Guyancourt, France
Guyancourt, France

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Peralta J.,Institute Astrofisica Of Andalucia Csic | Peralta J.,University of Lisbon | Imamura T.,Japan Aerospace Exploration Agency | Read P.L.,University of Oxford | And 3 more authors.
Astrophysical Journal, Supplement Series | Year: 2014

This paper is the first of a two-part study devoted to developing tools for a systematic classification of the wide variety of atmospheric waves expected on slowly rotating planets with atmospheric superrotation. Starting with the primitive equations for a cyclostrophic regime, we have deduced the analytical solution for the possible waves, simultaneously including the effect of the metric terms for the centrifugal force and the meridional shear of the background wind. In those cases when the conditions for the method of the multiple scales in height are met, these wave solutions are also valid when vertical shear of the background wind is present. A total of six types of waves have been found and their properties were characterized in terms of the corresponding dispersion relations and wave structures. In this first part, only waves that are direct solutions of the generic dispersion relation are studied - acoustic and inertia-gravity waves. Concerning inertia-gravity waves, we found that in the cases of short horizontal wavelengths, null background wind, or propagation in the equatorial region, only pure gravity waves are possible, while for the limit of large horizontal wavelengths and/or null static stability, the waves are inertial. The correspondence between classical atmospheric approximations and wave filtering has been examined too, and we carried out a classification of the mesoscale waves found in the clouds of Venus at different vertical levels of its atmosphere. Finally, the classification of waves in exoplanets is discussed and we provide a list of possible candidates with cyclostrophic regimes. © 2014. The American Astronomical Society. All rights reserved.

News Article | August 22, 2016

Lidar measurements for water vapor vertical profiles up to the stratosphere Measurements taken from a new lidar observation system could support studies of cloud formation. Using continuous laser radar (lidar) observations taken at different locations on Earth, it is possible to monitor the occurrence of cirrus clouds over discrete periods of up to 10 years (see Figure 1). By analyzing the lidar data, we can classify clouds1 according to the processes by which they were formed, such as isentropic transport in the vicinity of the tropopause (at 10–18km above the Earth's surface), contrails (frozen water and aerosol particles from aircraft exhaust), and tropical storms. To better understand these occurrences, and to simulate cloud formation, scientists require an assessment of cloud water vapor content. Figure 1. Monthly cirrus cloud occurrence (black dotted line) based on measurements taken using ground-based lidar at the Observatory of Haute-Provence (OHP), France. Cloud-aerosol lidar with orthogonal polarization (CALIOP) measurements (in red) were taken above the same site. (Reprinted with permission. Monthly cirrus cloud occurrence (black dotted line) based on measurements taken using ground-based lidar at the Observatory of Haute-Provence (OHP), France. Cloud-aerosol lidar with orthogonal polarization (CALIOP) measurements (in red) were taken above the same site. (Reprinted with permission. 1 At high altitude, water vapor exhibits large horizontal gradients and is distributed in long strips of air. Satellite images are unable to adequately capture these phenomena and the mixing processes by which they occur, and standard meteorological radiosondes suffer interruptions in operation and bias. Furthermore, existing lidar approaches are unable to reach very high altitudes. One approach to water vapor detection uses lidar with Raman spectroscopy, which exploits the Raman (inelastic) scattering of photons. Raman effects induce a frequency shift in light, the magnitude of which depends on the molecule. These spectral shifts enable identification of the scattering induced by several atmospheric constituents. However, compared with elastic (Rayleigh) scattering, Raman scattering is weak. Furthermore, the Raman method of water vapor detection is based on the ratio of Raman scattering by water vapor and by nitrogen. Thus, it does not provide a direct measurement of the water vapor mixing ratio. We sought an alternative approach to enable direct lidar water vapor detection, and have designed an instrument that is capable of detecting vertical water vapor with an accuracy of greater than 10% at up to 10km above the Earth's surface in standard conditions.2, 3 We developed and built a test platform at the Observatory of Versailles Saint-Quentin-en-Yvelines in France (see Figure 2), which was based on the results of preliminary tests on existing Rayleigh lidar at observatories on the island of La Réunion and at Haute-Provence in the south of France.3 We then installed the instrument at a new observatory in La Réunion (see Figure 3.). Figure 2. The lidar system being assembled in Guyancourt, France. Figure 3. The 532nm laser beam pointing vertically from the lidar station at the Maïdo observatory (20.8°S, 55.5°E) in La Réunion. The instrument's calibration depends on the stability of the detection system, which we ensured by taking ancillary GPS measurements of the total water vapor column. The lidar system has a coaxial configuration (which precludes parallax effects that may lead to a large ‘ blind’ zone in the small field of view), and therefore enables detection of water vapor from the ground. During dry periods, calibration is more challenging to achieve because the vapor column is very small and results in greater uncertainties in GPS measurements. We installed flash lamps atop the receiving telescope to enable internal calibration and to monitor any sudden instrumental changes. The lidar system uses two neodymium-doped yttrium aluminum garnet pulse lasers of 23W and a telescope with a diameter of 1.2m, and is designed to operate at 355 or 532nm. We used lidar in a UV configuration during our successive campaigns, since UV wavelengths enable the optimum measurement of water vapor.4 Over a two-year period we obtained data that revealed frequent transport of air from the stratosphere through complex layering effects. Our instrument was able to reach as far as the lower stratosphere when we integrated measurements taken over the course of several hours or days5 (see Figure 4). Figure 4. Water vapor profile constructed by integrating data collected over several days (black line) compared with Microwave Limb Sounder satellite measurements (blue). The red and green dashed lines correspond to statistical uncertainties and variance (95%), respectively. (Reprinted with permission. Water vapor profile constructed by integrating data collected over several days (black line) compared with Microwave Limb Sounder satellite measurements (blue). The red and green dashed lines correspond to statistical uncertainties and variance (95%), respectively. (Reprinted with permission. 4 In summary, detecting water vapor is essential in the study of cloud formation, and existing techniques—including the use of satellite imaging or Raman spectroscopy—are either ineffective or challenging to use. We have designed a lidar-based instrument that can detect water vapor up to the level of the stratosphere. In future, we plan to operate this instrument alongside collocated ozone and aerosols lidar to enable the study of inter-annual variability, vertical transport, and cirrus formation. A second lidar system (similar to ours) is planned for development at the Observatory of Haute-Provence. University of Versailles Saint-Quentin-en-Yvelines Philippe Keckhut is a professor and director of the LATMOS space research laboratory. He received his PhD at Paris VI University in 1991, and was a visiting scientist at the US National Oceanic and Atmospheric Administration in Washington DC between 1994 and 1995. La Réunion University Hélène Vérèmes is a graduate of Toulouse University, France, and is preparing her PhD on water vapor lidar and mesoscale modeling. Valentin Duflot obtained his PhD in 2011. He is currently an associate physicist and the scientific coordinator of the Réunion Observatory OPAR. He is the co-investigator of the H O Raman lidar and principal investigator of the tropospheric ozone lidar, mobile aerosols lidar, and handheld photometers operating at La Réunion. 1. C. Hoareau, P. Keckhut, V. Noël, H. Chepfer, J.-L. Baray, A decadal cirrus clouds climatology from ground-based and spaceborne lidars above the south of France (43.9°N-5.7°E), Atmos. Chem. Phys. 13, p. 6951-6963, 2013. doi:10.5194/acp-13-6951-2013 2. P. L. Keckhut, H. Vérèmes, C. Horeau, D. Dionisi, J.-L. Baray, V. Duflot, G. Payen, J.-P. Cammas, A. Hauchecorne, Monitoring the water cycle in the UT/LS with Raman lidar. Presented at SPIE Asia-Pacific Remote Sensing 2016. 3. C. Hoareau, P. Keckhut, J.-L. Baray, L. Robert, Y. Courcoux, J. Porteneuve, H. Vömel, B. Morel, A Raman lidar at La Réunion (20.8°S, 55.5°E) for monitoring water vapor and cirrus distributions in the subtropical upper troposphere: preliminary analyses and description of a future system, Atmos. Meas. Tech. 5, p. 1333-1348, 2012. doi:10.5194/amt-5-1333-2012 4. P. Keckhut, Y. Courcoux, J.-L. Baray, J. Porteneuve, H. Vérèmes, A. Hauchecorne, D. Dionisi, et al., Introduction to the Maïdo lidar calibration campaign dedicated to the validation of upper air meteorological parameters, J. Appl. Rem. Sens. 9, p. 094099, 2015. doi:10.1117/1.JRS.9.094099 5. D. Dionisi, P. Keckhut, Y. Courcoux, A. Hauchecorne, J. Porteneuve, J.-L. Baray, J. Leclair de Bellevue, et al., Water vapor observations up to the lower stratosphere through the Raman lidar during the Maïdo lidar calibration campaign, Atmos. Meas. Tech. 8, p. 1425-1445, 2015.

Richer E.,Ecole Polytechnique - Palaiseau | Modolo R.,LATMOS | Chanteur G.M.,Ecole Polytechnique - Palaiseau | Hess S.,LATMOS | Leblanc F.,French National Center for Scientific Research
Journal of Geophysical Research: Space Physics | Year: 2012

[1] The interaction of the solar wind (SW) with the magnetic field of Mercury is investigated by means of a three dimensional parallelized multispecies hybrid model. A comparison between two mathematical representations of Mercury's intrinsic magnetic field is studied. The first model is an Offset Dipole (OD) having the offset and dipolar moment reported by Anderson et al. (2011). The second model is a combination of a Dipole and a Quadrupole (DQ), the total field is fitted to the offset dipolar field, for northern latitudes greater than 50°. Simulations reproduce the features which characterize Mercury's interaction with the SW, encompassing the Bow Shock (BS), the magnetosheath, the magnetotail, the "cusps" region and the neutral current sheet. Global hybrid simulations of the Hermean magnetosphere run for the OD and DQ models demonstrate that the southern parts of the magnetospheres produced by the OD and DQ models differ greatly in topology and volume meanwhile their northern parts-are quite similar. In particular the DQ model exhibits a dome of closed field lines around the south pole in contrast to the OD. Without further information on the intrinsic magnetic field of the planet in the southern region which should be provided by BepiColombo after year 2020, we can only speculate on the influence of the different magnetic topologies on the magnetospheric dynamics. © 2012. American Geophysical Union. All Rights Reserved.

Cheruy F.,French National Center for Scientific Research | Campoy A.,University Pierre and Marie Curie | Dupont J.-C.,Ecole Polytechnique - Palaiseau | Ducharne A.,University Pierre and Marie Curie | And 4 more authors.
Climate Dynamics | Year: 2013

The identification of the land-atmosphere interactions as one of the key source of uncertainty in climate models calls for process-level assessment of the coupled atmosphere/land continental surface system in numerical climate models. To this end, we propose a novel approach and apply it to evaluate the standard and new parametrizations of boundary layer/convection/clouds in the Earth System Model (ESM) of Institut Pierre Simon Laplace (IPSL), which differentiate the IPSL-CM5A and IPSL-CM5B climate change simulations produced for the Coupled Model Inter-comparison Project phase 5 exercise. Two different land surface hydrology parametrizations are also considered to analyze different land-atmosphere interactions. Ten-year simulations of the coupled land surface/atmospheric ESM modules are confronted to observations collected at the SIRTA (Site Instrumental de Recherche par Télédection Atmosphérique), located near Paris (France). For sounder evaluation of the physical parametrizations, the grid of the model is stretched and refined in the vicinity of the SIRTA, and the large scale component of the modeled circulation is adjusted toward ERA-Interim reanalysis outside of the zoomed area. This allows us to detect situations where the parametrizations do not perform satisfactorily and can affect climate simulations at the regional/continental scale, including in full 3D coupled runs. In particular, we show how the biases in near surface state variables simulated by the ESM are explained by (1) the sensible/latent heat partitionning at the surface, (2) the low level cloudiness and its radiative impact at the surface, (3) the parametrization of turbulent transport in the surface layer, (4) the complex interplay between these processes. We also show how the new set of parametrizations can improve these biases. © 2012 Springer-Verlag.

Dib S.,Imperial College London | Piau L.,LATMOS | Mohanty S.,Imperial College London | Braine J.,French National Center for Scientific Research
Monthly Notices of the Royal Astronomical Society | Year: 2011

We explore how the star formation efficiency (SFE) in a protocluster clump is regulated by metallicity-dependent stellar winds from the newly formed massive OB stars (M*≥ 5M⊙) on their main sequence. The model describes the coevolution of the mass function of gravitationally bound cores and of the initial mass function in a protocluster clump. Dense cores are generated uniformly in time at different locations in the clump, and contract over lifetimes that are a few times their free-fall times. The cores collapse to form stars that power strong stellar winds whose cumulative kinetic energy evacuates the gas from the clump and quenches further core and star formation. This sets the final SFE, SFEf. Models are run with various metallicities in the range Z/Z⊙=[0.1, 2]. We find that the SFEf decreases strongly with increasing metallicity. The SFEf-metallicity relation is well described by a decaying exponential whose exact parameters depend weakly on the value of the core formation efficiency. We find that there is almost no dependence of the SFEf-metallicity relation on the clump mass. This is due to the fact that an increase (decrease) in the clump mass leads to an increase (decrease) in the feedback from OB stars which is opposed by an increase (decrease) in the gravitational potential of the clump. The clump mass-cluster mass relations we find for all of the different metallicity cases imply a negligible difference between the exponent of the mass function of the protocluster clumps and that of the young clusters' mass function. By normalizing the SFEs to their value for the solar metallicity case, we compare our results to SFE-metallicity relations derived on galactic scales and find a good agreement. As a by-product of this study, we also provide ready-to-use prescriptions for the power of stellar winds of main-sequence OB stars in the mass range [5, 80]M⊙ in the metallicity range we have considered. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Savoini P.,Ecole Polytechnique - Palaiseau | Lembege B.,LATMOS
Journal of Geophysical Research A: Space Physics | Year: 2015

A curved shock is analyzed in the whole quasi-perpendicular propagation region (90° ≥ θBn≥45°) in a supercritical regime with the help of a 2-D particle-in-cell code including self-consistent effects such as the shock front curvature and the time-of-flight effects. Two distinct ion populations are observed within the foreshock: a (gyrotropic) field-aligned beam population, hereafter named "FAB," and a (nongyrotropic) gyrophase bunched population, hereafter named "GPB." The origin of these high-energy particles and their corresponding acceleration mechanisms are analyzed in details in the present paper. Both FAB and GPB populations are shown to be produced by the shock front itself and more important, do have exactly the same origin. At the shock front, the two populations gain a nongyrotropic distribution, but FAB population loses its initial phase coherency after suffering several bounces along the curved front. This result has one main consequence: the time evolution of the two populations does not involve some distinct reflection processes as often claimed in the literature, but results only from the particle time history at the shock front. This important result was not expected and greatly simplifies the question of their origin. More precisely, a new parameter, the injection angle θinj has been defined between the shock normal direction and the ion gyrating velocity vector. We found that the FAB population is formed by ions injected almost along the shock front, while GPB population is formed by ions injected almost along the shock normal. ©2015. American Geophysical Union. All Rights Reserved.

Parent du Chatelet J.,Meteo - France | Boudjabi C.,LATMOS | Besson L.,Meteo - France | Caumont O.,Meteo - France
Journal of Atmospheric and Oceanic Technology | Year: 2012

Refractivity measurements in the boundary layer by precipitation radar could be useful for convection prediction. Until now such measurements have only been performed by coherent radars, but European weather radars are mostly equipped with noncoherent magnetron transmitters for which the phase and frequency may vary. In this paper, the authors give an analytical expression of the refractivity measurement by a noncoherent drifting-frequency magnetron radar and validate it by comparing with in situ measurements. The main conclusion is that, provided the necessary corrections are applied, the measurement can be successfully performed with a noncoherent radar. The correction factor mainly depends on the local-oscillator frequency variation, which is known perfectly. A second-order error, proportional to the transmitted frequency variation, can be neglected as long as this change remains small. © 2012 American Meteorological Society.

Wilquet V.,Belgian Institute for Space Aeronomy | Drummond R.,Belgian Institute for Space Aeronomy | Mahieux A.,Belgian Institute for Space Aeronomy | Robert S.,Belgian Institute for Space Aeronomy | And 3 more authors.
Icarus | Year: 2012

The variability of the aerosol loading in the mesosphere of Venus is investigated from a large data set obtained with SOIR, a channel of the SPICAV instrument suite onboard Venus Express. Vertical profiles of the extinction due to light absorption by aerosols are retrieved from a spectral window around 3.0. μm recorded in many solar occultations (~200) from September 2006 to September 2010. For this period, the continuum of light absorption is analyzed in terms of spatial and temporal variations of the upper haze of Venus. It is shown that there is a high short-term (a few Earth days) and a long-term (~80 Earth days) variability of the extinction profiles within the data set. Latitudinal dependency of the aerosol loading is presented for the entire period considered and for shorter periods of time as well. © 2011 Elsevier Inc.

Katushkina O.A.,Moscow State University | Izmodenov V.V.,Moscow State University | Quemerais E.,LATMOS | Sokol J.M.,Polish Academy of Sciences
Journal of Geophysical Research: Space Physics | Year: 2013

Recently Sokõł et al. (2012) have presented a reconstruction of heliolatitudinal and time variations of the solar wind speed and density. Method of the reconstruction was based on the following: (i) measurements of the interplanetary scintillations, (ii) OMNI-2 solar wind data in the ecliptic plane, and (iii) Ulysses solar wind data out of the ecliptic plane. In this paper we use hydrogen charge exchange rates derived from their results as input parameters to calculate the interstellar hydrogen distribution in the heliosphere in the frame of our 3-D time-dependent kinetic model. The hydrogen distribution is then used to calculate the backscattered solar Lyman-alpha intensity maps. The theoretical Lyman-alpha maps are compared with the SOHO/SWAN measurements during maximum and minimum of the solar cycle activity. We found that in the solar minimum there is a quite good agreement between the model results and the SWAN data, but in the solar maximum sky maps of the Lyman-alpha, intensities are qualitatively different for the model results and observations. Physical reasons of the differences are discussed. ©2013. American Geophysical Union. All Rights Reserved.

Battaglia A.,University of Leicester | Delanoe J.,LATMOS
Journal of Geophysical Research: Atmospheres | Year: 2013

Four years (2007-2010) of colocated 94 GHz CloudSat radar reflectivities and 532 nm CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) backscattering coefficients are used to globally characterize snow-precipitating clouds. CALIOP is particularly useful for the detection of mixed and supercooled liquid water (SLW) layers. Liquid layers are common in snow precipitating clouds: overall/over sea/over land 49%/ 57%/33% of the snowy profiles present SLW or mixed-phase layers. The spatial and seasonal dependencies of our results-with snowing clouds more likely to be associated with mixed phase during summer periods-are related to snow layer top temperatures. SLW occurs within the majority (>80%) of snow-precipitating clouds with cloud tops warmer than 250 K, and is present 50% of the time when the snow-layer top temperature is about 240 K. There is a marked tendency for such layers to occur close to the top of the snow-precipitating layer (75% of the times within 500 m). Both instruments can be synergetically used for profiling ice-phase-only snow, especially for light snow (Z<0 dBZ, S<0.16 mm/h) when CALIOP is capable of penetrating, on average, more than half of the snow layer depth. These results have profound impact for deepening our understanding of ice nucleation and snow growth processes, for improving active and passive snow remote sensing techniques, and for planning snow-precipitation missions. © 2012. American Geophysical Union.

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