Arctic warming is assumed to be four times the global warming. A published study by Ivanov et al. (2019) shows that the annual average temperature of Franz Joseph Land (the world’s northernmost island region, a Russian territory) has increased by 5.2 °C from 2000-2017. This result supported the idea of determining whether Svalbard (Norwegian territory) is experiencing similar warming. Svalbard has historically been an attractive research center for examining climate change in the Arctic. Due to easier accessibility, the vast majority of weather stations have been located on the western part of the main island, Spitsbergen, which does not provide a representative picture of the entire archipelago. Therefore, this project has focused on eastern Spitsbergen. Data from six stations have been processed to analyze the temperature changes based on linear regression (the same method as at Franz Joseph Land). As eastern Spitsbergen has never been a priority, only short datasets are available, with the longest one dating from 2009. Because of this, no statistically significant result could be elucidated. Instead, data from Longyearbyen, which is located southwest were implemented, allowing analysis over the same period as Franz Joseph Land (2000-2017). This result suggested a temperature increase of 5.6 °C for the same period, with a statistical significance of P = 0.13, as well as that the winters are extra vulnerable to warming. The stations from eastern Spitsbergen’s local variability were also examined, which showed that the local climate varies although the stations are relatively close. Among others, Pyramiden seemed to be most affected by the lapse rate feedback, meaning a significant strong warming at the surface.

In the course of this thesis, Arctic low-level mixed-phase clouds and their interaction with aerosol and radiation have been investigated. To do so, measurements with the shipborne remote sensing supersite OCEANET-Atmosphere were conducted during the PS106 expedition in the Arctic summer 2017. OCEANET-Atmosphere comprises among other instruments a multiwavelength polarization lidar PollyXT and a microwave radiometer HATPRO. For PS106 the OCEANET-Atmosphere facility was complemented for the first time with a motion-stabilized vertically pointing Doppler cloud radar Mira-35. The cloud radar Doppler velocity was corrected for the ship’s vertical movement. The stabilization and the correction enabled, e.g., the derivation of eddy dissipation rates from the Doppler velocities. A data set of cloud microphysical and macrophysical properties was derived by applying the synergistic Cloudnet algorithm to the combined measurements of cloud radar, lidar, and microwave radiometer. Within this thesis, the set of the Cloudnet retrievals was improved to account for the complex structure of the Arctic cloud system. A new detection approach for the frequently observed low-level stratus clouds was developed based on the lidar signal-to-noise ratio. These clouds, which were below the lowest range gate of the cloud radar were observed during 50 % of the observational time. A new approach for the continuous determination of the ice crystal effective radius was introduced. This new retrieval made the data set suitable to perform high-resolved radiative transfer simulations. The retrieved data set was utilized to derive the first temperature relationship for heterogeneous ice formation in Arctic mixed-phase clouds. A strong dependence of the surface coupling state for high subzero ice-formation temperatures was found. For an ice-formation temperature above -15 °C, surface-coupled ice-containing clouds occur more frequently by a factor of 5 in numbers of observed clouds and by a factor of 2 in frequency of occurrence. Possible causes of the observed effect were discussed by sensitivity studies and a literature survey. Instrumental and methodological effects, and previously published similar observations of an increased ice occurrence at such high subzero temperatures have been ruled out as a possible explanation. The most likely cause of the observed effect was attributed to a larger reservoir of biogenic ice-nucleating particles in the surface-coupled marine boundary layer. This larger reservoir led to a higher freezing efficiency in these clouds which had at least their base in that layer. Finally, the importance of the detailed classification of the low-level clouds was highlighted by the evaluation of radiative transfer simulations. A difference in the cloud radiative effect of up to 100 W m-2 was calculated when these clouds were considered.:1 Introduction 2 Arctic — Amplified climate change 2.1 The Arctic climate system 2.2 Cloud radiation budget 2.3 Arctic mixed-phase clouds 2.4 Heterogeneous ice formation in Arctic mixed-phase clouds — constraints and previous findings 2.5 Motivating research questions 3 Data set — Applied instrumentation, processing, and retrievals 3.1 Introduction to ground-based active remote sensing of aerosol and clouds 3.1.1 Lidar principle 3.1.2 Radio Detection and Ranging — Radar 3.2 The Arctic expedition PS106 3.3 Instrumentation 3.3.1 The OCEANET-Atmosphere observatory 3.3.2 Other instruments used in this study 3.4 Data processing and synergistic retrievals 3.4.1 Correction of vertical-stare cloud radar observations for ship motion 3.4.2 Retrieval of eddy dissipation rate from Doppler radar spectra 3.4.3 Cloud macro- and microphysical properties from instrument-synergies 3.5 Summary of the data processing for PS106 4 Cloud and aerosol observations during PS106 4.1 Meteorological conditions during PS106 4.2 Case studies 4.3 Cloud and aerosol statistics during PS106 4.4 Discussion of the observational data sets 5 Contrasting surface-coupling effects on heterogeneous ice formation 5.1 Methodology 5.1.1 Ice-containing cloud analysis 5.1.2 Surface-coupling state 5.2 Results: influence of surface coupling on heterogeneous ice formation temperature 5.3 Discussion of the observed surface-coupling effects 5.3.1 Methodological and instrumental effects 5.3.2 Possible causes for increased ice occurrence in surface-coupled clouds 6 Application of the data set in collaborative studies and radiative transfer simulations within (AC)3 6.1 Radiative transfer simulations and cloud radiative effect 6.2 LLS treatment for improved radiative transfer simulations 6.3 Discussion 7 Summary and outlook Appendices A Determination of a volume depolarization threshold forlidar-based ice detection Bibliography / Im Rahmen dieser Arbeit wurden niedrige arktische Mischphasenwolken und ihre Wechselwirkung mit Aerosolen und Strahlung untersucht. Dazu wurden Messungen mit der schiffsgestützten Fernerkundungs-Supersite OCEANET-Atmosphere während der PS106-Expedition im arktischen Sommer 2017 durchgeführt. OCEANET-Atmosphere vereint, u.a., ein Multiwellenlängen-Polarisations-Lidar PollyXT und ein Mikrowellen-Radiometer HATPRO. Für PS106 wurde OCEANET-Atmosphere erstmalig um ein stabilisiertes, vertikal ausgerichtetes Doppler-Wolkenradar Mira-35 erweitert. Die Doppler-Geschwindigkeit wurde in Bezug auf die Vertikalbewegung des Schiffes korrigiert. Dank Stabilisierung und Korrektur war, z.B., die Ableitung von Wirbeldissipationsraten aus den Doppler-Geschwindigkeiten möglich. Unter Anwendung des synergetischen Cloudnet-Algorithmus wurde aus den kombinierten Wolkenradar, Lidar und Mikrowellenradiometer Messungen ein Datensatz der mikro- und makrophysikalischen Wolkeneigenschaften für PS106 erstellt. Im Rahmen dieser Arbeit wurde Cloudnet verbessert, um der komplexen Struktur der arktischen Wolken Rechnung zu tragen. Ein neuer Ansatz zur Erkennung der häufig beobachteten niedrigen Stratuswolken wurde entwickelt, basierend auf dem Lidar-Signal-zu-Rausch-Verhältnis. Diese Wolken, die unterhalb des untersten Höhenlevels des Wolkenradars auftraten, wurden während 50% der Beobachtungszeit identifiziert. Ein neuer Ansatz für die kontinuierliche Bestimmung des effektiven Radius der Eiskristalle wurde eingeführt. Dank dieser neuen Methode eignet sich der erstellte Datensatz für die Durchführung von Strahlungstransfersimulationen. Zum ersten Mal wurde eine Temperaturbeziehung für heterogene Eisbildung in arktischen Mischphasenwolken in Abhängigkeit ihres Oberflächen-Kopplungsstatus abgeleitet. Bei Temperaturen über -15°C war die relative Häufigkeit von Eis beinhaltenden Wolken doppelt so hoch und die Anzahl fünf Mal höher wenn sie mxit der Oberfläche gekoppelt waren, als bei entkoppelte Wolken. Mögliche Ursachen für den beobachteten Effekt wurden anhand von Sensitivitätsstudien und einer Literaturanalyse diskutiert. Instrumentelle und methodische Effekte sowie früher veröffentlichte ähnliche Beobachtungen konnten als mögliche Erklärung ausgeschlossen werden. Die wahrscheinlichste Ursache für den beobachteten Effekt wurde auf ein größeres Reservoir an biogenen Eiskristallisationskeimen in der oberflächengekoppelten marinen Grenzschicht zurückgeführt. Dieses größere Reservoir hat zu einer höheren Gefriereffizienz in Wolken geführt, die zumindest ihre Basis in dieser Schicht hatten. Die Bedeutung der detaillierten Klassifizierung von tiefliegenden Wolken auf Strahlungstransfersimulationen wurde hervorgehoben. Der simulierte Effekt der Wolken auf den Strahlungshaushalt unterschied sich bis zu 100 W m-2, unter Berücksichtigung dieser Wolken.:1 Introduction 2 Arctic — Amplified climate change 2.1 The Arctic climate system 2.2 Cloud radiation budget 2.3 Arctic mixed-phase clouds 2.4 Heterogeneous ice formation in Arctic mixed-phase clouds — constraints and previous findings 2.5 Motivating research questions 3 Data set — Applied instrumentation, processing, and retrievals 3.1 Introduction to ground-based active remote sensing of aerosol and clouds 3.1.1 Lidar principle 3.1.2 Radio Detection and Ranging — Radar 3.2 The Arctic expedition PS106 3.3 Instrumentation 3.3.1 The OCEANET-Atmosphere observatory 3.3.2 Other instruments used in this study 3.4 Data processing and synergistic retrievals 3.4.1 Correction of vertical-stare cloud radar observations for ship motion 3.4.2 Retrieval of eddy dissipation rate from Doppler radar spectra 3.4.3 Cloud macro- and microphysical properties from instrument-synergies 3.5 Summary of the data processing for PS106 4 Cloud and aerosol observations during PS106 4.1 Meteorological conditions during PS106 4.2 Case studies 4.3 Cloud and aerosol statistics during PS106 4.4 Discussion of the observational data sets 5 Contrasting surface-coupling effects on heterogeneous ice formation 5.1 Methodology 5.1.1 Ice-containing cloud analysis 5.1.2 Surface-coupling state 5.2 Results: influence of surface coupling on heterogeneous ice formation temperature 5.3 Discussion of the observed surface-coupling effects 5.3.1 Methodological and instrumental effects 5.3.2 Possible causes for increased ice occurrence in surface-coupled clouds 6 Application of the data set in collaborative studies and radiative transfer simulations within (AC)3 6.1 Radiative transfer simulations and cloud radiative effect 6.2 LLS treatment for improved radiative transfer simulations 6.3 Discussion 7 Summary and outlook Appendices A Determination of a volume depolarization threshold forlidar-based ice detection Bibliography

Im Vergleich zum Rest der Erde erfährt die Arktis eine signifikant schnellere Klimaerwärmung, die unter dem Begriff Arktische Verstärkung bekannt ist und mit zahlreichen sich verstärkenden Prozessen und Mechanismen einhergeht. Wolken spielen aufgrund ihrer Mitwirkung in verschiedenen, gegensätzlichen Effekten eine zwiespältige und eine der unsichersten Rollen für die Veränderung des arktischen Klimasystems. Daher ist die Untersuchung arktischer Wolken und ihrer Effekte von essenzieller Bedeutung, um den arktischen Klimawandel besser zu verstehen und in Modellen repräsentieren zu können. Diese Arbeit quantifiziert den bodennahen Strahlungseffekt von Wolken (engl. cloud radiative effect, CRE) aus einer Kombination von flugzeuggetragenen Breitbandstrahlungsmessungen während tiefer Flugabschnitte unter meist bewölkten Bedingungen und Strahlungstransfersimulationen für wolkenlose Bedingungen. Die Flugzeugmessungen wurden über den gegensätzlichen offenen Ozean- und Meereisoberflächen während dreier jahreszeitlich unterschiedlicher Kampagnen in der Umgebung von Spitzbergen aufgenommen. Ziel dieser Arbeit ist die Untersuchung des Einflusses von Oberflächen-, Wolken- und thermodynamischen Eigenschaften sowie Sonnenzenitwinkel (SZW) auf Unterschiede des solaren, thermisch-infraroten (TIR) und gesamten (solar+TIR) CRE, in Bezug auf die verschiedenen Kampagnen und Oberflächentypen. Die Unterschiede des solaren CRE werden überwiegend vom Kontrast der Oberflächenalbedo und vom jahreszeitlich variierenden SZW angetrieben. Der stärkste Abkühlungseffekt wurde im Frühsommer festgestellt und die über offenem Ozean beobachtete Abkühlung war stärker als über Meereis (-259 W m-2 vs. -65 W m-2 im Frühsommer, -108 W m-2 vs. -17 W m-2 im Frühjahr). Außerdem beeinflussen von Wolken ausgelöste Veränderungen der Oberflächenalbedo den solaren CRE je nach SZW und Oberflächentyp. Der TIR CRE zeigte aufgrund der vorherrschenden opaken Wolken und einer Kompensierung von Effekten sich ändernder Temperatur und Feuchte nur schwache Veränderungen zwischen den Kampagnen und Oberflächentypen (etwa 75 W m-2). Daher wird die Variabilität des gesamten CRE von der Schwankung des solaren CRE bestimmt. Über offenem Ozean wurde ein Abkühlungseffekt während aller Kampagnen beobachtet, über Meereis konnte der solare Abkühlungs- den TIR Erwärmungseffekt hingegen nur im Frühsommer ausgleichen. Für die gesamte Region Framstraße resultiert ein mittäglicher erwärmender, neutraler und abkühlender Effekt in Frühjahr, Frühsommer und Spätsommer aus dem Jahresgang der Meereisbedeckung. Zusätzlich zur eher qualitativen Analyse der einzelnen Einflussfaktoren auf die Unterschiede des solaren CRE wird ein Ansatz zur quantitativen Bestimmung dieser Faktoren angeregt. Diese neue Methode basiert auf einer Fallstudie mit sich verändernden Wolken- und Oberflächeneigenschaften senkrecht zur Eiskante während eines Warmlufteinschubs. Trotz signifikant mangelnder Komplexität der dem Fall zugrundeliegenden Parametrisierung werden plausible relative Beiträge von 77 % und 23 % zum Unterschied des solaren CRE zwischen offenem Ozean und Meereis für Oberflächen- respektive Wolkeneigenschaften ermittelt.:Zusammenfassung Abstract 1 General Introduction 1.1 Arctic Amplification – The Accelerated Warming of the Arctic 1.2 Clouds in the Arctic Climate System 2 Definitions and Theory 2.1 Radiative Quantities 2.2 Radiative Energy Budget 2.3 Surface, Cloud, and Atmospheric Properties 3 Motivation and Objectives 3.1 Previous Research on Cloud Radiative Effect 3.2 Aims of the Thesis 4 Observations and Methods 4.1 Aircraft Measurements 4.1.1 Broadband Radiation Measurements 4.1.2 Complementary Observations 4.2 Radiative Transfer Simulations 4.3 Calculation of Cloud Radiative Effect 4.4 Retrieval of Surface Albedo in Cloud-Free Conditions 4.4.1 Sea Ice Albedo and Retrieval of Equivalent Liquid Water Path 4.4.2 Open Ocean Albedo 4.4.3 Albedo of Inhomogeneous Surfaces 4.5 Uncertainty Estimation 4.5.1 Broadband Radiation Measurements 4.5.2 Radiative Transfer Simulations 4.5.3 Cloud Radiative Effect 4.6 Overview of Employed Data sets from all Campaigns 5 Statistical Analysis of Surface Cloud Radiative Effect 5.1 Campaign Characteristics 5.1.1 Sea Ice Distribution and Solar Zenith Angle 5.1.2 Thermodynamic Profiles 5.1.3 Cloud Properties 5.2 Impact of Cloud-Induced Albedo Modification 5.2.1 Impact on Surface Albedo 5.2.2 Impact on Solar Cloud Radiative Effect 5.3 Impact of Flight Altitude 5.4 Observed Cloud Radiative Effect 5.4.1 Solar Cloud Radiative Effect 5.4.2 Thermal-Infrared Cloud Radiative Effect 5.4.3 Total Cloud Radiative Effect 6 Sensitivity of Solar Cloud Radiative Effect to Surface and Cloud Properties 6.1 Introduction of Case 6.1.1 Synoptic Situation 6.1.2 Surface and Cloud Characteristics 6.2 Contributions to Solar Cloud Radiative Effect 6.2.1 Relative Contributions along a Continuous Time Series 6.2.2 Relative Contributions between States 7 Conclusions and Outlook 7.1 Summary and Conclusions 7.2 Outlook A Appendix A.1 Uncertainty Estimation A.1.1 Broadband Radiation Measurements A.1.2 Radiative Transfer Simulations A.1.3 Cloud Radiative Effect List of Figures List of Tables List of Symbols and Acronyms Bibliography Acknowledgements Summary of the Dissertation List of Papers and Author’s Contribution Supervision Statement / Compared to the rest of the globe, the Arctic experiences a significantly more rapid climate warming, which is called Arctic amplification and linked to numerous intensifying processes and mechanisms. Due to their contribution to and modification of various counteracting effects, clouds play one of the most ambiguous and uncertain roles in the change of the Arctic climate system. Thus, the investigation of clouds and their effects in the Arctic is essential to better understand and represent the Arctic climate change. This thesis quantifies the near-surface cloud radiative effect (CRE), which is obtained from a combination of airborne broadband radiation measurements during low-level flight sections under mostly cloudy conditions and radiative transfer simulations for cloud-free conditions. The airborne measurements were acquired over the contrasting open ocean and sea ice surfaces during three seasonally distinct campaigns in the vicinity of Svalbard. The objective of this thesis is to analyze the impact of surface, cloud, and thermodynamic properties as well as solar zenith angle (SZA) on differences of the solar, thermal-infrared (TIR), and total (solar+TIR) CRE with respect to the particular campaigns and surface types. The difference of the solar CRE is found to be predominantly driven by the contrasting surface albedo and the seasonally varying SZA. The strongest solar cooling effect was detected in early summer and the cooling observed over open ocean was stronger compared to sea ice (-259 W m-2 vs. -65 W m-2 in early summer, -108 W m-2 vs. -17 W m-2 in spring). Additionally, modifications of the surface albedo induced by the cloud-related illumination changes affect the solar CRE depending on SZA and surface type. The TIR CRE varied only weakly between campaigns and surface types (around 75 W m-2) due to the predominant opaque clouds and a compensation effect of changing temperature and humidity. Consequently, the variability of the total CRE is driven by the solar CRE variation. While a total cooling effect was present over open ocean during all campaigns, the solar cooling could compensate the TIR warming effect over sea ice only during early summer. For the entire Fram Strait region, the seasonal cycle of the sea ice distribution results in a total warming, neutral, and cooling effect during solar noon in spring, early summer, and late summer. In addition to the rather qualitative analysis of the individual contributors to the CRE differences, an attempt to quantitatively disentangle the contributions is proposed. This new method is based on a case study with varying cloud and surface properties over the sea ice edge during a warm air intrusion. Although the underlying parameterization developed for the case significantly lacks complexity, plausible relative contributions of surface and cloud properties of about 77 % and 23 % to the solar CRE difference between sea ice and open ocean are retrieved.:Zusammenfassung Abstract 1 General Introduction 1.1 Arctic Amplification – The Accelerated Warming of the Arctic 1.2 Clouds in the Arctic Climate System 2 Definitions and Theory 2.1 Radiative Quantities 2.2 Radiative Energy Budget 2.3 Surface, Cloud, and Atmospheric Properties 3 Motivation and Objectives 3.1 Previous Research on Cloud Radiative Effect 3.2 Aims of the Thesis 4 Observations and Methods 4.1 Aircraft Measurements 4.1.1 Broadband Radiation Measurements 4.1.2 Complementary Observations 4.2 Radiative Transfer Simulations 4.3 Calculation of Cloud Radiative Effect 4.4 Retrieval of Surface Albedo in Cloud-Free Conditions 4.4.1 Sea Ice Albedo and Retrieval of Equivalent Liquid Water Path 4.4.2 Open Ocean Albedo 4.4.3 Albedo of Inhomogeneous Surfaces 4.5 Uncertainty Estimation 4.5.1 Broadband Radiation Measurements 4.5.2 Radiative Transfer Simulations 4.5.3 Cloud Radiative Effect 4.6 Overview of Employed Data sets from all Campaigns 5 Statistical Analysis of Surface Cloud Radiative Effect 5.1 Campaign Characteristics 5.1.1 Sea Ice Distribution and Solar Zenith Angle 5.1.2 Thermodynamic Profiles 5.1.3 Cloud Properties 5.2 Impact of Cloud-Induced Albedo Modification 5.2.1 Impact on Surface Albedo 5.2.2 Impact on Solar Cloud Radiative Effect 5.3 Impact of Flight Altitude 5.4 Observed Cloud Radiative Effect 5.4.1 Solar Cloud Radiative Effect 5.4.2 Thermal-Infrared Cloud Radiative Effect 5.4.3 Total Cloud Radiative Effect 6 Sensitivity of Solar Cloud Radiative Effect to Surface and Cloud Properties 6.1 Introduction of Case 6.1.1 Synoptic Situation 6.1.2 Surface and Cloud Characteristics 6.2 Contributions to Solar Cloud Radiative Effect 6.2.1 Relative Contributions along a Continuous Time Series 6.2.2 Relative Contributions between States 7 Conclusions and Outlook 7.1 Summary and Conclusions 7.2 Outlook A Appendix A.1 Uncertainty Estimation A.1.1 Broadband Radiation Measurements A.1.2 Radiative Transfer Simulations A.1.3 Cloud Radiative Effect List of Figures List of Tables List of Symbols and Acronyms Bibliography Acknowledgements Summary of the Dissertation List of Papers and Author’s Contribution Supervision Statement

Aerosols change the Earth's energy balance. Black carbon (BC) aerosols are a product of incomplete combustion of fossil fuels and biomass burning and cause a net warming through aerosol radiation interactions (ari) and aerosol cloud interactions (aci). BC aerosols have potentially strong implications on the Arctic climate, yet the net global climate effect of BC is very uncertain. Best estimates assume a net warming effect, roughly half to that of CO2. However, the time scales during which CO2 emissions affect the global climate are on the order of hundreds of years, while BC is a short-lived climate pollutant (SLCP) with atmospheric life times of days to weeks. Climate models or atmospheric transport models struggle to emulate the seasonality and amplitude of BC concentrations in the Arctic, which are low in summer and high in winter/spring during the so called Arctic haze season. The high uncertainties regarding BC's climate impact are not only related to ari and aci, but also due to model parameterizations of BC lifetime and transport, and the highly uncertain estimates of global and regional BC emissions. Given the high uncertainties in technology-based emission inventories (EI), there is a need for an observation-based assessment of sources of BC in the atmosphere. We study short-term and long-term observations of elemental carbon (EC), the mass-based analog of optically-defined BC. EC aerosol concentrations and carbon-isotope-based (δ13C and ∆14C) sources were constrained (top-down) for three Arctic receptor sites in Abisko (northern Sweden), Tiksi (East Siberian Russia), and Zeppelin (on Svalbard, Norway). The radiocarbon (∆14C) signature allows to draw conclusion on the EC sources (fossil fuels vs. biomass burning) with high accuracy (&lt;5% variation). Stable carbon isotopic fingerprints (δ13C) give qualitative information of the consumed fuel type, i.e. coal, C3-plants (wood), liquid fossil fuels (diesel) or gas flaring (methane and non-methane hydrocarbons). These fingerprints can be used in conjunction with Bayesian statistics, to estimate quantitative source contributions of the sources. Finally, our observations were compared to predictions from a state of the art atmospheric transport model (coupled to BC emissions), conducted by our collaborators at NILU (Norwegian Institute for Air Research). Observed BC concentrations showed a high seasonality throughout the year, with elevated concentrations in the winter, at all sites. The highest concentrations were measured on Svalbard during a short campaign (Jan-Mar 2009) focusing on BC pollution events. Long-term observations showed that Svalbard (2013) had overall the lowest annual BC concentrations, followed by Abisko (2012) and Tiksi (2013). Isotope constraints on BC combustion sources exhibited a high seasonality and big amplitude all across the Eurasian Arctic. Uniform seasonal trends were observed in all three year-round studies, showing fractions of biomass burning of 60-70% in summer and 10-40% in winter. Europe was the major source region (&gt;80%) for BC emissions arriving at Abisko and the main sources were liquid fossil fuels and biomass burning (wood). The model agreed very well with the Abisko observations, showing good model skill and relatively well constrained sources in the European regions of the EI. However, for the Svalbard and East Siberian Arctic observatories the model-observation agreement was not as good. Here, Russia, Europe and China were the major contributors to the mostly liquid fossil and biomass burning BC emissions. This showed that the EI still needs to be improved, especially in regions where emissions are high but observations are scarce (low ratio of observations to emitted pollutant quantity). Strategies for BC mitigation in the (Eurasian) Arctic are probably most efficient, if fossil fuel (diesel) emissions are tackled during winter and spring periods, all across Eurasia. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript.</p>

A new German research consortium is investigating the causes and effects of the rapid rise of near-surface air temperatures in the Artic. Within the last 25 years a remarkable increase of the Arctic near-surface air temperature exceeding the global warming by a factor of two to three has been observed. The phenomenon is commonly referred to as Arctic Amplification. The warming results in rather drastic changes of a variety of climate parameters. For example, the Arctic sea ice has declined significantly. This ice retreat has been well identified by satellite measurements. However, coupled regional and global climate models still fail to reproduce it adequately; they tend to systematically underestimate the observed sea ice decline. This model observation difference implies that the underlying physical processes and feedback mechanisms are not appropriately represented in Arctic climate models. Thus, the predictions of these models are also likely to be inadequate. It is mandatory to identify the origin of this disagreement. / Ein neu geschaffenes deutsches Forschungskonsortium untersucht die Ursachen und Effekte des rapiden Anstiegs der bodennahen Lufttemperatur in der Arktis. Innerhalb der letzten 25 Jahre wurde ein bemerkenswerter Anstieg der Bodenlufttemperatur in der Arktis beobachtet, welcher die globale Erwärmung um den Faktor 2 bis 3 übersteigt. Dieses Phänomen wird als arktische Verstärkung bezeichnet. Diese Erwärmung resultiert vielmehr in einer drastischen Änderung einer Vielzahl von Klimarparametern. Beispielsweise ist das arktische Meereis deutlich zurückgegangen. Dieser Eisrückgang wurde durch Satellitenbeobachtungen gut beobachtet. Dagegen haben regionale und globale Klimamodelle immer noch Probleme, den Rückgang entsprechend zu reproduzieren. Sie tendieren dazu, den Meereisrückgang systematisch zu unterschätzen. Die Unterschiede zwischen Modell und Beobachtungen legen nahe, dass die grundlegenden physikalischen Prozesse und Rückkopplungsmechanismen nicht entsprechend in arktischen Klimamodellen repräsentiert werden. Somit sind wahrscheinlich auch die Vorhersagen der Modelle unzureichend. Es ist notwendig, den Ursprung dieser Unstimmigkeit zu identifizieren.

The Arctic region shows a particularly high susceptibility to climate change, which historically manifests in an amplification of the near-surface warming in the Arctic relative to the global mean. This Arctic amplification (AA) has impacts on the climate system also beyond the northern polar regions, which highlights the importance to adequately represent it in numerical models. While state-of-the-art climate models widely agree on the presence of AA, they simulate a large spread in the magnitude of Arctic-amplified warming. This thesis addresses the need to evaluate the performance of global climate models in projecting AA and its most important drivers. For the latter, the focus is on the three amplifying climate feedbacks (ACFs) that largely drive the meridional warming structure leading to AA. The ACFs include the sea-ice-albedo feedback (SIAF), the Planck feedback, and the lapse-rate feedback (LRF). These feedbacks arise from the relevant changes in Arctic sea ice, near-surface temperatures, and the deviation from the near-surface temperature change through the atmosphere, respectively. In the thesis, two observational constraints are presented to narrow the range of climate models of the sixth Coupled Model Intercomparison Project (CMIP6) regarding their projection of AA and the ACFs in both past and future climate. While for the past, the models representation of near-surface processes can often be directly evaluated against observations, it is particularly the LRF that is difficult to constrain as it incorporates the entire atmospheric warming structure. As a consequence, the historical constraint focuses on the LRF, while the future constraint gives a prediction range for the evolution of AA and all three ACFs through the 21st century. The main results are highlighted in the view of the changing atmospheric energy budget (AEB) of the Arctic under anthropogenic climate forcing. The AEB provides a framework to address Arctic climate change at large scales, and further helps to decide on the relevant aspects that provide appropriate metrics for constraining both AA and the ACFs. In other words, the perspective of a changing Arctic AEB highlights important alterations of the energetics under climate change, that further link to changes in climate aspects that partly explain the inter-model spread in simulated AA and the ACFs. The main results of the cumulative thesis are formulated on the basis of three published research papers, papers I, II, and III. Paper I addresses the Arctic AEB which is typically characterised by an equilibrium between net radiative cooling and advective heating, and mostly an absence of convection. This radiative-advective equilibrium (RAE) approximates well the energy budget and thermal structure of the Arctic atmosphere. The main outcome of paper I is that with continuous warming as simulated by CMIP6 models in an idealised setup, a deviation from the RAE increasingly develops, resulting from sea ice retreat and increased ocean-to-atmosphere heat fluxes. These changes are further concomitant with a depletion of the typical surface-based temperature inversion and a decrease in advective heating, which is byword for the convergence of atmospheric energy transport in the Arctic. Since the RAE currently explains much of the basic thermal structure of the Arctic atmosphere, those changes have the potential to further mediate the LRF. Paper II builds on paper I and evaluates the performance of climate models in representing the key aspects of the Arctic LRF in CMIP6 historical simulations that have the best estimates of the transient climate forcings during the observational period. In particular it is found that CMIP6 models that realistically simulate both the lower thermal structure of the atmosphere and the poleward atmospheric energy transport are more trustworthy in informing about the LRF and how much it contributed to Arctic warming during the past few decades. The evaluation is based on observations of surface-based temperature inversions during the year-long Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, and atmospheric energy transport convergence computations from reanalyses. Paper III expands the constraint approach of paper II and carries out an emergent constraint (EC) on future AA and the ACFs that further elaborates on the physical relationships between the constraining metrics and future climate projections. Previous work has highlighted that parts of the inter-model spread in simulated AA is explained through the spread in contemporaneous sea ice loss across climate models. The thesis confirms this link by showing that CMIP6 models with a stronger climatological sea ice loss project a stronger AA in the future under the assumption of a high emission scenario. By further linking the degree of future ice loss to the current-climate sea ice amount in CMIP6 models, paper III facilitates an EC on the future evolution of AA and the ACFs. In particular, models with a lower contemporary sea ice amount project a larger magnitude of AA by setting the stage for stronger climatological ice loss and near-surface warming, linking to the relevant ACFs. From the corresponding prediction it is evident that AA is expected to continue at a warming rate that is more than twice or three times larger than global-mean warming. Furthermore, the three ACFs continue to contribute to Arctic warming, with the SIAF leading the warming contribution response. Lastly, the consideration of statistically strong and physically plausible relationships across climate models makes the EC a valuable technique to constrain climate model simulations in conjunction with observations. This thesis highlights the potential of combining the advantages of both presented constraints: Using multiple process-relevant aspects instead of one singular metric (paper II), but considering the mechanistic couplings between these metrics and the climate projection of interest (paper III) will improve our model-evaluation techniques and further help guiding the design of future climate simulations.:Summary of the dissertation List of papers Author’s contribution Supervision statement 1 Introduction 2 Research focus 3 The Arctic atmospheric energy budget 3.1 The atmospheric column model 3.2 The annual atmospheric energy budget 4 Arctic amplification and climate feedbacks 4.1 Amplifying climate feedbacks 4.2 A comment on process coupling 5 Methods and data 5.1 Energy budget equations 5.2 Quantifying Arctic amplification and climate feedbacks 5.3 Climate model data 5.3.1 CMIP6 idealised simulations 5.3.2 CMIP6 historical simulations 5.3.3 CMIP6 ssp585 simulations 5.4 Observational constraints 5.4.1 Constraint on historical Arctic lapse-rate feedback 5.4.2 Constraint on future Arctic amplification and relevant climate feedbacks 6 Results 6.1 Paper I - Deviations from the Arctic radiative-advective equilibrium under anthropogenic climate change 6.2 Paper II - Constraining the Arctic lapse-rate feedback during past decades by contemporary observations 6.3 Paper III - Constraining future Arctic amplification and the relevant climate feedbacks based on the recent sea ice climatology 7 Summary and outlook References Lists Acknowledgements Appendix A: Paper I Appendix B: Paper II Appendix C: Paper III