Guyancourt, France
Guyancourt, France

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Kirstetter P.-E.,Laboratoire Atmospheres | Andrieu H.,Laboratoire Central des Ponts et Chaussees | Delrieu G.,Laboratoire detude des Transferts en Hydrologie et Environnement | Boudevillain B.,Laboratoire detude des Transferts en Hydrologie et Environnement
Journal of Applied Meteorology and Climatology | Year: 2010

Nonuniform beam filling associated with the vertical variation of atmospheric reflectivity is an important source of error in the estimation of rainfall rates by radar. It is, however, possible to correct for this error if the vertical profile of reflectivity (VPR) is known. This paper presents a method for identifying VPRs from volumetric radar data. The method aims at improving an existing algorithm based on the analysis of ratios of radar measurements at multiple elevation angles. By adding a rainfall classification procedure defining more homogeneous precipitation patterns, the issue of VPR homogeneity is specifically addressed. The method is assessed using the dataset from a volume-scanning strategy for radar quantitative precipitation estimation designed in 2002 for the Bollène radar (France). The identified VPR is more representative of the rain field than are other estimated VPRs. It has also a positive impact on radar data processing for precipitation estimation: while scatter remains unchanged, an overall bias reduction at all time steps is noticed (up to 6% for all events) whereas performance varies with type of events considered (mesoscale convective systems, cold fronts, or shallow convection) according to the radar-observation conditions. This is attributed to the better processing of spatial variations of the vertical profile of reflectivity for the stratiform regions. However, adaptation of the VPR identification in the difficult radar measurement context in mountainous areas and to the rainfall classification procedure proved challenging because of data fluctuations. © 2010 American Meteorological Society.

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.

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.

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.

Lemaitre C.,Laboratoire Atmospheres | Flamant C.,Laboratoire Atmospheres | Cuesta J.,French National Center for Scientific Research | Raut J.-C.,Laboratoire Atmospheres | And 3 more authors.
Atmospheric Chemistry and Physics | Year: 2010

The radiative heating rate due to mineral dust over West Africa is investigated using the radiative code STREAMER, as well as remote sensing and in situ observations gathered during the African Monsoon Multidisciplinary Analysis Special Observing Period (AMMA SOP). We focus on two days (13 and 14 June 2006) of an intense and long lasting episode of dust being lifted in remote sources in Chad and Sudan and transported across West Africa in the African easterly jet region, during which airborne operations were conducted at the regional scale, from the southern fringes of the Sahara to the Gulf of Guinea. Profiles of heating rates are computed from airborne LEANDRE 2 (Lidar Embarqué pour l'étude de l'Atmosphère: Nuages Dynamique, Rayonnement et cycle de l'Eau) and space-borne CALIOP (Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations) lidar observations using two mineral dust model constrained by airborne in situ data and ground-based sunphotometer obtained during the campaign. Complementary spaceborne observations (from the Moderate-resolution Imaging Spectroradiometer-MODIS) and in-situ observations such as dropsondes are also used to take into account the infrared contribution of the water vapour. We investigate the variability of the heating rate on the vertical within a dust plume, as well as the contribution of both shortwave and longwave radiation to the heating rate and the radiative heating rate profiles of dust during daytime and nighttime. The sensitivity of the so-derived heating rate is also analyzed for some key variables for which the associated uncertainties may be large. During daytime, the warming associated with the presence of dust was found to be between 1.5 K day1 and 4 K day 1, on average, depending on altitude and latitude. Strong warming (i.e. heating rates as high as 8 K day1) was also observed locally in some limited part of the dust plumes. The uncertainty on the heating rate retrievals in the optically thickest part of the dust plume was estimated to be between 0.5 and 1.4 K day1. During nighttime much smaller values of heating/cooling are retrieved (less than ±1 K day1). Furthermore, cooling is observed as the result of the longwave forcing in the dust layer, while warming is observed below the dust layer, in the monsoon layer. © Author(s) 2010.

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.

Brogniez H.,Laboratoire Atmospheres | Clain G.,Laboratoire Atmospheres | Roca R.,CNRS Geophysical Research and Oceanographic Laboratory
Journal of Applied Meteorology and Climatology | Year: 2015

This paper describes the upper-tropospheric humidity (UTH) product derived from brightness temperature measurements of the Sondeur Atmosphérique du Profil d'Humidité Intertropicale par Radiométrie (SAPHIR) radiometer on board the Megha-Tropiques satellite. Under nonscattering conditions, the observations from three channels of SAPHIR-located at ±0.2, ±1.1, and ±2.8 GHz, respectively, around the 183.31-GHz strong water vapor absorption band-are interpreted into three different UTHs following a well-established method and thus describing the humidity content of the upper to midtroposphere. The evaluation of the UTHs is performed using reference UTHs defined from relative humidity (RH) profiles from radiosoundings of two field campaigns: the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011/Dynamics of the Madden-Julian Oscillation/Atmospheric Radiation Measurement Program Madden-Julian Oscillation Investigation Experiment (CINDY/DYNAMO/AMIE) and a Megha-Tropiques dedicated campaign in Ouagadougou, Burkina Faso, during the summer of 2012. A budget of the various uncertainties associated with each component of the evaluation method (such as the radiometric sensitivity and the radiative transfer computations) was created to achieve a more robust comparison between the two UTH estimates. The comparison between the reference UTHs and the SAPHIR UTHs reveals small global biases of lower than 2% RH on average, with correlation coefficients between 0.86 and 0.89. Taking into account the individual uncertainties gives root-mean-square errors of regressions that range between 0.92% and 4.71%. These three UTHs provide a vertical distribution of the RH that is suitable for studying various temporal and spatial scales of the tropical variability. The signature of a mesoscale convective system on its environment is briefly presented to illustrate the capability of this new dataset. © 2015 American Meteorological Society.

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.

Evaristo R.,Laboratoire Atmospheres | Scialom G.,Laboratoire Atmospheres | Viltard N.,Laboratoire Atmospheres | Lemaitre Y.,Laboratoire Atmospheres
Quarterly Journal of the Royal Meteorological Society | Year: 2010

During the AMMA campaign, the Doppler and polarimetric radar Ronsard was deployed in Kopargo, Benin. In the present paper, three squall lines sampled during that period are described. It was the first time this type of system had been observed with a polarimetric radar in the West African region. The strongest case occurred on 28 July 2006, and the others on 12 September and 30 June. The polarimetric characteristics of these systems are described and a hydrometeor classification algorithm is applied in order to identify the microphysics of the systems. Finally, the 3D wind field is retrieved for the 28 July event. The convective part of the squall lines is generally characterized by maxima of reflectivity, differential reflectivity (ZDR), and specific differential phase (KDP). The results of the hydrometeor classification show that the convective region is composed of strong (>30 mm h-1) to moderate rain (5-30 mm h-1) below the melting level, and graupel above. This zone seems to be characterized by relatively strong updraughts (5-10 m s-1), responsible both for the formation of graupel by riming and for the growth of drops due to coalescence. To the rear the systems are characterized by light rain (<5 mm h-1) below and snow or ice crystals above. These regions present downward flow below the melting layer, which prevents drop growth by condensation, consistent with the light rain observed. The stratiform part of these systems can be very non-uniform, due to the presence of old cells dissipating, associated with moderate rain and thus stronger reflectivities. © 2010 Royal Meteorological Society.

Funatsu B.M.,Laboratoire Atmospheres | Funatsu B.M.,University of Rennes 2 – Upper Brittany | Dubreuil V.,University of Rennes 2 – Upper Brittany | Claud C.,CNRS Dynamic Meteorology Laboratory | And 2 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2012

We present a characterization of convective activity at sub-regional scale from two sets of satellite-based microwave observations: the Advanced Microwave Sounding Unit (AMSU) and the combined Tropical Rainfall Measuring Mission (TRMM) microwave imager and precipitation radar data, for the period 2001 to 2011. We focus on the state of Mato Grosso, Brazil, located at the southern edge of the so-called "Legal Amazon" which has undergone intense land cover transformation in the last 4 decades. The annual cycle of mean convective activity described by AMSU and TRMM are in good agreement, with a correlation close to 0.80. The mean amplitude of convective activity is maximal early in the rainy season, except for AMSU deep convective area, which presents a maximum in January. The diurnal cycle of convection was examined for the period 2003 to 2007, and it was found that convection is maximal near 1500 local time (LT) and minimal around 0700 LT. Unlike the amplitude, the phase shows little intraseasonal and interannual variability. A slight decrease in convective activity in the studied period was found, possibly indicating an extension of the dry season. Comparisons of convective activity between deforested and forested areas showed no significant differences in the phase of the diurnal cycle, but our analysis shows a tendency for increase (decrease) in convection in deforested (forested) areas for the period considered. A longer time series is however necessary in order to strengthen the robustness of our results. © 2012. American Geophysical Union. All Rights Reserved.

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