Apport des mesures du radar à synthèse d'ouverture de Sentinel-1 pour l'étude des propriétés du manteau neigeux

Le suivi de l’évolution du manteau neigeux est directement lié à des enjeux socio-économiques majeurs en zone de montagne. Parmi ces enjeux figure la prévision du risque d’avalanche qui s’appuie principalement sur des observations et sur la connaissance de l’état du manteau neigeux et de son évolution dans le temps. Dans cette thèse, co-financée par le CNES et par Météo- France, nous avons évalué l’apport d’observations de télédétection spatiale active micro-ondes issues du radar à synthèse d’ouverture (SAR) de Sentinel-1, pour suivre l’évolution de certaines propriétés du manteau neigeux. Dans un premier temps, nous avons évalué la chaîne de modélisation SAFRAN-ISBA/Crocus-MEMLS par rapport aux données Sentinel-1 pré-traitées sur 3 saisons hivernales de 2014 à 2017, sur une zone de 2310 km2 à 20 m de résolution dans les Alpes du Nord françaises. Nous avons montré que les données SAR étaient pertinentes pour suivre l’évolution du manteau neigeux et, avons démontré la capacité de la chaîne de modélisation à reproduire les variations du signal observé dans le temps malgré de forts biais négatifs en cas de neige humide. Nous nous sommes intéressés à la valeur ajoutée des observations SAR de Sentinel-1 pour cartographier la neige humide, c’est-à-dire, la neige avec un taux élevé d’eau liquide. Des comparaisons ont été effectuées entre les produits neige humide obtenus par Sentinel-1 et les produits neige de Sentinel-2 distribués par Theia. Cette étude a été menée sur la saison hivernale 2017-2018, qui a connu un enneigement exceptionnel. Ces travaux ouvrent la voie à l’assimilation de données de télédétection SAR dans le modèle de neige Crocus ainsi qu’à une plus grande exploitation de ces données dans le cadre du suivi de l’enneigement pour de multiples applications

Categorisation of the length of bowhead whales from British Arctic whaling records

British whalers were the first and last from Europe to hunt bowhead whales (Balaena mysticetus) commercially from the Arctic whaling grounds of the Greenland Sea (East Greenland-Svalbard-Barents stock) and Davis Strait (East Canada-West Greenland stock). Thus, British Arctic whaling records are unique, as they include both the beginning and the final story of the near extirpation of the species from these waters. By consolidating, cross-checking, and updating the work of numerous colleagues over the years, a database of over 11,000 individual records of British whaling voyages to these grounds between 1725 and 1913 has been established. Using conversion algorithms, it has been possible to derive statistically robust information on the length of the bowheads caught from the amount of oil they yielded. Translating oil yield to whale length is an important step as oil yield is one of the most common parameters documented within historical whaling records. Analysis suggests the length of whales caught at these two whaling grounds, Greenland Sea and Davis Strait, were different. A higher proportion within the East Greenland-Svalbard-Barents stock, taken from the Greenland Sea grounds, measured less than 12.5m (classed as juveniles), whilst the East Canada-West Greenland stock, taken from Davis Strait grounds, were skewed towards larger whales, 13 to 14 m long (classed as sexually mature). Furthermore, there was clear evidence that a shift in the distribution of whale length occurred when the whalers extended their hunting grounds to encompass additional regions within the Greenland Sea and Davis Strait in 1814 and 1817 respectively. Prior to expansion, we find that that the vast majority (85%) of the East Canada-West Greenland stock were of the length that are classified as sexually mature (>13.0 m), whereas only 39% of East Greenland-Svalbard-Barents stock taken were of this size. After the enlargement of the whaling grounds, the length distribution shifted with a reduction to 50% of the East Canada-West Greenland stock and an increase to 44% of the East Greenland-Svalbard-Barents stock being categorised as sexually mature. These results show the important information that may be derived from historical whaling records. Since the commercial hunt of the bowheads ceased in the European Arctic there have been substantial changes in both the oceanographic and sea ice regime in the region, thus understanding the past through whaling records can help to understand the implications of future climate-induced changes in bowhead whale populations and their habitat

Sea-ice decline could keep zooplankton deeper for longer

As Arctic sea ice deteriorates, more light enters the ocean, causing largely unknown effects on the ecosystem. Using an autonomous biophysical observatory, we recorded zooplankton vertical distribution under Arctic sea ice from dusk to dawn of the polar night. Here we show that zooplankton ascend into the under-ice habitat during autumn twilight, following an isolume of 2.4 × 10−4 W m−2. We applied this trigger isolume to CMIP6 model outputs accounting for incoming radiation after sunset and before sunrise of the polar night. The models project that, in about three decades, the total time spent by zooplankton in the under-ice habitat could be reduced by up to one month, depending on geographic region. This will impact zooplankton winter survival, the Arctic foodweb, and carbon and nutrient fluxes. These findings highlight the importance of biological processes during the twilight periods for predicting change in high-latitude ecosystems

Under-ice light field analysis in the western Arctic Ocean during late summer

The Arctic is no longer a region dominated by thick multi-year ice (MYI), but by thinner, more dynamic, first-year-ice (FYI). This shift towards a seasonal ice cover has consequences for the under-ice light field, as sea-ice and its snow cover are a major factor influencing radiative transfer and thus, biological activity within- and under the ice. This work describes in situ measurements of light transmission through different types of sea-ice (MYI and FYI) performed during two expeditions to the Chukchi sea in August 2018 and 2019, as well as a simple characterisation of the biological state of the ice microbial system. Our analysis shows that, in late summer, two different states of FYI exist in this region: 1) FYI in an enhanced state of decay, and 2) robust FYI, more likely to survive the melt season. The two FYI types have different average ice thicknesses: 0.74 ± 0.07 m (N = 9) and 0.93 ± 0.11 m (N = 9), different average values of transmittance: 0.15 ± 0.04 compared to 0.09 ± 0.02, and different ice extinction coefficients: 1.49 ± 0.28 and 1.12 ± 0.19 m−1. The measurements performed over MYI present different characteristics with a higher average ice thickness of 1.56 ± 0.12 m, lower transmittance (0.05 ± 0.01) with ice extinction coefficients of 1.24 ± 0.26 m−1 (N = 12). All ice types show consistently low salinity, chlorophyll a concentrations and nutrients, which may be linked to the timing of the measurements and the flushing of melt-water through the ice. With continued Arctic warming, the summer ice will continue to retreat, and the decayed variant of FYI, with a higher scattering of light, but a reduced thickness, leading to an overall higher light transmittance, may become a more relevant ice type. Our results suggest that in this scenario, more light would reach the ice interior and the upper-ocean

Contribution of the synthetic aperture radar measurements of Sentinel-1 to study the snowpack properties

Le suivi de l’évolution du manteau neigeux est directement lié à des enjeux socio-économiques majeurs en zone de montagne. Parmi ces enjeux figure la prévision du risque d’avalanche qui s’appuie principalement sur des observations et sur la connaissance de l’état du manteau neigeux et de son évolution dans le temps. Dans cette thèse, co-financée par le CNES et par Météo- France, nous avons évalué l’apport d’observations de télédétection spatiale active micro-ondes issues du radar à synthèse d’ouverture (SAR) de Sentinel-1, pour suivre l’évolution de certaines propriétés du manteau neigeux. Dans un premier temps, nous avons évalué la chaîne de modélisation SAFRAN-ISBA/Crocus-MEMLS par rapport aux données Sentinel-1 pré-traitées sur 3 saisons hivernales de 2014 à 2017, sur une zone de 2310 km2 à 20 m de résolution dans les Alpes du Nord françaises. Nous avons montré que les données SAR étaient pertinentes pour suivre l’évolution du manteau neigeux et, avons démontré la capacité de la chaîne de modélisation à reproduire les variations du signal observé dans le temps malgré de forts biais négatifs en cas de neige humide. Nous nous sommes intéressés à la valeur ajoutée des observations SAR de Sentinel-1 pour cartographier la neige humide, c’est-à-dire, la neige avec un taux élevé d’eau liquide. Des comparaisons ont été effectuées entre les produits neige humide obtenus par Sentinel-1 et les produits neige de Sentinel-2 distribués par Theia. Cette étude a été menée sur la saison hivernale 2017-2018, qui a connu un enneigement exceptionnel. Ces travaux ouvrent la voie à l’assimilation de données de télédétection SAR dans le modèle de neige Crocus ainsi qu’à une plus grande exploitation de ces données dans le cadre du suivi de l’enneigement pour de multiples applications.Monitoring snowpack properties in moutainous areas is directly related to major socio-economic issues. Among these issues, avalanche prediction works through a range of tools based on meteorological and snow observations and modeling. In this thesis, co-funded by the CNES and Météo-France, we evaluated the contribution of Sentinel-1 synthetic aperture radar (SAR) remote sensing observations to study the snowpack properties and the quality of the simulations for assimilation in a snowpack model. As a first step, we evaluated the SAFRAN-ISBA/Crocus- MEMLS modeling chain against pre-processed Sentinel-1 data for 3 winter seasons from 2014 to 2017 over an area of 2310 km2 in the Northern French Alps. We have shown that SAR data are relevant for monitoring snowpack evolution and demonstrated the ability of the modeling chain to reproduce observed signal variations despite strong negative bias in wet snow conditions. We focused on wet snow products derived from Sentinel-1 SAR observations in synergy with snow absence/presence products derived from visible Sentinel-2 observations. This study was conducted on the winter season 2017-2018, which was remarkable for its snow and avalanche conditions. Such combined products make it possible to follow the spatio-temporal variability of mountain wet snow and dry snow at high elevation. This work opens the way for the assimilation of SAR remote sensing data into the Crocus snowpack model as well as greater exploitation of this data in the context of avalanche snow monitoring and prediction for a variety of purposes

Evaluation of Sub-Kilometric Numerical Simulations of C-Band Radar Backscatter over the French Alps against Sentinel-1 Observations

International audienceThis study compares numerical simulations and observations of C-band radar backscatter in a wide region (2300 km 2 ) in the Northern French Alps. Numerical simulations were performed using a model chain composed of the SAFRAN meteorological reanalysis, the Crocus snowpack model and the radiative transfer model Microwave Emission Model for Layered Snowpacks (MEMLS3&a), operating at a spatial resolution of 250-m. The simulations, without any bias correction, were evaluated against 141 Sentinel-1 synthetic aperture radar observation scenes with a resolution of 20 m over three snow seasons from October 2014 to June 2017. Results show that there is good agreement between observations and simulations under snow-free or dry snow conditions, consistent with the fact that dry snow is almost transparent at C-band. Under wet snow conditions, although the changes in time and space are well correlated, there is a significant deviation, up to 5 dB, between observations and simulations. The reasons for these discrepancies were explored, including a sensitivity analysis on the impact of the liquid water percolation scheme in Crocus. This study demonstrates the feasibility of performing end-to-end simulations of radar backscatter over extended geographical region. This makes it possible to envision data assimilation of radar data into snowpack models in the future, pending that deviations are mitigated, either through bias corrections or improved physical modeling of both snow properties and corresponding radar backscatter

Categorisation of the length of bowhead whales from British Arctic whaling records

Peer reviewed: TrueBritish whalers were the first and last from Europe to hunt bowhead whales (Balaena mysticetus) commercially from the Arctic whaling grounds of the Greenland Sea (East Greenland-Svalbard-Barents stock) and Davis Strait (East Canada-West Greenland stock). Thus, British Arctic whaling records are unique, as they include both the beginning and the final story of the near extirpation of the species from these waters. By consolidating, cross-checking, and updating the work of numerous colleagues over the years, a database of over 11,000 individual records of British whaling voyages to these grounds between 1725 and 1913 has been established. Using conversion algorithms, it has been possible to derive statistically robust information on the length of the bowheads caught from the amount of oil they yielded. Translating oil yield to whale length is an important step as oil yield is one of the most common parameters documented within historical whaling records. Analysis suggests the length of whales caught at these two whaling grounds, Greenland Sea and Davis Strait, were different. A higher proportion within the East Greenland-Svalbard-Barents stock, taken from the Greenland Sea grounds, measured less than 12.5m (classed as juveniles), whilst the East Canada-West Greenland stock, taken from Davis Strait grounds, were skewed towards larger whales, 13 to 14 m long (classed as sexually mature). Furthermore, there was clear evidence that a shift in the distribution of whale length occurred when the whalers extended their hunting grounds to encompass additional regions within the Greenland Sea and Davis Strait in 1814 and 1817 respectively. Prior to expansion, we find that that the vast majority (85%) of the East Canada-West Greenland stock were of the length that are classified as sexually mature (&amp;gt;13.0 m), whereas only 39% of East Greenland-Svalbard-Barents stock taken were of this size. After the enlargement of the whaling grounds, the length distribution shifted with a reduction to 50% of the East Canada-West Greenland stock and an increase to 44% of the East Greenland-Svalbard-Barents stock being categorised as sexually mature. These results show the important information that may be derived from historical whaling records. Since the commercial hunt of the bowheads ceased in the European Arctic there have been substantial changes in both the oceanographic and sea ice regime in the region, thus understanding the past through whaling records can help to understand the implications of future climate-induced changes in bowhead whale populations and their habitat.</jats:p

Monitoring Wet Snow Over an Alpine Region Using Sentinel-1 Observations

International audienceThe main objective of this study was to monitor wet snow conditions from Sentinel-1 over a season, to examine its variation over time by cross-checking wet snow with independent snow and weather estimates, and to study its distribution taking into account terrain characteristics such as elevation, orientation, and slope. One of our motivations was to derive useful representations of daily or seasonal snow changes that would help to easily identify wet snow elevations and determine melt-out days in an area of interest. In this work, a well-known approach in the literature is used to estimate the extent of wet snow cover continuously over a season and an analysis of the influence of complex mountain topography on snow distribution is proposed taking into account altitude, slope, and aspect of the terrain. The Sentinel-1 wet snow extent product was compared with Sentinel-2 snow products for cloud free scenes. We show that while there are good agreements between the two satellite products, differences exist, especially in areas of forests and glaciers where snow is underestimated. This underestimation must be considered alongside the areas of geometric distortion that were excluded from our study. We analysed retrievals at the scale of our study area by examining wet snow Altitude–Orientation diagrams for different classes of slopes and also wet snow Altitude–Time diagrams for different classes of orientations. We have shown that this type of representation is very useful to get an overview of the snow distribution as it allows to identify very easily wet snow lines for different orientations. For an orientation of interest, the Altitude–Time diagrams can be used to track the evolution of snow to locate altitudes and dates of snow loss. We also show that ascending/descending Sentinel-1 image time series are complementary to monitor wet snow over the French alpine areas to highlight wet snow altitude ranges and identify melt-out days. Links have also been made between Sentinel-1 responses (wet snow) and snow/meteorological events carefully listed over the entire 2017–2018 season