Construction de la station sismologique OGSM à Saint-Maurice-de-Rotherens (Savoie)

View of the building constructed to house the seismological station OGSM as part of the funding obtained from Europe for the INTERREG-RISE project. The station, operated by ISTerre, is part of the Permanent Broadband Network (RLBP) of Résif, a research infrastructure dedicated to the observation and understanding of the internal Earth structure and dynamics. It is based on observation networks of high technological level, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout the French territory. The data collected enable the study, with high spatio-temporal resolution, of ground deformation, surface and deep structures, local and global seismicity and natural hazards, particularly seismic, on French territory. Résif is integrated into the European (EPOS - European plate observatory system) and worldwide systems of instruments that allow us to image the interior of the Earth as a whole and to study numerous natural phenomena.Vue du bâtiment construit pour héberger la station sismologique OGSM dans le cadre du financement obtenu de l’Europe sur le projet INTERREG-RISE. La station, opérée par l’ISTerre, est intégrée au Réseau Large Bande Permanent (RLBP) de Résif, une infrastructure de recherche dédiée à l’observation et la compréhension de la structure et de la dynamique Terre interne. Il se base sur des réseaux d’observation de haut niveau technologique, composés d’instruments sismologiques, géodésiques et gravimétriques déployés de manière dense sur tout le territoire français. Les données recueillies permettent d’étudier avec une haute résolution spatio-temporelle la déformation du sol, les structures superficielles et profondes, la sismicité à l’échelle locale et globale et les aléas naturels, et plus particulièrement sismiques, sur le territoire français. Résif s’intègre aux dispositifs européens (EPOS - European plate observatory system) et mondiaux d’instruments permettant d’imager l’intérieur de la Terre dans sa globalité et d’étudier de nombreux phénomènes naturels

Quantification of seasonal and diumal dynamics of subglacial channels using seismic observations on an Alpine glacier

Water flowing below glaciers exerts a major control on glacier basal sliding. However, our knowledge of the physics of subglacial hydrology and its link with sliding is limited because of lacking observations. Here we use a 2-year-long dataset made of on-ice-measured seismic and in situ-measured glacier basal sliding speed on Glacier d'Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels' hydraulic radius and hydraulic pressure gradient. At seasonal timescales we find that hydraulic radius and hydraulic pressure gradient respectively exhibit a 2- and 6-fold increase from spring to summer, followed by comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels' behaviour at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behaviour that is consistent with channels behaving out of equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which prove that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speed, which supports that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speed) through an efficient hydraulic connection between the two systems. The present findings provide an essential basis for testing the physics represented in subglacial hydrology and glacier sliding models.ISSN:1994-0416ISSN:1994-042

Water vapor observation at 42 m height above ground at Dome C, East Antarctic plateau (2018-2020)

The data set provides 3 years of almost continuous observation of water vapor in the air at 42m height on the high antarctic plateau, 123° 21' E, 75° 06' S, 3233 m above sea level. Each data is an average over the previous ½ hour. The water vapor content is measured in a heated air flow to avoid that supersaturated air at ambient temperature deposits excess moisture (above 100% with respect to ice) before reaching the humidity sensor. In fact, many reports correspond to significant supersaturation (see references provided). HMP155 thermohygrometers are used, which for the hygrometer natively report relative humidity with respect to liquid water even below 0°C. This is the variable provided in the data set, along with temperature in the heated air flow and ambient temperature. There are several conversion formulae in the literature to convert to e.g. partial pressure and relative humidity with respect to ice. As there is no clear consensus on which should be preferred in the range of temperatures at Dome C, the user is left to carry our her/his own conversions

Water vapor observation in the lower atmospheric boundary layer at Dome C, East Antarctic plateau

The data set provides 3 years of almost continuous observation of water vapor in the air at 3 levels in the lowest 42 m above Dome C on the high antarctic plateau, 123° 21' E, 75° 06' S, 3233 m above sea level. Each data is an average over the previous ½ hour. The water vapor content is measured in a heated air flow to avoid that supersaturated air at ambient temperature deposits excess moisture (above 100% with respect to ice) before reaching the humidity sensor. In fact, many reports correspond to significant supersaturation (see references provided). HMP155 thermohygrometers are used, which for the hygrometer natively report relative humidity with respect to liquid water even below 0°C. This is the variable provided in the data set, along with temperature in the heated air flow and ambient temperature. There are several conversion formulae in the literature to convert to e.g. partial pressure and relative humidity with respect to ice. As there is no clear consensus on which should be preferred in the range of temperatures at Dome C, the user is left to carry our her/his own conversions

Water vapor observation at 3 m height above ground at Dome C, East Antarctic plateau (2018-2020)

The data set provides 3 years of almost continuous observation of water vapor in the air at 3m height on the high antarctic plateau, 123° 21' E, 75° 06' S, 3233 m above sea level. Each data is an average over the previous ½ hour. The water vapor content is measured in a heated air flow to avoid that supersaturated air at ambient temperature deposits excess moisture (above 100% with respect to ice) before reaching the humidity sensor. In fact, many reports correspond to significant supersaturation (see references provided). HMP155 thermohygrometers are used, which for the hygrometer natively report relative humidity with respect to liquid water even below 0°C. This is the variable provided in the data set, along with temperature in the heated air flow and ambient temperature. There are several conversion formulae in the literature to convert to e.g. partial pressure and relative humidity with respect to ice. As there is no clear consensus on which should be preferred in the range of temperatures at Dome C, the user is left to carry our her/his own conversions

Water vapor observation at 18 m height above ground at Dome C, East Antarctic plateau (2018-2020)

The data set provides 3 years of almost continuous observation of water vapor in the air at 18m height on the high antarctic plateau, 123° 21' E, 75° 06' S, 3233 m above sea level. Each data is an average over the previous ½ hour. The water vapor content is measured in a heated air flow to avoid that supersaturated air at ambient temperature deposits excess moisture (above 100% with respect to ice) before reaching the humidity sensor. In fact, many reports correspond to significant supersaturation (see references provided). HMP155 thermohygrometers are used, which for the hygrometer natively report relative humidity with respect to liquid water even below 0°C. This is the variable provided in the data set, along with temperature in the heated air flow and ambient temperature. There are several conversion formulae in the literature to convert to e.g. partial pressure and relative humidity with respect to ice. As there is no clear consensus on which should be preferred in the range of temperatures at Dome C, the user is left to carry our her/his own conversions

Water vapor in cold and clean atmosphere: a 3-year data set in the boundary layer of Dome C, East Antarctic Plateau

Abstract. The air at the surface of the high Antarctic Plateau is very cold, dry and clean. In such conditions the atmospheric moisture can significantly deviate from thermodynamic equilibrium conditions, and supersaturation with respect to ice can occur. Most conventional humidity sensors for meteorological applications cannot report supersaturation in this environment. A simple approach for measuring supersaturation using conventional instruments, one being operated in a heated airflow, is presented. Since 2018, this instrumental setup was deployed at 3 levels in the lower ~40 m above the surface at Dome C on the high Antarctic Plateau. The 3-year 2018–2020 record (Genthon et al. 2021) is presented and analyzed for features such as the frequency of supersaturation with respect to ice, diurnal and seasonal variability, and vertical distribution. As supercooled liquid water droplets are frequently observed in clouds at the temperatures met on the high Antarctic Plateau, the distribution of relative humidity with respect to liquid water at Dome C is also discussed. It is suggested that, while not strictly mimicking the conditions of the high troposphere, the surface atmosphere on the Antarctic Plateau is a convenient natural laboratory to test parametrizations of cold microphysics predominantly developed to handle the genesis of high tropospheric clouds. Data are distributed on the PANGAEA data repository at https://doi.pangaea.de/10.1594/PANGAEA.939425 (Genthon et al., 2021)

Water vapor in cold and clean atmosphere: a 3-year data set in the boundary layer of Dome C, East Antarctic Plateau

International audienceThe air at the surface of the high Antarctic Plateau is very cold, dry and clean. Under such conditions, the atmospheric moisture can significantly deviate from thermodynamic equilibrium, and supersaturation with respect to ice can occur. Most conventional humidity sensors for meteorological applications cannot report supersaturation in this environment. A simple approach for measuring supersaturation using conventional instruments, with one being operated in a heated airflow, is presented. Since 2018, this instrumental setup has been deployed at three levels in the lower ∼40 m above the surface at Dome C on the high Antarctic Plateau. A resulting 3-year (2018-2020) record (Genthon et al., 2021a) is presented and analyzed for features such as the frequency of supersaturation with respect to ice, diurnal and seasonal variability, and vertical distribution. As supercooled liquid water droplets are frequently observed in clouds at the temperatures experienced on the high Antarctic Plateau, the distribution of relative humidity with respect to liquid water at Dome C is also discussed. It is suggested that, while not strictly mimicking the conditions of the high troposphere, the surface atmosphere on the Antarctic Plateau is a convenient natural laboratory to test parametrizations of cold microphysics predominantly developed to handle the genesis of high tropospheric clouds. Data are available from the PANGAEA data repository at https://doi.org/10.1594/PANGAEA.939425 (Genthon et al., 2021a)

Atmospheric moisture supersaturation in the near-surface atmosphere at Dome C, Antarctic Plateau

International audienceSupersaturation often occurs at the top of the tro-posphere where cirrus clouds form, but is comparatively unusual near the surface where the air is generally warmer and laden with liquid and/or ice condensation nuclei. One exception is the surface of the high Antarctic Plateau. One year of atmospheric moisture measurement at the surface of Dome C on the East Antarctic Plateau is presented. The measurements are obtained using commercial hygrometry sensors modified to allow air sampling without affecting the moisture content, even in the case of supersaturation. Supersaturation is found to be very frequent. Common unadapted hygrometry sensors generally fail to report supersaturation, and most reports of atmospheric moisture on the Antarctic Plateau are thus likely biased low. The measurements are compared with results from two models implementing cold microphysics parame-terizations: the European Center for Medium-range Weather Forecasts through its operational analyses, and the Model At-mosphérique Régional. As in the observations, supersatura-tion is frequent in the models but the statistical distribution differs both between models and observations and between the two models, leaving much room for model improvement. This is unlikely to strongly affect estimations of surface sub-limation because supersaturation is more frequent as temperature is lower, and moisture quantities and thus water fluxes are small anyway. Ignoring supersaturation may be a more serious issue when considering water isotopes, a tracer of phase change and temperature, largely used to reconstruct past climates and environments from ice cores. Because observations are easier in the surface atmosphere, longer and more continuous in situ observation series of atmospheric su-persaturation can be obtained than higher in the atmosphere to test parameterizations of cold microphysics, such as those used in the formation of high-altitude cirrus clouds in meteorological and climate models