Arctic low-level mixed-phase clouds and their complex interactions with aerosol and radiation: Remote sensing of the Arctic troposphere with the shipborne supersite OCEANET-Atmosphere

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 BibliographyIm 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 Bibliograph

Asymmetries in cloud microphysical properties ascribed to sea ice leads via water vapour transport in the central Arctic

To investigate the influence of sea ice openings like leads on wintertime Arctic clouds, the air mass transport is exploited as a heat and humidity feeding mechanism which can modify Arctic cloud properties. Cloud microphysical properties in the central Arctic are analysed as a function of sea ice conditions during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in 2019–2020. The Cloudnet classification algorithm is used to characterize the clouds based on remote sensing observations and the atmospheric thermodynamic state from the observatory on board the research vessel (RV) Polarstern. To link the sea ice conditions around the observational site with the cloud observations, the water vapour transport (WVT) being conveyed towards RV Polarstern has been utilized as a mechanism to associate upwind sea ice conditions with the measured cloud properties. This novel methodology is used to classify the observed clouds as coupled or decoupled to the WVT based on the location of the maximum vertical gradient of WVT height relative to the cloud-driven mixing layer. Only a conical sub-sector of sea ice concentration (SIC) and the lead fraction (LF) centred on the RV Polarstern location and extending up to 50 km in radius and with an azimuth angle governed by the time-dependent wind direction measured at the maximum WVT is related to the observed clouds. We found significant asymmetries for cases when the clouds are coupled or decoupled to the WVT and selected by LF regimes. Liquid water path of low-level clouds is found to increase as a function of LF, while the ice water path does so only for deep precipitating systems. Clouds coupled to WVT are found to generally have a lower cloud base and larger thickness than decoupled clouds. Thermodynamically, for coupled cases the cloud-top temperature is warmer and accompanied by a temperature inversion at the cloud top, whereas the decoupled cases are found to be closely compliant with the moist adiabatic temperature lapse rate. The ice water fraction within the cloud layer has been found to present a noticeable asymmetry when comparing coupled versus decoupled cases. This novel approach of coupling sea ice to cloud properties via the WVT mechanism unfolds a new tool to study Arctic surface–atmosphere processes. With this formulation, long-term observations can be analysed to enforce the statistical significance of the asymmetries. Furthermore, our results serve as an opportunity to better understand the dynamic linkage between clouds and sea ice and to evaluate its representation in numerical climate models for the Arctic system.</p

Case study of a humidity layer above Arctic stratocumulus and potential turbulent coupling with the cloud top

Specific humidity inversions (SHIs) above low-level cloud layers have been frequently observed in the Arctic. The formation of these SHIs is usually associated with large-scale advection of humid air masses. However, the potential coupling of SHIs with cloud layers by turbulent processes is not fully understood. In this study, we analyze a 3 d period of a persistent layer of increased specific humidity above a stratocumulus cloud observed during an Arctic field campaign in June 2017. The tethered balloon system BELUGA (Balloon-bornE moduLar Utility for profilinG the lower Atmosphere) recorded vertical profile data of meteorological, turbulence, and radiation parameters in the atmospheric boundary layer. An in-depth discussion of the problems associated with humidity measurements in cloudy environments leads to the conclusion that the observed SHIs do not result from measurement artifacts. We analyze two different scenarios for the SHI in relation to the cloud top capped by a temperature inversion: (i) the SHI coincides with the cloud top, and (ii) the SHI is vertically separated from the lowered cloud top. In the first case, the SHI and the cloud layer are coupled by turbulence that extends over the cloud top and connects the two layers by turbulent mixing. Several profiles reveal downward virtual sensible and latent heat fluxes at the cloud top, indicating entrainment of humid air supplied by the SHI into the cloud layer. For the second case, a downward moisture transport at the base of the SHI and an upward moisture flux at the cloud top is observed. Therefore, the area between the cloud top and SHI is supplied with moisture from both sides. Finally, large-eddy simulations (LESs) complement the observations by modeling a case of the first scenario. The simulations reproduce the observed downward turbulent fluxes of heat and moisture at the cloud top. The LES realizations suggest that in the presence of a SHI, the cloud layer remains thicker and the temperature inversion height is elevated

Atmospheric temperature, water vapour and liquid water path from two microwave radiometers during MOSAiC

The microwave radiometers HATPRO (Humidity and Temperature Profiler) and MiRAC-P (Microwave Radiometer for Arctic Clouds - Passive) continuously measured radiation emitted from the atmosphere throughout the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition on board the research vessel Polarstern. From the measured brightness temperatures, we have retrieved atmospheric variables using statistical methods in a temporal resolution of 1 s covering October 2019 to October 2020. The integrated water vapour (IWV) is derived individually from both radiometers. In addition, we present the liquid water path (LWP), temperature and absolute humidity profiles from HATPRO. To prove the quality and to estimate uncertainty, the data sets are compared to radiosonde measurements from Polarstern. The comparison shows an extremely good agreement for IWV, with standard deviations of 0.08–0.19 kg m−2 (0.39–1.47 kg m−2) in dry (moist) situations. The derived profiles of temperature and humidity denote uncertainties of 0.7–1.8 K and 0.6–0.45 gm−3 in 0–2 km altitude

Application of the shipborne remote sensing supersite OCEANET for profiling of Arctic aerosols and clouds during Polarstern cruise PS106

From 25 May to 21 July 2017, the research vessel Polarstern performed the cruise PS106 to the high Arctic in the region north and northeast of Svalbard. The mobile remote-sensing platform OCEANET was deployed aboard Polarstern. Within a single container, OCEANET houses state-of-the-art remote-sensing equipment, including a multiwavelength Raman polarization lidar PollyXT and a 14-channel microwave radiometer HATPRO (Humidity And Temperature PROfiler). For the cruise PS106, the measurements were supplemented by a motion-stabilized 35 GHz cloud radar Mira-35. This paper describes the treatment of technical challenges which were immanent during the deployment of OCEANET in the high Arctic. This includes the description of the motion stabilization of the cloud radar Mira-35 to ensure vertical-pointing observations aboard the moving Polarstern as well as the applied correction of the vessels heave rate to provide valid Doppler velocities. The correction ensured a leveling accuracy of ±0.5◦ during transits through the ice and an ice floe camp. The applied heave correction reduced the signal induced by the vertical movement of the cloud radar in the PSD of the Doppler velocity by a factor of 15. Low-level clouds, in addition, frequently prevented a continuous analysis of cloud conditions from synergies of lidar and radar within Cloudnet, because the technically determined lowest detection height of Mira-35 was 165 m above sea level. To overcome this obstacle, an approach for identification of the cloud presence solely based on data from the near-field receiver of PollyXT at heights from 50 m and 165 m above sea level is presented. We found low-level stratus clouds, which were below the lowest detection range of most automatic ground-based remote-sensing instruments during 25 % of the observation time. We present case studies of aerosol and cloud studies to introduce the capabilities of the data set. In addition, new approaches for ice crystal effective radius and eddy dissipation rates from cloud radar measurements and the retrieval of aerosol optical and microphysical properties from the observations of PollyXT are introduced. © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License

Tethered balloon-borne profile measurements of atmospheric properties in the cloudy atmospheric boundary layer over the Arctic sea ice during MOSAiC: Overview and first results

The tethered balloon-borne measurement system BELUGA (Balloon-bornE moduLar Utility for profilinG the lower Atmosphere) was deployed over the Arctic sea ice for 4 weeks in summer 2020 as part of the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition. Using BELUGA, vertical profiles of dynamic, thermodynamic, aerosol particle, cloud, radiation, and turbulence properties were measured from the ground up to a height of 1,500 m. BELUGA was operated during an anomalously warm period with frequent liquid water clouds and variable sea ice conditions. Three case studies of liquid water phase, single-layer clouds observed on 3 days (July 13, 23, and 24, 2020) are discussed to show the potential of the collected data set to comprehensively investigate cloud properties determining cloud evolution in the inner Arctic over sea ice. Simulated back-trajectories show that the observed clouds have evolved within 3 different air masses (“aged Arctic,” “advected over sea ice,” and “advected over open ocean”), which left distinct fingerprints in the cloud properties. Strong cloud top radiative cooling rates agree with simulated results of previous studies. The weak warming at cloud base is mostly driven by the vertical temperature profile between the surface and cloud base. In-cloud turbulence induced by the cloud top cooling was similar in strength compared to former studies. From the extent of the mixing layer, it is speculated that the overall cloud cooling is stronger and thus faster in the warm oceanic air mass. Larger aerosol particle number concentrations and larger sizes were observed in the air mass advected over the sea ice and in the air mass advected over the open ocean

Ozone depletion in the Arctic and Antarctic stratosphere induced by wildfire smoke

A record-breaking stratospheric ozone loss was observed over the Arctic and Antarctica in 2020. Strong ozone depletion occurred over Antarctica in 2021 as well. The ozone holes developed in smoke-polluted air. In this article, the impact of Siberian and Australian wildfire smoke (dominated by organic aerosol) on the extraordinarily strong ozone reduction is discussed. The study is based on aerosol lidar observations in the North Pole region (October 2019-May 2020) and over Punta Arenas in southern Chile at 53.2°S (January 2020-November 2021) as well as on respective NDACC (Network for the Detection of Atmospheric Composition Change) ozone profile observations in the Arctic (Ny-Ålesund) and Antarctica (Neumayer and South Pole stations) in 2020 and 2021. We present a conceptual approach on how the smoke may have influenced the formation of polar stratospheric clouds (PSCs), which are of key importance in the ozone-depleting processes. The main results are as follows: (a) the direct impact of wildfire smoke below the PSC height range (at 10-12 km) on ozone reduction seems to be similar to well-known volcanic sulfate aerosol effects. At heights of 10-12 km, smoke particle surface area (SA) concentrations of 5-7 μm2 cm-3 (Antarctica, spring 2021) and 6-10 μm2 cm-3 (Arctic, spring 2020) were correlated with an ozone reduction in terms of ozone partial pressure of 0.4-1.2 mPa (about 30 % further ozone reduction over Antarctica) and of 2-3.5 mPa (Arctic, 20 %-30 % reduction with respect to the long-term springtime mean). (b) Within the PSC height range, we found indications that smoke was able to slightly increase the PSC particle number and surface area concentration. In particular, a smoke-related additional ozone loss of 1-2 mPa (10 %-20 % contribution to the total ozone loss over Antarctica) was observed in the 14-23 km PSC height range in September-October 2020 and 2021. Smoke particle number concentrations ranged from 10 to 100 cm-3 and were about a factor of 10 (in 2020) and 5 (in 2021) above the stratospheric aerosol background level. Satellite observations indicated an additional mean column ozone loss (deviation from the long-term mean) of 26-30 Dobson units (9 %-10 %, September 2020, 2021) and 52-57 Dobson units (17 %-20 %, October 2020, 2021) in the smoke-polluted latitudinal Antarctic belt from 70-80°S. Copyright

Wildfire smoke, Arctic haze, and aerosol effects on mixed-phase and cirrus clouds over the North Pole region during MOSAiC: an introduction

An advanced multiwavelength polarization Raman lidar was operated aboard the icebreaker Polarstern during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition to continuously monitor aerosol and cloud layers in the central Arctic up to 30gkm height. The expedition lasted from September 2019 to October 2020 and measurements were mostly taken between 85 and 88.5ggN. The lidar was integrated into a complex remote-sensing infrastructure aboard the Polarstern. In this article, novel lidar techniques, innovative concepts to study aerosol-cloud interaction in the Arctic, and unique MOSAiC findings will be presented. The highlight of the lidar measurements was the detection of a 10gkm deep wildfire smoke layer over the North Pole region between 7-8gkm and 17-18gkm height with an aerosol optical thickness (AOT) at 532gnm of around 0.1 (in October-November 2019) and 0.05 from December to March. The dual-wavelength Raman lidar technique allowed us to unambiguously identify smoke as the dominating aerosol type in the aerosol layer in the upper troposphere and lower stratosphere (UTLS). An additional contribution to the 532gnm AOT by volcanic sulfate aerosol (Raikoke eruption) was estimated to always be lower than 15g%. The optical and microphysical properties of the UTLS smoke layer are presented in an accompanying paper . This smoke event offered the unique opportunity to study the influence of organic aerosol particles (serving as ice-nucleating particles, INPs) on cirrus formation in the upper troposphere. An example of a closure study is presented to explain our concept of investigating aerosol-cloud interaction in this field. The smoke particles were obviously able to control the evolution of the cirrus system and caused low ice crystal number concentration. After the discussion of two typical Arctic haze events, we present a case study of the evolution of a long-lasting mixed-phase cloud layer embedded in Arctic haze in the free troposphere. The recently introduced dual-field-of-view polarization lidar technique was applied, for the first time, to mixed-phase cloud observations in order to determine the microphysical properties of the water droplets. The mixed-phase cloud closure experiment (based on combined lidar and radar observations) indicated that the observed aerosol levels controlled the number concentrations of nucleated droplets and ice crystals

Overview of the MOSAiC expedition - Atmosphere

With the Arctic rapidly changing, the needs to observe, understand, and model the changes are essential. To support these needs, an annual cycle of observations of atmospheric properties, processes, and interactions were made while drifting with the sea ice across the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from October 2019 to September 2020. An international team designed and implemented the comprehensive program to document and characterize all aspects of the Arctic atmospheric system in unprecedented detail, using a variety of approaches, and across multiple scales. These measurements were coordinated with other observational teams to explore cross-cutting and coupled interactions with the Arctic Ocean, sea ice, and ecosystem through a variety of physical and biogeochemical processes. This overview outlines the breadth and complexity of the atmospheric research program, which was organized into 4 subgroups: atmospheric state, clouds and precipitation, gases and aerosols, and energy budgets. Atmospheric variability over the annual cycle revealed important influences from a persistent large-scale winter circulation pattern, leading to some storms with pressure and winds that were outside the interquartile range of past conditions suggested by long-term reanalysis. Similarly, the MOSAiC location was warmer and wetter in summer than the reanalysis climatology, in part due to its close proximity to the sea ice edge. The comprehensiveness of the observational program for characterizing and analyzing atmospheric phenomena is demonstrated via a winter case study examining air mass transitions and a summer case study examining vertical atmospheric evolution. Overall, the MOSAiC atmospheric program successfully met its objectives and was the most comprehensive atmospheric measurement program to date conducted over the Arctic sea ice. The obtained data will support a broad range of coupled-system scientific research and provide an important foundation for advancing multiscale modeling capabilities in the Arctic