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Search Results: 1 - 10 of 144416 matches for " F. Dalaudier "
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Anisotropy of small-scale stratospheric irregularities retrieved from scintillations of a double star α-Cru observed by GOMOS/ENVISAT
V. Kan, V. F. Sofieva,F. Dalaudier
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 2012,
Abstract: In this paper, we discuss estimating anisotropy of air density irregularities (ratio of characteristic horizontal and vertical scales) from satellite observations of bi-chromatic scintillations of a double star whose components are not resolved by the detector. The analysis is based on fitting experimental auto- and cross-spectra of scintillations by those computed using the 3-D spectral model of atmospheric irregularities consisting of anisotropic and isotropic components. Application of the developed method to the scintillation measurements of the double star α-Cru by GOMOS (Global Ozone Monitoring by Occultation of Stars) fast photometers results in estimates of anisotropy coefficient of ~15–20 at altitudes 30–38 km, as well as other parameters of atmospheric irregularities. The obtained estimates of the anisotropy coefficient correspond to small-scale irregularities, close to the buoyancy scale.
Anisotropy of small-scale stratospheric irregularities retrieved from scintillations of a double star α-Cru observed by GOMOS/ENVISAT
V. Kan,V. F. Sofieva,F. Dalaudier
Atmospheric Measurement Techniques Discussions , 2012, DOI: 10.5194/amtd-5-4881-2012
Abstract: In this paper, we discuss estimating anisotropy of air density irregularities (ratio of characteristic horizontal and vertical scales) from satellite observations of bi-chromatic scintillations of a double star whose components are not resolved by the detector. The analysis is based on fitting experimental auto- and cross-spectra of scintillations by those computed using the 3-D spectral model of atmospheric irregularities consisting of anisotropic and isotropic components. Application of the developed method to the scintillation measurements of the double star α-Cru by GOMOS (Global Ozone Monitoring by Occultation of Stars) fast photometers results in estimates of anisotropy coefficient of ~15–20 at altitudes 30–38 km, as well as other parameters of atmospheric irregularities. The obtained estimates of the anisotropy coefficient correspond to small-scale irregularities, close to the buoyancy scale.
Can one detect small-scale turbulence from standard meteorological radiosondes?
R. Wilson, F. Dalaudier,H. Luce
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 2011,
Abstract: It has been recently proposed by Clayson and Kantha (2008) to evaluate the climatology of atmospheric turbulence through the detection of overturns in the free atmosphere by applying a Thorpe analysis on relatively low vertical resolution (LR) profiles collected from standard radiosoundings. Since then, several studies based on this idea have been published. However, the impact of instrumental noise on the detection of turbulent layers was completely ignored in these works. The present study aims to evaluate the feasibility of overturns detection from radiosoundings. For this purpose, we analyzed data of two field campaigns during which high-resolution (HR) soundings (10–20 cm) were performed simultaneously with standard LR soundings. We used the raw data of standard meteorological radiosondes, the vertical resolution ranging from 5 to 9 m. A Thorpe analysis was applied to both LR and HR potential temperature profiles. A denoising procedure was first applied in order to reduce the probability of occurrence of artificial overturns, i.e. potential temperature inversions due to instrumental noise only. We then compared the empirical probability density functions (pdf) of the sizes of the selected overturns from LR and HR profiles. From HR profiles measured in the troposphere, the sizes of the detected overturns range from 4 to ~1000 m. The shape of the size pdf of overturns is found to sharply decrease with increasing scales. From LR profiles, the smallest size of detected overturns is ~32 m, a similar decrease in the shape of the pdf of sizes being observed. These results suggest that overturns, resulting either from small-scale turbulence or from instabilities, can indeed be detected from meteorological radiosonde measurements in the troposphere and in the stratosphere as well. However they are rather rare as they belong to the tail of the size distribution of overturns: they only represent the 7 % largest events in the troposphere, and 4 % in the stratosphere.
Temperature sheets and aspect sensitive radar echoes
H. Luce,M. Crochet,F. Dalaudier
Annales Geophysicae (ANGEO) , 2003,
Abstract: here have been years of discussion and controversy about the existence of very thin and stable temperature sheets and their relationship to the VHF radar aspect sensitivity. It is only recently that very high-resolution in situ temperature observations have brought credence to the reality and ubiquity of these structures in the free atmosphere and to their contribution to radar echo enhancements along the vertical. Indeed, measurements with very high-resolution sensors are still extremely rare and rather difficult to obtain outside of the planetary boundary layer. They have only been carried out up to the lower stratosphere by Service d’A′ eronomie (CNRS, France) for about 10 years. The controversy also persisted due to the volume resolution of the (Mesosphere)-Stratosphere-Troposphere VHF radars which is coarse with respect to sheet thickness, although widely sufficient for meteorological or mesoscale investigations. The contribution within the range gate of many of these structures, which are advected by the wind, and decay and grow at different instants and could be distorted either by internal gravity waves or turbulence fields, could lead to radar echoes with statistical properties similar to those produced by anisotropic turbulence. Some questions thus remain regarding the manner in which temperature sheets contribute to VHF radar echoes. In particular, the zenithal and azimuthal angular dependence of the echo power may not only be produced by diffuse reflection on stable distorted or corrugated sheets, but also by extra contributions from anisotropic turbulence occurring in the stratified atmosphere. Thus, for several years, efforts have been put forth to improve the radar height resolution in order to better describe thin structures. Frequency interferometric techniques are widely used and have been recently further developed with the implementation of high-resolution data processings. We begin by reviewing briefly some characteristics of the ST radar echoes with a particular emphasis on recent works. Their possible coupling with stable sheets is then presented and their known characteristics are described with some hypotheses concerning their generation mechanisms. Finally, measurement campaigns that took recently place or will be carried out in the near future for improving our knowledge of these small-scale structures are presented briefly. Key words. Meteorology and atmospheric dynamics (turbulence; instruments and techniques) – Radio Science (remote sensing)
Can one detect small-scale turbulence from standard meteorological radiosondes?
R. Wilson,F. Dalaudier,H. Luce
Atmospheric Measurement Techniques Discussions , 2011, DOI: 10.5194/amtd-4-969-2011
Abstract: It has been recently proposed by Clayson and Kantha (2008) to evaluate the climatology of atmospheric turbulence in the free atmosphere by applying a Thorpe analysis on standard radiosoundings obtained with relatively low resolution (LR) in the vertical. Since then, several studies based on this idea have been published. However, the impact of instrumental noise on the detection of turbulent layers was completely ignored in these works. The present study aims to evaluate the feasibility of turbulence detection from radiosoundings. For this purpose, we analyzed data of two field campaigns during which high-resolution (HR) soundings (10–20 cm) were performed simultaneously with standard LR soundings. We here used the raw data of standard radiosondes, the vertical resolution ranging from 5 to 8 m. A Thorpe analysis was performed on both LR and HR potential temperature profiles. A denoising procedure was first applied in order to reduce the probability of occurrence of artificial inversions, i.e. inversions due to instrumental noise only. We then compare the empirical probability of the sizes of the selected overturns from LR and HR profiles. From HR profiles in the troposphere, the scales of the detected turbulent overturns range from 4 to ~1000 m. The shape of the distribution of the size of overturns is found to sharply decrease with increasing scales. From LR profiles, the smallest scale of detected overturns is ~32 m, a similar decrease in the shape of the size distribution being observed. These results suggest that turbulent events can indeed be detected from standard radiosondes measurements in the troposphere. However these events are rather rare as they belong to the tail of the size distribution of the turbulent overturns: they only represent the 7% largest events. Similar conclusions are obtained from radiosondes data collected in the lower stratosphere, but the fraction of the detectable events is even smaller than in the troposphere since they are the 4% largest events.
Technical Note: Scintillations of the double star α Cru observed by GOMOS/Envisat
V. F. Sofieva,F. Dalaudier,V. Kan,A. S. Gurvich
Atmospheric Chemistry and Physics Discussions , 2009,
Abstract: In this paper, we discuss scintillation time-spectra of the double star α Cru, which were measured by the GOMOS/Envisat photometer. The components of α Cru are not resolved by the angular field of view of the detector. The double structure of the light source reveals itself in the modulation of the observed scintillation spectra; this modulation is caused by anisotropic irregularities of stratospheric air density. We present qualitative and quantitative explanation of properties of the double-star scintillation spectra. Possibilities of using double star scintillations for studying atmospheric air density irregularities are also discussed in the paper.
Influence of scintillation on GOMOS ozone retrievals
V. F. Sofieva,V. Kan,F. Dalaudier,E. Kyr?l?
Atmospheric Chemistry and Physics Discussions , 2009,
Abstract: The stellar light passed through the Earth atmosphere is affected by refractive effects, which should be taken into account in retrievals from stellar occultation measurements. Scintillation caused by air density irregularities is a nuisance for retrievals of atmospheric composition. In this paper, we consider the influence of scintillation on stellar occultation measurements and on quality of ozone retrievals from these measurements, based on experience of the GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument on board the Envisat satellite. In the GOMOS retrievals, the scintillation effect is corrected using scintillation measurements by the fast photometer. We present quantitative estimates of the current scintillation correction quality and of the impact of scintillation on ozone retrievals by GOMOS. The analysis has shown that the present scintillation correction efficiently removes the distortion of transmission spectra caused by anisotropic scintillations. The impact of errors of dilution and anisotropic scintillation correction on quality of ozone retrievals is negligible. However, the current scintillation correction is not able to remove the wavelength-dependent distortion of transmission spectra caused by isotropic scintillations, which can be present in off-orbital-plane occultations. This distortion may result in error of ozone retrievals of 0.5–1.5% at altitudes 20–40 km. This contribution to the error budget is significant for bright stars. The advanced inversion methods that can minimize the influence of scintillation correction error are also discussed.
Influence of scintillation on quality of ozone monitoring by GOMOS
V. F. Sofieva,V. Kan,F. Dalaudier,E. Kyr?l?
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2009,
Abstract: Stellar light passing through the Earth atmosphere is affected by refractive effects, which should be taken into account in retrievals from stellar occultation measurements. Scintillation caused by air density irregularities is a nuisance for retrievals of atmospheric composition. In this paper, we consider the influence of scintillation on stellar occultation measurements and on the quality of ozone retrievals from these measurements, based on experience of the GOMOS (Global Ozone Monitoring by Occultation of Stars) instrument on board the Envisat satellite. In GOMOS retrievals, the scintillation effect is corrected using scintillation measurements by the fast photometer. We present quantitative estimates of the current scintillation correction quality and of the impact of scintillation on ozone retrievals by GOMOS. The analysis has shown that the present scintillation correction efficiently removes the distortion of transmission spectra caused by scintillations, which are generated by anisotropic irregularities of air density. The impact of errors of dilution and anisotropic scintillation correction on the quality of ozone retrievals is negligible. However, the current scintillation correction is not able to remove the wavelength-dependent distortion of transmission spectra caused by isotropic scintillations, which can be present in off-orbital-plane occultations. This distortion may result in ozone retrieval errors of 0.5–1.5% at altitudes 20–40 km. This contribution constitutes a significant percentage of the total error for bright stars. The advanced inversion methods that can minimize the influence of scintillation correction error are also discussed.
Technical note: Scintillations of the double star α Cru observed by GOMOS/Envisat
V. F. Sofieva,F. Dalaudier,V. Kan,A. S. Gurvich
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2009,
Abstract: In this paper, we discuss scintillation time-spectra of the double star α Cru, which were measured by the GOMOS/Envisat photometer. The components of α Cru are not resolved by the angular field of view of the detector. The double structure of the light source reveals itself in the modulation of the observed scintillation spectra; this modulation is caused by anisotropic irregularities of the stratospheric air density. We present a qualitative and quantitative explanation of the properties of the double-star scintillation spectra. Possibilities of using double star scintillations for studying atmospheric air density irregularities are also discussed in the paper.
On the effect of moisture on the detection of tropospheric turbulence from in situ measurements
R. Wilson, H. Luce, H. Hashiguchi, M. Shiotani,F. Dalaudier
Atmospheric Measurement Techniques (AMT) & Discussions (AMTD) , 2013,
Abstract: The present paper addresses the detection of turbulence based on the Thorpe (1977) method applied to an atmosphere where saturation of water vapor occurs. The detection method proposed by Thorpe relies on the sorting in ascending order of a measured profile of a variable conserved through adiabatic processes, (e.g. potential temperature). For saturated air, the reordering should be applied to a moist-conservative potential temperature, θm, which is analogous to potential temperature for a dry (subsaturated) atmosphere. Here, θm is estimated from the Brunt–V is l frequency derived by Lalas and Einaudi (1974) in a saturated atmosphere. The application to balloon data shows that the effective turbulent fraction of the troposphere can dramatically increase when saturation is taken into account. Preliminary results of comparisons with data simultaneously collected from the VHF Middle and Upper atmosphere radar (MUR, Japan) seem to give credence to the proposed approach.
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