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Thieblemont R.,French National Center for Scientific Research | Huret N.,French National Center for Scientific Research | Orsolini Y.J.,Norwegian Institute For Air Research | Hauchecorne A.,Laboratoire Atmospheres | Drouin M.-A.,French National Center for Scientific Research
Journal of Geophysical Research: Atmospheres | Year: 2011

During winter, the polar vortex forms a dynamical barrier in the arctic stratosphere which prevents large scale exchanges between the high latitude and tropical regions. Nevertheless, the occurrence of thin tropical air mass intrusions at the edge of the polar vortex have in fact been detected and modeled. These structures could play an important role in improving our knowledge of the balance between chemical and dynamical processes associated with the ozone budget. After the final stratospheric warming in springtime, the breakdown of the polar vortex occurs and the summer circulation starts to develop. Air mass intrusions from the tropics can be trapped at polar latitudes and persist until August in the anticyclone, advected by summer easterlies. These structures, named "frozen-in anticyclones" (FrIACs), have already been observed in 2003 and 2005 by MIPAS-ENVISAT and MLS-AURA tracer measurements. We present here a new case of FrIAC in 2007 highlighted using MLS-AURA measurements. In order to better understand the dynamical conditions required for such events and the associated processes, we performed a climatology of tropical air mass intrusions using a potential vorticity contour advection model. This climatology reveals a preferred path for exchanges between the polar and tropical stratospheres. Using ERA-Iterim wind and temperature reanalysis from ECMWF, we have established links between FrIAC occurrences and Rossby wave activity. There is evidence that FrIACs can exist if no major sudden stratospheric warming occurs during the polar vortex phase and their development seems favorable if the tropical quasi-biennial oscillation is in the easterly phase. Copyright 2011 by the American Geophysical Union. Source


Le Gall A.,Jet Propulsion Laboratory | Le Gall A.,Laboratoire Atmospheres | Hayes A.G.,California Institute of Technology | Ewing R.,California Institute of Technology | And 4 more authors.
Icarus | Year: 2012

Dune fields dominate ~13% of Titan's surface and represent an important sink of carbon in the methane cycle. Herein, we discuss correlations in dune morphometry with altitude and latitude. These correlations, which have important implications in terms of geological processes and climate on Titan, are investigated through the microwave electromagnetic signatures of dune fields using Cassini radar and radiometry observations. The backscatter and emissivity from Titan's dune terrains are primarily controlled by the amount of interdune area within the radar footprint and are also expected to vary with the degree of the interdunal sand cover. Using SAR-derived topography, we find that Titan's main dune fields (Shangri-La, Fensal, Belet and Aztlan) tend to occupy the lowest elevation areas in Equatorial regions occurring at mean elevations between ~-400 and ~0. m (relative to the geoid). In elevated dune terrains, we show a definite trend towards a smaller dune to interdune ratio and possibly a thinner sand cover in the interdune areas. A similar correlation is observed with latitude, suggesting that the quantity of windblown sand in the dune fields tends to decrease as one moves farther north. The altitudinal trend among Titan's sand seas is consistent with the idea that sediment source zones most probably occur in lowlands, which would reduce the sand supply toward elevated regions. The latitudinal preference could result from a gradual increase in dampness with latitude due to the asymmetric seasonal forcing associated with Titan's current orbital configuration unless it is indicative of a latitudinal preference in the sand source distribution or wind transport capacity. © 2011 Elsevier Inc. Source


Roca R.,University Pierre and Marie Curie | Guzman R.,University Pierre and Marie Curie | Lemond J.,University Pierre and Marie Curie | Meijer J.,University Pierre and Marie Curie | And 2 more authors.
Surveys in Geophysics | Year: 2012

Free tropospheric humidity (FTH) is a key parameter of the radiation budget of the Earth. In particular, its distribution over the intertropical belt has been identified as an important contributor to the water vapour feedback. Idealized radiative transfer computations are performed to underscore the need to consider the whole probability distribution function (PDF) rather than the arithmetical mean of the FTH. The analysis confirmed the overwhelming role of the dry end of the PDF in the radiative perturbation of the top of atmosphere longwave budget. The physical and dynamical processes responsible for the maintenance of this dry part of the FTH distribution are reviewed, and the lateral mixing between the tropics and the extra-tropics is revealed as a major element of the dry air dynamics. The evolution of this lateral mixing in the framework of the global warming is discussed, and perspectives of work are listed as a mean of a conclusion. © 2012 Springer Science+Business Media B.V. Source


Bouilloud L.,Laboratoire dEtude des Transferts en Hydrologie et Environnement LTHE | Delrieu G.,Laboratoire dEtude des Transferts en Hydrologie et Environnement LTHE | Boudevillain B.,Laboratoire dEtude des Transferts en Hydrologie et Environnement LTHE | Kirstetter P.-E.,Laboratoire Atmospheres
Journal of Hydrology | Year: 2010

A method to estimate rainfall from radar data for post-event analysis of flash-flood events has been developed within the EC-funded HYDRATE project. It follows a pragmatic approach including careful analysis of the observation conditions for the radar system(s) available for the considered case. Clutter and beam blockage are characterised by dry-weather observations and simulations based on a digital terrain model of the region of interest. The vertical profile of reflectivity (VPR) is either inferred from radar data if volume scanning data are available or simply defined using basic meteorological parameters (idealised VPR). Such information is then used to produce correction factor maps for each elevation angle to correct for range-dependent errors. In a second step, an effective Z- R relationship is optimised to remove the bias over the hit region. Due to limited data availability, the optimisation is carried out with reference to raingauge rain amounts measured at the event time scale. Sensitivity tests performed with two well-documented rain events show that a number of Z= aRb relationships, organised along hyperbolic curves in the (a and b) parameter space, lead to optimum assessment results in terms of the Nash coefficient between the radar and raingauge estimates. A refined analysis of these equifinality patterns shows that the "total additive conditional bias" can be used to discriminate between the Nash coefficient equifinal solutions. We observe that the optimisation results are sensitive to the VPR description and also that the Z- R optimisation procedure can largely compensate for range-dependent errors, although this shifts the optimal coefficients in the parameter space. The time-scale dependency of the equifinality patterns is significant, however near-optimal Z- R relationships can be obtained at all time scales from the event time step optimisation. © 2010 Elsevier B.V. Source


Peng Z.,University Paris - Sud | Dobrijevic M.,University of Bordeaux 1 | Hebrard E.,University of Bordeaux 1 | Carrasco N.,Laboratoire Atmospheres | Pernot P.,University Paris - Sud
Faraday Discussions | Year: 2010

Titan's atmospheric chemistry modeling is presently limited by the lack of knowledge about many reaction rate coefficients at low temperature (50-200 K). Considering the difficulty of measuring such data, the only way to improve this situation is to identify key reactions as the ones for which better estimations of reaction rates is guaranteed to have a strong influence on the precision of model predictions. This is a slow iterative process, the limit of which has never been clearly defined in terms of model precision. The fact is that this limit is not a fully deterministic simulation, since one should not expect all reaction rate coefficients ever to become available with null uncertainty. The present study considers a quite optimistic scenario, in which all reaction rate coefficients in the chemical model are assumed to be known with a 10% relative uncertainty. The implications for chemical growth modeling are discussed. © 2010 The Royal Society of Chemistry. Source

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