Browsing by Subject "Geofysiikka"

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  • Leinonen, Katri (Helsingin yliopisto, 2019)
    Globaalille valtamerten pinnanousulle on viimeisen 40 vuoden aikana julkaistu suuri määrä mahdollisia tulevaisuuskuvia, skenaarioita. Tässä pro gradu -työssä tarkastellaan kahdeksan 1980-luvulla tehdyn merenpinnan noususkenaarion osuvuutta vertaamalla skenaarioita toteutuneeseen valtamerten pinnannousuun. Globaalia merenpinnan muutosta on mitattu viimeisten parin vuosisadan aikana mareografimittauksilla eri puolella maailmaa. Vuodesta 1993 alkaen globaalin merenpinnan korkeusaineistoa on tarjolla myös satelliittimittauksista. Tässä työssä merenpinnan nousun vertailutasona on käytetty kummallakin mittausmenetelmällä tehtyjä merenpinnan nousuaikasarjoja. Jotta noususkenaariot ja mittausaikasarjat olisivat keskenään vertailukelpoisia, valittiin yhteiseksi aloitusvuodeksi 1980. Kaikkien merenpinnan noususkenaarioiden ja mittausaikasarjojen nollataso siirrettiin alkamaan tästä vuodesta. Globaalit ilman lämpötilan noususkenaariot ovat kiihtyviä, ja yleisesti myös merenpinnan noususkenaarioiden oletetaan olevan kiihtyviä. Merenpinnan noususkenaarioihin, joille ei ollut annettu merenpinnan nousun väliarvioita, sovitettiin toisen asteen yhtälön mukainen kiihtyvyys. Muihin skenaarioihin sovitettiin kaikkien annettujen välipisteiden kautta kulkeva käyrä. Tässä työssä tarkastellut merenpinnan noususkenaariot poikkeavat toisistaan huomattavasti alimpien skenaarioiden ennustaessa parinkymmenen sentin merenpinnan nousua ja ylimpien skenaarioiden jopa useiden metrien nousua. Vuonna 2018 kaikkien skenaarioiden keskiarvo on noin 6 cm toteutuneen merenpinnan nousun yläpuolella, nousua vuodesta 1980 on tapahtunut noin 10 cm. Toteutunut merenpinnan nousu on myös selvästi skenaarioiden mediaanin alapuolella. Vajaa 70 % skenaarioista nousee nopeammin kuin vedenkorkeus on todellisuudessa noussut. Suomen rannikolla, Hangossa, merenpinnan noususkenaarioiden perusteella tehty ennuste on 2010luvulla muutamia senttimetriä mitatun merenpinnan yläpuolella. Merenpinnan noususkenaariot ovat vuosien varrella kehittyneet, mutta perustuvat edelleen paljolti samoihin tai samankaltaisiin menetelmiin kuin 1980-luvulla tehdyt merenpinnan noususkenaariot. Tämän takia on hyödyllistä tarkastella 1980-luvulla käytettyjä menetelmiä, niiden taustalla olevia oletuksia ja niistä saatujen ennusteiden osuvuutta. Tässä työssä tarkastellut merenpinnan noususkenaariot jaettiin tekomenetelmän mukaan kahteen pääryhmään. Toinen ryhmä koostui merenpinnan nousuun vaikuttavien osatekijöiden arviointiin perustuvista noususkenaarioista. Toisessa ryhmässä olivat semi-empiiriset skenaariot, joissa merenpinnan muutosta arvioidaan jonkin ilmastollisen suureen perusteella. Merenpinnan noususkenaarioista valittiin parhaiten vuosien 1980 - 2018 merenpinnan nousua kuvaavat skenaariot. Parhaiten osuvissa skenaarioissa oli laaja kirjo eri menetelmillä tehtyjä ja erilaisiin oletuksiin perustuvia skenaarioita. Merkittävää osuvuuseroa ei kahdella eri päämenetelmällä tehtyjen skenaarioiden välillä löydetty.
  • Jutila, Arttu (Helsingfors universitet, 2017)
    Studying and modelling the snow distribution processes is important because snow influences the ground, flora, and fauna by affecting among other things the energy balance both in large and small scales and the near-surface atmospheric conditions due to its highly reflective and insulating properties. The aim of the study was to use the spatially distributed high-resolution snow-evolution modelling system SnowModel to simulate the snow conditions in winter 2015-2016 in the Saariselkä region in Northern Finland and assess the model's performance. SnowModel has not been used to study a domain in Finland before, and the model gives information about variables that are hardly measured in Finland, such as snow sublimation. The simulations were first run without snow water equivalent assimilation and then assimilating the available snow water equivalent (SWE) observations. The simulation results show that in the default mode the model needs assimilation and SWE observations, preferably more frequent observations towards the spring, to produce physically sensible results. The domain averaged simulated end-of-winter maximum SWE value of 220 mm was reached on 21 April 2016. The simulated SWE patterns match with known elevation and vegetation dependencies. Timing of the first snow, the beginning of the snow season and the end-of-winter SWE are simulated well, whereas the melt and the snowfree date depend on the amount of snow. The assimilation run suggests that the needed summed precipitation is as much as 18 % larger than the observed increasing towards the northeast. Similarly, the simulated summed melt reaches locally up to 70 % larger values compared to the non-assimilation run. Blowing-snow sublimation takes place in open areas and its simulated summed value is up to 27 mm. Simulated static-surface sublimation varies between 4-22 mm. The simulated sublimation from the canopy-intercepted snow peaks at 110 mm. Up to 16 % of the precipitation is returned to the atmosphere by sublimation. The simulation results could be improved by utilizing more detailed data of the study domain and modifying the hard-coded variables to suit the surroundings, which could in turn decrease the need for assimilating SWE observations.
  • Kanarik, Hedi (Helsingin yliopisto, 2018)
    Ilmatieteen laitoksella on runsaasti hankekohtaisesti tehtyjä virtausmittauksia akustisella Dopplerilmiöön perustuvalla ADCP -laitteella. Tällaiset akustiset mittarit pystyvät muita virtausmittareita paremmin mittaamaan laajoja merialueita, joten ne ovatkin maailmanlaajuisesti yksi suosituimmista menetelmistä tarkkailla merien virtauksia. Tärkeimmät ehdot mittausten onnistumiselle on mitatun virtauksen horisontaalinen homogeenisuus, joka ei aina toteudu muun muassa vedessä olevien äänisignaalin sirottajien itsenäisen liikkeen seurauksena. Laite pyrkii jatkuvasti tarkistamaan olosuhteiden riittävän sopivuuden ja poistaa tehokkaasti esimerkiksi mittausalueelle osuneiden kalojen liikkeet. Mikäli laitteen sisäinen laaduntarkkailu on kuitenkin liian tiukka, se saattaa liian helposti hylätä poikkeuksellisempia ilmiöitä, joten tiukempi laaduntarkkailu jätetään usein erikseen tehtäväksi. Tässä tutkielmassa kehitin laaduntarkastusohjelmiston merenpohjaan ankkuroidulle ADCP:lle. Työssä keskitytään erityisesti Ilmatieteen laitoksen käyttämään Teledyne RD Instrument’s -valmistajan Workhorse Sentinel -laitteeseen. Kynnysarvot datan laadulle on määritelty erityisesti tämän valmistajan mittareille ja testit perustuvat laitteen tallentamaan tietoon mittausprosessista. Lähestymistapa perustuu oletukseen, että jos valtaosa virtausnopeuden määrittämisen yhteydessä tehdyistä mittauksista eivät olleet riittävän luotettavia, niin luultavasti loput näistä näennäisesti onnistuneista mittauksista eivät myöskään edusta todellista virtaustilannetta. Laaduntarkistusohjelmisto kehitettiin käyttämällä esimerkkimateriaalina Saaristomerellä Lövskärin risteyksessä vuonna2013 suoritettuja mittauksia. Lövskärin datasetti oli erittäin hyvälaatuista ja epähomogeenisuuden seurauksena datasetistä poistettiin noin 0,3 % mittauksista. Meren ylintä 5 metrin kerrosta ei pystytty mittaamaan voimakkaan sivukeilan aiheuttaman häiriön takia (13 % mittauksista). Datasetissä on huomattavissa selkeää mittausten epävarmuuden kasvua termokliinissä ja yöaikaan, mikä johtuu sirottajina toimivan eläinplanktonin aktiivisuudesta. Yleisesti alueen virtaukset olivat termokliinin seurauksena vahvasti kerrostuneet ja alueella ilmeni syksyllä lyhytkestoisia voimakkaita (lähes 50 cm/s) virtauksia.
  • Jalli, Heini (Helsingin yliopisto, 2020)
    Aallokon mittaamiseen Itämerellä on vakiintunut käytettäväksi aaltopoiju, joka on ankkuroitava pintapoiju. Kyseinen havaintotapa aiheuttaa mittauskauden lyhenemistä jäätalven vuoksi. Jotta mittauskautta pystyttäisiin pidentämään, tarvitaan mittaussysteemi, jota ei tarvitse nostaa ylös vedestä ennen jäätalvea. Pintavirtauksia mitataan yleisesti pohjaan asennettavilla akustisilla Doppler virtaus profiilimittalaitteilla (ADCP), joissa ei ole reaaliaikaista tiedonsiirtoa. Viimeisten vuosien aikana lähes reaaliaikaista aineistoa lähettävään aaltopoijuun on lisätty pintavirtauksen havainnoinnin mahdollistavat anturit. Tässä tutkielmassa arvioidaan ADCP:n havaintojen luotettavuutta aallokon mittaamisessa verrattuna aaltopoijuun ja vertaillaan aaltopoijun ja ADCP:n virtaushavaintoja toisiinsa. Tässä tutkielmassa käytetty havaintoaineisto on saatu kahden eri vuoden, kesien 2017 ja 2018, aikana toteutetuista mittalaitevertailuista. Mittausjaksot tehtiin Suomenlahdella Hankoniemen itäpuolella. Havaintoaineistolle on tehty laadunvarmistusta ennen kuin niitä on vertailtu. Laaduntarkastuksen kriteerit on saatu mittalaitteiden valmistajien ilmoittamista raja-arvoista, suosituksista ja kirjallisuudessa olevista Suomenlahden aallokko-olosuhteiden raja-arvoista. Havaintoaineistoa on analysoitu ja verrattu toisiinsa aikasarjojen, hajontakuvaajien ja tilastollisten arvojen kautta. ADCP:n ja aaltopoijun merkitsevän aallonkorkeuden vastaavuus on hyvä, mutta ADCP ei pysty havaitsemaan alle 0,5 metrin aallokkoa luotettavasti. Syvemmälle asennettu ADCP aliarvioi suhteellisen systemaattisesti merkitsevää aallonkorkeutta verrattuna aaltopoijun havaintoihin. Aliarviointia on teoriassa mahdollista korjata ja näin parantaa mittauksien vastaavuutta, mutta käytännössä se ei ole järkevää koska se vaatisi uusien vertailujen tekemistä muuan muassa jokaiselle mittalaitteelle ja -paikalle. Huipun periodin ja aallokon suunnan vastaavuus ei ollut tilastollisesti merkittävää ja ADCP:n mittauksia näistä suureista voisi käyttää, tarvittaisiin tarkempaa spektrien analysointia. Aaltopoijun ja ADCP:n pintavirtaushavaintoja vertaillessa on vastaavuutta arvojen välillä, mutta aallokon kasvaessa erot mittauksissa kasvavat. Havaittuja eroja ei voi selittää pelkästään vertailtavien laitteiden mittaussyvyyden erolla, joka oli keskimäärin 1 metri.
  • Honkanen, Martti (Helsingfors universitet, 2017)
    Carbon dioxide is a key compound both in climate change and marine biological productivity. In the oceans, the sea-air exchange of carbon dioxide is driven by large-scale currents and changes in solubility, whereas in coastal seas, such as the Baltic Sea, biological activity has a significant effect on the aquatic carbonate system. However, direct measurements of the sea-air exchange of carbon dioxide are difficult to carry out due to the small magnitude of the fluxes, waves, sea ice and sea spray that influence sensitive instruments. Eddy covariance method is a widely used direct method for measuring the fluxes of carbon dioxide. The sea-air fluxes of carbon dioxide are also commonly calculated using a parametrization based on wind speed. Finnish Marine Institute in collaboration with Finnish Environment Institute began to construct a new atmospheric and marine research station on Utö in the Archipelago Sea in 2012. The objective of this study is to determine the suitability of the new station to measure air-sea exchange of carbon dioxide and aquatic carbonate system. The air-sea fluxes of carbon dioxide were measured with three infrared gas analyzer set-ups during October-December 2016, and the fluxes were also calculated using a parametrization of gas transfer velocity. The aquatic carbonate system of the surface water during July-October 2016 was studied using dissolved carbon dioxide concentration and pH of the seawater measured in a flow-through pumping system. The dissolved carbon dioxide concentration was measured by using an equilibration chamber together with an infrared gas analyzer. The micrometeorological tower erected on the shore can be applied for measuring the sea-air fluxes of carbon dioxide in an open sea wind sector, which was determined based on the roughness length. The closed-path infrared gas analyzer (Licor LI-7000) works well in the hard coastal conditions, whereas an open-path analyzer (Licor LI-7500) had difficulties in measuring fluxes during showers and high relative humidity. The Pearson correlation coefficient between the fluxes measured with two closed-path gas analyzer set-ups was 0.91. The flux calculated using a parametrization of gas transfer velocity showed only a small correlation with the measured flux, with a Pearson correlation coefficient of 0.13. The flow-through pumping system can be used to calculate the components of the aquatic inorganic carbonate system. The measured pH was linked to the dissolved carbon dioxide concentration of the sea water. Additionally, it was discovered that the LI-7000 is sensitive to changes in instrument temperature.
  • Siponen, Joula (Helsingin yliopisto, 2019)
    Changes in sea ice cover are one of the most visible signs of climate change. Long time series of thickness observations are needed to study climatological changes. The knowledge of sea ice thickness is also important for human activities in the Arctic. Need for better predictions of sea ice conditions is increasing due to opening of the Arctic sea routes for longer operational season among other things. Improving predictions requires high-quality observations. However, measuring sea ice thickness with adequate resolution, accuracy and coverage over the Arctic is a major challenge. In this thesis a new product of sea ice thickness, ESA-CCI (Climate Change Initiative), based on satellite radar altimetry is used for assessment of sea ice thickness from ocean reanalysis ORAS5. The CCI product combines two satellite missions, CryoSat-2 and ENVISAT, which leads to 15-year time series of sea ice thickness over the Arctic. The new CCI product performs well in validation of the reanalysis. Overall average difference (RMSE) between sea ice thickness in the CCI product and reanalysis is below 1 m but seasonal and interannual variation during the time series is from 0.5 m to 1.3 m. There are strong regional differences. The results of this thesis support previous research. Differences are a sum of reanalysis biases, such as incorrect physics or forcing, as well as uncertainties in satellite altimetry, such as the snow product used in thickness retrieval. Monthly separated time series of sea ice volume for the CCI coverage reveal years of extremely low volume and recovery during the season. The trends in sea ice volume are clearly negative. Monthly CCI trends are statistically significant. ORAS5 trends have larger interannual variability and therefore show no significance. The observed negative trends are connected to changes in both, atmospheric and oceanic forcing.
  • Haavisto, Noora (Helsingfors universitet, 2017)
    Hydrografian seuranta Itämerellä on hoidettu perinteisesti tutkimusaluksilla, joiden operoiminen on kallista ja joilla päästään seurantapisteille tekemään luotauksia vain muutaman kerran vuodessa. Ilmatieteen laitos on vuodesta 2011 testannut ja operoinut automaattisia profiloivia Argo-poijuja Selkämerellä. Poijut tarjoavat uudenlaisen menetelmän hydrografian ja syvien virtausten mittaamiseen, mutta Itämeren mataluus ja vähäsuolainen vesi aiheuttavat haasteita niiden operointiin. Tässä tutkielmassa käsitellään Ilmatieteen laitoksen Argo-poijujen viiden ensimmäisen operatiivisen vuoden (2012--2016) profiilimittauksia. Tärkeimpiä tutkimuskysymyksiä ovat, mitä poijujen data kertoo Selkämerestä itsenäisenä datasettinä, missä poijut toimivat paremmin suhteessa perinteiseen seurantaan ja missä eivät, ja miten poijuja kannattaisi käyttää osana olemassa olevaa havaintoverkkoa. Argo-poijujen datan perusteella lasketut keskimääräiset lämpötilan ja suolaisuuden arvot Selkämerellä olivat lähellä kirjallisuudessa esitettyjä, joskin pohjakerroksen suolaisuus oli noin 0,5\,g\,kg$^{-1}$ matalampi. Tämän arveltiin johtuvan osin siitä, etteivät kaikki profiilit ulottuneet pohjaan asti, ja osittain lyhyestä aikasarjasta verrattuna kirjallisuuden klimatologisiin keskiarvoihin. Vuosi 2014 havaittiin poikkeukselliseksi sekä pintalämpötilan että pohjakerroksen suolaisuuden osalta. Korkean suolaisuuden todettiin olevan todennäköisesti merkki varsinaiselta Itämereltä Selkämerelle tulleesta tavallista suuremmasta vesimäärästä. Argo-poijut tuottavat profiileja vesipatsaan hydrografiasta huomattavasti tiheämmin, kuin perinteiset seurantamatkat, joten niiden vahvuus on laivaseurantaan verrattuna lyhytaikaisten muutosten seurannassa. Virtauksia on myös mahdollista arvioida Argo-poijujen ajelehtimisnopeuden perusteella. Selkämeren syvänteelle lasketut virtausnopeudet olivat lähellä kirjallisuudessa esitettyjä (1,4--4,8\,cm\,s$^{-1}$). Virtausnopeudet ovat ennemminkin hydrografiamittausten sivutuote, kun Argo-poijujen käyttötarkoitus, nopeuksien arvioinnissa on paljon virhelähteitä, jotka yhdistettynä matalaan merialueeseen hankaloittavat tulosten tulkintaa. Kaiken kaikkiaan Argo-poijut todettiin Selkämerellä toimiviksi merialueen haaseista huolimatta. Poijut tuottavat profiilidataa tiheydellä, joka on aiemmin ollut saavuttamattomissa laivaseurannan kalleuden takia. Ensimmäisinä operatiivisina vuosina Selkämeren Argo-poijut ovat tuottaneet syvännealueen profiilidatasta jopa 80\,\% (verrattuna HELCOMin seuranta-asemilla tehtyihin mittauksiin). Tulevaisuudessa olisi kiinnostavaa kokeilla poijujen ohjautumista esimerkiksi Ahvenanmerellä, ja tutkia, miten tällä hetkellä (10/2017) Selkämerellä yhtäaikaa mittaavat kolme poijua kuvaavat koko Selkämerta ja olisiko tämä sopiva määrä poijuja yhtäaikaisessa operatiivisessa seurannassa.
  • Lindgren, Elisa (Helsingfors universitet, 2015)
    Keväällä auringonsäteilyn lisääntyessä alkaa jääpeitteisillä järvillä sulamiskausi, jolloin ensin lumi ja sen jälkeen jääkansi sulavat. Lumeton jää läpäisee jonkin verran auringonsäteilyn näkyvää valoa, joka lämmittää jään alla olevaa vettä ja aiheuttaa siten epästabiilin tiheyskerrostuneisuuden sekä konvektiivisen sekoittumisen alkamisen. Koska jäätyvien järvien jääpeitekauden kesto on lyhentynyt edellisen vuosisadan aikana, mikä voi vaikuttaa esimerkiksi arktisten maa-alueiden ilmastoon ja järvien ekologiaan, tarvitaan jäidenlähdön ja järvien sulamiskauden fysikaalisten prosessien tutkimusta, jotta veden, jään ja ilmakehän välinen vuorovaikutus ymmärrettäisiin ja voitaisiin mallintaa paremmin. Kilpisjärvi (69 03'N 20 50'E) on Suomen Käsivarren Lapissa sijaitseva arktinen tundra-järvi. Sen jäätalvi kestää tavallisesti marraskuusta kesäkuuhun, ja järven jää saavuttaa vuosittain noin 90 cm paksuuden. Keväällä 2013 tehtiin Kilpisjärvelle kenttämatka, jonka tavoitteena oli tutkia arktisten järvien fysikaalisia prosesseja sulamiskaudella, ja jonka aikana kerättyjä mittauksia jään ominaisuuksista sekä veden lämpötilasta analysoidaan tässä tutkielmassa. Kilpisjärvellä havainnoitiin jään sulamista ja mitattiin jään läpi tunkeutuvan PAR-säteilyn määrää sekä tutkittiin järven jäästä otetusta näytteestä kiderakennetta. Järveen asennetut termistoriketjut mittasivat veden lämpötilan kehitystä järven eri syvyyksillä, ja CTD-luotauksilla seurattiin lämpötilan alueellista vaihtelua järven eri osissa sekä säteilyn vaimenemista vesirungossa. Helsingin Yliopiston Kilpisjärven biologiselta asemalta ja Ilmatieteenlaitoksen Kilpisjärven kyläkeskuksen sääasemalta saatiin lisäksi säätietoja, joiden perusteella laskettiin jään lämpötase. Kenttäjakson aikana 25.5.-4.6. Kilpisjärvellä vallitsivat poikkeukselliset olosuhteet, sillä korkeimmillaan ilman lämpötila kohosi +25 Celsiusasteeseen. Järven jää suli nopeasti 4-5 cm vuorokaudessa, mikä johti aikaiseen jäidenlähtöön 3. kesäkuuta. Vaikka lämpötasetta hallitsi auringonsäteily, olivat myös havaittavan lämmön vuo ja pitkäaaltoisen säteilyn netto jäätä lämmittäviä tekijöitä. Pelkästään kohvajäästä koostuvan jään läpäisykyky oli korkea, 0,6-0,9, ja valon vaimenemiskertoimelle saatiin arvoja väliltä 0,2 m−1 ja 0,8 m−1. Jään alla irradianssin taso oli keskimäärin 155 W m−2, ja tämä lämmitysteho aiheutti veden lämpötilan nopean kasvun jään alla jo ennen jäidenlähtöä. Alueellinen vaihtelu oli kuitenkin selkeä, sillä litoraalialueet lämpenivät nopeammin kuin pelagiaalialueet. Tutkimus osoitti, että lähellä jäidenlähtöä auringonsäteily on merkittävin vettä lämmittävä tekijä, jolloin sen ajama konvektio aiheuttaa veden tehokkaan sekoittumisen. Jään läpäisseen säteilyn määrä ja veden lämpeneminen vaihtelevat kuitenkin todennäköisesti järven eri osien välillä epähomogeenisen jääpeitteen takia, mistä voi seurata tiheyserojen ajamien konvektiivisten solujen syntyminen ja siten monimutkainen virtausrakenne jään alla. Tästä ei saatu havaintoja käytetyillä menetelmillä, joten vastaavissa myöhemmissä tutkimuksissa olisi mittaukset suunniteltava kattamaan laajempi alue järveä.
  • Kiuru, Risto (Helsingfors universitet, 2016)
    Physical and mechanical properties of drill core specimens were determined as a part of investigations into excavation damage in the dedicated study area in ONK-TKU-3620. The main goal of this study was to find indicators of excavation damage in the form of anomalous physical properties linked to increased porosity or lower mechanical strength. Geophysical indicators are desired for their ease, speed and cost-effectiveness. The secondary goal was to find associations between dynamic and static elastic properties, to allow estimation of rock mechanical properties using geophysical measurements. The parameters most sensitive to the presence of (saline) pore space fluid showed depth dependencies. Resistivity showed abnormally low values in the first 0.2 m, and an increase with depth in the first 0.7 m from the study area surface. S-velocity, shear impedance, shear modulus and Young's modulus all showed abnormally low values in the first 0.2 m from the study area surface. In addition to clear depth dependencies, other indicators of excavation damage were found. Specimens in the first 0.7 m from the study area surface showed increased proportion of high (> 0.5 %) porosity values. Combinations of high porosity/shallow depth, low resistivity/shallow depth, high porosity/low resistivity, low IP/shallow depth and low IP/high porosity also seem to separate anomalous specimens. S-velocity, P/S -ratio, Poisson's ratio and all three impedances in respect to depth separated anomalous specimens. Abnormally high S-velocity in respect to other elastic properties also seemed to separate anomalous specimens. On one of the anomalous specimens, the presence of an EDZ feature was confirmed by Posiva geologist. This specimen could be identified based on S-velocity, P/S -ratio, Poisson's ratio and all three impedances. Best indicators for excavation damage based on this study would appear to be resistivity, S-velocity, shear impedance, shear modulus and Young's modulus. Most of the other elastic parameters in conjunction with other parameters could be used to identify anomalous specimens. The results support the use of electrical and seismic methods to identify excavation damage. Estimation of static elastic properties based on dynamic elastic properties does not appear possible based on this study. The views and opinions presented here are those of the author, and do not necessarily reflect the views of Posiva.
  • Kokkonen, Iiro (Helsingin yliopisto, 2018)
    The Kara Sea is part of the seasonal sea ice zone in the Arctic, where the warming climate is rapidly changing the sea ice regime. The warm Atlantic water transported through the Barents Sea has a strong influence on the ice conditions in the northern Kara Sea. In this thesis, trends and interannual variability in sea ice conditions in the Kara Sea area studied. For this purpose, coupled sea ice-ocean model NEMO-LIM3 and sea ice concentration datasets derived from passive microwave satellite observations (SMMR, SSM/I and SSMIS) are used. Additionally, the model performance is assessed by comparing its output with the observations. The ice coverage examined in regional and local scales shows negative trends in all months in 1978-2015. The interannual variability of the total ice covered fraction increased in winter and spring when the ice regime shifted from full to partial ice cover over the sea. Meanwhile the variability in summer and autumn decreased. The annual ice free time rapidly extended in the area north of Novaya Zemlya where the warm Atlantic water enters the Kara Sea. The mean sea ice thickness, based on the sea ice-ocean model data in 1997-2015, has become thinner in all months. The model is generally in good agreement with the observations, with the exception of the northern Kara Sea where the model underestimated heat advection. The findings confirm that the sea ice conditions in the Kara Sea have changed towards a new regime with shorter and more variable ice seasons.
  • Särkikoski, Sanna (Helsingin yliopisto, 2019)
    The idea for this study was to unify the heights systems in the tide gauge data available from freely available sources and to make a surface interpolation of the Baltic Sea surface heights. There were some data availability problems in the south-east coast of the Baltic Proper which lead us to limit the study area to the northern Baltic Sea. We compared the surface interpolation made for the years 2007–2016 with the NEMO-Nordic reanalysis product sea levels in order to determine if the interpolated surface could be a used as an approximation of the sea surface heights in the Baltic Sea. Correlation between the interpolated surface and the model were 0.59–0.77 depending on the location. The model sea levels had a sudden change in the overall sea levels in mid-winter 2013–2014 and also a different trend before and after this jump. We also calculated correlations separately for each year from 2007 to 2016 for three mid-basin points. The correlations were between 0.91–0.98. We found the interpolation to be quite susceptible to errors and missing measurements, which makes it difficult to create the surface interpolation for the southern Baltic Proper with the current data availability problems. Therefore, other methods are needed in order to approximate sea levels in that area.
  • Nummelin, Aleksi (Helsingfors universitet, 2012)
    The Meridional overturning circulation (MOC) is one crucial component in Earth's climate system, redistributing heat round the globe. The abyssal limb of the MOC is fed by the deep water formation near the poles. A basic requirement for any successful climate model simulation is the ability to reproduce this circulation correctly. The deep water formation itself, convection, occurs on smaller scales than the climate model grid size. Therefore the convection process needs to be parameterized. It is, however, somewhat unclear how well the parameterizations which are developed for turbulence can reproduce the deep convection and associated water mass transformations. The convection in the Greenland Sea was studied with 1-D turbulence model GOTM and with data from three Argo floats. The model was run over the winter 2010-2011 with ERA-Interim and NCEP/NCAR atmospheric forcings and with three different mixing parameterizations, k-e, k-kL (Mellor-Yamada) and KPP. Furthermore, the effects of mesoscale spatial variations in the atmospheric forcing data were tested by running the model with forcings taken along the floats' paths (Lagrangian approach) and from the floats' median locations (Eulerian approach). The convection was found to happen by gradual mixed layer deepening. It caused salinity decrease in the Recirculating Atlantic Water (RAW) layer just below the surface while in the deeper layers salinity and density increase was clearly visible. A slight temperature decrease was observed in whole water column above the convection depth. Atmospheric forcing had the strongest effect on the model results. ERA-interim forcing produced model output closer to the observations, but the convection begun too early with both forcings and both generated too low temperatures in the end. The salinity increase at mid-depths was controlled mainly by the RAW layer, but also atmospheric freshwater flux was found to affect the end result. Furthermore, NCEP/NCAR freshwater flux was found to be large enough (negative) to become a clear secondary driving factor for the convection. The results show that mixing parameterization mainly alters the timing of convection. KPP parameterization produced clearly too fast convection while k-e parameterization produced output which was closest to the observations. The results using Lagrangian and Eulerian approaches were ambiguous in the sense that neither of them was systematically closer to the observations. This could be explained by the errors in the reanalyzes arising from their grid size. More conclusive results could be produced with the aid of finer scale atmospheric data. The results, however, clearly indicate that atmospheric variability in scales of 100 km produces quantifiable differences in the results.
  • Soini, Assi-Johanna (Helsingfors universitet, 2017)
    Comparing meteorite densities with the densities of small solar system bodies provides clues to the nature of asteroid interiors, especially accretional and collisional processes of asteroids, which reflects the evolution of the early solar nebula. Bjurböle is a L/LL4 ordinary chondrite. Bjurböle meteorites have high friability and porosity compared to other ordinary chondrites. Bjurböle meteorites are compositionally homogeneous and any density variations ascribe their internal structure. In addition, Bjurböle meteorite shower consists of numerous recovered meteorites, thus sampling a large volume of Bjurböle meteoroid. Volumes of ten Bjurböle meteorites ranging in mass from 17.27 g to 13.48 kg were determined using non-contaminating and non-destructive 3D laser scanner and pycnometer. Masses were determined using different scales. Densities were calculated based on the volumes, and porosities were derived from the acquired densities. No trend in density and porosity as a function of meteorite mass was found. Absence of a trend in Bjurböle meteorites can be interpreted based on distribution of strength and porosity within the parent meteoroid body. It suggests that density and porosity are inhomogeneously distributed within parent body and weaker parts are fragmented and disintegrated during atmospheric entry. Only the parts above certain strength survive, and their sizes vary within the parent body forming meteorites ranging in size from grams to tens of kilograms.
  • Juva, Katriina (Helsingfors universitet, 2016)
    The temperature and the salinity fields (i.e. the hydrography) of the Baltic Sea determine the density and hence the stratification and density depended circulation of the sea. These features are affected by the changes in the hydrologic circulation, most importantly by the changes in the atmospheric circulation and in the water exchange with the North Sea. The aims of this thesis are to study the hydrographical conditions and changes for the period 1971 - 2007 of the surface and bottom layers of the Baltic Sea and the model sensitivity to number of variables. The surface layer is well studied, but on the whole Baltic Sea scale, the bottom layer studies are rare in number. The halocline and thermocline depths are also included, since they provide information about the mixing. By combining the information from the surface and the bottom, the overview for the whole hydrographical state is provided. For the analysis, three hindcast simulations based on the three-dimensional North-Baltic Sea model are used. The simulations differ in the number of vertical layers, initial conditions and the strength of the bottom drag coefficient. The results show that the vertical stratification is weaker in model than what is observed in in-situ measurements. The simulations differ remarkably in the salinity level and in its evolution. On average, the salinity is decreasing 0.1 - 0.4 ppt per decade except on the deepest parts of the Baltic Proper. The temperature is increasing at the surface and above the permanent halocline on average 0.2 - 0.4 degree Celsius per decade. Large regional differences between the west and east coast of the basins were found. The bottom temperature increase up to 1 degree Celsius per decade was found in the eastern coast of the eastern Gotland Basin, whereas on the Swedish coast the changes are more moderate and during some months, opposite. On the opposite site of the Bothnian Sea and the Gotland Basin, monthly anomalies up to degree Celsius were found for autumn months. In the deeper layers, the temperature decreases 0.2 - 0.4 degree Celsius per decade. The study showed that the Baltic Sea is undergoing a rapid change. In order to get a more detailed view of the changes in stratification and circulation, the changes in density should be studied next.
  • Heinilä, Kirsikka (Helsingin yliopisto, 2019)
    Optical snow monitoring methods have tendency to underestimate snow cover beneath the evergreen forest canopy due to the masking effect of trees. There is need to develop method for providing more reliable snow products and enhance their use e.g. in hydrological and climatological models. The main objective of this thesis is to provide information to improve the accuracy of snow mapping by algorithm development and its regional parameterization. This thesis exploits reflectance data derived from ground-based, mast-borne, airborne and space-borne sensors. Each datatype with different ground resolutions has specific strengths and weaknesses. Together this dataset provides valuable information to advance knowledge of reflectance properties of snow-covered forests and supports the interpretation of satellite-borne reflectance observations. Improvement of satellite-based snow cover mapping is essential because it is the only way to monitor snow cover spatially, temporally and economically effectively. To obtain information about certain geophysical variable using satellite data, a model for interpreting the satellite signal must be developed. The feasibility of satellite-borne observations in describing geophysical variables depends on the reliability of the model used. Here simple reflectance models based on the zeroth order radiative transfer equation and lineal mixing models are investigated. They are found to reliably describe the observed surface reflectances from snow-covered terrain, both in forests and in open areas. Additionally, to improve methods for seasonal snow cover monitoring in forests, the high spatial resolution observations are required to describe spectral properties and their temporal behaviour of different targets inside the investigated scene. It is also important to combine these target-specific reflectances with the in situ data to describe the characteristics of the target area. In this thesis the datasets complement each other so that while mast-borne data provides information on the temporal behaviour of the scene reflectance of the specific location where measurement conditions are well known, the airborne data provides information during a very short time (~1 hour) on the spatial variation of scene reflectance from the areas where land cover, forest characteristics and snow conditions are well defined. The results demonstrate the notable effect of forest on observed reflectance in both the temporal (changes in illumination geometry) and on the spatial (changes in forest structure) scale. The presence of tree canopy also weakens the capability of the Normalized Difference Snow Index (NDSI) to detect snow-covered areas. Additionally, the effect of melting snow cover on reflectances and NDSI is significant in all land covers producing high variation inside individual land cover types too.
  • Piipponen, Kaiu (Helsingfors universitet, 2017)
    Geothermal energy is a growing industry and with Enhanced Geothermal System (EGS) technology it is possible to utilize geothermal energy in low heat flow areas. The ongoing EGS project in Southern Finland provides a great opportunity to learn and explore EGS technologies in a complex environment: hard crystalline rock, high pressure and low hydraulic permeability. This work describes physics behind an EGS plant, as well as basic concept of EGS, give examples of some existing plants and make calculations of how much power a plant in Finland can produce. In order to plan and build a successful plant, suitable parameters for the system are determined by modelling. The modelling is done analytically and numerically. Physical properties governing the EGS models are conductive and convective heat transfer and rock hydraulic properties that allow fluid flow. Hydraulic permeability is discussed in detail, because it is the key parameter in EGS: rock is stimulated in order to enhance permeability in order to make fluid flow possible through interconnected fractures. It is a spatially correlated parameter and it is distributed lognormally making fluid flow highly channelled. Modelling of heat and mass transfer aims to parametrize an EGS plant in the conditions of Southern Finland. The parameters governing heat transfer with fluid flowing in the geothermal reservoir are size of the reservoir and fluid velocity, which depends on matrix permeability. The larger the reservoir the more hot contact area fluid encounters and the better it heats up, the slower the flow, the longer time fluid stays in the reservoir and therefore heats up more. High flow rates cool the reservoir rapidly. However, a large reservoir is difficult to achieve, maintaining enhanced permeability requires relatively high fluid flow rates and the higher the flow rate, the more power the plant produces, so slow flow is not economically feasible. Analytical models are done with Matlab and numerical models are done with finite-element software COMSOL Multiphysics. Numerical models benchmark the analytical solutions and use spatially correlated permeability to modify fluid flow pattern and see how temperature in the reservoir changes with changes in fluid flow. The results show that creating large reservoir that could operate for 20 years with desired power production is unrealistic. Total output fluid flow required to produce over 1 MW of power is 10 kg/s. At such rate there is a risk that the reservoir cools and output fluid temperature is not sufficient for power production. In case of heterogeneous permeability connectivity of the reservoir is not as good as in case of homogeneous permeability and there is a risk that total fluid flow in the reservoir is slower and therefore less power produced.
  • Hellqvist, Niina (Helsingfors universitet, 2013)
    Seismisiä heijastusluotausaineistoja käytetään yleisesti kuoren rakenteiden tulkinnassa. Voimakkaimpien heijastusamplitudivaihtelujen oletetaan liittyvän kuoren litologisiin muutoksiin sekä tektonisiin ja deformaatiorakenteisiin. Tässä tutkielmassa vahvistettiin seismisen attribuuttianalyysin avulla seismisen aineiston epäjatkuvia taustarakenteita, jotka usein jäävät voimakkaampien heijastuksien peittämiksi. Aineistona ovat FIRE 2 ja 2A – heijastusluotauslinjat, jotka sijaitsevat paleoproterotsooisella svekofennialaisella pääalueella. Alue on osa Etelä-Suomen kaarikompleksia, ja se antaa hyvän mahdollisuuden svekofennialaisen orogenian rakenteen tutkimiseen. Seisminen aineisto suodatettiin kaadeohjatulla mediaanisuodattimella, joka parantaa aineiston taustalla olevien heijastajien jatkuvuutta vähentämällä satunnaista kohinaa. Suodatus aloitetaan määrittelemällä tutkimusikkuna, joka sisältää useita rekisteröintejä. Fourierin muunnoksella lasketaan rekisteröinneille paikalliset kaadearvot. Kaadeohjaus seuraa seismistä tapausta rekisteröinnistä toiseen käyttäen hyväksi näitä kaadearvoja. Kaadeohjattu mediaanisuodatin poimii ikkunan sisällä olevien rekisteröintien amplitudien mediaaniarvon ja korvaa kaikki ikkunan sisälle sattuvat rekisteröinnit tällä mediaaniarvolla. Tällöin aineistosta poistuvat yksittäiset korkean amplitudin heijastajat ja taustalla olevat rakenteet korostuvat. Kaadeohjattu mediaanisuodatin paransi heijastusluotausaineistossa olevien yksityiskohtaisten rakenteiden havaittavuutta. Attribuuttianalysoitujen tulosten tulkinnassa käytettiin seismistä fasies – menetelmää. Suodatetuista sektioista tulkittiin läpikotaista S-C' – deformaatiorakennetta, joka indikoi ekstensionaalista ympäristöä. Ekstensio tulkitaan liittyvän Svekofennidien romahtamiseen.
  • Pellikka, Hilkka (Helsingfors universitet, 2011)
    Sea level rise is among the most worrying consequences of climate change, and the biggest uncertainty of sea level predictions lies in the future behaviour of the ice sheets of Greenland and Antarctica. In this work, a literature review is made concerning the future of the Greenland ice sheet and the effect of its melting on Baltic Sea level. The relation between sea level and ice sheets is also considered more generally from a theoretical and historical point of view. Lately, surprisingly rapid changes in the amount of ice discharging into the sea have been observed along the coastal areas of the ice sheets, and the mass deficit of Greenland and West Antarctic ice sheets which are considered vulnerable to warming has been increasing from the 1990s. The changes are probably related to atmospheric or oceanic temperature variations which affect the flow speed of ice either via meltwater penetrating to the bottom of the ice sheet or via changes in the flow resistance generated by the floating parts of an ice stream. These phenomena are assumed to increase the mass deficit of the ice sheets in the warming climate; however, there is no comprehensive theory to explain and model them. Thus, it is not yet possible to make reliable predictions of the ice sheet contribution to sea level rise. On the grounds of the historical evidence it appears that sea level can rise rather rapidly, 1-2 metres per century, even during warm climate periods. Sea level rise projections of similar magnitude have been made with so-called semiempirical methods that are based on modelling the link between sea level and global mean temperature. Such a rapid rise would require considerable acceleration of the ice sheet flow. Stronger rise appears rather unlikely, among other things because the mountainous coastline restricts ice discharge from Greenland. The upper limit of sea level rise from Greenland alone has been estimated at half a metre by the end of this century. Due to changes in the Earth s gravity field, the sea level rise caused by melting ice is not spatially uniform. Near the melting ice sheet the sea level rise is considerably smaller than the global average, whereas farther away it is slightly greater than the average. Because of this phenomenon, the effect of the Greenland ice sheet on Baltic Sea level will probably be rather small during this century, 15 cm at most. Melting of the Antarctic ice sheet is clearly more dangerous for the Baltic Sea, but also very uncertain. It is likely that the sea level predictions will become more accurate in the near future as the ice sheet models develop.
  • Pöntinen, Mikko (Helsingin yliopisto, 2018)
    One of the main factors currently limiting geophysical and geological studies of asteroids is the lack of visual and near-infrared (Vis-NIR) spectra. European Space Agency’s upcoming Euclid mission will observe up to 150,000 asteroids and gather a large amount of spectral data of them in the Vis-NIR wavelength range. Asteroids will appear as faint streaks in the images. In order to exploit the spectra, the asteroids have to first be found in the massive amounts of data to be obtained by Euclid. In this work we tested two methods for detecting asteroid streaks in simulated Euclid images. The first method is StreakDet, a software originally developed to detect streaks caused by space debris. We optimized the parameters of StreakDet, and developed a comprehensive analysis software that can visualize and give statistics of the StreakDet results. StreakDet was tested by feeding 4096×4136 pixel images to the software, which then returned the coordinates of the asteroids found. The second method is machine learning. We programmed a deep neural network, which was then trained to distinguish between asteroid images and non-asteroid images. Smaller images were used for this binary classification task, but we also developed a sliding window method for analyzing larger images with the neural network. After optimizing the program parameters, StreakDet was able to detect approximately 60% of asteroids with apparent magnitude V < 22.5. StreakDet worked better for long streaks, up to 125 pixels (corresponding to an asteroid with a sky motion of 80 "/h) while streaks shorter than 15 pixels (10 "/h) were typically not found. The neural network was able to classify the brightest (20 < V < 21) streaks with up to 98% accuracy when using very small images. When analyzing larger images, the sliding window algorithm produced heat maps as output, from which the asteroids could easily be spotted. The machine learning algorithm utilized was fairly simple, so even better results may be obtained with more advanced algorithms.
  • Kokkonen, Tom (Helsingin yliopisto, 2019)
    The processes related to urban development (urbanisation, densification, irrigation and worsened air quality) are assumed to affect the urban hydrological cycle, but little is known about the impact of the individual processes. One of the reasons for the knowledge gaps is the lack of measurements for the needed resolution or for the period of interest. Reanalysis products can provide the needed data, but those have not been evaluated in urban areas. Furthermore, model evaluation is commonly made against eddy covariance (EC) measurements, but little is known of the uncertainties related to often non-ideal instrument location at urban areas. To answer the uncertainties in urban hydrological cycle, Surface Urban Energy and Water Balance Scheme (SUEWS) forced with the WATCH Forcing Data is used. The analyses are performed in cities located in different climate conditions (Vancouver, London and Beijing) and above varying urban land covers (dense city centre and suburban areas). To understand better the uncertainties related to model evaluation, uncertainties of EC method are analysed using two identical EC systems at the same level close to each other in central Helsinki. The most crucial reanalysis variables to correct are precipitation due to coarse spatial resolution and the incoming solar radiation due to haze. SUEWS performs well when forced with corrected WATCH data, which allows detailed analysis of urban hydrological cycle. The irrigation has the dominant effect over densification and urbanisation on suburban hydrological cycle. The densification increases the runoff as much as the initial urbanisation, even though the increase of impervious surfaces is much smaller. The haze decreases evaporation which increases runoff and soil infiltration especially at smaller daily precipitation totals. This is expected to flush pollutants from surfaces and increase the pollutant loads of urban waters. After the post-processing of the EC data, systematic uncertainties in latent heat flux originating from a single-point observation above dense city centre due to removal of large fraction of wind disturbed data are only 8%. Thus, useful and representative EC observations can be obtained from urban areas, despite the errors induced by the non-ideal location. The results of this thesis answer the uncertainties in urban hydrological cycle by bringing new knowledge of the dominant factors in the urban water balance and the representativeness of reanalysis data in urban areas. The results can help urban planners to design sustainable cities being able to mitigate and adapt to the common problems in urban hydrology.