Sainte-Foy-lès-Lyon, France
Sainte-Foy-lès-Lyon, France

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Le Gall A.,University Pierre and Marie Curie | Malaska M.J.,Jet Propulsion Laboratory | Lorenz R.D.,Johns Hopkins University | Janssen M.A.,Jet Propulsion Laboratory | And 7 more authors.
Journal of Geophysical Research E: Planets | Year: 2016

For the last decade, the passive radiometer incorporated in the Cassini RADAR has recorded the 2.2 cm wavelength thermal emission from Titan's seas. In this paper, we analyze the radiometry observations collected from February 2007 to January 2015 over one of these seas, Ligeia Mare, with the goal of providing constraints on its composition, bathymetry, and dynamics. In light of the depth profile obtained by Mastrogiuseppe et al. (2014) and of a two-layer model, we find that the dielectric constant of the sea liquid is <1.8, and its loss tangent is <3.6-2.1+4.3×10-5. Both results point to a composition dominated by liquid methane rather than ethane. A high methane concentration suggests that Ligeia Mare is primarily fed by methane-rich precipitation and/or ethane has been removed from it (e.g., by crustal interaction). Our result on the dielectric constant of the seafloor is less constraining <2.9-0.9+0.9, but we favor a scenario where the floor of Ligeia Mare is covered by a sludge of compacted and possibly nitrile-rich organic material formed by the deposition of photochemical haze or by rain washing of the nearby shores. We use these results to produce a low-resolution bathymetry map of the sea. We also estimate the temperature variation of the bulk sea between February 2007 and July 2013 to be <2 K, which provides a constraint on its net evaporative cooling currently being explored in ocean circulation models. Lastly, we suggest a lag in the summer warming of the northern polar terrains. ©2016. American Geophysical Union. All Rights Reserved.

Dedieu J.P.,French National Center for Scientific Research | Beninca De Farias G.,CEA Grenoble | Castaings T.,CNRS GIPSA Laboratory | Allain-Bailhache S.,CNRS Institute of Electronics and Telecommunications, Rennes | And 3 more authors.
Canadian Journal of Remote Sensing | Year: 2012

The aim of the SOAR #1341 project is to perform temporal analyses of changes in RADARSAT-2 full-polarimetry parameters on snow cover in a mountainous area. The objective of the present study was to determine whether there is a correlation between changes in radar statistics and changes in physical snow parameters during winter and spring. This paper focuses on the preprocessing of the images and presents the methodological steps and first results obtained in full polarimetry mode. Six RADARSAT-2 quad-pol images were acquired between January 2009 and January 2010, five in different snow conditions and one snow-free image in summer used as a reference. The fine acquisition mode was selected with a medium incidence angle (398). Acombination of LANDSAT-7 and SPOToptical images and field measurements was used for the validation step. First, RADARSAT-2 images had to be pre-processed due to the influence of high mountain topography on the polarimetric signal: the planned incidence angle was computed using a fine digital elevation model (DEM). Next, the DEM and optical dataset were reprojected onto the slant range mode of RADARSAT-2, the configuration required to preserve the phase signal and polarimetric statistics. Polarimetric analysis was performed using the PolSARpro software from ESA/IETR. The coherency matrices were calculated for each RADARSAT-2 image. Polarimetric descriptors based on the eigenvector-eigenvalue decomposition theorem of this coherency matrix were obtained. The behavior of the Single Eigenvalue Relative Difference, a polarimetric parameter that depends on scattering mechanisms, was analyzed. The polarimetric analysis showed an increase in the multiple scattering mechanism with a dry snow cover compared with the snow-free image. With a wet snow cover, there was an increase in contribution of the single scattering compared with the snow-free image. Our results showed that it was possible to identify temporal changes in dry, wet, or no snow characteristics throughout the winter season by analyzing primary polarimetric decomposition parameters and by comparing them with measurements made at 10 field sites. © 2012 CASI.

Rousselot M.,Center Detudes Of La Neige | Durand Y.,Center Detudes Of La Neige | Giraud G.,Center Detudes Of La Neige | Merindol L.,Center Detudes Of La Neige | Daniel L.,Center Detudes Of La Neige
Journal of Glaciology | Year: 2010

Snow and weather conditions typical of exceptional cycles of fresh-snow avalanches in thenorthern Alps are investigated using the numerical avalanche-hazard forecasting procedure of Météo-France. Sensitivity tests are performed on the events of February 1999 in the Chamonix France region,and resulting snowpack instability modeled at the massif scale is compared using adapted new indicesand maps. Our results complete conclusions of earlier observation-based studies by providing newinsights into the snow and weather conditions of February 1999. The large avalanches mainly resulted from large and very unstable fresh-snow accumulations. Moreover, the snowpack instability was increased locally by wind transport of light and fresh snow in February. The mechanical weaknesses resulting from the weather conditions prior to February were a key factor in explaining the unusual volumes of these avalanches. This study suggests that the operational numerical SAFRAN/Crocus/ MÉPRA (SCM) chain provides reliable forecasts of extreme new-snow avalanche situations at the massifscale, but that local-scale simulations are still needed to improve the efficiency of risk mitigation andcivil protection policies.

Eckert N.,IRSTEA | Coleou C.,Center Detudes Of La Neige | Castebrunet H.,IRSTEA | Castebrunet H.,Center Detudes Of La Neige | And 3 more authors.
Cold Regions Science and Technology | Year: 2010

In December 2008, an intense avalanche cycle occurred in the eastern part of the southern French Alps. Southerly atmospheric fluxes that progressively evolved into an easterly return caused important snowfalls with return periods up to 10. years. Cold temperatures and drifting snow had important aggravating effects. The return period for the number of avalanches was above 50. years in two massifs and some of the avalanche had very long runouts that exceeded historical limits recorded in the French avalanche atlas. Using this case study, this paper illustrates and discusses how avalanche reports, snow and weather data and results from numerical modelling of the snow cover can be combined to analyse abnormal temporal clusters of snow avalanches. For instance, it is shown how statistical techniques developed in other fields can be used to test the significance of different explanatory factors, extract spatio-temporal patterns, compare them with previous cycles and quantify the magnitude/frequency relationship at different scales. © 2010 Elsevier B.V.

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