Browsing by discipline "Meteorology"
Now showing items 81-93 of 93
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(2015)This study aims to connect air-sea ice turbulent carbon dioxide (CO2) exchange and the surface energy balance over melting fjord ice. Recent studies have shown that sea ice melt might act as a significant CO2 sink in Arctic waters. Melt process have been suggested to dilute both brine and surface water partial pressures of carbon dioxide ( pCO2). Also biological activity and carbonate chemistry changes the air-ocean CO2 concentration gradient. Even small fluxes might be potentially significant as the maximum sea ice extent covers approximately 7 % of Earth's surface. As multi-year ice diminishes with the on-going climate change a bigger portion of the ice cover will experience melt in the summer season and thus the melt induced changes on the carbon cycling in the Arctic will have a greater effect. Surface energy balance consists of net radiation, turbulent fluxes of latent and sensible heat and conductive heat flux. During melt the sea ice surface transforms from a dry snow cover to melt ponds. Surface melt leads to a decrease in the surface albedo controlling the surface energy balance. Sea ice temperature affects both air-ice-ocean energy exchange and the permeability of the ice. My thesis is based on a 30 day measurement campaign in June 2014 from The Young Sound fjord, in North-East Greenland (74° 18' N, 20° 13' W). Turbulent fluxes of CO2 and H2O were measured with 10 Hz with two open-path infrared gas-analysers and two sonic anemometers at approximately 3 m height. One mast was used to measure basic meteorology (temperature, humidity, radiation, wind). Continuous measurements of surface water pCO2 were made 2.5 m below the ice. Conductive heat flux was determined from ice cores. The turbulent fluxes were calculated with the Eddy Covariance-methodology. Weak winds decreased the number of good quality measurements and created gaps in the time series. The measured CO2 flux ranged between 1.92 ja -3.2 µmol m-2 s-1 (positive fluxes being efflux) and sea ice was a net sink during the campaign. Sea ice temperature rose steadily with time, being above the permeability threshold in all measurements. Surface water pCO2 was lower than the atmospheric throughout the study and the under saturation grew from 50 µatm to 140 µatm in 10 days (13. to 23. June). Despite the strong pCO2 difference indicating ocean sink the CO2 absorption at the ice surface did not show an increase over time. A possible explanation for this is the equilibration of the air-melt pond pCO2 gradient. The strongest CO2 uptake was recorded as the melt ponds formed. Two strong efflux events ( flux > 1 µmol m-2 s-1) were recorded on June 11 and 18. No strong correlations were found between the surface energy budget and the air-surface CO2 exchange. However, the CO2 flux has a weak positive correlation both with wind speed and net radiation. The weak correlations are probably explained by the co-existence of different processes simultaneously affecting the air-ice-ocean CO2 gradients. The results support the hypothesis of melting sea ice acting as a sink for atmospheric CO2. The measurement quality could be improved by using a closed-path gas analyser. Despite the challenges Eddy Covariance is currently the state-of-art methodology for measuring ecosystem-scale turbulent exchange.
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(Helsingin yliopistoUniversity of HelsinkiHelsingfors universitet, 2003)
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(2017)Ukkosen voimakkuuden luokittelusta on tehty vain vähän tutkimusta. Rajuilman voimakuutta voidaan määritellä kaikkien siihen liittyvien sääilmiöiden (rankkasade ja rakeet, salamointi, syöksyvirtaukset, trombit) perusteella. Usein voimakkuusluokitus tehdään ilmiön aiheuttamien tuhojen perusteella (syöksyvirtaukset, trombit), joka vaatii ihmishavainnot tuhoalueelta: tällainen luokitus ei ole reaaliaikainen. Ukkosen tapauksessa mielekäs ilmiö voimakkuuden luokittelemiseksi on salamointi. Salamoinnin perusteella on tehty aiemminkin ukkosen voimakkuusluokitusta, mutta luokitukset perustuvat salamoiden vuosi- tai vuorokausimääriin. Hetkellisen ukkosen voimakkuuden määrittelemiseksi on tässä työssä tutkittu maasalamamääriä Suomen alueella 15 minuutin aikana 20 x 20 km havaintoalueilla. Tämän työn tarkoitus oli selvittää, että voidaanko salamahavaintojen perusteella tehdä objektiivinen luokitus ukkosen hetkelliselle voimakkuudelle, sekä mikä on järkevä luokkajako voimakkuudelle. Tutkimus toteutettiin käymällä läpi NORDLIS-salamanpaikannusverkon havainnot vuosilta 2002–2016. Havaintoaineistosta lasketaan maasalamamäärät 15 minuutin aika-askelissa 2 584 hilaruudussa Suomen alueella ja sen lähiympäristössä. Havaitut salamamäärät vaihtelivat välillä 1–325 salamaa 400 km^-2 15 min^-1. Saaduista tuloksista lasketaan ukkosen hetkellisen salamoinnin todennäköisyysjakauma. Tämän työn tulosten pohjalta on määritetty ukkosen hetkelliselle voimakkuudelle logaritmisesti tasavälinen, 5-portainen luokitus, jossa alin luokka L1 (1–3 maasalamaa 400 km^2 15 min^-1) kuvaa heikkoa ukkosta ja ylin luokka L5 ( >100 maasalamaa 400 km^2 15 min^-1 ) erittäin rajua ukkosta. Määritettyä ukkosen hetkellisen voimakkuuden luokitusta voidaan käyttää Suomen oloja vastaavilla alueilla, mikäli salamahavaintoja saadaan riittävällä tarkkuudella. Alueilla, joilla ukkoset ovat selvästi Suomen oloja voimakkaampia tämä luokitus antaa hetkelliselle ukkosen voimakkuudelle luultavasti liian alhaisia arvoja.
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(2017)This thesis is a study of the uncertainties related to the eddy covariance measurement technique on a forest ecosystem that is located in Hyytiälä, Southern Finland. The aim of this study is to analyze carbon dioxide and energy fluxes measured at two vertically displaced eddy covariance set-ups. In particular, to determine if the observed deviations between the set-ups could be linked with micrometeorological or biological variations or if they are resulted just by the stochastic nature of turbulence. The magnitude of uncertainties linked to eddy covariance technique are still under discussion and this thesis attempts to shed a light on these questions. The analysis is done to half hourly mean flux and meteorological data that was measured at the Hyytiälä SMEAR II –site in 2015 at the heights of 23.3 m and 33.0 m. Monthly, diurnal and cumulative variations of the fluxes are analyzed. A footprint model is used to analyze the flux correlation with the underlying vegetation. The flux dependence on atmospheric stability is also determined. The analysis shows that the annual cumulative difference of net ecosystem exchange (CO_2 exchange) between the two measurement heights is estimated to be 49 gC m^(-2) year^(-1) (17 % difference). The annual cumulative evapotranspiration difference is estimated to be 105 mm (29 % difference). There are no significant differences between the sensible heat fluxes. The difference between the measurement heights does not seem to influence significantly the flux estimations made with the eddy covariance method. However, the measurement results for latent heat flux acquired from the 33.0 m set-up are continuously smaller than those of the 23.3 m set-up.
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(2015)A semi-automatic method for detecting the tops of the mixed layer in day time and the stable and residual layers in night time is presented. Automatic algorithms to detect gradients in the ceilometer data are utilized, in combination with a stability criteria, provided by an eddy covariance system as well as manual layer detection and quality control. The observations were carried out at Welgegund, a regional background site on the South African savannah. One year of observations was analysed, and the method is shown to work well considering existing knowledge of the continental boundary layer structure and previous observations in southern Africa. Despite having some limitations, the method provided notably high data coverage. The frequency at which each layer was detected showed an annual cycle being lowest in the summer and highest in the winter for all the three layers studied, combined with a diurnal cycle with day time providing lower coverage. A clear diurnal cycle of the boundary layer evolution was observed, however the average heights of the tops of different types of layers showed modest or non-existing annual variation. The day-to-day variation was profound. The strongest seasonal characteristic was present in the summer, when occasional deep convective layers were observed increasing the variability of the mixed layer top compared with other seasons. The effects of conditional sampling were tested by separating the observations in five data sets based on weather conditions and the applicability of the method, and various reasons with potential of causing bias in the results are discussed. The result underlines the need for representative observations of all conditions wished to be included in the study. Some examples of the implications of boundary layer structure on particle concentration are considered in explaining phenomena observed in particle number distribution measurements.
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(Helsingin yliopistoHelsingfors universitetUniversity of Helsinki, 2009)Inadvertent climate modification has led to an increase in urban temperatures compared to the surrounding rural area. The main reason for the temperature rise is the altered energy portioning of input net radiation to heat storage and sensible and latent heat fluxes in addition to the anthropogenic heat flux. The heat storage flux and anthropogenic heat flux have not yet been determined for Helsinki and they are not directly measurable. To the contrary, turbulent fluxes of sensible and latent heat in addition to net radiation can be measured, and the anthropogenic heat flux together with the heat storage flux can be solved as a residual. As a result, all inaccuracies in the determination of the energy balance components propagate to the residual term and special attention must be paid to the accurate determination of the components. One cause of error in the turbulent fluxes is the fluctuation attenuation at high frequencies which can be accounted for by high frequency spectral corrections. The aim of this study is twofold: to assess the relevance of high frequency corrections to water vapor fluxes and to assess the temporal variation of the energy fluxes. Turbulent fluxes of sensible and latent heat have been measured at SMEAR III station, Helsinki, since December 2005 using the eddy covariance technique. In addition, net radiation measurements have been ongoing since July 2007. The used calculation methods in this study consist of widely accepted eddy covariance data post processing methods in addition to Fourier and wavelet analysis. The high frequency spectral correction using the traditional transfer function method is highly dependent on relative humidity and has an 11% effect on the latent heat flux. This method is based on an assumption of spectral similarity which is shown not to be valid. A new correction method using wavelet analysis is thus initialized and it seems to account for the high frequency variation deficit. Anyhow, the resulting wavelet correction remains minimal in contrast to the traditional transfer function correction. The energy fluxes exhibit a behavior characteristic for urban environments: the energy input is channeled to sensible heat as latent heat flux is restricted by water availability. The monthly mean residual of the energy balance ranges from 30 Wm-2 in summer to -35 Wm-2 in winter meaning a heat storage to the ground during summer. Furthermore, the anthropogenic heat flux is approximated to be 50 Wm-2 during winter when residential heating is important.
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(2018)Ilmakehän aerosolihiukkasilla on merkittäviä vaikutuksia ilmastoon ja ihmisten terveyteen. Suuri osa näistä aerosolihiukkasista on peräisin uusien pienhiukkasten muodostumistapahtumista eli NPF-tapahtumista (New Particle Formation). NPF-tapahtumissa ilmakehän höyryistä muodostuu pienhiukkasia, jotka alkavat kasvaa. Tämä tutkielma käsittelee läntisessä Saudi-Arabiassa tehdyissä aerosolimittauksissa havaittuja NPF-tapahtumia. Tarkoituksena on määrittää NPF-tapahtumien yleisyys sekä näiden voimakkuutta ja siten ilmasto- ja terveysvaikutuksellista potentiaalia kuvaavat muodostumis- ja kasvunopeudet. Lisäksi erityisen mielenkiinnon kohteena ovat nk. kutistumistapahtumat, joissa NPF-tapahtumissa syntyneiden hiukkasten moodin keskimääräisen halkaisijan havaitaan pienenevän kasvuvaiheen jälkeen. NPF-tapahtumien tunnistaminen sekä muodostumis- ja kasvunopeuksien määrittäminen perustuivat aerosolihiukkasten lukumääräkokojakaumamittausten analysointiin. Muodostumisnopeudet laskettiin vuomenetelmällä ja kasvunopeudet seuraamalla automaattisesta moodisovitusalgoritmista saatujen halkaisijoiden aikakehitystä. NPF-tapahtumien tunnistaminen perustui lukumääräkokojakaumien visuaaliseen tarkasteluun. NPF-tapahtumien piirteisiin vaikuttavia tekijöitä selvitettiin lähinnä käytettävissä olevien meteorologisten suureiden mittausten sekä dispersiomallilla laskettujen ilmamassojen lähdealueiden avulla. Lukumääräkokojakaumamittausten analysointi osoitti, että NPF-tapahtumat ovat tarkastellulla mittauspaikalla hyvin yleisiä ja voimakkaita. NPF-päivien esiintymistiheys oli 73 % luokitelluista päivistä (454 kpl) ja keskimääräiset muodostumis- ja kasvunopeudet olivat 13,5 cm-3 s-1 (J7nm) ja 8,2 nm h-1 (GR7-12nm). 76 %:ssa NPF-tapahtumista havaittiin lisäksi kutistumistapahtuma. Kutistumistapahtumien esiintymistiheys sekä muodostumis- ja kasvunopeudet olivat hieman suurempia kesäkuukausina, kun taas NPF-tapahtumien yleisyydellä ei ollut selkeää vuodenaikaisvaihtelua. NPF-tapahtumat ja niiden voimakkuus ovat mitä todennäköisimmin riippuvaisia rannikon puolelta kulkeutuvista antropogeenisistä päästöistä. Tähän viittaa erityisesti se, että pienhiukkasten muodostumista ei havaittu lainkaan sisämaasta peräisin olevissa ilmamassoissa. Kutistumistapahtumat eivät vaikuttaneet olevan yhteydessä hiukkasten haihtumista lisääviin tekijöihin (lämpötilan kasvu, ilmamassojen sekoittuminen). Ilmamassojen lähdealueiden ja antropogeenisten päästöjen horisontaalisen jakauman tarkastelu viittasivat puolestaan näennäiseen kutistumisprosessiin. Tässä moodin halkaisijan pieneneminen aiheutuisi haihtumisen sijaan heikommista NPF-tapahtumista peräisin olevien, ja siten vähemmän kasvaneiden hiukkasten havaitsemisesta.
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(2012)Regional climate models are important tools in climate change impact studies due to their high horizontal resolution. On the other hand, regional simulations still include considerable uncertainties and can have substantial biases in comparison to observations. Thus, before the data can be used for deriving climate projections, these biases have to be identified and, to the extent possible, eliminated. There are two approaches to combine the information from observations and simulations: either to adjust observations with the simulated change (delta-change approach) or to correct the biases in the simulations relative to observations during a control period. In this thesis, seven projection methods for daily precipitation were tested in a cross-validation framework. Model simulations taken from the ENSEMBLES data set were used to test the relative performance of these methods. In addition to traditional delta change method that scales only time mean precipitation, three algorithms which take daily variability into account were used. Two of these (Engen-Skaugen and power transformation algorithms) scale the standard deviation, while the most flexible one does the correction/adjustment percentilewise (analogy algorithm), so that changes in the shape of the distribution are also taken into account. The algorithms were applied both using delta change and bias correction approaches. The performances of the projection methods depend on time, location and also the part of the distribution considered. Bias correction done with the analogy-algorithm worked well in a large part of the distribution, especially in north Europe. Due to smaller fraction of wet days and larger intermodel differences in the simulations, delta change methods performed relatively better in south Europe than in north Europe. On the other hand, bias correction with the power transformation algorithm has the best ability to adjust heavy precipitation, apparently due to the strong scaling it applies to the upper tail of the distribution. The results improved when the projections for the best performing methods were combined. The reason is the same as with multi model mean projections: errors in different projections tend to cancel each other out. To assess the uncertainty due to intermethod differences, methods were applied directly to observations taken from the data set gathered by European Climate Assessment & Data. The results showed that most of the overall uncertainty (if only one emission scenario is used) comes from intermodel differences that are large especially for bias correction methods. Uncertainty related to intermethod differences is smallest in the middle parts of the distribution, but increases towards the tails of the distribution and tends to be largest in summer. Thus, the intermethod differences are non-negligible and should be taken into account when calculating daily precipitation projections.
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(2012)Säähavainnonteon automatisoinnin kasvava trendi on ollut havaittavissa jo useiden vuosien ajan. Automatisoinnin hyötyjä ovat kustannustehokkuus, havainnonteon objektiivisuus sekä havainnointitiheyden helppo kasvattaminen. Automatisointi vaatii kuitenkin myös laadullista seurantaa. Havaintolaitteistojen sisäisiin ohjelmistoihin ei useimmiten ole pääsyä, joten havaintoihin joudutaan soveltamaan jälkikäteen määriteltyjä korjausluokitteluita ja testejä. Tutkielmassa perehdytään eri sääolosuhteissa vallitseviin olosuhteisiin ja niiden vallitsevan sään automaattiselle havainnoinnille tuomiin haasteisiin. Tutkimuksen lähestymistapa pohjautuu Alankomaiden ilmatieteen laitoksessa (KNMI) tehtyyn tutkimukseen. Tutkittavana havaintolaitteena on Vaisalan valmistama vallitsevan sään anturi, FD12P, jonka mittaustekniikka perustuu optisen eteenpäinsironnan, sadeanturin kapasitanssin, sekä lämpötilamittausten yhteiskäyttöön. Tavoitteena on rajata eri sääilmiöissä vallitsevia tilastollisia sääolosuhteita ja implementoida niitä automaattisen säähavainnoinnin havainnontarkastukseen. Samoja periaatteita noudattaen pyritään kehittelemään uudenlainen luokittelumenettely anturin mittaaman tuntemattoman sateen olomuodon sääluokalle. Aineistona käytettiin Helsinki-Vantaan ja Rovaniemen lentoasemien havaintoja vuosilta 2003-2012. WMO:n koodistojen mukaan ilmoitetut vallitsevan sään havainnot yhdenmukaistettiin sääilmiön mukaisiin säätyyppeihin. Luokittelun jälkeen automaatin mittaussarjoihin sovellettiin automaattisia lämpötilan, kostealämpötilan, FD12P:n mittaaman sateen intensiteetin sekä näkyvyyden, suhteellisen kosteuden, pilvisyyden ja pilvenkorkeuden havainnoista johdettuja korjausalgoritmeja. Automaatin mittaamaan alkuperäisdataan sovellettiin 23:a eri korjausalgoritmia parhaimmillaan neljällä eri modifikaatiolla. Korjausalgoritmien laatua tarkkailtiin viiden mm. sääennusteiden verifioinnissa laajalti käytetyn tunnusluvun sekä osuvuusmatriisien avulla. Manuaaliset havainnot toimivat referenssinä. Mittausten laatua parantaneita algoritmeja kertyi lopulta 16 kappaletta. Havaintoasemien yhdistetyn aineiston sääilmiöiden osuvuus parani kokonaisuudessaan 8,1 prosenttiyksikköä. Tuntemattoman sadeluokan uusi menettely tarkensi tuntemattoman sateen luokittelua 11,6 prosenttiyksikköä. Virheelliset havainnot pienentyivät molemmissa tapauksissa merkittävästi. Kokonaisuudessaan uudenlaisten korjausalgoritmien sekä tuntemattoman sadeluokan uusi korjausmenettely antoi rohkaisevia tuloksia. Optimaalisten korjausten saavuttaminen vaatinee joka tapauksessa vielä hieman lisätutkimusta. Lopullista operatiiviseen käyttöön soveltamista ennen tulee määritellä myös käytettävien korjausalgoritmien haluttu aggressiivisuus sekä niiden sovellettavuusaste.
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(2014)Tässä kirjallisuuskatsaus-tyyppisessä tutkielmassa kerrotaan vuoristojäätiköissä tapahtuvien muutosten syistä, luodaan katsaukset viimeaikaisiin ja odotettavissa oleviin jäätikkömuutoksiin sekä esitellään jäätikkömuutosten seurauksia ja hyödyntämistä ilmastotutkimuksessa. Aiheen tärkeys liittyy muun muassa jäätiköiden toimimiseen makean veden varastoina, jotka sitovat ja vapauttavat vettä moninaisissa aikaskaaloissa, vaikuttaen näin ollen niitä sisältävien alueiden valuntaan sekä merenpinnan korkeuteen. Jäätiköt toimivat myös eräänlaisina ilmastonmuutosten ilmaisimina, sillä jäätikkömuutokset ovat yleensä voimakkaasti kytköksissä ilmakehän olosuhteissa tapahtuvaan vaihteluun. Viimeisten reilun sadan vuoden aikana vuoristojäätiköt näyttäisivät yleisesti pienentyneen, ja useat tutkimukset vihjaavat vuoristojäätiköiden keskimäärin menettävän massaansa myös kuluvalla vuosisadalla, vaikkakin ennustetut muutokset riippuvat paljon muun muassa oletuksista, joita tuleviin ilmasto-oloihin liittyen joudutaan tällaisissa tarkasteluissa tekemään.
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(2014)Cavity ring-down spectroscopy is a laser absorption technique based on the principle of measuring the rate of exponential decay of light intensity inside the ring-down cavity. When the absorption spectrum of a gas is known, it is possible to determine the mole fraction of this gas by measuring the height of the absorption peak, which can be acquired from the rate of decay of light. This technique is used in G1301, G2301 and G2401 (Picarro Inc.) gas analyzers which measure carbon dioxide (CO2), methane (CH4) and water vapor. However, measured gas mole fractions are diluted from their actual value; mostly due to variations in atmospheric water vapor. This effect causes large errors and it has to be corrected either by drying the sample or applying a water vapor correction. A default water vapor correction is included in Picarro gas analyzers, but it might not be accurate enough for use in some measurements. In this study, determination of water vapor correction coefficients was carried out by doing several droplet tests for seven different gas analyzers, which included one G2401, two G1301, four G2301gas analyzers. Mean correction functions determined for the analyzers were compared to the Picarro default correction. In addition, the comparison was made with time series data for one of the analyzers. Also, the water vapor measurement of the gas analyzers was calibrated to acquire the actual water vapor mole fraction. As a result, the factory correction for CO2 was proved sufficient for high accuracy measurements only up to 0.7 % water vapor mole fraction. For CH4, the factory coefficient was enough up to 2.0 %, which corresponds to dew point temperature of 18 °C. In conclusion, neither of factory corrections is enough for use all year round. So, the water vapor correction should be made for each gas analyzer when making high accuracy measurements. Due to cyclic drift of water vapor measurement, the correction should remain stable over time, but this needs further verification. Currently, the correction should be made at least once per year.
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(Helsingin yliopistoHelsingfors universitetUniversity of Helsinki, 2010)This study evaluates how the advection of precipitation, or wind drift, between the radar volume and ground affects radar measurements of precipitation. Normally precipitation is assumed to fall vertically to the ground from the contributing volume, and thus the radar measurement represents the geographical location immediately below. In this study radar measurements are corrected using hydrometeor trajectories calculated from measured and forecasted winds, and the effect of trajectory-correction on the radar measurements is evaluated. Wind drift statistics for Finland are compiled using sounding data from two weather stations spanning two years. For each sounding, the hydrometeor phase at ground level is estimated and drift distance calculated using different originating level heights. This way the drift statistics are constructed as a function of range from radar and elevation angle. On average, wind drift of 1 km was exceeded at approximately 60 km distance, while drift of 10 km was exceeded at 100 km distance. Trajectories were calculated using model winds in order to produce a trajectory-corrected ground field from radar PPI images. It was found that at the upwind side from the radar the effective measuring area was reduced as some trajectories exited the radar volume scan. In the downwind side areas near the edge of the radar measuring area experience improved precipitation detection. The effect of trajectory-correction is most prominent in instant measurements and diminishes when accumulating over longer time periods. Furthermore, measurements of intensive and small scale precipitation patterns benefit most from wind drift correction. The contribution of wind drift on the uncertainty of estimated Ze (S) - relationship was studied by simulating the effect of different error sources to the uncertainty in the relationship coefficients a and b. The overall uncertainty was assumed to consist of systematic errors of both the radar and the gauge, as well as errors by turbulence at the gauge orifice and by wind drift of precipitation. The focus of the analysis is error associated with wind drift, which was determined by describing the spatial structure of the reflectivity field using spatial autocovariance (or variogram). This spatial structure was then used with calculated drift distances to estimate the variance in radar measurement produced by precipitation drift, relative to the other error sources. It was found that error by wind drift was of similar magnitude with error by turbulence at gauge orifice at all ranges from radar, with systematic errors of the instruments being a minor issue. The correction method presented in the study could be used in radar nowcasting products to improve the estimation of visibility and local precipitation intensities. The method however only considers pure snow, and for operational purposes some improvements are desirable, such as melting layer detection, VPR correction and taking solid state hydrometeor type into account, which would improve the estimation of vertical velocities of the hydrometeors.
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(2014)Preliminary estimation of wind speed at the wind turbine hub height is critically important when planning new wind farms. Wind turbine power output is proportional to the cube of wind speed which means that even small uncertainties in wind speed estimation will greatly affect the estimated energy yield. Wind resource estimation is usually based on wind measurements using meteorological masts and SODAR (SOnic Detection And Ranging) instruments. Modern wind turbine hub height typically ranges from 100 to 140 meters. Wind speeds measured with a mast must often be extrapolated to turbine hub height, while SODAR measures a continuous wind profile up to approximately 200 meters height. The goal of this study is to assess the uncertainty of SODAR measurements and to analyse the vertical wind profile variability in Finnish conditions from a wind power perspective. Mast and SODAR data collected at 17 sites across Finland covering a total of 381 months of measurements was available for this study. Both the amount and type of equipment at the sites as well as temporal data coverage varied greatly among the sites. Both coastal and inland sites were represented. Mast and SODAR data were quality controlled using manual inspection. The main reason for this was to identify periods with anemometer freezing, which results in erroneous wind data. Quality controlled data was used to calculate various parameters utilized in the wind resource assessment such as annual wind speed, wind shear represented by the power law exponent (alpha), atmospheric stability category and turbulence intensity. SODAR uncertainty was studied using the difference of wind speed measured by co-located mast and SODAR in relation to wind speed, humidity, alpha and turbulence intensity. Wind profile variability was studied with emphasis on hub height wind speed extrapolation, particularly in terms of profile shape and wind speed magnitude. Based on the results, the most common stability categories are neutral and slightly stable, which are related to relatively strong wind shear. Wind profiles were found to vary significantly with season. Therefore wind speed extrapolation should not be performed based on seasonal data when aiming for statistical representativeness of the wind resource. As a result of this study a statistical method to account for the seasonality is presented. SODAR was observed to overestimate horizontal wind speed in rainfall as well as in high wind speed and weakly turbulent conditions. Due to these tendencies, conducting short SODAR measurement campaigns in which the afore-mentioned conditions prevail is not recommended. As winters have generally higher wind speeds in Finland, short wintertime SODAR campaigns are discouraged. A verification measurement period for mast-SODAR intercomparison ought to be carried out before using SODAR as a standalone instrument. This should be conducted close to a meteorological mast and the period should include as much meteorological variability as possible. This would ensure some level of certainty in SODAR measurements, especially in the absence of an acknowledged calibration standard.
Now showing items 81-93 of 93