind direction. The clouds can be seen during certain atmospheric conditions and are often called ”cloud streets” because of their characteristic appearance. By performing a linear instability analysis, the underlying mechanisms causing the onset of boundary layer rolls has been analysed in this study. There are two governing mech- anisms that cause the boundary layer rolls to form, the thermal instability and the dynamic instability. The thermal instability is caused by convection in an unstable airmass, while the dynamic instability usually is associated with neutral or stable conditions. The dynamic instability arise due to an inflection point in the wind profile, around which eddies develop. In a previous study by Svensson et al. (2017), rolls were observed over the Swedish east-coast, stretching out over sea during four days; 2 of May 1997, 3 of May 1997, 17 of May 2011 and 25 of May 2011. The aim of this study is to simulate the rolls on these four dates, analyse the underlying mechanisms and establish what type of instability that primarily causes the rolls to form. The linear stability analysis performed in this study indicate that the dynamic instability is the main mechanism giving rise to the rolls on all four studied dates. The rolls are found to arise over the Swedish mainland and are advected out over the sea. Both the orientation of the rolls and the modeled wind direction are in accordance with the observations. A qualitative agreement is found for the wavelength, the amplitude and the altitude of the rolls, when comparing the results of this study with the observations.