Fast missing value imputation using ensemble of SOMs

This report presents a methodology for missing value imputation. The methodology is based on an ensemble of Self-Organizing Maps (SOM), which is weighted using Nonnegative Least Squares algorithm. Instead of a need for lengthy validation procedure as when using single SOMs, the ensemble proceeds straight into final model building. Therefore, the methodology has very low computational time while retaining the accuracy. The performance is compared to other state-of-the-art methodologies using two real world databases from different fields

Methodologies for time series prediction and missing value imputation

The amount of collected data is increasing all the time in the world. More sophisticated measuring instruments and increase in the computer processing power produce more and more data, which requires more capacity from the collection, transmission and storage. Even though computers are faster, large databases need also good and accurate methodologies for them to be useful in practice. Some techniques are not feasible to be applied to very large databases or are not able to provide the necessary accuracy. As the title proclaims, this thesis focuses on two aspects encountered with databases, time series prediction and missing value imputation. The first one is a function approximation and regression problem, but can, in some cases, be formulated also as a classification task. Accurate prediction of future values is heavily dependent not only on a good model, which is well trained and validated, but also preprocessing, input variable selection or projection and output approximation strategy selection. The importance of all these choices made in the approximation process increases when the prediction horizon is extended further into the future. The second focus area deals with missing values in a database. The missing values can be a nuisance, but can be also be a prohibiting factor in the use of certain methodologies and degrade the performance of others. Hence, missing value imputation is a very necessary part of the preprocessing of a database. This imputation has to be done carefully in order to retain the integrity of the database and not to insert any unwanted artifacts to aggravate the job of the final data analysis methodology. Furthermore, even though the accuracy is always the main requisite for a good methodology, computational time has to be considered alongside the precision. In this thesis, a large variety of different strategies for output approximation and variable processing for time series prediction are presented. There is also a detailed presentation of new methodologies and tools for solving the problem of missing values. The strategies and methodologies are compared against the state-of-the-art ones and shown to be accurate and useful in practice.Maailmassa tuotetaan koko ajan enemmän ja enemmän tietoa. Kehittyneemmät mittalaitteet, nopeammat tietokoneet sekä kasvaneet siirto- ja tallennuskapasiteetit mahdollistavat suurien tietomassojen keräämisen, siirtämisen ja varastoinnin. Vaikka tietokoneiden laskentateho kasvaa jatkuvasti, suurten tietoaineistojen käsittelyssä tarvitaan edelleen hyviä ja tarkkoja menetelmiä. Kaikki menetelmät eivät sovellu valtavien aineistojen käsittelyyn tai eivät tuota tarpeeksi tarkkoja tuloksia. Tässä työssä keskitytään kahteen tärkeään osa-alueeseen tietokantojen käsittelyssä: aikasarjaennustamiseen ja puuttuvien arvojen täydentämiseen. Ensimmäinen näistä alueista on regressio-ongelma, jossa pyritään arvioimaan aikasarjan tulevaisuutta edeltävien näytteiden pohjalta. Joissain tapauksissa regressio-ongelma voidaan muotoilla myös luokitteluongelmaksi. Tarkka aikasarjan ennustaminen on riippuvainen hyvästä ja luotettavasta ennustusmallista. Malli on opetettava oikein ja sen oikeellisuus ja tarkkuus on varmistettava. Lisäksi aikasarjan esikäsittely, syötemuuttujien valinta- tai projektiotapa sekä ennustusstrategia täytyy valita huolella ja niiden soveltuvuus mallin yhteyteen on varmistettava huolellisesti. Tehtyjen valintojen tärkeys kasvaa entisestään mitä pidemmälle tulevaisuuteen ennustetaan. Toinen tämän työn osa-alue käsittelee puuttuvien arvojen ongelmaa. Tietokannasta puuttuvat arvot voivat heikentää data-analyysimenetelmän tuottamia tuloksia tai jopa estää joidenkin menetelmien käytön, joten puuttuvien arvojen arviointi ja täydentäminen esikäsittelyn osana on suositeltavaa. Täydentäminen on kuitenkin tehtävä harkiten, sillä puutteellinen täydentäminen johtaa hyvin todennäköisesti epätarkkuuksiin lopullisessa käyttökohteessa ja ei-toivottuihin rakenteisiin tietokannan sisällä. Koska kyseessä on esikäsittely, eikä varsinainen datan hyötykäyttö, puuttuvien arvojen täydentämiseen käytetty laskenta-aika tulisi minimoida säilyttäen laskentatarkkuus. Tässä väitöskirjassa on esitelty erilaisia tapoja ennustaa pitkän ajan päähän tulevaisuuteen ja keinoja syötemuuttujien valintaan. Lisäksi uusia menetelmiä puuttuvien arvojen täydentämiseen on kehitetty ja niitä on vertailtu olemassa oleviin menetelmiin

Mapping of Sea Surface Nutrients in the North Pacific: Basin-wide Distribution and Seasonal to Interannual Variability

Monthly maps of sea surface nutrient (phosphate, nitrate and silicate) concentrations were produced for the North Pacific (10-60°N, 120°E-90°W) for the years 2001 to 2010 using a self-organizing map trained with temperature, salinity, chlorophyll-a concentration and mixed layer depth. Nutrient sampling was carried out mainly by ships of opportunity, providing good seasonal coverage of the surface ocean. Using the mapping results, we investigated the spatio-temporal variability of surface North Pacific nutrient and dissolved inorganic carbon (DIC) distributions on seasonal and interannual time scales. Nutrient and DIC concentrations were high in the subarctic in winter and low in the subtropics. In the summer, substantial amount of nutrients remained unutilized in subarctic and the northern part of the subarctic-subtropical boundary region while that was not the case in the southern part of the boundary region. In the subtropics, nutrients were almost entirely depleted throughout the year, while DIC concentrations showed a north-south gradient and significant seasonal change. Nutrients and DIC show a large seasonal drawdown in the western subarctic region, while the drawdown in the eastern subarctic region was weaker, especially for silica. The subarctic-subtropical boundary region also showed a large seasonal drawdown, which was most prominent for DIC and less obvious for nitrate and silicate. In the interannual time scale, the Pacific Decadal Oscillation was related to a seesaw pattern between the subarctic-subtropical boundary region and the Alaskan Gyre through the changes in horizontal advection, vertical mixing and biological production

Data-based estimates of the ocean carbon sink variability – First results of the Surface Ocean pCO2 Mapping intercomparison (SOCOM)

Using measurements of the surface-ocean CO2 partial pressure (pCO2) and 14 different pCO2 mapping methods recently collated by the Surface Ocean pCO2 Mapping intercomparison (SOCOM) initiative, variations in regional and global sea–air CO2 fluxes are investigated. Though the available mapping methods use widely different approaches, we find relatively consistent estimates of regional pCO2 seasonality, in line with previous estimates. In terms of interannual variability (IAV), all mapping methods estimate the largest variations to occur in the eastern equatorial Pacific. Despite considerable spread in the detailed variations, mapping methods that fit the data more closely also tend to agree more closely with each other in regional averages. Encouragingly, this includes mapping methods belonging to complementary types – taking variability either directly from the pCO2 data or indirectly from driver data via regression. From a weighted ensemble average, we find an IAV amplitude of the global sea–air CO2 flux of 0.31 PgC yr−1 (standard deviation over 1992–2009), which is larger than simulated by biogeochemical process models. From a decadal perspective, the global ocean CO2 uptake is estimated to have gradually increased since about 2000, with little decadal change prior to that. The weighted mean net global ocean CO2 sink estimated by the SOCOM ensemble is −1.75 PgC yr−1 (1992–2009), consistent within uncertainties with estimates from ocean-interior carbon data or atmospheric oxygen trend

Arctic Sea Level Reconstruction

Rekonstruktion af historisk arkisk havniveau er meget vanskeligt på grund afden begrænsede dækning og kvalitet af data fra tidevandsstationer og altimetri iområdet. Denne afhandling omhandler mange af disse problemer, og diskutererstrategier til at opnå en stabil og plausibel rekonstruktion af arktisk havniveaufra 1950 til i dag. Den primære kilde til data om historisk havniveau, i hvert fald i størrelsesordenen årtier til århundreder, er tidevandsstationer. Observationer fra tidevandsstationer verden over indsamles i databasen Permanent Service for Mean Sea Level (PSMSL), og inkluderer data langs de arktiske kyster. En rimeligmængde data er tilgængeligt langs de norske og russiske kyster fra ca. 1950og frem, og de fleste publicerede resultater har hidtil holdt sig nær disse. Kunganske lidt data fra tidevandsstationer andre steder er tilgængeligt, og tidsseriermed en længde på flere årtier, sådan som det generelt anbefales til havniveaurekonstruktion, er slet ikke tilgængelige uden for de norske og russiske sektorer. Siden begyndelsen af 1990’erne har altimetri-satellitmissioner givet observationer af havniveau med mere spatielt komplet dækning. Dette gør det muligt at udtrække de primære variationsmønstre, hvilket kan bruges som kalibrering for en rekonstruktionsmetode. Til oceanografiske formål er altimetridataene over det Arktiske Ocean ikke af så god kvalitet som på lavere breddegrader, men udgør ikke desto mindre en uvurderlig datakilde. I løbet af dette projekt er nyprocesseret arktisk altimetri blevet tilgængeligt, hvilket muliggør mere detaljerede analyser. Meget af det tidlige arbejde med projektet har dog været baseret på data fra havmodeller som substitut for altimetrien.Ligesom andre publicerede havniveaurekonstruktioner er dette projekt baseret på kombinationen af data fra tidevandsstationer og mønstre fra altimetri. Detviser sig, at selv om det er muligt at rekonstruere differenserne mellem hverttidsskridt og kumulere disse for at få et rekonstrueret havniveaudatasæt, så kandenne metode give vidt forskellige resultater og er svær at stabilisere pga. demange huller i dataene. En mere robust metode, beskrevet i Ray og Douglas(2011), tager højde for hele tidsrækken for hver tidevandsstation og er væsentligt mindre tilbøjelig til at drive væk vertikalt. Desværre stopper mange optegnelser fra russiske tidevandsstationer omkring 1990, hvorefter antallet af tilgængelige tidevandsstationer er temmelig begrænset. I dette projekt undersøges virkningen af at introducere en del af altimetridatasættet som “virtuelle tidevandsstationer” for at imødegå denne mangel på data, og dette lader til at stabilisere rekonstruktionen yderligere. Dog er denne tilgang i et vist omfang afhængig af et relativt stationært havniveau før altimetriæraen, men tidligere resultater indikerer at mængderne af ferskvand i området har været nogenlunde stationære indtil 1980’erne. Det var oprindeligt hensigten i dette projekt at opnå en robust rekonstruktion ved brug af alternative dekompositionsteknikker som erstatning for de almindeligt brugte empiriske ortogonale funktioner (EOF’er) til kalibreringen. Om end en alternativ dekomposition (maksimal-autokorrelationsfaktorer, MAF’er), er blevet undersøgt, så viser det sig at præprocessering og håndtering af huller i data (gennem omhyggeligt metodevalg) er den primære udfordring mht. at opnårobuste havniveaurekonstruktioner i det arktiske område. Rekonstruktionerne i dette projekt dækker perioden 1950 til 2010 med månedlige data. Alle havområder nord for 68°N indgår (op til 90°N med Drakkar-kalibrering, og op til 82°N med altimetrikalibrering).Reconstruction of historical Arctic sea level is very difficult due to the limited coverage and quality of tide gauge and altimetry data in the area. This thesis addresses many of these issues, and discusses strategies to help achieve a stable and plausible reconstruction of Arctic sea level from 1950 to today.The primary record of historical sea level, on the order of several decades to a few centuries, is tide gauges. Tide gauge records from around the world are collected in the Permanent Service for Mean Sea Level (PSMSL) database, and includes data along the Arctic coasts. A reasonable amount of data is available along the Norwegian and Russian coasts since 1950, and most published research on Arctic sea level extends cautiously from these areas. Very little tide gauge data is available elsewhere in the Arctic, and records of a length of several decades,as generally recommended for sea-level reconstruction, are completely absent outside the Norwegian and Russian sectors. Since the early 1990s, altimetric satellite missions have provided more spatially complete observations of sea level. This allows extraction of the primary variation patterns, which can be used as calibration for a reconstruction method.For oceanographic purposes, the altimetric record over the Arctic Ocean is inferiorin quality to that of moderate latitudes, but nonetheless an invaluable set of observations. During this project, newly processed Arctic altimetry from the ERS-1/-2 and Envisat missions has become available, allowing analysis ingreater detail, though much early progress on the project was based on ocean model data.Like other published sea level reconstructions, this project is based on the combination of tide gauge records and altimetry patterns. It is found that while it is possible to reconstruct the timestep differences and cumulate these to obtain a reconstructed sea-level record, this approach may yield widely variable results and is difficult to stabilize due to the many gaps in the data. A more robust approach, as described by Ray and Douglas (2011), takes into account the entirety of each tide gauge record and makes the reconstruction much less prone to drifting away over time.Unfortunately, many of the Russian-sector tide gauge records end around 1990,leaving a fairly sparse record after this. This project examines the effect of introducing a subset of the altimetric dataset as “virtual tide gauges” to remedy this sparsity, and appears to further stabilize the reconstruction. As Arcticsea level changes are particularly concentrated in the Beaufort Gyre area, this also introduces observations in an important area. However, this approach to some extent relies on relatively stationary conditions before the altimetric era,though previous research indicates largely stationary amounts of freshwater until the 1980s. This project initially aimed to obtain a robust reconstruction through the use of alternative decompositions, rather than the commonly used empirical orthogonal functions (EOFs), for the calibration. While one alternative decomposition,maximum auto correlation factors (MAFs), is investigated, it is found that preprocessing and handling of gaps (through appropriate method choice) in the tide gauge record is the primary concern for obtaining robust sea level reconstructions in the Arctic area.The reconstructions obtained in this project concern the period 1950 to 2010 using monthly data. The spatial coverage is all ocean areas above 68°N, limited to the north depending on the calibration dataset used (90°N for Drakkarcalibrated reconstructions, 82°N for altimetry-based reconstructions)

Diagnosis of the atmospheric hydrological cycle and its variability in the present-day climate

This thesis investigates some important aspects of the atmospheric branch of the hydrological cycle in the modern day climate from an observational perspective. Data quality is evaluated, focusing on two state-of-the-art reanalysis products, ERA-I and JRA-55. Regional-scale discrepancies among reanalyses and observations, especially in their annual cycles, are found in the warm pool, Amazon, Gulf stream and Indian subcontinent regions. In the tropics, oceanic evaporation and its temporal variability are notably greater in JRA-55 than in ERA-I and satellite-based estimates, while both reanalyses overestimate precipitation. Higher tropical precipitation and evaporation, accompanied by a slightly lower level of total column water (TCW), might suggest a more intense hydrological cycle, but this can be an ill-defined concept especially when analysis increments mask “spin-down” errors in reanalysis models. Analysis increments arise to remove unphysical residuals in the atmospheric water budget, and these are explored via a cluster analysis to identify regimes with common behavior. Consistent for ERA-I and JRA-55, the regime with the largest negative residuals (greater moisture outputs than inputs) exceeding 50% of mean precipitation occurs during the dry season of some low latitude regions that feature strong seasonality, high evapotranspiration and high moisture divergence. Errors in the moisture divergence are likely responsible because they correlate strongly with the budget residual. Empirical Orthogonal Function (EOF) and Self Organizing Map (SOM) analyses are applied to identify the dominant inter-annual patterns of vertically-integrated moisture divergence variability. They reveal that the transition from strong La Niña through to extreme El Niño events is not a linear one and that the EOF orthogonality constraint results in the patterns being split between leading EOFs that are non-linearly related. The SOM analysis captures the range of responses to the El Niño Southern Oscillation (ENSO), indicating that the distinction between the moderate and extreme El Niños can be as great as the difference between La Niña and moderate El Niños, from a moisture divergence point of view. On diurnal time scales, horizontal moisture fluxes vary in response to thermodynamic and dynamic effects. TCW shows a global scale diurnal cycle that peaks around 1800 - 2100 local time with a peak-to-trough magnitude of 0.4mm. Semi-diurnal variations in surface winds and pressure, consistent with atmospheric tidal theory, create a westward propagating moisture convergence/divergence wave along the equator. Finally, the importance of Tropical Cyclones (TCs) as a source of freshwater for the North American continent is estimated using an ensemble of schemes designed to attribute onshore moisture fluxes to TCs. Averaged over the 2004–2012 hurricane seasons and integrated over the western, southern and eastern coasts of North America, the seven schemes attribute 7 to 18% (mean 14 %) of total net onshore flux to Atlantic TCs. A reduced contribution of 10% (range 9 to 11 %) was found for the 1980–2003 period, though only two schemes could be applied to this earlier period. Over the whole 1980–2012 period, a further 8% (range 6 to 9% from two schemes) was attributed to East Pacific TCs, resulting in a total TC contribution of 19% (range 17 to 22 %) to the ocean-to-land moisture transport onto the North American continent between May and November. The inter-annual variability does not appear to be strongly related to ENSO

Investigating the link between southern African droughts and global atmospheric teleconnections using regional climate models

Includes bibliographical referencesDrought is one of the natural hazards that threaten the economy of many nations, especially in Southern Africa, where many socio-economic activities depend on rain-fed agriculture. This study evaluates the capability of Regional Climate Models (RCMs) in simulating the Southern African droughts. It uses the Standardized Precipitation-Evapotranspiration Index (SPEI, computed using rainfall and temperature data) to identify 3-month droughts over Southern Africa, and compares the observed and simulated drought patterns. The observation data are from the Climate Research Unit (CRU), while the simulation data are from 10 RCMs (ARPEGE, CCLM, HIRHAM, RACMO, REMO, PRECIS, RegCM3, RCA, WRF, and CRCM) that participated in the Regional Climate Downscaling Experiment (CORDEX) project. The study also categorizes drought patterns over Southern Africa, examines the persistence and transition of these patterns, and investigates the roles of atmospheric teleconnections on the drought patterns. The results show that the drought patterns can occur in any season, but they have preference for seasons. Some droughts patterns may persist up to three seasons, while others are transient. Only about 20% of the droughts patterns are induced solely by El Niño Southern Oscillation (ENSO), other drought patterns are caused by complex interactions among the atmospheric teleconnections. The study also reveals that the Southern Africa drought pattern is generally shifting from a wet condition to a dry condition, and that the shifting can only be captured with a drought monitoring index that accounts for temperature influence on drought. Only few CORDEX RCMs simulate the Southern African droughts as observed. In this regard, the ARPEGE model shows the best simulation. The best performance may be because the stretching capability of ARPEGE helps the model to eliminate boundary condition problems, which are present in other RCMs. In ARPEGE simulations, the stretching capability would allow a better interaction between large and small scale features, and may lead to a better representation of the rain producing systems in Southern Africa. The results of the study may be applied to improve monitoring and prediction of regionally-extensive drought over Southern Africa, and to reduce the socio-economic impacts of drought in the region

Delayed Ionospheric Response to Solar EUV/UV Radiation Variations

The variability of the thermosphere-ionosphere (T-I) system and its complex behavior is strongly dependent on the continuously changing solar extreme ultraviolet (EUV) and ultraviolet (UV) radiation. The ionospheric electron density (or ion density) is mainly controlled by photoionization, loss by recombination, and transport processes. Transport processes play a significant role in the T-I composition and are responsible for the plasma distribution. The ionospheric response to solar activity has been investigated using total electron content (TEC) and solar EUV observations, as well as various solar proxies. An ionospheric delay of about 1-2 days in the daily TEC on the time scale of 27 days solar rotation period has been reported. It has also been shown that the He-II index is one of the best solar proxies to represent the solar activity at different time scales. The ionospheric delay in relation to solar radiation variations has attracted less attention in the past, especially with respect to its possible mechanisms. However, such studies, are of great importance for a better understanding of the complex interactions between solar radiation and the ionosphere that affect radio communications and navigation systems such as GNSS. Since the T-I region is affected not only by solar radiation, but also by lower atmospheric forcings, geomagnetic activity, and space weather events. Therefore, numerical modeling provides an opportunity to interpret the possible physical mechanism. To shed more light on this issue, a global, 3-D, time-dependent, physics-based numerical model was used in this thesis. It is a comprehensive numerical study to investigate the ionospheric response to solar flux changes during the 27 days solar rotation period. Satellite observations were used for comparison with the model simulations. The average delay for the observed (modeled) TEC is about 17 (16) h againest high-resolution solar EUV flux. The study confirms the capabilities of the model to reproduce the delayed ionospheric response with daily and hourly resolution. These results are in close agreement with previous studies. For the first time, the model simulations were performed to understand the role of eddy diffusion. The study shows that eddy diffusion is an important factor affecting the ionospheric delay and highlights the influence of the lower atmospheric forcing. Eddy diffusion was found to cause a change in thermospheric composition, which induces changes in atomic oxygen by modifying loss and photoionization rates. Atomic oxygen contributes significantly to ionization. Enhanced eddy diffusion leads to a decrease in atomic oxygen ion density and consequently TEC. Therefore, TEC decreases due to enhanced eddy diffusion, showing that the ionospheric delay is reduced. Thus, slow transport leads to maximum ionospheric delay.:Bibliographische Beschreibung Bibliographic Description Acronyms 1 General introduction 1.1 Introduction: Ionospheric delayed response 1.2 Objectives and structure of the thesis 1.3 Model description and data 1.3.1 CTIPe model description 1.3.2 Data 2 Paper 1: Ionospheric delayed response: preliminary results Vaishnav, R., Jacobi, C., Berdermann, J., Schmölter, E., and Codrescu, M.: Ionospheric response to solar EUV variations: Preliminary results 3 Paper 2: Long term trends of ionospheric response to solar EUV variations Vaishnav, R., Jacobi, C., and Berdermann, J.: Long-term trends in the iono- spheric response to solar extreme-ultraviolet variations 4 Paper 3: Comparison between CTIPe model simulations and satellite measurements Vaishnav, R., Schmölter, E., Jacobi, C., Berdermann, J., and Codrescu, M.: Ionospheric response to solar extreme ultraviolet radiation variations: com- parison based on CTIPe model simulations and satellite measurements 5 Paper 4: Role of eddy diffusion in the ionospheric delayed response Vaishnav, R., Jacobi, C., Berdermann, J., Codrescu, M., and Schmölter, E.: Role of eddy diffusion in the delayed ionospheric response to solar flux changes 6 Conclusions 7 Outlook References Acknowledgements Curriculum Vitae AffirmationDie Veränderungen des Thermosphäre-Ionosphäre (T-I) Systems und dessen Komplexität werden entscheidend durch die sich ständig ändernde extreme ultraviolette (EUV) und ultraviolette (UV) Sonnenstrahlung geprägt. Hierbei wird die ionosphärische Elektronendichte (oder Ionendichte) hauptsächlich durch Photoionisation, Rekombination und Transportprozesse gesteuert. Insbesondere Transportprozesse spielen eine wichtige Rolle für die Zusammensetzung des T-I-Systems und sind für die Plasmaverteilung verantwortlich. Die ionosphärische Reaktion auf Veränderungen der Sonnenaktivität wurde mithilfe des Gesamtelektronengehalts (englisch total electron content, TEC) und Messdaten des solaren EUV-Spektrums sowie solaren Proxys untersucht. Eine ionosphärische Verzögerung von 1 bis 2 Tagen für Tageswerte von TEC wurde für die 27-Tage-Sonnenrotation gefunden. Es wurde auch gezeigt, dass der He-II-Index einer der besten solaren Proxys ist, um die Sonnenaktivität auf verschiedenen Zeitskalen zu beschreiben. Die ionosphärische Verzögerung in Bezug auf Variationen der Sonnenstrahlung wurde in der Vergangenheit wenig Aufmerksamkeit gewidmet. Insbesondere die zugrundenliegenden Mechanismen wurden nicht untersucht. Solche Studien sind jedoch von entscheidender Bedeutung für ein besseres Verständnis der komplexen Wechselwirkungen zwischen Sonnenstrahlung und Ionosphäre, die unteranderem die Leistung von Radiokommunikation und globalen Navigationssystemen beeinflussen. Das T-I-System wird jedoch nicht nur von der solaren EUV-Strahlung kontrolliert. Prozesse der unteren Atmosphäre, geomagnetische Aktivität und Weltraumwettereignisse haben ebenfalls einen Einfluss auf diese Region. Daher bietet sich numerische Modellierung als Möglichkeit für die Interpretation der physikalischen Prozesse an. Zur Klärung der offenen Fragen wurde in dieser Arbeit ein globales, dreidimensionales, zeitabhängiges physikalisches Modell verwendet und eine umfangreiche Studie der ionosphärischen Reaktion auf Veränderungen der Sonnenstrahlungen während der 27-Tage-Sonnenrotation wurde durchgeführt. Hierfür wurden Messdaten von Satellitenmissionen mit den Modellsimulationen verglichen. Im Mittel ergibt sich eine Verzögerung von 16 Stunden aus der Analyse der Messdaten und eine Verzögerung von 17 Stunden aus den Modellsimulationen. Die Studie bestätigt demnach die Fähigkeit des Modells, die verzögerte ionosphärische Reaktion in stündlicher und täglicher Auflösung zu simulieren. Diese Ergebnisse stimmen gut mit vorangegangenen Studien überein. Im Rahmen dieser Arbeit wurden zum ersten Mal Simulationen zum Einfluss der Eddy-Diffusion durchgeführt. Diese Analyse zeigt, dass die Eddy-Diffusion ein wichtiger Faktor für die Ausprägung der ionosphärischen Verzögerung ist und dass der Einfluss von Prozessen der unteren Atmosphäre eine entscheidende Rolle spielt. Es wurde festgestellt, dass die Eddy-Diffusion eine erhebliche Veränderung der thermosphärischen Zusammensetzung verursacht, was wiederum zu Veränderung der Menge des atomaren Sauerstoffs führt. Dies beeinflusst dann die Ionisations- und Verlustrate. Da der atomare Sauerstoff erheblich zur Ionisierung beiträgt. Zunehmender Eddy-Diffusion folgen damit auch verkleinert der atomarer Sauerstoff Ionendichte und TEC. Daher nimmt TEC mit zunehmender Eddy-Diffusion ab und auch die Verzögerung wird kleiner. Andersherum führt ein langsamer Transport zu einem Maximum der ionosphärischen Verzögerung. Diese Dissertation gibt eine umfangreiche Zusammenfassung für das Verständnis der ionosphärischen Verzögerung zu Variationen der solaren EUV-Strahlung. Dafür werden TEC-Messungen mit numerischen Simulationen kombiniert. Weiterhin werden durch Vergleich die besten solaren Proxys für die Beschreibung der solaren Aktivität in T-I-Modellen bestimmt. Dies ist von entscheidender Bedeutung, um den Fokus auf die Verbesserung dieser Modelle zu lenken.:Bibliographische Beschreibung Bibliographic Description Acronyms 1 General introduction 1.1 Introduction: Ionospheric delayed response 1.2 Objectives and structure of the thesis 1.3 Model description and data 1.3.1 CTIPe model description 1.3.2 Data 2 Paper 1: Ionospheric delayed response: preliminary results Vaishnav, R., Jacobi, C., Berdermann, J., Schmölter, E., and Codrescu, M.: Ionospheric response to solar EUV variations: Preliminary results 3 Paper 2: Long term trends of ionospheric response to solar EUV variations Vaishnav, R., Jacobi, C., and Berdermann, J.: Long-term trends in the iono- spheric response to solar extreme-ultraviolet variations 4 Paper 3: Comparison between CTIPe model simulations and satellite measurements Vaishnav, R., Schmölter, E., Jacobi, C., Berdermann, J., and Codrescu, M.: Ionospheric response to solar extreme ultraviolet radiation variations: com- parison based on CTIPe model simulations and satellite measurements 5 Paper 4: Role of eddy diffusion in the ionospheric delayed response Vaishnav, R., Jacobi, C., Berdermann, J., Codrescu, M., and Schmölter, E.: Role of eddy diffusion in the delayed ionospheric response to solar flux changes 6 Conclusions 7 Outlook References Acknowledgements Curriculum Vitae Affirmatio

Spatial-Temporal Data Mining for Ocean Science: Data, Methodologies, and Opportunities

With the increasing amount of spatial-temporal~(ST) ocean data, numerous spatial-temporal data mining (STDM) studies have been conducted to address various oceanic issues, e.g., climate forecasting and disaster warning. Compared with typical ST data (e.g., traffic data), ST ocean data is more complicated with some unique characteristics, e.g., diverse regionality and high sparsity. These characteristics make it difficult to design and train STDM models. Unfortunately, an overview of these studies is still missing, hindering computer scientists to identify the research issues in ocean while discouraging researchers in ocean science from applying advanced STDM techniques. To remedy this situation, we provide a comprehensive survey to summarize existing STDM studies in ocean. Concretely, we first summarize the widely-used ST ocean datasets and identify their unique characteristics. Then, typical ST ocean data quality enhancement techniques are discussed. Next, we classify existing STDM studies for ocean into four types of tasks, i.e., prediction, event detection, pattern mining, and anomaly detection, and elaborate the techniques for these tasks. Finally, promising research opportunities are highlighted. This survey will help scientists from the fields of both computer science and ocean science have a better understanding of the fundamental concepts, key techniques, and open challenges of STDM in ocean