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Browsing by master's degree program "Ilmakehätieteiden maisteriohjelma (Atmospheric Sciences)"

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  • Zhang, Tinghan (2022)
    Air ions can play an important role in new particle formation (NPF) process and consequently influence the atmospheric aerosols, which affect climate and air quality as potential cloud condensation nuclei. However, the air ions and their role in NPF have not been comprehensively investigated yet, especially in polluted area. To explore the air ions in polluted environment, we compared the air ions at SORPES site, a suburban site in polluted eastern China, with those at SMEAR II, a well-studied boreal forest site in Finland, based on the air ion number size distribution (0.8-42 nm) measured with Neutral Cluster and Air Ion Spectrometer (NAIS) during 7 June 2019 to 31 August 2020. Air ions were size classified into three size ranges: cluster (0.8-2 nm), intermediate (2-7 nm), and large (7-20 nm). Median concentration of cluster ions at SORPES (217 cm−3) was about 6 times lower than that at SMEAR II (1268 cm−3) due to the high CS and pre-existing particle loading in polluted area, whereas the median large ion concentration at SORPES (197 cm−3) was about 3 times higher than that of SMEAR II (67 cm−3). Seasonal variations of ion concentration differed with ion sizes and ion polarity at two sites. High concentration of cluster ions was observed in the evening in the spring and autumn at SMEAR II, while the cluster ion concentration remained at a high level all day in the same seasons. The NPF events occurred more frequently at SORPES site (SMEAR II 16% ; SORPES: 39%), and the highest values of NPF frequency at both sites were in spring ((SMEAR II: spring: 43%; SORPES: spring: 56%). During the noon time on NPF event day, the concentration of intermediate ions were 8-14 times higher than same ours on non-event days, indicating that can be used as an indicator for NPF in SMEAR II and SORPES. The median formation rate of 1.5 nm at SMEAR II were higher then that at SORPES, while higher formation rate of 3 nm ions were observed at SORPES. At 3 nm, the formation rate of charged particles was only 11% and 1.6% of the total rate at SMEAR II and SORPES respectively, which supports the current view that neutral ways dominate the new particle process in continental boundary. However, higher ratio between charged and total formation rate of 3 nm particle at SMEAR II indicates ion-induced nucleation can have a bigger contribution to NPF in clear area in comparison to polluted area. Higher median GR of 3-7 nm (SMEAR II: 3.1 nm h−1; SORPES: 3.7 nm h−1) and 7-20 nm (SMEAR II: 5.5 nm h−1; SORPES: 6.9 nm h−1) ions at SORPES were found in comparison to SMEAR II, suggesting the higher availability of condensing vapors at SORPES. This study presented a comprehensive comparison of air ions in completely different environments, and highlighted the need for long-term ion measurements to improve the understanding of air ions and their role in NPF in polluted area like eastern China
  • Sholeninova, Polina (2021)
    The Arctic Ocean is known to be inhabited with energetic mesoscale eddies commonly detected in depths from 200 m to 1200 m. Due to their high energetics and ability to transfer momentum, heat, salt and biochemical properties for long distances from their origin, eddies may considerably affect the structure of a water column in the Arctic Ocean. This study investigated an anticyclonic eddy event detected at one of the mooring stations deployed under the Nansen and Amundsen Basins Observational System project. The mooring located at the deep part of the continental slope of the Laptev Sea and conducted autonomous measurements during the years 2013–2015. The conductivity-temperature-depth, as well as current measurements from the Acoustic Doppler Current Profiler in the upper ocean (24–82 m) and from the McLane Moored Profiler in the intermediate layer (88–760 m), were examined. Spectral analysis of the currents and calculation of the eddy available potential energy were performed. This study revealed a mesoscale eddy with the core centred deeper than 750 m drifted past the mooring for 2 months. Its horizontal length scale was ∼128 km. The water properties typical for the Fram Strait Branch of the Atlantic water carried by the subsurface boundary current were trapped in the eddy. This study suggests that the eddy was originated from the baroclinic instability of the front between the Fram Strait Branch and the Barents Sea Branch of the Atlantic water flow.
  • Santillo, Jordan (2022)
    Research in radar technology requires readily accessible data from weather systems of varying properties. Lack of real-world data can delay or stop progress in development. Simulation aids this problem by providing data on demand. In this publication we present a new weather radar signal simulator. The algorithm produces raw time series data for a radar signal using physically based methodology with statistical techniques incorporated for computational efficiency. From a set of user-defined scatterer characteristics and radar system parameters, the simulator solves the radar range equation for individual, representative precipitation targets in a virtual weather cell. The model addresses the question of balancing utility and performance in simulating signal that contains all the essential weather information. For our applications, we focus on target velocity measurements. Signal is created with respect to the changing position of targets, leading to a discernable Doppler shift in frequency. We also show the operation of our simulator in generating signal using multiple pulse transmission schemes. First, we establish the theoretical basis for our algorithm. Then we demonstrate the simulator's capability for use in experimentation of advanced digital signal processing techniques and data acquisition, focusing on target motion. Finally, we discuss possible future developments of the simulator and their importance in application.
  • Rannisto, Henri (2022)
    Suomen lentosäähavainnot käyvät läpi murrosta kohti automaatiota. Automaattisiin havaintoihin liittyy laatuongelmia, joten syntyi idea tehdä aiheesta laajempi tutkimus. Tutkimusaineistona käytettiin Rovaniemen lentoaseman havainnontekijöiden vuodesta 2011 lähtien täyttämää verifiointitaulukkoa, jossa ideana on kirjata manuaalisen havainnon tekohetkellä ylös automaattijärjestelmän tarjoamat arvot eri sääsuureille. Vertailtavat parametrit ovat näkyvyys, pilven alaraja ja vallitseva sää. Parametrien automaatin ja ihmisen määrittämät arvot ristiintaulukoitiin jokaiselle kolmelle parametrille erikseen. Tulokset eivät antaneet kovin hyvää kuvaa automaattihavaintojen nykyisestä laadusta, sillä kaikkien kolmen parametrin osalta havainnoista löytyi merkittäviä puutteita arvojen tarkkuudessa ja ajantasaisuudessa. Erot tarkaksi oletettuihin ihmishavaintoihin olivat niin suuria, että esiin nousi kysymyksiä lentoturvallisuuteen ja automaattihavaintojen käytön järkevyyteen liittyen. Tulosten pohjalta esitetään ratkaisuksi merkittäviä parannuksia havaintojärjestelmään sekä havaintojen tilapäistä manualisointia parannusprosessin ajaksi. Tutkielmassa käydään varsinaisen tutkimusosion lisäksi läpi Suomen lentosäähavaintojen teoriaa. Tekstissä pureudutaan syvemmin manuaalisen ja automaattisen havaintomenetelmän perusperiaatteisiin sekä esitellään Suomen lentosäähavaintojen historiaa pääpiirteittäin.
  • Laasonen, Asta (2021)
    Carbon monoxide (CO) is a chemically reactive trace gas in the atmosphere, indirectly affecting radiative balance. The oxidation of CO with hydroxyl radical (OH) is the large sink of atmospheric CO. The reactions of CO and OH decrease the atmospheric capacity to oxidize atmospheric methane (CH4), hence indirectly extends the lifetime of CH4 in the atmosphere. In addition, CO oxidation increases the abundance of tropospheric ozone (O3). CH4 and O3 are both very strong greenhouse gases, and it has been estimated that the cumulative indirect radiative forcing of CO can be even more significant than the third most powerful greenhouse gas, nitrous oxide. This study studied CO fluxes in four different ecosystems: a boreal forest, a boreal fen, a cropland in the boreal region, and a sisal plantation in the semi-arid tropical zone. All the ecosystems were CO sources during the growing season from May to August, and ecosystems showed strong seasonal variation. Fluxes had a regular diurnal cycle, peaking at noon and zero flux or small uptake at night. The main drivers for the CO emissions were radiation and air temperature. The strong correlation between radiation and CO flux proved that photodegradation was an important process in biogenic CO emissions. Radiation and air temperature were used in a simple linear regression model to estimate the biogenic CO emissions in the study sites. The model was trained for Hyytiälä data in 2016, tested for the rest of the data from Hyytiälä in 2015 and 2017 and other sites. The chamber measurements showed that soils were CO sinks and CO emissions were mainly from vegetation. Generally, in many upscaling models of CO, soil consumption is considered significantly larger than photodegradation. This study showed that many terrestrial ecosystems can be sources of CO, even though there are generally considered as a sink of CO. There is a need for ecosystem-scale flux measurements in other ecosystems and latitudes to understand better the global CO budget.
  • Sorsa, Jani (2022)
    Ilmatieteen laitoksella on otettu käyttöön eri säämallien ennusteita yhdistelevä, niin sanotun konsensusennusteperiaatteen mukainen jälkikäsittelymenetelmä, joka tunnetaan nimellä Blend. Tämä tutkielman tarkoituksena on selvittää Blend-menetelmällä tuotetun tuuliennusteen toimivuutta Suomen merialueilla käyttämällä muutamaa yleiseen käyttöön vakiintunutta sääennusteiden verifiointimenetelmää. Verifiointi on toteutettu vertaamalla Blend-ennusteen tuulennopeusarvoja niin ikään jälkikäsittelyllä tuotettuihin potentiaalituuliarvoihin 25:llä Suomen merialueilla sijaitsevalla havaintoasemalla. Potentiaalituulta on päätetty käyttää alkuperäisten tuulihavaintojen sijasta, koska se parantaa eri sääasemilta tulevien mittaustulosten keskinäistä vertailukelpoisuutta ja näin ollen tekee verifiointituloksista paremmin koko alueelle yleistettäviä. Tulokset osoittavat odotetusti, että merkittävimmät Blend-tuuliennusteen toimivuuteen vaikuttavat tekijät ovat tuulennopeus ja ennustepituus – ennustevirhe kasvaa yleisesti suuremmilla tuulennopeuksilla ja pidemmillä ennustepituuksilla. Myös muilla muuttujilla, kuten vuorokauden- ja vuodenajalla sekä tuulen suunnalla, havaittiin olevan jonkin verran vaikutusta ennustevirheeseen. Useimmissa säätilanteissa Blend-ennusteen voidaan todeta olevan toimivuudeltaan varsin hyvä ja tasalaatuinen. Blend-ennusteen merkittävin ongelma on etenkin suurilla tuulennopeuksilla huomattavan suuri negatiivinen harha (bias), eli ennustetut tuulennopeudet ovat havaintoihin nähden selvästi liian heikkoja. Tästä johtuen Blend ei useimmissa tapauksessa kykene ennustamaan kovimpia tuulia, jotka ovat harvinaisuudestaan huolimatta operatiivisen sääennustamisen kannalta kaikista tärkeimpiä mm. merialueille annettavien tuulivaroitusten vuoksi. Menetelmä on kuitenkin kehityskelpoinen, ja jos ennusteharha pystytään jatkossa minimoimaan laskennassa paremmin, se saattaa kyetä tuottamaan jopa varsinaisia säämalleja parempia tuuliennusteita.
  • Vainio, Tomi (2022)
    Small arctic glaciers have in general been consistently neglected with respect to the collection of long time-series observations. Available data is often a product of multiple independent and separate studies, thus gaps in the data sets are common. Numerical modelling provides one solution to alleviate existing gaps in knowledge, while historical observations can be used to assess model accuracy. The Foxfonna ice cap and associated glacier were investigated with the aid of the numerical modelling software, Elmer/Ice. The goal was to reproduce core glaciological characteristics of the entire glacier system from a 3D simulation based on multiple digital elevation models (DEMs) between the years 1961-2021. The methods proved capable of providing additional information on the glaciological characteristics of a small glacier system, such as Foxfonna. Issues primarily arose from the steady state assumption and the difficulty of producing simulations for a dynamically varying glacier system.
  • Paakkanen, Elias (2022)
    Tässä työssä on tutkittu Euroopan ja Pohjois-Atlantin talvi-ilmaston muuttumista 30-vuotisjaksojen 1961–1990 ja 1991–2020 välillä. Aineistona on käytetty Euroopan keskipitkien sääennusteiden keskuksen (ECMWF) kehittämää ERA5-uusanalyysidataa, jossa on assimiloitu havaintoja sääennustusmallin tuottamaan alkuarvauskenttään. Karttakuvat on piirretty niin ikään ECMWF:n kehittämällä ohjelmistolla, Metviewillä. Lämpötilan muutoksen pystyleikkauskuvan piirtämiseen on puolestaan käytetty Pythonin numpy- ja matplotlib.pyplot -kirjastoja. Työssä on tarkasteltu ilmanpaineessa, suihkuvirtauksessa, lämpötilassa, pystyliikkeissä, kosteudessa ja sademäärässä tapahtuneita muutoksia. Ennen varsinaisia tuloksia tutkielmassa on selitetty meteorologisiin suureisiin liittyvää fysikaalista teoriaa: miten paine, lämpötila ja tiheys ovat riippuvaisia toisistaan, kuinka geostrofinen tuuli syntyy sekä mitkä tekijät vaikuttavat sateen syntyyn. Kaikissa tarkasteltavissa suureissa on havaittu muutoksia. Lämpötilat ovat nousseet lähes koko Euroopan ja Pohjois-Atlantin alueella: eniten Pohjois-Euroopassa ja Pohjoisella jäämerellä sekä vähemmän Etelä-Euroopassa. Ilmanpaine on noussut Pohjois-Euroopassa ja Pohjois-Atlantin pohjoisosassa sekä laskenut Etelä-Euroopassa ja Pohjois-Atlantin eteläosassa. 250 hPa:n painepinnan Pohjois-Atlantin keskimääräinen suihkuvirtausmaksimi on voimistunut ja liikahtanut hieman pohjoisemmaksi. Sademäärät ja ilman sisältämän vesihöyryn määrä ovat kasvaneet Pohjois-Euroopassa ja pienentyneet Etelä-Euroopassa. Nousu- ja laskuliikkeet ovat monin paikoin voimistuneet. Muutosten tilastollisen merkitsevyyden tutkimiseen on käytetty Studentin kaksisuuntaista t-testiä. Alatroposfäärin lämpötilan muutos on eniten tilastollisesti merkitsevä, mutta muidenkin suureiden muutoksissa tilastollista merkitsevyyttä havaittiin laajalti. Tämä on loogista, sillä lämpötilan muutokset ovat kytköksissä myös muiden suureiden muutoksiin. Aiheesta on tehty myös aiemmin tutkimuksia, joiden tulokset ovat pääosin yhteensopivia tämän työn tulosten kanssa. Ainoastaan 500 hPa:n painepinnan geopotentiaalikorkeuden trendissä oli pientä eroavaisuutta. Tässä tutkielmassa muutosten tilastollinen merkitsevyys oli suurempaa kuin aiemmissa tutkimuksissa.
  • Nieminen, Elina (2022)
    The legislation of the Paris Agreement obliges Finland to pursue actions that keep the global average temperature rise below 2°C and aim to limit the average temperature rise to 1.5°C. The current Finnish government has aligned the national goal of carbon neutrality by 2035. The role of municipalities in promoting or compensating carbon sinks has not yet been defined, although municipalities play an important role as a platform for climate work at local and regional levels. However, it is already known that the Finnish National Climate Act, which is being reformed at this moment, will be subject to an obligation to produce their own climate programs at municipal, regional or provincial level. Environmental competence and environmental development have been important in Lahti for several decades already. The City of Lahti has set its target for carbon neutrality for 2025 and it includes targets for reducing, compensating, and increasing carbon sinks. This work focused on the examination of carbon sequestration and sinks in an urban environment in Lahti, in the example area of approximately 82 hectares, through which a wider understanding of the city's potential to grow coal stocks and sinks in a tight urban structure within different land use classes and different ground cover between them. Based on the Finnish Environment Agency's CORINE land cover classification, the current potential of carbon sequestration for urban land use classes were calculated in this work and the actions to increase carbon sequestration capacity were identified. The work examined the availability of the finished spatial data and to supplement incomplete information, existing literature on the topic was used, as well as other existing spatial records of the city of Lahti and previously made surveys. The largest carbon sink was observed in forest areas, of which in mixed forests representing the largest forest type in the area. Through the calculations and literature carbon sinks and stocks in residential areas were also found to be significant in terms of vegetation, as well as in terms of soil based on the literature review. In planting street and park trees for the purpose of increasing the carbon sink, the most important thing was found to be the long lifetime of trees and securing it. Growing of carbon sinks is most effective in areas where carbon sequestration is already at a high level but increasing vegetation cover in all urban land covers will increase the carbon sink in the long run. One major conclusion of the work was that Lahti's current method of determining carbon sinks and stocks has been inadequate at least for the determining them in built areas, and future measures to maintain, preserve and increase carbon stocks and sinks would not be seen by the same calculation method in the computing. In general, the research data and methods are still largely based on observations and results from the operational processes of natural ecosystems, and these are utilized in urban planning, construction, and maintenance of urban green areas. An incomplete knowledge of the ecological processes in urban areas is a problem that produced challenges in this work as well. More research data is needed on carbon sinks in urban land use classes to gain a more secure understanding of carbon sinks and stocks, although the common importance of vegetation in urban areas is already clear. Although the work focused on carbon in an urban environment, it is necessary to remember the diversity of the urban environment and the other ecosystem services it produces. Land use planning, as well as the management of green spaces in the urban environment, can enhance both the size of carbon storages and sinks and biodiversity and they do not have to be entirely separate from each other.
  • Leino, Henrik (2022)
    Low-level wind shear is a significant aviation hazard. A sudden reduction in the headwind along an aircraft flight path can induce a loss of lift, from which an aircraft may not be able to recover when it is close to the ground. Airports therefore use low-level wind shear alert systems to monitor wind velocities within the airport terminal area and alert of any detected hazardous wind shear. There exist three ground-based sensor systems capable of independently observing low-level wind shear: a Doppler weather radar-based, a Doppler wind lidar-based, and an anemometer-based system. However, as no single sensor system is capable of all-weather wind shear observations, multiple alert systems are used simultaneously, and observations from each system are integrated to produce one set of integrated wind shear alerts. Algorithms for integrating Doppler weather radar and anemometer wind shear observations were originally developed in the early 1990s. However, the addition of the Doppler wind lidar-based alert system in more recent years warrants updates to the existing radar/anemometer integration algorithms. This thesis presents four different replacement candidates for the original radar/anemometer integration algorithms. A grid-based integration approach, where observations from different sensor systems are mapped onto a common grid and integrated, is found to best accommodate central integration considerations, and is recommended as the replacement to the original radar/anemometer algorithms in operational use. The grid-based approach is discussed in further detail, and a first possible implementation of the algorithm is presented. In addition, ways of validating the algorithm and adopting it for operational use are outlined.
  • Aldana, Miguel Francisco (2021)
    Accuracy and general performance of weather radar measurements are of great importance to society due to their use in quantitative precipitation estimation and its role on flood hazard risks prevention, agriculture or urban planning, among others. However, radars normally suffer from systematic errors such as attenuation, misscalibration in Z field or bias in Zdr field, or random errors such as clutter, beam blockage, noise, non-meteorological echoes or non-uniform beam filling, which affect directly the rain rate estimates or any other relevant product to meteorologists. Impact of random errors is reduced by exploiding the polarimetric properties of polarimetric radars by identifying and classifying measurements according to their signature and a classification scheme based on the available polarimetric variables, but systematic errors are more difficult to address as they require a ’’true’’ or reference value in order to be corrected. The reference value can either be absolute or obtained from another radar variable. In reality, an absolute reference value is not feasible because we normally do not know what we are observing with the radar. Therefore, a way of assesing this issue is by elaborating theoretical relations between radar variables based on their consistency when measuring a volume with hydrometeors of known characteristics such as size and concentration. This procedure is known as self-consistency theory and it is a powerful tool for checking radar measurements quality and correcting offsets causing bias, misscalibration or attenuation. The theoretical radar variables themselves can be simulated using available T-Matrix scattering algorithms, that estimate the scattered phase and amplitude for a given distribution of drops of a given size. Information of distribution of drops of a given size, commonly referred as drop size distributions, can be obtained, for instance, from gauge or disdrometer measurements. Once the theoretical relations among radar variables are established, it is possible to check the consistency of, for instance, measured differential reflectivity with respect to differential reflectivity calculated as function of measured reflectivity, assuming the latter has been filtered properly, and any discrepancy between the observed and theoretical differential reflectivity can be thus attributed to offsets in the radar. This work thus presents a methodology for the revision of radar measurements filtering and quality for their improvement by correcting bias and calibration, using theoretical relations between radar variables through self-consistency theory. Furthermore, as the aforementioned issues are easier to track and resolve in the liquid rain regime of precipitation, this work presents a detailed description of methodologies to exclude ice-phased hydrometeors such as the melting layer detection algorithm and its operational implementation along with other complementary filters suggested in the literature. Examples of the melting layer detection and filtering as well as self-consistency curves for radar measurement performance evaluation are also provided.
  • Leino, Joonas (2022)
    Mars-planeetan kaasukehä koostuu enimmäkseen hiilidioksidista, kun taas vesihöyryä on hyvin vähän. Kaasukehän lämpötila vaihtelee noin +10 ja -130 Celsius-asteen välillä ja pintapaine on vain noin sadasosa Maan ilmakehän paineesta. Marsin kaasukehässä on usein paljon hienojakoista pölyä, joka absorboi tehokkaasti auringonsäteilyä ja täten vaikuttaa kaasukehän toimintaan. Marsin pinnan reagoidessa erittäin nopeasti auringonsäteilyn määrän muutoksiin sekä kaasukehässä olevan pölyn vuoksi rajakerroksen mallinnuksessa käytettävissä malleissa säteilyn parametrisaatioiden täytyy olla mahdollisimman hyviä. Helsingin yliopisto ja Ilmatieteen laitos ovat kehittäneet Marsin kaasukehän tutkimukseen tarkoitetun 1-ulotteisen pylväsmallin. Malli on erittäin nopea ja helposti muokattavissa, joten sillä voidaan testata uusia ilmakehäfysiikan lainalaisuuksia ja algoritmeja, joita voidaan mahdollisesti lisätä kolmiulotteisiin Marsin kaasukehän malleihin. Tämä työ tehtiin osana Ilmatieteen laitoksen Marsin tutkimusryhmää ja työssä tutustutaan Marsin kaasukehän rajakerrokseen sekä pylväsmalliin. Lisäksi mallin antamia tuloksia esitellään ja verrataan Curiosity mönkijän (toiselta nimeltään Mars Science Laboratory, MSL) havaintoihin sekä tutkitaan mallin herkkyyttä sen alustusparametreihin. Mallin ennustamia lämpötilan, vesihöyryn tilavuuden sekoitussuhteen ja suhteellisen kosteuden vuorokausisyklejä verrattiin MSL:n havaintoihin eri vuodenaikoina. MSL laskeutui vuonna 2012 lähelle Marsin päiväntasaajaa Gale-kraatterin pohjalle ja se sisältää Ilmatieteen laitoksen suunnittelemat ja rakentamat mittalaitteet paineelle ja suhteelliselle kosteudelle. Mallin ennustamat vuorokausisyklit vastasivat hyvin mönkijän havaintoja ja tuloksista nähtiin myös lämpötilan suuri vuorokausivaihtelu kaasukehän reagoidessa nopeasti auringonsäteilyn muutoksiin. MSL:n paineen mittauksista (yli 3000 Marsin vuorokautta) nähtiin selvästi hiilidioksidin vuodenaikaiskierto etelänavalta pohjoisnavalle ja päinvastoin. Lisäksi vuoden 2018 globaali pölymyrsky näkyi monissa eri mittaustuloksissa. Mallin herkkyyttä tutkittiin muuttamalla neljää eri alustusparametria: pinnan lämpötilaa ja painetta, ilmapylvään vesisisältöä (PWC) sekä pölyn optista paksuutta (tau). Näiden testien perusteella mallin ennustamiin vuorokauden lämpötilaprofiileihin eniten vaikuttivat pinnan lämpötilan ja pölyn optisen paksuuden alustus, kun taas kosteusprofiileihin eniten vaikuttivat PWC:n ja pölyn optisen paksuuden alustus. Näistä parametreista pinnan paineen alustuksella oli vähiten vaikutusta mallin ennustamiin profiileihin.
  • Hasu, Mikael (2022)
    This thesis investigates how the Lorenz model state sensitivity appears on the prior state error of the Extended Kalman Filter (EKF) process. The Lorenz model is a well-known ordinary differential equation system. Its simple nonlinear equations show that a chaotic system, like the atmosphere, does not have a single deterministic solution. Edward N. Lorenz also showed that the predictability of the state depends on the flow itself, and numerical weather prediction models, therefore, cannot always be trusted equally. For this reason, when computing a forecast, it is necessary to consider both the model and observations with their weight uncertainties to get the most probabilistic analysis state. The EKF is an algorithm that provides a powerful data assimilation method for nonlinear systems. Its operating principle is based on the evolution of prior state (model evolution) and observation updates. Each observation update calculates the most likely state based on the prior state and observation errors. The process continues from the new analysis state by evolving the model until the next observation update. In this study, I made the EKF utilizing the Lorenz model and sent ensembles from the analysis states on the Lorenz attractor. I calculated the variance of evolved ensembles and compared them to the magnitude of prior state error at the observation update time levels. The results showed that these two parameters are positively correlated. For the 18-timestep observation interval, Pearson’s correlation coefficient was 0.850, which indicates a very high correlation. Therefore, it can be concluded that when the prior state error is small, the ensemble on the Lorenz attractor indicates good predictability (i.e., dispersion of ensemble members is small) and vice versa.
  • Lobo, Hannah (2021)
    The lidar depolarisation ratio is used for aerosol categorisation as it is indicative of aerosol shape. Commonly, depolarisation ratio is measured in short term studies at short wavelengths such as 355 nm and 532 nm. The depolarisation ratio has a spectral dependency and so exploring values at longer wavelengths could be valuable for future studies. Here, aerosol depolarisation ratio at 1565 nm is measured across Finland’s ground based remote sensing network over a four year period. The Halo Photonics StreamLine Doppler lidars instruments were found to be stable over long time periods and cloud based calibration was used to correct for the bleed though. The depolarisation ratio of elevated aerosol layers was compared to boundary layer aerosol. A higher average depolarisation ratio was found for elevated aerosol with the exception of boreal forest sites in the summer months where values were similar. Elevated aerosols over Finland were found to originate mostly from the Arctic, Europe, Russia and North America using aerosol transport models. Four case studies were looked at in more detail: Saharan dust with a depolarisation ratio of 0.249 ± 0.018, pollen with a depolarisation ratio of 0.207 ± 0.013, anthropogenic pollution with a depolarisation ratio of 0.067 ± 0.009, and a mixed layer with a depolarisation ratio of 0.152 ± 0.019 thought to be pollen and smoke. Based on this study, Halo Doppler Lidar can be used to measure elevated aerosol at 1565 nm in the long term. Future studies could use 1565 nm depolarisation ratio alongside commonly used shorter wavelengths to aid aerosol categorisation.