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Search Results: 1 - 10 of 189827 matches for " G. Barrot "
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Le paysage en peinture: espace rural et culture esthétique en Europe occidentale (XVIe - XVIIIe siècle)
Mappemonde , 1997,
Abstract: L'invention de la perspective autorise à porter un regard de géographe sur les tableaux du XVIe au XVIIIe siècle. Le souci de réalisme permet de saisir des évolutions, selon le temps et les lieux, dans le paysage rural. Mais les choix de représentation traduisent bien davantage le contenu idéologique du regard que porte la clientèle du peintre sur la campagne qui reste dans cette période la base productive essentielle de la société.
Retrieval of ozone profiles from GOMOS limb scattered measurements
S. Tukiainen, E. Kyr l , P. T. Verronen, D. Fussen, L. Blanot, G. Barrot, A. Hauchecorne,N. Lloyd
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 2011,
Abstract: The GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument on board the Envisat satellite measures the vertical composition of the atmosphere using the stellar occultation technique. While the night-time occultations of GOMOS have been proven to be of good quality, the daytime occultations are more challenging due to weaker signal-to-noise ratio. During daytime GOMOS measures limb scattered solar radiation in addition to stellar radiation. In this paper we introduce a retrieval method that determines ozone profiles between 20–60 km from GOMOS limb scattered solar radiances. GOMOS observations contain a considerable amount of stray light at high altitudes. We introduce a method for removing stray light and demonstrate its feasibility by comparing the corrected radiances against those measured by the OSIRIS (Optical Spectrograph & Infra Red Imaging System) instrument. For the retrieval of ozone profiles, a standard onion peeling method is used. The first comparisons with other data sets suggest that the retrieved ozone profiles in 22–50 km are within 10% compared with the GOMOS night-time occultations and within 15% compared with OSIRIS. GOMOS has measured about 350 000 daytime profiles since 2002. The retrieval method presented here makes this large amount of data available for scientific use.
Mid-latitude ozone monitoring with the GOMOS-ENVISAT experiment version 5: the noise issue
P. Keckhut, A. Hauchecorne, L. Blanot, K. Hocke, S. Godin-Beekmann, J.-L. Bertaux, G. Barrot, E. Kyr l , J. A. E. van Gijsel,A. Pazmino
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2010,
Abstract: The GOMOS ozone profiles have been analysed to evaluate the GOMOS ability to capture the long-term ozone evolution at mid-latitudes during the expected recovery phase of the ozone layer. Version 5 of the operational GOMOS ozone data has been compared with data from two of the longest ground-based instruments based on different techniques and already involved with many other previous space instrument validations. Comparisons between ground-based and GOMOS data confirm the occurrence of spurious retrievals mainly occurring since 2006. Using a selected set of data it is shown that some bad retrievals are induced by the increasing dark charge of the detectors combined with an inadequate method for its correction. This effect does not only induce a continuous bias, but is rather exhibiting a bimodal distribution including the correct profiles and the bad retrievals. For long-term analyses it is recommended filtering the data according to background light conditions and star temperature (spectrum shape). The new method of the dark charge estimate proposed to be implemented in the version 6 of the ESA algorithm seems to significantly reduce the occurrence of such effects and should allow to monitor stratospheric ozone using GOMOS data with greater confidence.
Retrieval of atmospheric parameters from GOMOS data
E. Kyr l , J. Tamminen, V. Sofieva, J. L. Bertaux, A. Hauchecorne, F. Dalaudier, D. Fussen, F. Vanhellemont, O. Fanton d'Andon, G. Barrot, M. Guirlet, A. Mangin, L. Blanot, T. Fehr, L. Saavedra de Miguel,R. Fraisse
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2010,
Abstract: The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European Space Agency's ENVISAT satellite measures attenuation of stellar light in occultation geometry. Daytime measurements also record scattered solar light from the atmosphere. The wavelength regions are the ultraviolet-visible band 248–690 nm and two infrared bands at 755–774 nm and at 926–954 nm. From UV-Visible and IR spectra the vertical profiles of O3, NO2, NO3, H2O, O2 and aerosols can be retrieved. In addition there are two 1 kHz photometers at blue 473–527 nm and red 646–698 nm. Photometer data are used to correct spectrometer measurements for scintillations and to retrieve high resolution temperature profiles as well as gravity wave and turbulence parameters. Measurements cover altitude region 5–150 km. Atmospherically valid data are obtained in 15–100 km. In this paper we present an overview of the GOMOS retrieval algorithms for stellar occultation measurements. The low signal-to-noise ratio and the refractive effects due to the point source nature of stars have been important drivers in the development of GOMOS retrieval algorithms. We present first the Level 1b algorithms that are used to correct instrument related disturbances in the spectrometer and photometer measurements The Level 2 algorithms deal with the retrieval of vertical profiles of atmospheric gaseous constituents, aerosols and high resolution temperature. We divide the presentation into correction for refractive effects, high resolution temperature retrieval and spectral/vertical inversion. The paper also includes discussion about the GOMOS algorithm development, expected improvements, access to GOMOS data and alternative retrieval approaches.
Retrievals from GOMOS stellar occultation measurements using characterization of modeling errors
V. F. Sofieva, J. Vira, E. Kyr l , J. Tamminen, V. Kan, F. Dalaudier, A. Hauchecorne, J.-L. Bertaux, D. Fussen, F. Vanhellemont, G. Barrot,O. Fanton d'Andon
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 2010,
Abstract: In this paper, we discuss the development of the inversion algorithm for the GOMOS (Global Ozone Monitoring by Occultation of Star) instrument on board the Envisat satellite. The proposed algorithm takes accurately into account the wavelength-dependent modeling errors, which are mainly due to the incomplete scintillation correction in the stratosphere. The special attention is paid to numerical efficiency of the algorithm. The developed method is tested on a large data set and its advantages are demonstrated. Its main advantage is a proper characterization of the uncertainties of the retrieved profiles of atmospheric constituents, which is of high importance for data assimilation, trend analyses and validation.
Global ozone monitoring by occultation of stars: an overview of GOMOS measurements on ENVISAT
J. L. Bertaux, E. Kyr l , D. Fussen, A. Hauchecorne, F. Dalaudier, V. Sofieva, J. Tamminen, F. Vanhellemont, O. Fanton d'Andon, G. Barrot, A. Mangin, L. Blanot, J. C. Lebrun, K. Pérot, T. Fehr, L. Saavedra, G. W. Leppelmeier,R. Fraisse
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2010,
Abstract: GOMOS on ENVISAT (launched in February, 2002) is the first space instrument dedicated to the study of the atmosphere of the Earth by the technique of stellar occultations (Global Ozone Monitoring by Occultation of Stars). Its polar orbit makes good latitude coverage possible. Because it is self-calibrating, it is particularly well adapted to long time trend monitoring of stratospheric species. With 4 spectrometers, the wavelength coverage of 248 nm to 942 nm enables monitoring ozone, H2O, NO2, NO3, air density, aerosol extinction, and O2. Two additional fast photometers (with 1 kHz sampling rate) enable the correction of the effects of scintillations, as well as the study of the structure of air density irregularities resulting from gravity waves and turbulence. A high vertical resolution profile of the temperature may also be obtained from the time delay between the red and the blue photometer. Noctilucent clouds (Polar Mesospheric Clouds, PMC) are routinely observed in both polar summers and global observations of OClO and sodium are achieved. The instrument configuration, dictated by the scientific objectives' rationale and technical constraints, is described, together with the typical operations along one orbit, along with the statistics from over 6 years of operation. Typical atmospheric transmission spectra are presented and some retrieval difficulties are discussed, in particular for O2 and H2O. An overview is presented of a number of scientific results already published or found in more detail as companion papers in the same ACP GOMOS special issue. This paper is particularly intended to provide an incentive for the exploitation of GOMOS data available to the whole scientific community in the ESA data archive, and to help GOMOS data users to better understand the instrument, its capabilities and the quality of its measurements, thus leading to an increase in the scientific return.
Response of tropical stratospheric O3, NO2 and NO3 to the equatorial Quasi-Biennial Oscillation and to temperature as seen from GOMOS/ENVISAT
A. Hauchecorne, J. L. Bertaux, F. Dalaudier, P. Keckhut, P. Lemennais, S. Bekki, M. Marchand, J. C. Lebrun, E. Kyr l , J. Tamminen, V. Sofieva, D. Fussen, F. Vanhellemont, O. Fanton d'Andon, G. Barrot, L. Blanot, T. Fehr,L. Saavedra de Miguel
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2010,
Abstract: The stellar occultation spectrometer GOMOS (Global Ozone Monitoring by Occultation of Stars) on ESA's Envisat satellite measures vertical profiles O3, NO2 and NO3 with a high long-term stability due to the self-calibrating nature of the technique. More than 6 years of GOMOS data from August 2002 to end 2008 have been analysed to study the inter-annual variation of O3, NO2 and NO3 in the tropics. It is shown that the QBO of the equatorial wind induces variations in the local concentration larger than 10% for O3 and larger than 25% for NO2. Quasi-Biennial Oscillation signals can be found in the evolution of the three constituents up to at least 40 km. We found that NO3 is positively correlated with temperature up to 45 km in the region where it is in chemical equilibrium with O3. Our results confirm the existence of a transition from a dynamical control of O3 below 28 km with O3 correlated with temperature and a chemical/temperature control between 28 and 38 km with O3 anti-correlated with NO2 and temperature. Above 38 km and up to 50 km a different regime is found with O3 and NO2 correlated with each other and anti-correlated with temperature. For the NO2/temperature anti-correlation in the upper stratosphere, our proposed explanation is the modulation of the N2O ascent by the QBO up to 45 km. The oxidation of N2O is the main source of NOy in this altitude region. An enhancement of the ascending motion will cool adiabatically the atmosphere and will increase the amount of N2O concentration available for NOy formation.
A 2003 stratospheric aerosol extinction and PSC climatology from GOMOS measurements on Envisat
F. Vanhellemont, D. Fussen, C. Bingen, E. Kyr l , J. Tamminen, V. Sofieva, S. Hassinen, P. Verronen, A. Sepp l , J. L. Bertaux, A. Hauchecorne, F. Dalaudier, O. Fanton d'Andon, G. Barrot, A. Mangin, B. Theodore, M. Guirlet, J. B. Renard, R. Fraisse, P. Snoeij, R. Koopman,L. Saavedra
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2005,
Abstract: Stratospheric aerosols play an important role in a number of atmospheric issues such as midlatitude ozone depletion, atmospheric dynamics and the Earth radiative budget. Polar stratospheric clouds on the other hand are a crucial factor in the yearly Arctic and Antarctic ozone depletion. It is therefore important to quantify the stratospheric aerosol/PSC abundance. In orbit since March 2002, the GOMOS instrument onboard the European Envisat satellite has provided a vast aerosol extinction data set. In this paper we present aerosol/PSC zonal median values that were constructed from this data set, together with a discussion of the results.
GOMOS data characterisation and error estimation
J. Tamminen, E. Kyr l , V. F. Sofieva, M. Laine, J.-L. Bertaux, A. Hauchecorne, F. Dalaudier, D. Fussen, F. Vanhellemont, O. Fanton-d'Andon, G. Barrot, A. Mangin, M. Guirlet, L. Blanot, T. Fehr, L. Saavedra de Miguel,R. Fraisse
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2010,
Abstract: The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument uses stellar occultation technique for monitoring ozone, other trace gases and aerosols in the stratosphere and mesosphere. The self-calibrating measurement principle of GOMOS together with a relatively simple data retrieval where only minimal use of a priori data is required provides excellent possibilities for long-term monitoring of atmospheric composition. GOMOS uses about 180 of the brightest stars as its light source. Depending on the individual spectral characteristics of the stars, the signal-to-noise ratio of GOMOS varies from star to star, resulting also in varying accuracy of retrieved profiles. We present here an overview of the GOMOS data characterisation and error estimation, including modeling errors, for O3, NO2, NO3, and aerosol profiles. The retrieval error (precision) of night-time measurements in the stratosphere is typically 0.5–4% for ozone, about 10–20% for NO2, 20–40% for NO3 and 2–50% for aerosols. Mesospheric O3, up to 100 km, can be measured with 2–10% precision. The main sources of the modeling error are incompletely corrected scintillation, inaccurate aerosol modeling, uncertainties in cross sections of trace gases and in atmospheric temperature. The sampling resolution of GOMOS varies depending on the measurement geometry. In the data inversion a Tikhonov-type regularization with pre-defined target resolution requirement is applied leading to 2–3 km vertical resolution for ozone and 4 km resolution for other trace gases and aerosols.
Spatio-temporal observations of the tertiary ozone maximum
V. F. Sofieva, E. Kyr l , P. T. Verronen, A. Sepp l , J. Tamminen, D. R. Marsh, A. K. Smith, J.-L. Bertaux, A. Hauchecorne, F. Dalaudier, D. Fussen, F. Vanhellemont, O. Fanton d'Andon, G. Barrot, M. Guirlet, T. Fehr,L. Saavedra
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2009,
Abstract: We present spatio-temporal distributions of the tertiary ozone maximum (TOM), based on GOMOS (Global Ozone Monitoring by Occultation of Stars) ozone measurements in 2002–2006. The tertiary ozone maximum is typically observed in the high-latitude winter mesosphere at an altitude of ~72 km. Although the explanation for this phenomenon has been found recently – low concentrations of odd-hydrogen cause the subsequent decrease in odd-oxygen losses – models have had significant deviations from existing observations until recently. Good coverage of polar night regions by GOMOS data has allowed for the first time to obtain spatial and temporal observational distributions of night-time ozone mixing ratio in the mesosphere. The distributions obtained from GOMOS data have specific features, which are variable from year to year. In particular, due to a long lifetime of ozone in polar night conditions, the downward transport of polar air by the meridional circulation is clearly observed in the tertiary ozone maximum time series. Although the maximum tertiary ozone mixing ratio is achieved close to the polar night terminator (as predicted by the theory), TOM can be observed also at very high latitudes, not only in the beginning and at the end, but also in the middle of winter. We have compared the observational spatio-temporal distributions of the tertiary ozone maximum with that obtained using WACCM (Whole Atmosphere Community Climate Model) and found that the specific features are reproduced satisfactorily by the model. Since ozone in the mesosphere is very sensitive to HOx concentrations, energetic particle precipitation can significantly modify the shape of the ozone profiles. In particular, GOMOS observations have shown that the tertiary ozone maximum was temporarily destroyed during the January 2005 and December 2006 solar proton events as a result of the HOx enhancement from the increased ionization.
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