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Interprétation des séries temporelles altimétriques sur la calotte polaire Antarctique

S. Partouty 1
1 CRYO - Cryosphère satelittaire
LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales
Abstract : This work aims at improving our understanding of the altimetric time series acquired over the Antarctic Ice Sheet. Dual frequency data (S Band - 3.2GHz and Ku Band - 13.6GHz) from the altimeter onboard the ENVISAT satellite are used, during a five year time period from january 2003 until december 2007. These data cover around 80% of the surface of the Antarctic continent, up to 82°S. Having data in two different frequencies is valuable when it comes to better estimate the altimeter sensitivity regarding snow surface property changes. Over the Antarctic ice sheet, snow surface changes with respect to space and time, being affected by meteorological conditions close to the surface, and especially winds. The altimetric wave penetrates more or less deeply beneath the surface, depending on snow surface and subsurface properties. As a result, when the wave comes back to the satellite, the recorded signal, named waveform, is more or less distorted. The accuracy of the ice sheet topographic changes computed thanks to satellite altimetric techniques depends on our knowledge of the processes inducing this distortion. The purpose of the present work is to better understand the effect of changing wind conditions on altimetric data. Winds in Antarctica are indeed famous for their strength and their impact on the snow surface state. First, spatial and temporal variability of the altimetric data on the one hand, and of wind speed reanalysis fields (from ERA-Interim, NCEP/NCAR and NCEP/DOE projects) on the other hand are studied. We estimate spatial and temporal typical length scales for all datasets. As a result, we are able to smooth the data, so that all datasets have the same spatial and temporal caracterictic length scales. Furthermore, we note that our time series are well described by an annual signal. This annual cycle shows that whereas wind speed would always be maximum in austral winter, altimetric seasonal cycles have very different behaviors depending on the location. Basically, two main large areas arise that cover almost all the Antarctic continent with really different properties when it comes to temporal variations of the altimetric signal. We then use the altimetric waveform caracteristics, and explode how this waveform changes with respect to time to reconstruct wind speed variations. The reconstructed winds and the winds from reanalysis are highly consistent, which tends to attest that altimertic signals are indeed affected by changing snow surface conditions due to wind speed variations. We lastly use a regional model (MAR for Modèle Atmosphérique Régional / LGGE) which has a higher spatial resolution than global climate models and takes into account feedback mecanisms between snow surface and winds. This last part allows us to confirm previous results, and highlights the specificities of the MAR model. Methods employed in this work are empirical, and results are mainly qualitative. However, we managed to detect the occurence of two main areas, which obviously are a result of different feedback mecanisms between atmospherical processes and snow surface state, leading to different roughness formation that affect the altimetric data.
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S. Partouty. Interprétation des séries temporelles altimétriques sur la calotte polaire Antarctique. Océan, Atmosphère. Université Paul Sabatier - Toulouse III, 2009. Français. ⟨tel-01018319⟩

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