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C. M. Carmagnola, F. Domine, M. Dumont, P. Wright, B. Strellis, M. Bergin, J. Dibb, G. Picard, Q. Libois, L. Arnaud, S. Morin. (2013). Snow spectral albedo at Summit, Greenland: measurements and numerical simulations based on physical and chemical properties of the snowpack (Vol. 7).
Abstract:
Abstract. The broadband albedo of surface snow is determined both by the near-surface profile of the physical and chemical properties of the snowpack and by the spectral and angular characteristics of the incident solar radiation. Simultaneous measurements of the physical and chemical properties of snow were carried out at Summit Camp, Greenland (72°36´ N, 38°25´ W, 3210 m a.s.l.) in May and June 2011, along with spectral albedo measurements. One of the main objectives of the field campaign was to test our ability to predict snow spectral albedo by comparing the measured albedo to the albedo calculated with a radiative transfer model, using measured snow physical and chemical properties. To achieve this goal, we made daily measurements of the snow spectral albedo in the range 350–2200 nm and recorded snow stratigraphic information down to roughly 80 cm. The snow specific surface area (SSA) was measured using the DUFISSS instrument (DUal Frequency Integrating Sphere for Snow SSA measurement, Gallet et al., 2009). Samples were also collected for chemical analyses including black carbon (BC) and dust, to evaluate the impact of light absorbing particulate matter in snow. This is one of the most comprehensive albedo-related data sets combining chemical analysis, snow physical properties and spectral albedo measurements obtained in a polar environment. The surface albedo was calculated from density, SSA, BC and dust profiles using the DISORT model (DIScrete Ordinate Radiative Transfer, Stamnes et al., 1988) and compared to the measured values. Results indicate that the energy absorbed by the snowpack through the whole spectrum considered can be inferred within 1.10%. This accuracy is only slightly better than that which can be obtained considering pure snow, meaning that the impact of impurities on the snow albedo is small at Summit. In the near infrared, minor deviations in albedo up to 0.014 can be due to the accuracy of radiation and SSA measurements and to the surface roughness, whereas deviations up to 0.05 can be explained by the spatial heterogeneity of the snowpack at small scales, the assumption of spherical snow grains made for DISORT simulations and the vertical resolution of measurements of surface layer physical properties. At 1430 and around 1800 nm the discrepancies are larger and independent of the snow properties; we propose that they are due to errors in the ice refractive index at these wavelengths. This work contributes to the development of physically based albedo schemes in detailed snowpack models, and to the improvement of retrieval algorithms for estimating snow properties from remote sensing data. Programme: 1088
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Dominic Saunderson, Andrew Mackintosh, Felicity McCormack, Richard Selwyn Jones, Ghislain Picard. (2022). Surface melt on the Shackleton Ice Shelf, East Antarctica (2003–2021).
Abstract:
Abstract. Many ice shelves in Antarctica experience surface melt each summer, with potentially severe consequences for sea level rise. However, large interannual and regional variability in surface melt increases uncertainty in predictions of how ice shelves will react to climate change. Previous studies of surface melt have usually focused on either a process-level understanding of surface melt through energy balance investigations, or used regional melt metrics to quantify interannual variability in satellite observations of surface melt. Here, we use an approach that helps bridge the gap between work at these two scales. Using daily passive microwave observations from the AMSR-E and AMSR-2 sensors, and the machine learning approach of a self-organising map, we identify nine representative spatial distributions (“patterns”) of surface melt on the Shackleton Ice Shelf, East Antarctica, over the previous two decades (2002/03–2020/21). Our results point to a significant role for surface air temperatures in controlling the interannual variability of summer melt, and also reveal the influence of local controls on driving melt. In particular, prolonged melt in the south-east of the shelf and along the grounding line shows the importance of katabatic winds and surface albedo. Our approach highlights the necessity of understanding both local and large-scale controls on surface melt, and demonstrates that self-organising maps can be used to investigate the variability of surface melt on Antarctic ice shelves. Programme: 1110
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Ghislain Picard, Marion Leduc-Leballeur, Alison F. Banwell, Ludovic Brucker, Giovanni Macelloni. (2022). The sensitivity of satellite microwave observations to liquid water in the Antarctic snowpack.
Abstract:
Abstract. Surface melting on the Antarctic Ice Sheet has been monitored by satellite microwave radiometery for over 40 years. Despite this long perspective, our understanding of the microwave emission from wet snow is still limited, preventing the full exploitation of these observations to study supraglacial hydrology. Using the Snow Microwave Radiative Transfer (SMRT) model, this study investigates the sensitivity of microwave brightness temperature to snow liquid water content at frequencies from 1.4 to 37 GHz. We first determine the snowpack properties for 8 selected coastal sites by retrieving profiles of density, grain size and ice layers from microwave observations when the snowpack is dry during winter time. Second, a series of brightness temperature simulations is run with added water. The results show that: i) a small amount of liquid water (≈0.5 kg m-2 can be detected, but the actual amount can not be retrieved in the full range of possible water contents, ii) the detection of a buried wet layer is possible up to a maximum 1 to 6 m depth depending on the frequency (6–37 GHz) and on the site, iii) surface ponds and water-saturated areas may prevent melt detection, but the current coverage of these water bodies in the large satellite field of view is presently too small in Antarctica to have noticeable effects, iv) at 1.4 GHz, while the simulations are less reliable, we found a weaker sensitivity to liquid water and the maximal depth of detection is relatively shallow (<10 m) compared to the typical radiation penetration depth in dry firn (≈1000 m) at this low frequency. These numerical results pave the way for the development of improved multi-frequency algorithms to detect melt intensity and depth in the Antarctic snowpack. Programme: 1110
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M. Casado, A. Landais, G. Picard, T. Münch, T. Laepple, B. Stenni, G. Dreossi, A. Ekaykin, L. Arnaud, C. Genthon, A. Touzeau, V. Masson-Delmotte, J. Jouzel. (2016). Archival of the water stable isotope signal in East Antarctic ice cores (Vol. 2016).
Abstract: The oldest ice core records are obtained from the East Antarctic plateau. Water stable isotopes records are key for reconstructions of past climatic conditions both over the ice sheet and at the evaporation source. The accuracy of such climate reconstructions crucially depends on the knowledge of all the processes affecting the water vapour, precipitation and snow isotopic composition. Atmospheric fractionation processes are well understood and can be integrated in Rayleigh distillation and complex isotope enabled climate models. However, a comprehensive quantitative understanding of processes potentially altering the snow isotopic composition after the deposition is still missing, especially for exchanges between vapour and snow. In low accumulation sites such as found on the East Antarctic Plateau, these poorly constrained processes are especially likely to play a significant role. This limits the interpretation of isotopic composition from ice core records, specifically at short time scales. Here, we combine observations of isotopic composition in the vapour, the precipitation, the surface snow and the buried snow from various sites of the East Antarctic Plateau. At the seasonal scale, we highlight a significant impact of metamorphism on surface snow isotopic signal compared to the initial precipitation isotopic signal. In particular, in summer, exchanges of water molecules between vapour and snow are driven by the sublimation/condensation cycles at the diurnal scale. Using highly resolved isotopic composition profiles from pits in five East Antarctic sites, we identify a common 20 cm cycle which cannot be attributed to the seasonal variability of precipitation. Altogether, the smaller range of isotopic compositions observed in the buried and in the surface snow compared to the precipitation, and also the reduced slope between surface snow isotopic composition and temperature compared to precipitation, constitute evidences of post-deposition processes affecting the variability of the isotopic composition in the snow pack. To reproduce these processes in snow-models is crucial to understand the link between snow isotopic composition and climatic conditions and to improve the interpretation of isotopic composition as a paleoclimate proxy.
Programme: 1013
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Young, D. A., Roberts, J. L., Ritz, C., Frezzotti, M., Quartini, E., Cavitte, M. G. P., Tozer, C. R., Steinhage, D., Urbini, S., Corr, H. F. J., Van Ommen, T., and Blankenship, D. D.. (2016). High resolution boundary conditions of an old ice target near Dome C, Antarctica,. The cryosphere discuss., 2016, 1–16.
Abstract: Abstract. A high resolution (1 km line spacing) aerogeophysical survey was conducted over a region near the East Antarctic Ice Sheet's Dome C that may hold a 1.5 million year old climate record. New ice thickness data derived from an airborne coherent radar sounder was combined with unpublished data that was unavailable for earlier compilations. We find under the primary candidate region elevated rough topography, near a number of subglacial lakes, but also regions of smoother bed. The high resolution of this ice thickness dataset also allows us to explore the nature of ice thickness uncertainties in the context of radar geometry and processing.
Programme: 902
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Sara Arioli, Ghislain Picard, Laurent Arnaud, Vincent Favier. (2023). Dynamics of the snow grain size in a windy coastal area of Antarctica from continuous in situ spectral-albedo measurements (Vol. 17).
Abstract: The grain size of the superficial snow layer is a key determinant of the surface albedo in Antarctica. Its evolution is the result of multiple interacting processes, such as dry and wet metamorphism, melt, snow drift, and precipitation. Among them, snow drift has the least known and least predictable impact. The goal of this study is to relate the variations in surface snow grain size to these processes in a windy location of the Antarctic coast. For this, we retrieved the daily grain size from 5-year-long in situ observations of the spectral albedo recorded by a new multi-band albedometer, unique in terms of autonomy and described here for the first time. An uncertainty assessment and a comparison with satellite-retrieved grain size were carried out to verify the reliability of the instrument, and an RMSE up to 0.16 mm in the observed grain size was found. By relating these in situ measurements to time series of snow drift, surface temperature, snow surface height and snowfall, we established that the evolution of the grain size in the presence of snow drift is complex and follows two possible pathways: (1) a decrease in the grain size (about half of our measurements) resulting from the deposition of small grains advected by the wind (surprisingly, this decrease is often – 2/3 of the cases– associated with a decrease in the surface height, i.e., a net erosion over the drift episode), (2) an increase in the grain size (the other half) due to either the removal of the surface layer or metamorphism. However, we note that this increase is often limited with respect to the increase predicted by a theoretical metamorphism model, suggesting that a concomitant deposition of small grains is likely. At last, we found that wind also completely impedes the deposition of snowfall during half of the observed precipitation events. When this happens, the grain size evolves as if precipitation were not occurring. As a result of all these processes, we conclude that the grain size in a windy area remains more stable than it would be in the absence of snow drift, hence limiting the variations in the albedo and in the radiative energy budget.
Programme: 1110
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Ghislain Picard, Marion Leduc-Leballeur, Alison F. Banwell, Ludovic Brucker, Giovanni Macelloni. (2022). The sensitivity of satellite microwave observations to liquid water in the Antarctic snowpack (Vol. 16).
Abstract: Surface melting on the Antarctic Ice Sheet has been monitored by satellite microwave radiometry for over 40 years. Despite this long perspective, our understanding of the microwave emission from wet snow is still limited, preventing the full exploitation of these observations to study supraglacial hydrology. Using the Snow Microwave Radiative Transfer (SMRT) model, this study investigates the sensitivity of microwave brightness temperature to snow liquid water content at frequencies from 1.4 to 37 GHz. We first determine the snowpack properties for eight selected coastal sites by retrieving profiles of density, grain size and ice layers from microwave observations when the snowpack is dry during wintertime. Second, a series of brightness temperature simulations is run with added water. The results show that (i) a small quantity of liquid water (≈0.5 kg m−2) can be detected, but the actual quantity cannot be retrieved out of the full range of possible water quantities; (ii) the detection of a buried wet layer is possible up to a maximum depth of 1 to 6 m depending on the frequency (6–37 GHz) and on the snow properties (grain size, density) at each site; (iii) surface ponds and water-saturated areas may prevent melt detection, but the current coverage of these waterbodies in the large satellite field of view is presently too small in Antarctica to have noticeable effects; and (iv) at 1.4 GHz, while the simulations are less reliable, we found a weaker sensitivity to liquid water and the maximal depth of detection is relatively shallow (<10 m) compared to the typical radiation penetration depth in dry firn (≈1000 m) at this low frequency. These numerical results pave the way for the development of improved multi-frequency algorithms to detect melt intensity and the depth of liquid water below the surface in the Antarctic snowpack.
Programme: 1110
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Dominic Saunderson, Andrew Mackintosh, Felicity McCormack, Richard Selwyn Jones, Ghislain Picard. (2022). Surface melt on the Shackleton Ice Shelf, East Antarctica (2003–2021) (Vol. 16).
Abstract: Melt on the surface of Antarctic ice shelves can potentially lead to their disintegration, accelerating the flow of grounded ice to the ocean and raising global sea levels. However, the current understanding of the processes driving surface melt is incomplete, increasing uncertainty in predictions of ice shelf stability and thus of Antarctica's contribution to sea-level rise. Previous studies of surface melt in Antarctica have usually focused on either a process-level understanding of melt through energy-balance investigations or used metrics such as the annual number of melt days to quantify spatiotemporal variability in satellite observations of surface melt. Here, we help bridge the gap between work at these two scales. Using daily passive microwave observations from the AMSR-E and AMSR-2 sensors and the machine learning approach of a self-organising map, we identify nine representative spatial distributions (“patterns”) of surface melt on the Shackleton Ice Shelf in East Antarctica from 2002/03–2020/21. Combined with output from the RACMO2.3p3 regional climate model and surface topography from the REMA digital elevation model, our results point to a significant role for surface air temperatures in controlling the interannual variability in summer melt and also reveal the influence of localised controls on melt. In particular, prolonged melt along the grounding line shows the importance of katabatic winds and surface albedo. Our approach highlights the necessity of understanding both local and large-scale controls on surface melt and demonstrates that self-organising maps can be used to investigate the variability in surface melt on Antarctic ice shelves.
Programme: 1110
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Gauthier Vérin, Florent Domine, Marcel Babin, Ghislain Picard, Laurent Arnaud. (2022). Metamorphism of snow on Arctic sea ice during the melt season: impact on spectral albedo and radiative fluxes through snow (Vol. 16).
Abstract: The energy budget of Arctic sea ice is strongly affected by the snow cover. Intensive sampling of snow properties was conducted near Qikiqtarjuaq in Baffin Bay on typical landfast sea ice during two melt seasons in 2015 and 2016. The sampling included stratigraphy, vertical profiles of snow specific surface area (SSA), density and irradiance, and spectral albedo (300–1100 nm). Both years featured four main phases: (I) dry snow cover, (II) surface melting, (III) ripe snowpack, and (IV) melt pond formation. Each phase was characterized by distinctive physical and optical properties. A high SSA value of 49.3 m2 kg−1 was measured during phase I on surface wind slabs together with a corresponding broadband albedo (300–3000 nm) of 0.87. Phase II was marked by alternating episodes of surface melting, which dramatically decreased the SSA below 3 m2 kg−1, and episodes of snowfall re-establishing pre-melt conditions. Albedo was highly time-variable, with minimum broadband values around 0.70. In phase III, continued melting led to a fully ripe snowpack composed of clustered rounded grains. Albedo began to decrease in the visible as snow thickness decreased but remained steady at longer wavelengths. Moreover, significant spatial variability appeared for the first time following snow depth heterogeneity. Spectral albedo was simulated by radiative transfer using measured SSA and density vertical profiles and estimated impurity contents based on limited measurements. Simulations were most of the time within 1 % of measurements in the visible and within 2 % in the infrared. Simulations allowed the calculations of albedo and of the spectral flux at the snow–ice interface. These showed that photosynthetically active radiation fluxes at the bottom of the snowpack durably exceeded 5 W m−2 (∼9.2 µmol m−2 s−1) only when the snowpack thickness started to decrease at the end of phase II.
Programme: 1042
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Georg Lackner, Florent Domine, Daniel F. Nadeau, Matthieu Lafaysse, Marie Dumont. (2022). Snow properties at the forest–tundra ecotone: predominance of water vapor fluxes even in deep, moderately cold snowpacks (Vol. 16).
Abstract: The forest–tundra ecotone is a large circumpolar transition zone between the Arctic tundra and the boreal forest, where snow properties are spatially variable due to changing vegetation. The extent of this biome through all circumpolar regions influences the climate. In the forest–tundra ecotone near Umiujaq in northeastern Canada (56∘33′31′′ N, 76∘28′56′′ W), we contrast the snow properties between two sites, TUNDRA (located in a low-shrub tundra) and FOREST (located in a boreal forest), situated less than 1 km apart. Furthermore, we evaluate the capability of the snow model Crocus, initially developed for alpine snow, to simulate the snow in this subarctic setting. Snow height and density differed considerably between the two sites. At FOREST, snow was about twice as deep as at TUNDRA. The density of snow at FOREST decreased slightly from the ground to the snow surface in a pattern that is somewhat similar to alpine snow. The opposite was observed at TUNDRA, where the pattern of snow density was typical of the Arctic. We demonstrate that upward water vapor transport is the dominant mechanism that shapes the density profile at TUNDRA, while a contribution of compaction due to overburden becomes visible at FOREST. Crocus was not able to reproduce the density profiles at either site using its standard configuration. We therefore implemented some modifications for the density of fresh snow, the effect of vegetation on compaction, and the lateral transport of snow by wind. These adjustments partly compensate for the lack of water vapor transport in the model but may not be applicable at other sites. Furthermore, the challenges using Crocus suggest that the general lack of water vapor transport in the snow routines used in climate models leads to an inadequate representation of the density profiles of even deep and moderately cold snowpacks, with possible major impacts on meteorological forecasts and climate projections.
Programme: 1042
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