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A Case Study of Offshore Advection of Boundary Layer Rolls over a Stably Stratified Sea Surface
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (Vindenergi)ORCID iD: 0000-0003-2857-3700
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. (Vindenergi)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
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2017 (English)In: Advances in Meteorology, ISSN 1687-9309, E-ISSN 1687-9317, Vol. 2017, article id 9015891Article in journal (Refereed) Published
Abstract [en]

Streaky structures of narrow (8-9 km) high wind belts have been observed from SAR images above the Baltic Sea during stably stratified conditions with offshore winds from the southern parts of Sweden. Case studies using the WRF model and in situ aircraft observations indicate that the streaks originate from boundary layer rolls generated over the convective air above Swedish mainland, also supported by visual satellite images showing the typical signature cloud streets. The simulations indicate that the rolls are advected and maintained at least 30-80 km off the coast, in agreement with the streaks observed by the SAR images. During evening when the convective conditions over land diminish, the streaky structures over the sea are still seen in the horizontal wind field; however, the vertical component is close to zero. Thus advected feature from a land surface can affect the wind field considerably for long times and over large areas in coastal regions. Although boundary layer rolls are a well-studied feature, no previous study has presented results concerning their persistence during situations with advection to a strongly stratified boundary layer. Such conditions are commonly encountered during spring in coastal regions at high latitudes.

Place, publisher, year, edition, pages
2017. Vol. 2017, article id 9015891
Keyword [en]
boundary layer rolls, coastal transition, stable conditions
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
URN: urn:nbn:se:uu:diva-306939DOI: 10.1155/2017/9015891ISI: 000414082500001OAI: oai:DiVA.org:uu-306939DiVA, id: diva2:1044822
Projects
StandUp for Wind
Funder
National Supercomputer Centre (NSC), Sweden, 2015/1-224StandUp for Wind
Available from: 2016-11-07 Created: 2016-11-07 Last updated: 2018-04-12Bibliographically approved
In thesis
1. Wind and atmospheric stability characteristics over the Baltic Sea
Open this publication in new window or tab >>Wind and atmospheric stability characteristics over the Baltic Sea
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Abstract [sv]

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.

Place, publisher, year, edition, pages
Uppsala: Department of Earth Sciences, Uppsala University, 2016. p. 37
Keyword
coastal transition, stable conditions, boundary layer rolls, low level jet, WRF, Baltic Sea
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:uu:diva-306948 (URN)
Presentation
2016-12-02, Norrland 1, Villavägen 16, Uppsala, 17:15 (English)
Opponent
Supervisors
Projects
StandUp for Wind
Funder
StandUp for WindNational Supercomputer Centre (NSC), Sweden, 2015/1-224
Available from: 2017-02-07 Created: 2016-11-07 Last updated: 2017-02-07Bibliographically approved
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