Radiative Effects of Clouds in the Arctic

In this thesis, the radiative effect of Arctic clouds during early summer is investigated based on observations collected aboard the research vessel Polarstern during the expedition PS106 conducted in 2017 in combination with passive satellite observations. The interactions of clouds with radiation, and the relevance of several macro- and microphysical properties of clouds and surface conditions are analyzed. An investigation of the small-scale variability of solar radiation on an ice floe based on a network of autonomous pyranometers covering an area of 0.83 km x 1.59 km, and the period from 4-16 June 2017 is given. Five distinct sky conditions are identified, and the mean and variance of atmospheric transmittance of global radiation are determined. Using a wavelet-based multi-resolution analysis, a comparison of individual station records and spatially averaged observations indicates that the absolute magnitude and scale-dependence of variability contain characteristic features for different sky conditions. For overcast conditions, distinctive patterns are identified in the diurnal variability and spatial distribution of the network observations, presumably caused by multiple reflection radiation between surface and cloud base in combination with the inhomogeneous surface conditions. A sensitivity analysis of radiative fluxes is performed for clear-sky and cloudy conditions using a 1-dimensional radiative transfer model, and is used as a basis to investigate how well state-of-the-art shipborne and passive satellite remote sensing observations can constrain the radiative effect of clouds and can serve to quantify the Arctic radiation budget. Cloud properties derived from the shipborne remote sensing observations with the Cloudnet algorithm are used as input for radiative transfer simulations. Simulated fluxes are compared to shipborne observations of the downward-terrestrial and solar fluxes as well as satellite products from CERES (Clouds and the Earth's Radiant Energy System, SYN1deg Ed. 4.1) to test closure of simulated and observed radiative fluxes, and to analyse the cloud radiative effect. Closure is achieved for clear-sky conditions. Based on selected case studies and an analysis for the entire PS106 period, the largest discrepancies are identified for low-level stratus, precipitation and ice clouds. Moreover, the cloud radiative effect inferred along the cruise track is compared to the entire Arctic to expand the regional context, making use of the wide spatial coverage of the CERES products. The results indicate a strong contribution of the solar flux to the radiation budget for the study period. Due to the reduction of solar radiation by clouds, a cooling effect of -8.8 W/m² and -9.3 W/m² is found at the surface for the PS106 cruise and the central Arctic, respectively. The similarity of local and regional CRE suggests that the PS106 cloud observations can be considered as representative of Arctic cloud conditions during the early summer of 2017.:Contents 1 Introduction 1.1 Motivation 1.2 Characteristics of Arctic Clouds 1.3 Effect of Arctic Clouds on the Radiation Budget 1.4 Link Between Arctic Clouds and Surface Conditions 1.5 Objectives of (AC)3 and this Thesis 1.6 Outline 2 Theoretical Background 2.1 Radiative Quantities 2.2 Radiative Interactions 2.2.1 Absorption 2.2.2 Scattering and Extinction 2.3 Radiative Transfer Equation 2.4 Radiative Transfer in the Arctic 2.4.1 Surface Reflection and Transmission 2.4.2 Clear-sky Conditions 2.4.3 Optical Properties of Clouds 2.5 Radiative Transfer Modelling 2.5.1 Two-stream Approximation 2.5.2 Correlated k -distribution 2.5.3 RRTMG 2.6 Energy Budget and Cloud Radiative Effect 3 PS106 Expedition, Instrumentation, Data sets, and Methods 3.1 Instrumentation 3.1.1 Pyranometer Network 3.1.2 Ship-borne Instrumentation 3.2 Data sets 3.2.1 Cloudnet 3.2.2 CERES data set 3.2.3 Ancillary data set 3.3 General Conditions During PS106 3.3.1 Synoptic and Surface Conditions 3.3.2 Atmospheric Temperature and Humidity Conditions 3.3.3 Statistical Analysis of Cloud Properties 3.4 Radiative Transfer Simulation Setup 4 Sensitivity Analysis of Arctic Fluxes 4.1 Clear-sky Perturbations 4.1.1 Atmosphere 4.1.2 Surface 4.2 Clear-sky Radiative Flux Uncertainty 4.3 Cloud Perturbations 4.3.1 Cloud Water Path 4.3.2 Cloud Particle Effective Radius 4.3.3 Liquid Fraction and Surface Albedo 4.3.4 Cloud Base Height 4.3.5 Cloud Geometrical Thickness 4.4 Synopsis 5 Cloud Induced Spatiotemporal Variability of Solar Radiation 5.1 Data Analysis 5.1.1 Data Processing 5.1.2 Sky Classification 5.2 Case Studies 5.2.1 Clear-sky Case 5.2.2 Overcast Case 5.2.3 Thin Cloud Case 5.2.4 Multilayer Case 5.2.5 Broken Cloud Case 5.3 Wavelet-based Multiresolution Analysis 5.4 Synopsis and Discussion 6 Radiation Closure 6.1 Radiative Flux Comparison Between CERES and T-CARS 6.2 Radiative Closure for Clear-sky Atmosphere 6.3 Radiative Closure for Cloudy Atmosphere 6.4 Synopsis and Discussion 7 Case Studies 7.1 Clear-sky Case 7.2 Single and Multilayer Ice Cloud Case 7.3 Mixed Phase Cloud Case 7.4 Synopsis 8 Radiation Budget and Cloud Radiative Effects 8.1 Cloud Radiative Effect (CRE) Analysis 8.2 Radiation Budget 8.3 Synopsis 9 Summary, Conclusions and Outlook 9.1 Summary and Conclusions 9.2 Outlook Appendix A Cloud Microphysical Properties During PS106 B CRE of Sensitivity Analysis C CERES Aerosol Products D Additional Observations Literature List of Abbreviations List of Symbols List of Figures List of Tables Acknowledgement

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

Radiative Effects of Clouds in the Arctic

In this thesis, the radiative effect of Arctic clouds during early summer is investigated based on observations collected aboard the research vessel Polarstern during the expedition PS106 conducted in 2017 in combination with passive satellite observations. The interactions of clouds with radiation, and the relevance of several macro- and microphysical properties of clouds and surface conditions are analyzed. An investigation of the small-scale variability of solar radiation on an ice floe based on a network of autonomous pyranometers covering an area of 0.83 km x 1.59 km, and the period from 4-16 June 2017 is given. Five distinct sky conditions are identified, and the mean and variance of atmospheric transmittance of global radiation are determined. Using a wavelet-based multi-resolution analysis, a comparison of individual station records and spatially averaged observations indicates that the absolute magnitude and scale-dependence of variability contain characteristic features for different sky conditions. For overcast conditions, distinctive patterns are identified in the diurnal variability and spatial distribution of the network observations, presumably caused by multiple reflection radiation between surface and cloud base in combination with the inhomogeneous surface conditions. A sensitivity analysis of radiative fluxes is performed for clear-sky and cloudy conditions using a 1-dimensional radiative transfer model, and is used as a basis to investigate how well state-of-the-art shipborne and passive satellite remote sensing observations can constrain the radiative effect of clouds and can serve to quantify the Arctic radiation budget. Cloud properties derived from the shipborne remote sensing observations with the Cloudnet algorithm are used as input for radiative transfer simulations. Simulated fluxes are compared to shipborne observations of the downward-terrestrial and solar fluxes as well as satellite products from CERES (Clouds and the Earth's Radiant Energy System, SYN1deg Ed. 4.1) to test closure of simulated and observed radiative fluxes, and to analyse the cloud radiative effect. Closure is achieved for clear-sky conditions. Based on selected case studies and an analysis for the entire PS106 period, the largest discrepancies are identified for low-level stratus, precipitation and ice clouds. Moreover, the cloud radiative effect inferred along the cruise track is compared to the entire Arctic to expand the regional context, making use of the wide spatial coverage of the CERES products. The results indicate a strong contribution of the solar flux to the radiation budget for the study period. Due to the reduction of solar radiation by clouds, a cooling effect of -8.8 W/m² and -9.3 W/m² is found at the surface for the PS106 cruise and the central Arctic, respectively. The similarity of local and regional CRE suggests that the PS106 cloud observations can be considered as representative of Arctic cloud conditions during the early summer of 2017.:Contents 1 Introduction 1.1 Motivation 1.2 Characteristics of Arctic Clouds 1.3 Effect of Arctic Clouds on the Radiation Budget 1.4 Link Between Arctic Clouds and Surface Conditions 1.5 Objectives of (AC)3 and this Thesis 1.6 Outline 2 Theoretical Background 2.1 Radiative Quantities 2.2 Radiative Interactions 2.2.1 Absorption 2.2.2 Scattering and Extinction 2.3 Radiative Transfer Equation 2.4 Radiative Transfer in the Arctic 2.4.1 Surface Reflection and Transmission 2.4.2 Clear-sky Conditions 2.4.3 Optical Properties of Clouds 2.5 Radiative Transfer Modelling 2.5.1 Two-stream Approximation 2.5.2 Correlated k -distribution 2.5.3 RRTMG 2.6 Energy Budget and Cloud Radiative Effect 3 PS106 Expedition, Instrumentation, Data sets, and Methods 3.1 Instrumentation 3.1.1 Pyranometer Network 3.1.2 Ship-borne Instrumentation 3.2 Data sets 3.2.1 Cloudnet 3.2.2 CERES data set 3.2.3 Ancillary data set 3.3 General Conditions During PS106 3.3.1 Synoptic and Surface Conditions 3.3.2 Atmospheric Temperature and Humidity Conditions 3.3.3 Statistical Analysis of Cloud Properties 3.4 Radiative Transfer Simulation Setup 4 Sensitivity Analysis of Arctic Fluxes 4.1 Clear-sky Perturbations 4.1.1 Atmosphere 4.1.2 Surface 4.2 Clear-sky Radiative Flux Uncertainty 4.3 Cloud Perturbations 4.3.1 Cloud Water Path 4.3.2 Cloud Particle Effective Radius 4.3.3 Liquid Fraction and Surface Albedo 4.3.4 Cloud Base Height 4.3.5 Cloud Geometrical Thickness 4.4 Synopsis 5 Cloud Induced Spatiotemporal Variability of Solar Radiation 5.1 Data Analysis 5.1.1 Data Processing 5.1.2 Sky Classification 5.2 Case Studies 5.2.1 Clear-sky Case 5.2.2 Overcast Case 5.2.3 Thin Cloud Case 5.2.4 Multilayer Case 5.2.5 Broken Cloud Case 5.3 Wavelet-based Multiresolution Analysis 5.4 Synopsis and Discussion 6 Radiation Closure 6.1 Radiative Flux Comparison Between CERES and T-CARS 6.2 Radiative Closure for Clear-sky Atmosphere 6.3 Radiative Closure for Cloudy Atmosphere 6.4 Synopsis and Discussion 7 Case Studies 7.1 Clear-sky Case 7.2 Single and Multilayer Ice Cloud Case 7.3 Mixed Phase Cloud Case 7.4 Synopsis 8 Radiation Budget and Cloud Radiative Effects 8.1 Cloud Radiative Effect (CRE) Analysis 8.2 Radiation Budget 8.3 Synopsis 9 Summary, Conclusions and Outlook 9.1 Summary and Conclusions 9.2 Outlook Appendix A Cloud Microphysical Properties During PS106 B CRE of Sensitivity Analysis C CERES Aerosol Products D Additional Observations Literature List of Abbreviations List of Symbols List of Figures List of Tables Acknowledgement

Radiative Effects of Clouds in the Arctic

In this thesis, the radiative effect of Arctic clouds during early summer is investigated based on observations collected aboard the research vessel Polarstern during the expedition PS106 conducted in 2017 in combination with passive satellite observations. The interactions of clouds with radiation, and the relevance of several macro- and microphysical properties of clouds and surface conditions are analyzed. An investigation of the small-scale variability of solar radiation on an ice floe based on a network of autonomous pyranometers covering an area of 0.83 km x 1.59 km, and the period from 4-16 June 2017 is given. Five distinct sky conditions are identified, and the mean and variance of atmospheric transmittance of global radiation are determined. Using a wavelet-based multi-resolution analysis, a comparison of individual station records and spatially averaged observations indicates that the absolute magnitude and scale-dependence of variability contain characteristic features for different sky conditions. For overcast conditions, distinctive patterns are identified in the diurnal variability and spatial distribution of the network observations, presumably caused by multiple reflection radiation between surface and cloud base in combination with the inhomogeneous surface conditions. A sensitivity analysis of radiative fluxes is performed for clear-sky and cloudy conditions using a 1-dimensional radiative transfer model, and is used as a basis to investigate how well state-of-the-art shipborne and passive satellite remote sensing observations can constrain the radiative effect of clouds and can serve to quantify the Arctic radiation budget. Cloud properties derived from the shipborne remote sensing observations with the Cloudnet algorithm are used as input for radiative transfer simulations. Simulated fluxes are compared to shipborne observations of the downward-terrestrial and solar fluxes as well as satellite products from CERES (Clouds and the Earth's Radiant Energy System, SYN1deg Ed. 4.1) to test closure of simulated and observed radiative fluxes, and to analyse the cloud radiative effect. Closure is achieved for clear-sky conditions. Based on selected case studies and an analysis for the entire PS106 period, the largest discrepancies are identified for low-level stratus, precipitation and ice clouds. Moreover, the cloud radiative effect inferred along the cruise track is compared to the entire Arctic to expand the regional context, making use of the wide spatial coverage of the CERES products. The results indicate a strong contribution of the solar flux to the radiation budget for the study period. Due to the reduction of solar radiation by clouds, a cooling effect of -8.8 W/m² and -9.3 W/m² is found at the surface for the PS106 cruise and the central Arctic, respectively. The similarity of local and regional CRE suggests that the PS106 cloud observations can be considered as representative of Arctic cloud conditions during the early summer of 2017.:Contents 1 Introduction 1.1 Motivation 1.2 Characteristics of Arctic Clouds 1.3 Effect of Arctic Clouds on the Radiation Budget 1.4 Link Between Arctic Clouds and Surface Conditions 1.5 Objectives of (AC)3 and this Thesis 1.6 Outline 2 Theoretical Background 2.1 Radiative Quantities 2.2 Radiative Interactions 2.2.1 Absorption 2.2.2 Scattering and Extinction 2.3 Radiative Transfer Equation 2.4 Radiative Transfer in the Arctic 2.4.1 Surface Reflection and Transmission 2.4.2 Clear-sky Conditions 2.4.3 Optical Properties of Clouds 2.5 Radiative Transfer Modelling 2.5.1 Two-stream Approximation 2.5.2 Correlated k -distribution 2.5.3 RRTMG 2.6 Energy Budget and Cloud Radiative Effect 3 PS106 Expedition, Instrumentation, Data sets, and Methods 3.1 Instrumentation 3.1.1 Pyranometer Network 3.1.2 Ship-borne Instrumentation 3.2 Data sets 3.2.1 Cloudnet 3.2.2 CERES data set 3.2.3 Ancillary data set 3.3 General Conditions During PS106 3.3.1 Synoptic and Surface Conditions 3.3.2 Atmospheric Temperature and Humidity Conditions 3.3.3 Statistical Analysis of Cloud Properties 3.4 Radiative Transfer Simulation Setup 4 Sensitivity Analysis of Arctic Fluxes 4.1 Clear-sky Perturbations 4.1.1 Atmosphere 4.1.2 Surface 4.2 Clear-sky Radiative Flux Uncertainty 4.3 Cloud Perturbations 4.3.1 Cloud Water Path 4.3.2 Cloud Particle Effective Radius 4.3.3 Liquid Fraction and Surface Albedo 4.3.4 Cloud Base Height 4.3.5 Cloud Geometrical Thickness 4.4 Synopsis 5 Cloud Induced Spatiotemporal Variability of Solar Radiation 5.1 Data Analysis 5.1.1 Data Processing 5.1.2 Sky Classification 5.2 Case Studies 5.2.1 Clear-sky Case 5.2.2 Overcast Case 5.2.3 Thin Cloud Case 5.2.4 Multilayer Case 5.2.5 Broken Cloud Case 5.3 Wavelet-based Multiresolution Analysis 5.4 Synopsis and Discussion 6 Radiation Closure 6.1 Radiative Flux Comparison Between CERES and T-CARS 6.2 Radiative Closure for Clear-sky Atmosphere 6.3 Radiative Closure for Cloudy Atmosphere 6.4 Synopsis and Discussion 7 Case Studies 7.1 Clear-sky Case 7.2 Single and Multilayer Ice Cloud Case 7.3 Mixed Phase Cloud Case 7.4 Synopsis 8 Radiation Budget and Cloud Radiative Effects 8.1 Cloud Radiative Effect (CRE) Analysis 8.2 Radiation Budget 8.3 Synopsis 9 Summary, Conclusions and Outlook 9.1 Summary and Conclusions 9.2 Outlook Appendix A Cloud Microphysical Properties During PS106 B CRE of Sensitivity Analysis C CERES Aerosol Products D Additional Observations Literature List of Abbreviations List of Symbols List of Figures List of Tables Acknowledgement

Spatial and temporal variability of broadband solar irradiance during POLARSTERN cruise PS106/1 Ice Floe Camp (June 4th-16th 2017)

The dataset is part of the expedition PS106/1 of the Research Vessel POLARSTERN to the Arctic Ocean in 2017. During the ice floe camp (draft period, June 4th-16th 2017) 15 pyranometer stations were deployed over the ice floe covering an area of about 1 Km². Each station measured broadband solar irradiance and temperature at 1Hz resolution. Relative humidity was also measured, however its use it is not recommended due to technical problems of the sensors. Each file contains level and cleanliness flag describing the status of the pyranometer dome per day. The criterion is as follows. Cleanliness clean =1, drops =2, frozen =3, no observation = 4 Leveling leveled =1, partially leveled =2, unleveled =3, no observation =

Spatiotemporal variability of solar radiation introduced by clouds over Arctic sea ice

The role of clouds in recent Arctic warming is not fully understood, including their effects on the solar radiation and the surface energy budget. To investigate relevant small-scale processes in detail, the intensive Physical feedbacks of Arctic planetary boundary layer, Sea ice, Cloud and AerosoL (PASCAL) drifting ice floe station field campaign was conducted during early summer in the central arctic. During this campaign, the small-scale spatiotemporal variability of global irradiance was observed for the first time on an ice floe with a dense network of autonomous pyranometers. A total of 15 stations were deployed covering an area of 0.83 km×1.59 km from 4–16 June 2017. This unique, open-access dataset is described here, and an analysis of the spatiotemporal variability deduced from this dataset is presented for different synoptic conditions. Based on additional observations, five typical sky conditions were identified and used to determine the values of the mean and variance of atmospheric global transmittance for these conditions. Overcast conditions were observed 39.6 % of the time predominantly during the first week, with an overall mean transmittance of 0.47. The second most frequent conditions corresponded to multilayer clouds (32.4 %), which prevailed in particular during the second week, with a mean transmittance of 0.43. Broken clouds had a mean transmittance of 0.61 and a frequency of occurrence of 22.1 %. Finally, the least frequent sky conditions were thin clouds and cloudless conditions, which both had a mean transmittance of 0.76 and occurrence frequencies of 3.5 % and 2.4 %, respectively. For overcast conditions, lower global irradiance was observed for stations closer to the ice edge, likely attributable to the low surface albedo of dark open water and a resulting reduction of multiple reflections between the surface and cloud base. Using a wavelet-based multi-resolution analysis, power spectra of the time series of atmospheric transmittance were compared for single-station and spatially averaged observations and for different sky conditions. It is shown that both the absolute magnitude and the scale dependence of variability contains characteristic features for the different sky conditions

Increasing the spatial resolution of cloud property retrievals from Meteosat SEVIRI by use of its high-resolution visible channel: implementation and examples

The modification of an existing cloud property retrieval scheme for the Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument on board the geostationary Meteosat satellites is described to utilize its highresolution visible (HRV) channel for increasing the spatial resolution of its physical outputs. This results in products with a nadir spatial resolution of 1 × 1 km2 compared to the standard 3 × 3 km2 resolution offered by the narrowband channels. This improvement thus greatly reduces the resolution gap between current geostationary and polar-orbiting meteorological satellite imagers. In the first processing step, cloudiness is determined from the HRV observations by a threshold-based cloud masking algorithm. Subsequently, a linear model that links the 0.6 µm, 0.8 µm, and HRV reflectances provides a physical constraint to incorporate the spatial high-frequency component of the HRV observations into the retrieval of cloud optical depth. The implementation of the method is described, including the ancillary datasets used. It is demonstrated that the omission of high-frequency variations in the cloud-absorbing 1.6 µm channel results in comparatively large uncertainties in the retrieved cloud effective radius, likely due to the mismatch in channel resolutions. A newly developed downscaling scheme for the 1.6 µm reflectance is therefore applied to mitigate the effects of this scale mismatch. Benefits of the increased spatial resolution of the resulting SEVIRI products are demonstrated for three example applications: (i) for a convective cloud field, it is shown that significantly better agreement between the distributions of cloud optical depth retrieved from SEVIRI and from collocated MODIS observations is achieved. (ii) The temporal evolution of cloud properties for a growing convective storm at standard and HRV spatial resolutions are compared, illustrating an improved contrast in growth signatures resulting from the use of the HRV channel. (iii) An example of surface solar irradiance, determined from the retrieved cloud properties, is shown, for which the HRV channel helps to better capture the large spatiotemporal variability induced by convective clouds. These results suggest that incorporating the HRV channel into the retrieval has potential for improving Meteosat-based cloud products for several application domains

Rapid growth of Aitken-mode particles during Arctic summer by fog chemical processing and its implication

In the Arctic, new particle formation (NPF) and subsequent growth processes are the keys to produce Aitken-mode particles, which under certain conditions can act as cloud condensation nuclei (CCNs). The activation of Aitken-mode particles increases the CCN budget of Arctic low-level clouds and, accordingly, affects Arctic climate forcing. However, the growth mechanism of Aitken-mode particles from NPF into CCN range in the summertime Arctic boundary layer remains a subject of current research. In this combined Arctic cruise field and modeling study, we investigated Aitken-mode particle growth to sizes above 80 nm. A mechanism is suggested that explains how Aitken-mode particles can become CCN without requiring high water vapor supersaturation. Model simulations suggest the formation of semivolatile compounds, such as methanesulfonic acid (MSA) in fog droplets. When the fog droplets evaporate, these compounds repartition from CCNs into the gas phase and into the condensed phase of nonactivated Aitken-mode particles. For MSA, a mass increase factor of 18 is modeled. The postfog redistribution mechanism of semivolatile acidic and basic compounds could explain the observed growth of >20 nm h(-1) for 60-nm particles to sizes above 100 nm. Overall, this study implies that the increasing frequency of NPF and fog-related particle processing can affect Arctic cloud properties in the summertime boundary layer.Peer reviewe

The Arctic Cloud Puzzle: Using ACLOUD/PASCAL Multiplatform Observations to Unravel the Role of Clouds and Aerosol Particles in Arctic Amplification

Clouds play an important role in Arctic amplification. This term represents the recently observed enhanced warming of the Arctic relative to the global increase of near-surface air temperature. However, there are still important knowledge gaps regarding the interplay between Arctic clouds and aerosol particles, and surface properties, as well as turbulent and radiative fluxes that inhibit accurate model simulations of clouds in the Arctic climate system. In an attempt to resolve this so-called Arctic cloud puzzle, two comprehensive and closely coordinated field studies were conducted: the Arctic Cloud Observations Using Airborne Measurements during Polar Day (ACLOUD) aircraft campaign and the Physical Feedbacks of Arctic Boundary Layer, Sea Ice, Cloud and Aerosol (PASCAL) ice breaker expedition. Both observational studies were performed in the framework of the German Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC) project. They took place in the vicinity of Svalbard, Norway, in May and June 2017. ACLOUD and PASCAL explored four pieces of the Arctic cloud puzzle: cloud properties, aerosol impact on clouds, atmospheric radiation, and turbulent dynamical processes. The two instrumented Polar 5 and Polar 6 aircraft; the icebreaker Research Vessel (R/V) Polarstern; an ice floe camp including an instrumented tethered balloon; and the permanent ground-based measurement station at Ny-Ålesund, Svalbard, were employed to observe Arctic low- and mid-level mixed-phase clouds and to investigate related atmospheric and surface processes. The Polar 5 aircraft served as a remote sensing observatory examining the clouds from above by downward-looking sensors; the Polar 6 aircraft operated as a flying in situ measurement laboratory sampling inside and below the clouds. Most of the collocated Polar 5/6 flights were conducted either above the R/V Polarstern or over the Ny-Ålesund station, both of which monitored the clouds from below using similar but upward-looking remote sensing techniques as the Polar 5 aircraft. Several of the flights were carried out underneath collocated satellite tracks. The paper motivates the scientific objectives of the ACLOUD/PASCAL observations and describes the measured quantities, retrieved parameters, and the applied complementary instrumentation. Furthermore, it discusses selected measurement results and poses critical research questions to be answered in future papers analyzing the data from the two field campaigns