The Earth's atmosphere is expected to warm in response to increasing atmospheric concentrations of greenhouse gases (GHG). The response of the Earth's complex and chaotic climate system to the GHG emissions is, however, difficult to assess. In this thesis, two issues of importance for the assessment of this response are studied. The first concerns the magnitude of the natural and anthropogenic emissions of CO2. An atmospheric transport model is used, combined with inventories of anthropogenic CO2 emissions and estimates of natural emissions, to compare modelled and observed variations in the concentration of CO2 at an Arctic monitoring site. The anthropogenic and natural emissions are shown to exert approximately equal influence on Arctic CO2 variations during winter. The primary focus of this thesis is the response of the climate system to the enhanced GHG forcing. It has been proposed that this response may project onto the leading modes of variability. In the present thesis, this hypothesis is tested against the alternative that the spatial patterns of variability change in response to the enhanced forcing. The response of the atmospheric circulation to the enhanced GHG forcing as simulated by a specific coupled global climate model (CGCM) is studied. The response projects strongly onto the leading modes of present-day variability. The spatial patterns of the leading modes are however changed in response to the enhanced GHG forcing. These changes in the spatial patterns are associated with a strengthening of the waveguide for barotropic Rossby waves in the Southern Hemisphere. The Northern Hemisphere waveguide is however unchanged. The magnitude of the global mean responses to an enhanced GHG forcing as simulated by CGCMs vary. Moreover, the regional responses vary considerably among CGCMs. In this thesis, it is hypothesised that the inter-CGCM differences in the spatial patterns of the response to the enhanced GHG forcing are partially explained by inter-CGCM differences in zonal-mean properties of the atmospheric flow. In order to isolate the effect of these differences in the zonal-mean background state from the effects of other sensitivities, a simplified model with idealised forcing is employed. The model used is a global three-level quasi-geostrophic model. The sensitivity of the stationary wave pattern (SWP) to changes in the zonal-mean wind and tropopause height of similar magnitude as those found in response to the enhanced GHG forcing in CGCMs is investigated. The SWP in the simplified model shows a sensitivity of comparable magnitude to the analogous response in CGCMs. These results indicate that the CGCM-simulated response is sensitive to relatively small differences in the zonal-mean background state. To assess the uncertainties in the regional response to the enhanced forcing associated with this sensitivity, ensemble simulations of climate change are of great importance.

The Earth's middle atmosphere at about 10-100 km has shown a substantial sensitivity to human activities. First, the ozone layer has been reduced since the the early 1980s due to man-made emissions of halogenated hydrocarbons. Second, the middle atmosphere has been identified as a region showing clear evidence of climate change due to increased emissions of greenhouse gases. While increased CO2 abundances are expected to lead to a warmer climate near the Earth's surface, observations show that the middle atmosphere has been cooling by up to 2-3 degrees per decade over the past few decades. This is partly due to CO2 increases and partly due to ozone depletion. Predicting the future development of the middle atmosphere is problematic because of strong feedbacks between temperature and ozone. Ozone absorbs solar ultraviolet radiation and thus warms middle atmosphere, and also, ozone chemistry is temperature dependent, so that temperature changes are modulated by ozone changes. This thesis examines the middle atmospheric response to a doubling of the atmospheric CO2 content using a coupled chemistry-climate model. The effects can be separated in the intrinsic CO2-induced radiative response, the radiative feedback through ozone changes and the response due to changes in the climate of the underlying atmosphere and surface. The results show, as expected, a substantial cooling throughout the middle atmosphere, mainly due to the radiative impact of the CO2 increase. Model simulations with and without coupled chemistry show that the ozone feedback reduces the temperature response by up to 40%. Further analyses show that the ozone changes are caused primarily by the temperature dependency of the reaction O+O2+M->O3+M. The impact of changes in the surface climate on the middle atmosphere is generally small. In particular, no noticeable change in upward propagating planetary wave flux from the lower atmosphere is found. The temperature response in the polar regions is non-robust and thus, for the model used here, polar ozone loss does not appear to be sensitive to climate change in the lower atmosphere as has been suggested recently. The large interannual variability in the polar regions suggests that simulations longer than 30 years will be necessary for further analysis of the effects in this region. The thesis also addresses the long-standing dilemma that models tend to underestimate the ozone concentration at altitudes 40-75 km, which has important implications for climate change studies in this region. A photochemical box model is used to examine the photochemical aspects of this problem. At 40-55 km, the model reproduces satellite observations to within 10%, thus showing a substantial reduction in the ozone deficit problem. At 60-75 km, however, the model underestimates the observations by up to 35%, suggesting a significant lack of understanding of the chemistry and radiation in this region.

The entry of satellite-derived decadal cloud datasets with homogeneous coverage in time and space enables studies not possible before. This thesis presents two such applications. The first study deals with cloud-aerosol processes and the second with an evaluation of cloud simulations from a regional climate model. The first part of the thesis describes the used satellite-derived dataset based on imagery from the Advanced Very High Resolution Radiometer (AVHRR) on the polar orbiting NOAA satellites. A method for cloud retrieval and the compilation of a 1991-2000 Scandinavian cloud climatology are described. The second part reveals an intriguing anti-correlation between monthly mean satellite-derived cloudiness and the concentration of the cosmogenetic isotope Beryllium-7 in near-surface aerosol samples for three measurement sites in Sweden. Large-scale transport processes are suggested as the most likely physical mechanism for this behaviour but more complex relations to cloud microphysical processes are not ruled out. The final part presents a thorough evaluation of cloud simulations of the SMHI Rossby Centre regional atmospheric model (RCA3). Several model-to-satellite adaptations are applied to avoid artificial biases of results. The study stresses the necessity to account for initial differences between observed and modelled clouds caused by satellite cloud detection limitations. Results show good agreement of modelled and observed cloud amounts while the vertical distribution of clouds appears largely different. RCA3 underestimates medium-level clouds while overestimating low- and high-level clouds. Also, the current use of the Maximum cloud overlap approach in the radiation scheme and an indicated excess of cloud condensate in modelled clouds appear to create excessive cloud optical thicknesses with serious implications for the surface radiation budget. Future applications are outlined based on greatly enhanced satellite-derived cloud and radiation budget datasets.

The boundary layer is the part of the atmosphere that is in direct contact with the ground via turbulent motion. At mid-latitudes the boundary layer is usually one or a few kilometers deep, while in the Arctic it is much more shallow, typically a few hundred meters or less. The reason is that here the absolute temperature increases in the lowest kilometer, making the boundary layer semi-permanently stably stratified. The exchange of heat, momentum and tracers between the atmosphere, ocean and ground under stable stratification is discussed from an observational, modeling and climate-change point of view. A compilation of six observational datasets, ordered by the Richardson number (rather than the widely used Monin-Obukhov length) reveals new information about turbulence in the very stably stratified regime. An essentially new turbulence closure model, based on the total turbulent energy concept and these observational datasets, is developed and tested against large-eddy simulations with promising results. The role of mesoscale motion in the exchange between the atmosphere and surface is investigated both for observations and in idealized model simulations. Finally, it is found that the stably stratified boundary layer is more sensitive to external surface forcing than its neutral and convective counterparts. It is speculated that this could be part of the explanation for the observed Arctic amplification of climate change.

Water vapour is the most important greenhouse gas and plays a fundamental role in the climate system and for the chemistry of the Earth's atmosphere. This thesis presents observations of water vapour in the middle atmosphere with a particular focus on the mesosphere. The majority of these observations presented in this thesis have been performed by the Swedish satellite Odin, providing global observations since 2001. Further observations come from the Hygrosonde-2 campaign in December 2001 based on balloon and rocket-borne measurements. A general overview of Odin's water vapour measurements in the middle atmosphere is given. The optimisation of the mesospheric water vapour retrieval is presented in detail. The analysis of the observations has focused mainly on different dynamical aspects utilising the characteristic of water vapour as a dynamical tracer in the middle atmosphere. One application is the mesospheric part of the semi-annual oscillation (SAO). The observations reveal that this oscillation is the dominant pattern of variability between 30°S and 10°N in the mesosphere up to an altitude of 80 km. Above 90 km the SAO is dominating at all latitudes in the tropics and subtropics. It is shown that the SAO exhibits a distinct phase change between 75 km and 80 km in the tropical region. This thesis also presents the first satellite observations of water vapour in the altitude range between 90 km and 110 km, extending the observational database up into the lower thermosphere. In the polar regions water vapour exhibits the annual maximum during winter time above 95 km, mainly caused by upwelling during this season. This behaviour is different from that observed in the subjacent part of the mesosphere where the annual maximum occurs during summer time. The Hygrosonde-2 campaign provided a high resolution measurement of water vapour in the vicinity of the polar vortex edge. This edge prevents horizontal transport causing different water vapour characteristics inside and outside the polar vortex. The observations show that this separating behaviour extends high up into the mesosphere. Small scale transitions in the Hygrosonde-2 profile between conditions inside and outside the vortex coincided with wind shears caused by gravity waves.

Referat Klimatologisk studie av cyklonbanor över Europa med koppling till snöförhållanden i norra Sverige Jakob Inghammar Under de senaste decennierna har temperaturen i atmosfären ökat. En sannolik effekt av detta är en förändring av förekomst och styrka för de utomtropiska cyklonerna. Deras uppträdande spelar en betydande roll för väder och klimat över de områden där de förekommer, därför är det relevant att undersöka om samvariationer för dem existerar med till exempel snöförhållanden och temperaturer. För att utforska detta har lågtryck över norra Atlanten och Europa identifierats ur återanalysdata (ERA-40) under månaderna oktober-mars för åren 1960-1999. Denna studie visar på en signifikant ökning av lågtrycksförekomst norr om 60°N och ett signifikant avtagande söder om 60°N. För lågtryckens styrka påvisas en signifikant positiv trend för båda dessa områden. Vid en analys för var och en av månaderna oktober-mars visas att det är under framförallt månaderna januari och februari som lågtrycken med tiden förflyttats norrut och att ingen sådan trend finns för oktober och november. Mellan årens högsta uppmätta snödjup i norra Sverige och lågtrycksfrekvensen kring norra Skandinavien finns en positiv korrelation. För medeltemperaturen på norra halvklotet och över vilka breddgrader cykloner mestadels rör sig under december-mars finns en samvariation, vid varmare förhållanden förflyttas cyklonbanorna norrut. En positiv korrelation finns mellan antal lågtryck norr om 60°N och temperaturen i Abisko. För vintrar, som i Abisko hade vitt skilda snöförhållanden, märks en avvikelse för vilka månader som lågtrycksfrekvensen kring norra Skandinavien var hög. Då snötäcket innehöll många isiga lager var lågtrycksfrekvensen hög i början och i slutet av vintern medan den var låg för månaderna i mitten. Det omvända skedde då snötäcket istället var ovanligt poröst. En stark samvariation finns mellan den nordatlantiska oscillationen (NAO) och lågtrycksfrekvens i de olika delarna norr och söder om 60°N av det undersökta området. Vid höga NAO-index ökar frekvensen i det norra området medan det minskar i det södra. Vid låga NAO-index sker det omvända.   Nyckelord: cykloner, lågtryck, återanalysdata, ERA-40, snö, Abisko / Abstract Climatologic study of cyclone tracks over Europe and linkage to snow conditions in northern Sweden Jakob Inghammar In the last decades the temperature in the atmosphere has been increasing. One plausible effect of this is a change in frequency and intensity of the extratropical cyclones. The appearance of the extratropical cyclones is crucial for the weather conditions and climate at the latitudes where they exist. Hence it is relevant to examine if the frequency of them are related to snow conditions and temperatures. This has been investigated over the north part of the Atlantic Ocean and Europe through detection of local minima in reanalysis data (ERA 40) regarding the sea level pressure for the months October-Mars for the years 1960-1999. The result for cyclone frequency display a significant increase in the region north of 60°N and at the same time a significant decrease in the region south of 60°N. In both regions there is a significant increase for the cyclone intensity. This trend with shifting cyclone tracks to the north is most pronounced for the months January and February while no trend can be seen for the months October and November. The maximum snow depth in the northern part of Sweden every year and the frequency of cyclones around northern Scandinavia are positively correlated. The mean temperature of the northern hemisphere and at which latitudes the cyclone tracks mostly exist during December-Mars co-varies. For the same months a positive correlation exists for the number of cyclones in the region north of 60°N and the mean temperature in Abisko. Different winters in Abisko with very diverse snow conditions also experienced diverseness concerning cyclone frequency around northern Scandinavia. During the winters when the snow cover was holding many icy layers; the frequency was high in the beginning and in the end of the winter seasons while the cyclone frequency was low in the middle. When the snow cover instead was very porous, the cyclone frequency occurs in the opposite way. A strong covariance exists between the North Atlantic Oscillation (NAO) and the number of cyclones in each region north and south of 60°N of the examined area. When the NAO-index is positive the frequency of cyclones is elevated in the north region and at the same time reduced in the south region, when the NAO-index is negative the opposite occur. Keywords: cyclones, reanalysis data, ERA-40, snow, Abisko

Vindkraft är ett miljövänligt energislag med stor utvecklingspotential, både i Sverige ochutomlands. I det här examensarbetet sammanfattas de viktigaste bitarna av kunskapen sombehövs för att förstå sig på vindkraft, med fokus på meteorologi och kommunikation medkonsultföretag i branschen. På grundval av litteratur i ämnet, konsultrapporter beställda av ettsvenskt företag i branschen och diskussion med experter ges här en grundläggande översikt imeteorologi för vindkraft, vilken innefattar kunskaper om klimatet i allmänhet ochvindklimatet i synnerhet, förståelse för luftens flöde, den geostrofiska vinden och hurtopografi och dylikt påverkar luftens strömning i gränsskiktet. Vidare diskuteras hur energin ivinden tas till vara, med en sammanfattning av viktiga tekniska detaljer; vindkraftsspecifikastorheter och uttryck, energiförluster och effekterna av att placera många vindkraftverk i engrupp; vakeffekten. En av de dominerande delarna av alla konsultrapporter som studerats ärhur man analyserar och behandlar vinddata för maximering av tillförlitlighet och relevans. Föratt en mätserie ska kunna användas för att förutsäga framtida vindar måste dennormalårskorrigeras, antingen med hjälp av andra mätserier, avancerade fysikaliska modellerav strömningen, eller både och. Osäkerheter uppkommer alltid, och en diskussion kring hur deolika konsultbolagen behandlar dem görs. Slutligen diskuteras olika sätt att förbättrakvaliteten på både beställning av rapport och slutprodukten från konsulten, utifrån ett antalfrågor ställda av ovan nämnda företag och författaren själv. Resultatet sammanfattas nedan.En mycket viktig del av utvecklingen står vindkraftprojekteringsföretag för. Företagen ibranschen behöver ha goda kunskaper i meteorologi för att bli framgångsrika. De måste ocksåkunna kommunicera sina kunskaper på ett klart och precist sätt. Företagen måste också kunnalära av andra i samma bransch och av andra med specialistkunskaper. För företag utan egnameteorologer anställda är det viktigt att kommunikationen och det ömsesidiga lärandet mellankonsult- och beställarbolag sker på ett professionellt, men samtidigt stöttande och lärande, sätt.Beställaren måste lära sig vad man ska beställa och vara tydlig med vad man kräver avkonsulten. Konsulten måste i sin tur vara tydlig med val av metod, tillvägagångssätt ochbegärande av rätt material som underlag för sina studier. En ny infallsvinkel som intediskuteras i rapporterna fast den kanske borde det är klimatförändringarnas påverkan på detlångsiktiga klimatet. Faktum är att trenden under 1900-talet inte visar någon större långsiktigförändring av medelvinden, men de årliga variationerna kan vara stora och tenderar att varierai cykler som skulle kunna utnyttjas för att maximera framgången för vindkraften. / Wind power is an environmentally friendly energy source with a considerable growthpotential, in Sweden as well as abroad. In this Bachelor’s thesis work, the most importantparts of the knowledge needed to understand wind power, is summarized. The focus is onmeteorology and communication with wind power consultants by a Swedish wind powercompany. With the basis of relevant literature, consultancy reports and discussion withsubject experts, is here provided a basic synopsis of wind power meteorology, which includesknowledge of the climate in general, and the wind climate specifically, understanding of thegeneral flow of the wind, the geostrophic wind in particular, and how topography and similarground features affect the boundary layer flow. Furthermore it is discussed how the windenergy content is harnessed, briefly going over some technical details, wind power specificquantities and expressions, energy losses and the effects of bundling wind power plants closetogether: wake effects. One of the dominant parts of all wind consultancy reports studied hereis how to analyze wind data series to maximize accuracy and relevance. A trustworthymeasuring series good enough to predict future wind energy content needs to be long termcorrected using other, longer measuring series as reference, or checked against an advancedphysically accurate mesoscale flow model, or both. Uncertainties are always a part of theequation, and a discussion over how the different wind power consultants treat theseuncertainties is made. Finally there is a discussion about different measures that can be madeto improve quality on both detailing orders for wind power consultancy reports and the finalproduct delivered to the client. The results are summarized in the next paragraph.Wind power developers are a major influence on how wind power research is shaped. A goodknowledge of meteorology is a key to success for wind power industry businesses.Communication with industry expert companies is also paramount. Clearly and precisely,companies need to learn from, and communicate with, other wind power enterprises, scientificexperts and researchers. Wind power companies without their own meteorologists have aneven bigger need of this professional contact with other businesses providing themeteorological expertise they need. Contacts need to be supportive and teaching. The clientneeds to learn what to order in detail and what they expect from the report. The providercompany needs to be detailed about their selection of methods and procedure. Consultantsshould be equally detailed in requisition of base data from the client. A point of view notpresent in the studied consult reports is climate change and how it affects the long term windclimate. As a matter of fact the Scandinavian 20th century average wind trend is neutral, butyear-to-year variations can be quite significant and tend to vary in cycles possible to exploitfor wind power success.

The value of an easy to install, accurate, far-reaching measurement instrument is easily understood when dealing with setting up and monitoring wind power parks. However the use of remote sensing equipment when doing this is a subject of much discussion amongst professional experts and scholars. This is especially the case when the environmental conditions are difficult, such as areas with complex terrain or cold climate (or both), for example forested, hilly or mountainous regions in north Europe and North America. The remote sensing technology SODAR provides for detailed 3-dimensional datasets, but need a skilled analyst to make sure the measurements are realistic. This thesis is aimed at verifying the accuracy of the SODAR wind measuring equipment at two potential wind power parks in Sweden. It has been done by comparing the wind speed, wind direction and turbulence measured by an AQ Systems AQ500 Wind Finder SODAR to the wind speed, wind direction and turbulence measured by in situ anemometers mounted in nearby meteorological masts. The comparison was made by calculation of statistical parameters such as correlation and root mean square error. The results of this investigation shows that the differences in the measured quantities are site specific and that i t‟s very difficult to distinguish between differences arising from ambient conditions, the measurement method and the method of calculating comparable values. It is clear, however, that conditions such as temperature, inhomogeneous fetch and wind speed are contributors to the observed discrepancies. When compensating for these various sources of error; such as eliminating data from malfunctioning equipment, icing conditions, mast wake and forest flow disturbance, the SODAR wind speed data shows a very high correlation with the mast anemometer data, giving a correlation coefficient of around 0.90 – 0.95. / Värdet av ett mätinstrument som är lätt att installera, noggrant och som kan mäta högt upp i troposfären är lätt att inse när man har att göra med att installera och övervaka vindkraftparker. Det har dock funnits en debatt om huruvida fjärranalytiska mätningar är tillräckliga för sådana ändamål i expert- och akademikerkretsar. Debatten spetsas ytterligare till då omgivningarna är svåra, exempelvis i områden med komplex terräng eller kallt klimat (eller både och), till exempel skogbeklädda, kulliga eller bergiga områden i norra Europa och Nordamerika som är aktuella för exploatering av vindkraft. Det fjärranalytiska redskapet SODAR förser exploatören med detaljerade tredimensionella datasamlingar, men kräver en erfaren och skicklig analytiker för att verkligen verifiera att mätresultaten är realistiska och tillförlitliga. Detta examensarbete har som mål att verifiera noggrannheten hos SODAR-utrustning vid två potentiella vindkraftparker i Sverige. Det har gjorts genom att jämföra vindhastighet, vindriktning och turbulens mätt med en AQ Systems AQ500 Wind Finder SODAR med vindhastighet, vindriktning och turbulens mätt med anemometrar i närbelägna meteorologiska mätmaster. Jämförelsen gjordes genom att beräkna statistiska parametrar som korrelation och standardavvikelse. Resultatet av undersökningen visar att skillnaderna i de uppmätta storheterna är specifika för varje mätplats och att det är mycket svårt att särskilja bidragen till skillnaderna mellan omgivande förhållanden, mätmetoder och beräkningsmetoder. Det står dock klart att förhållanden så som temperatur, inhomogen omgivande skrovlighet på anloppssträckan och vindhastigheten i sig själv bidrar till de observerade skillnaderna i mätresultat. Kompenseras felaktig och störd data bort så visar vindhastigheten mätt med SODAR en mycket god korrelation med mastanemometerdata, med en korrelationskoefficient omkring 0,90 – 0,95.

Long time average wind conditions at potential wind turbine sites are of great importance when deciding if an investment will be economically safe. Wind climate estimates such as these are traditionally done with in situ measurements for a number of months. During recent years, a wind climate database has been developed at the Department of Earth Sciences, Meteorology at Uppsala University. The database is based on model runs with the higher order closure mesoscale MIUU-model in combination with long term statistics of the geostrophic wind, and is now used as a complement to in situ measurements, hence speeding up the process of turbine siting. With this background, a study has been made investigating how well actual power productions during the years 2004-2006 from 21 Swedish wind turbines correlate with theoretically derived power productions for the corresponding sites. When comparing theoretically derived power productions based on long term statistics with measurements from a shorter time period, correction is necessary to be able to make relevant comparisons. This normal year correction is a main focus, and a number of different wind energy indices which are used for this purpose are evaluated. Two publicly available (Swedish and Danish Wind Index) and one derived theoretically from physical relationships and NCEP/NCAR reanalysis data (Geostrophic Wind Index). Initial testing suggests in some cases very different results when correcting with the three indices and further investigation is necessary. An evaluation of the Geostrophic Wind Index is made with the use of in situ measurements. When correcting measurement periods limited in time to a long term average, a larger statistical dispersion is expected with shorter measurement periods, decreasing with longer periods. In order to investigate this assumption, a wind speed measurement dataset of 7 years were corrected with the Geostrophic Wind Index, simulating a number of hypothetical measurement periods of various lengths. When normal year correcting a measurement period of specific length, the statistical dispersion decreases significantly during the first 10 months. A reduction to about half the initial statistical dispersion can be seen after just 5 months of measurements. Results show that the theoretical normal year corrected power productions in general are around 15-20% lower than expected. A probable explanation for the larger part of this bias is serious problems with the reported time-not-in-operation for wind turbines in official power production statistics. This makes it impossible to compare actual power production with theoretically derived without more detailed information. The theoretically derived Geostrophic Wind Index correlates well to measurements, however a theoretically expected cubed relationship of wind speed seem to account for the total energy of the wind. Such an amount of energy can not be absorbed by the wind turbines when wind speed conditions are a lot higher than normal. / Vindklimatet vid tänkbara platser för uppförande av vindkraftverk är avgörande när det beslutas huruvida det är en lämplig placering eller ej. Bedömning av vindklimatet görs vanligtvis genom vindmätningar på plats under ett antal månader. Under de senaste åren har en vindkarteringsdatabas utvecklats vid Institutionen för Geovetenskaper, Meteorologi vid Uppsala universitet. Databasen baseras på modellkörningar av en högre ordningens mesoskale-modell, MIUU-modellen, i kombination med klimatologisk statistik för den geostrofiska vinden. Denna används numera som komplement till vindmätningar på plats, vilket snabbar upp bedömningen av lämpliga platser. Mot denna bakgrund har en studie genomförts som undersöker hur bra faktisk energiproduktion under åren 2004-2006 från 21 vindkraftverk stämmer överens med teoretiskt härledd förväntad energiproduktion för motsvarande platser. Om teoretiskt härledd energiproduktion baserad på långtidsstatistik ska jämföras med mätningar från en kortare tidsperiod måste korrektion ske för att kunna göra relevanta jämförelser. Denna normalårskorrektion genomförs med hjälp av olika vindenergiindex. En utvärdering av de som finns allmänt tillgängliga (Svenskt vindindex och Danskt vindindex) och ett som härletts teoretiskt från fysikaliska samband och NCEP/NCAR återanalysdata (Geostrofiskt vindindex) görs. Inledande tester antyder att man får varierande resultat med de tre indexen och en djupare utvärdering genomförs, framförallt av det Geostrofiska vindindexet där vindmätningar används för att söka verifiera dess giltighet. När kortare tidsbegränsade mätperioder korrigeras till ett långtidsmedelvärde förväntas en större statistisk spridning vid kortare mätperioder, minskande med ökande mätlängd. För att undersöka detta antagande används 7 års vindmätningar som korrigeras med det Geostrofiska vindindexet. I detta simuleras ett antal hypotetiskt tänkta mätperioder av olika längd. När en mätperiod av specifik längd normalårskorrigeras minskar den statistiska spridningen kraftigt under de första 10 månaderna. En halvering av den inledande statistiska spridningen kan ses efter endast 5 månaders mätningar. Resultaten visar att teoretiskt härledd normalårskorrigerad energiproduktion generellt är ungefär 15-20% lägre än väntat. En trolig förklaring till merparten av denna skillnad är allvarliga problem med rapporterad hindertid för vindkraftverk i den officiella statistiken. Något som gör det omöjligt att jämföra faktisk energiproduktion med teoretiskt härledd utan mer detaljerad information. Det teoretiskt härledda Geostrofiska vindindexet stämmer väl överens med vindmätningar. Ett teoretiskt förväntat förhållande där energi är proportionellt mot kuben av vindhastigheten visar sig rimligen ta hänsyn till den totala energin i vinden. En sådan energimängd kan inte tas till vara av vindkraftverk när vindhastighetsförhållandena är avsevärt högre än de normala.

In this study the flux attenuation due to sensor displacement has been investigated over sea using an extensive set of data from the "Ocean Horizontal Array Turbulence Study". All previous investigations of the flux attenuation have been performed over land. A function developed for correcting fluxes in the homogenous surface layer was compared to measured flux attenuation. This investigation revealed the possibility to find new functions describing the flux attenuation when measurements are carried out over sea. From the measured flux attenuation studied here a change in the form of correction functions was required to improve the estimated flux loss. The most significant difference found in this report compared to the previous landbased study Horst (2006) is for stable conditions, where significantly less flux loss is found over sea. Two new functions describing the attenuation due to sensor displacement over sea have been constructed. One of these expressions has a discontinuity at z/L = 0. This is supported by measured flux attenuation. A reasonable interpretation is; however, that this discontinuity is caused by two separate turbulence regimes near neutrality on the stable and unstable side respectively. The discontinuity is thus not believed to be an effect merely of stability. A second correction function which is continuous over all stabilities has therefore also been constructed. These two functions and the correction function from Horst (2006) have been compared to measured flux loss. Based on this comparison the continuous correction function is recommended for correcting scalar fluxes measured over sea. It should be noted, however, that this expression only describes the mean attenuation and has been constructed from measurements at 5 and 5.5 m above mean sea level. The theoretical basis used in the development of the function for flux attenuation over land allows for a direct link between a spectral shape and the attenuation expression. This link has been preserved for the new expressions presented in this report. The spectral shape corresponding to the continuous correction function has been compared to measured mean cospectra and also to the cospectra from Horst (2006) corresponding to crosswind displacements. At a height of 10 m and a sensor displacement of 0.2 m the mean flux attenuation is about 1.3-4% in the stability interval −1 < z/L < 1.5 when using the new correction functions presented in this report.