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Oral presentation at the 2nd MOSAiC science conference showing the MOSAiC airborne laser scanner product overview and first results of seasonal surface roughness

Sea-ice thickness is a key factor and indicator in understanding the impact of the global climate change. Deriving basin-wide sea-ice thickness estimates from satellite laser and radar altimetry relies on freeboard measurements. The freeboard-to-thickness conversion in turn requires information of snow mass and the density of the sea-ice layer that have unknown spatio-temporal variabilities and trends directly translating into the uncertainty of decadal sea-ice thickness data records. In addition, inter-mission biases arise from, e.g., different sensor types and frequencies as well as varying footprint sizes affected by surface roughness across regions and seasons. Therefore, carrying out validation and inter-calibration studies is crucial for reliable and continuous observation of the Earth’s cryosphere. To achieve this, it is beneficial to have simultaneous measurements of freeboard, snow depth, and sea-ice thickness, which provide reference data for both direct satellite observations and geophysical target parameters. Here, we present Alfred Wegener Institute’s (AWI) IceBird program, which is a series of fixed-wing aircraft campaigns to measure Arctic sea ice and to monitor its change. During two late-winter campaigns in the western Arctic Ocean in 2017 and 2019, we have carried out surveys with the unique scientific instrument configuration including an airborne laser scanner (ALS) for surface topography and freeboard measurements, a tethered electromagnetic induction sounding instrument (EM-Bird) for total (snow+ice) thickness measurements, and an ultrawideband frequency-modulated continuous-wave microwave radar to measure snow thickness. Therefore, we are able to observe all three bounding interfaces in the sea-ice–snow system in high resolution along survey tracks on regional scales. During the ship-based drift expedition Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) between October 2019 and September 2020, helicopter surveys were carried out in high spatio-temporal resolution throughout the year, including the polar night, to measure freeboard and roughness with the ALS both in local grid pattern and in larger scale. Coincident EM-Bird ice thickness data and information from snow measurements on the ground will help linking these parameters and monitor them and their effect on satellite retrievals for a full seasonal cycle. The individual parameters are important for describing and monitoring the state of the Arctic sea ice and validating retrievals from satellite data, but combined they offer further possibilities to characterise sea ice. By assuming isostatic equilibrium, we are able to estimate up-to-date bulk density values for different sea-ice types from the IceBird data and to derive a parametrisation of sea-ice bulk density based on sea-ice freeboard. These data allow us to explore spatio-temporal variations in sea-ice parameters observable from space and to evaluate the validity of the freeboard-to-thickness conversion in satellite altimetry through comparison against dedicated satellite overpasses and orbit collections.

An integrated sensor platform including an inertial navigation system (INS) and a commercial airborne laser scanner (ALS) among other sensor was mounted in the cargo compartment in one of the Polarstern helicopters during MOSAiC. ALS data was acquired from more than 60 flights between October 2019 and September 2020 with a range of survey types intended to map changes of the sea ice surface during the full annual cycle at high spatial resolution and coverage. Here, we provide an overview of the collected data, the challenge of achieving centimeter elevation accuracy with a helicopter platform at high polar latitudes as well as the content and specifications of ALS data products. The high spatial resolution and repeated coverage of the larger area around Polarstern allow studying various surface features (e.g. pressure ridges, floes, melt ponds, snow drifts, etc.), their seasonal evolution, and their impact on atmosphere and ocean. Finally, we outline methods for planned applications, such as identifying individual floes and surface types using both measured freeboard and surface reflectance. Collocated helicopter-based optical and infrared imagery allow analyzing sea ice properties in further applications and to upscale comparable in-situ observations.

In order to investigate influences on the reflectance of snow covered Arctic sea ice, a discrete ordinate method and Mie-Theory based radiative transfer model has been set up. This model, the Snow on Sea Ice Model (SoSIM), is able to investigate changes in spectral and spectrally integrated (broadband) albedo of a multi-layer snow cover on sea ice due to varying snow microphysical parameters, atmospheric composition and incoming solar radiation. For typical conditions in the Arctic sea-ice area, it was found that the size of spherical snow grains and the angle of the sun above the horizon ultimately determine both spectral and broadband albedo of a thick snow layer. At 1300 nm, doubling the snow grain size decreases the albedo by about 20%, while a lower incident angle of solar light can offset this effect. The light absorbing impurity black carbon (soot) has a distinct influence on the albedo in the ultra-violet and visible range of the solar spectrum. However, it likely only lowers the albedo by less than 2%, for present concentrations of black carbon in Arctic snow. The spectral signature of black carbon is very similar to a thinning snow cover on top of a darker surface. SoSIM was also tested against other models and parametrisations for the spectral and broadband albedo of snowpacks as well as against field measurements of the spectral albedo of snow covered sea ice. The test proved the plausibility of the model results. Further, broadband albedo data from an airborne measurement campaign has been evaluated together with accompanying data such as sea ice thickness. This data demonstrated that sea-ice dynamics cause strong local surface heterogeneities. As a consequence, a strong variation is found in the spatially averaged surface albedo. To quantify this surface heterogeneity, an algorithm has been developed that automatically classifies typical freeze-up season surface covers of the Arctic ocean from photographs. As a combination of the findings from model study and evaluation of the campaign data, a model could be developed able to re-analyse the spatial distribution of broadband surface albedo of the Arctic ocean. The model utilises spatial information based on satellite observation of sea-ice concentration and thickness as well as climatological data of snow thickness. It is an alternative approach to derive the surface albedo based on SoSIM and not relying on satellite measurements in the visible range of the spectrum. First validations with air and satellite borne measurements of albedo distributions showed that the modelled albedo is plausible, yet has no better accuracy than ±5%. The model was used to predict the surface forcing of changes in the albedo as caused by the deposition of anthropogenic light absorbing substances onto the snow cover. It was found that depositing 40 ppbw black carbon into a pure snowpack causes an extra absorption of 1.58±0.83 W/m2 on average for the sea ice covered Arctic. The high relative uncertainty is caused by the uncertainty involved in the enhancement of light absorption by BC particles due to ageing processes.

Grain size of the surface snow is the key parameter for albedo in interior Antarctica, as impurity content is very small. The snow surface at the end of austral winter is characterized by very small grains. The small snow grains consist of broken precipitation particles and partially sublimated or mechanically fractured older ice particles. The albedo is consequently very high. The size of snow grains can be determined quite accurately by measuring its specific surface area (SSA), which is equivalent to the optical grain radius. The SSA as a material property used for albedo estimates typically shows an annual cycle. During the summer it decreases due to grain coarsening caused by snow metamorphism. However the surface layer is affected by a variety of processes including wind driven redistribution, precipitation or surface hoar formation. To discriminate the influence of the different processes on the evolution of the SSA during the summer we measured SSA along a 50m transect on a daily basis at Kohnen Station (75◦00′ S, 00◦00′ E, 2892 m a.s.l) in Dronning Maud Land (DML) indirectly by reflectance at 1310 nm. We found that the SSA was not reducing steadily but showed a cascading decrease. The peaks corresponded to precipitation events and frost formation lasting for several days and inhibiting the general expected decrease during the summer period. Our study indicates that even small amounts of precipitation during the summer period can affect the decrease of SSA, respectively the albedo, in the DML region on the East Antarctic Plateau.

The Arctic is a key player in the climate system because of the strong modification of the surface energy budget through snow and ice cover, which is tightly coupled to the global circulation of the atmosphere and the ocean. AWI (Alfred Wegener Institute) initiated therefore together with EC (Environment Canada) a special airborne program, as the Polar Airborne Measurements and Arctic Regional Climate Model Simulation Project (PAMARCMiP). The past two campaigns with POLAR 5 took place during April 2009 as well as April 2011. The Instrumentation, included a tethered electromagnetic (EM) sensor for sea ice thickness measurements [Haas et al, 2010], analyzers for ozone, gaseous elementary mercury, bromine monoxide, aerosol light scattering and aerosol light absorption and refractory black carbon, aerosol number concentration and aerosol size distribution, and aerosol optical depth (AOD). In addition, aerosol and ozone LIDAR were operated, and drop sondes were launched to characterize atmospheric state variables and to use it for combined LIDAR and aerosol data analysis [Hoffmann et al., 2012]. The traverses were completed within about a month, providing 3-D snapshots of aerosol, trace gases, atmospheric condition and sea ice thickness

Year-round observations of the physical snow and ice properties and processes that govern the ice pack evolution and its interaction with the atmosphere and the ocean were conducted during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition of the research vessel Polarstern in the Arctic Ocean from October 2019 to September 2020. This work was embedded into the interdisciplinary design of the 5 MOSAiC teams, studying the atmosphere, the sea ice, the ocean, the ecosystem, and biogeochemical processes. The overall aim of the snow and sea ice observations during MOSAiC was to characterize the physical properties of the snow and ice cover comprehensively in the central Arctic over an entire annual cycle. This objective was achieved by detailed observations of physical properties and of energy and mass balance of snow and ice. By studying snow and sea ice dynamics over nested spatial scales from centimeters to tens of kilometers, the variability across scales can be considered. On-ice observations of in situ and remote sensing properties of the different surface types over all seasons will help to improve numerical process and climate models and to establish and validate novel satellite remote sensing methods; the linkages to accompanying airborne measurements, satellite observations, and results of numerical models are discussed. We found large spatial variabilities of snow metamorphism and thermal regimes impacting sea ice growth. We conclude that the highly variable snow cover needs to be considered in more detail (in observations, remote sensing, and models) to better understand snow-related feedback processes. The ice pack revealed rapid transformations and motions along the drift in all seasons. The number of coupled ice-ocean interface processes observed in detail are expected to guide upcoming research with respect to the changing Arctic sea ice.