Polarized light scattering by inhomogeneous hexagonal monocrystals. Validation with ADEOS-POLDER measurements

Various in situ measurements of the light-scattering diagram in ice clouds were performed with a new nephelometer during several airborne campaigns. These measurements were favorably compared with a theoretical scattering model called Inhomogeneous Hexagonal Monocrystal (IHM) model. This model consists in computing the scattering of light by an ensemble of randomly oriented hexagonal ice crystals containing spherical impurities of soot and air bubbles. It is achieved by using a combination of ray tracing, Mie theory, and Monte Carlo techniques and enables to retrieve the six independent elements of the scattering matrix. This good agreement between nephelometer measurements and IHM model provides an opportunity to use this model in order to analyze ADEOS-POLDER total and polarized reflectance measurements over ice clouds. POLDER uses an original concept to observe ice cloud properties, enabling to measure reflectances and polarized reflectances, for a given scene, under several (up to 14) viewing directions. A first analysis of ice cloud spherical albedoes over the terrestrial globe for November 10, 1996, and April 23, 1997, shows a rather good agreement between measurements and modeling. Moreover, polarized reflectances are also calculated and show a satisfactory agreement with measurements

Etude des propriétés optiques et radiatives des cirrus à l'aide de modèles microphysiques élaborés (analyse de mesures in situ (Néphélomètre) et satellitaire (POLDER))

LILLE1-BU (590092102) / SudocSudocFranceF

Description des propriétés macrophysiques et microphysiques des nuages par télédétection active et passive (application à la campagne aéroportée FRENCH/DIRAC)

Les nuages sont l'une des principales sources d'incertitudes de la prévision du climat futur de la Terre. L'étude présentée vise à développer des méthodes affinant la description tant macrophysique que microphysique des nuages à partir de mesures aéroportées. Ces méthodes peuvent être à terme appliquées à des mesures spatiales du type de celles du train de l'espace, l'A -train. A cet effet, les mesures de deux radiomètres (POLDER et MiniMIR) et d'un lidar (LEANDRE) acquises pendant la campagne aéroportée FRENCH/DIRAC sont exploitées. Une méthode d'absorption différentielle est développée et utilisée pour corriger les mesures des deux radiomètres POLDER et MiniMIR de l'absorption par les gaz atmosphériques. POLDER, avec son large champ de vue, effectue une mesure radiométrique multidirectionnelle. La luminance multidirectionnelle d'une scène nuageuse est obtenue en la suivant sur plusieurs acquisitions successives de l'instrument. POLDER et LEANDRE permettent une évaluation de l'altitude des nuages. Le lidar fournit l'information la plus précise et peut permettre de discriminer les sommets et les bases de plusieurs couches nuageuses.Par des méthodes stéréoscopiques ou basées sur l'absorption par le dioxygène, POLDER fournit une information sur l'altitude des nuages. Celle-ci est moins précise mais présente l'avantage de rendre compte du large champ spatial couvert par l'instrument. Les mesures polarisées de POLDER ou de LEANDRE permettent une estimation de la phase thermodynamique des nuages, mais par commodité, une méthode simple basée sur le rapport des luminances mesurées dans le visible et le moyen infrarouge par MiniMIR est développée et privilégiée dans cette étude. Finalement, une méthode d'estimation optimale est développée et mise en œuvre pour déterminer l'épaisseur optique des nuages et la dimension des particules qui les constituent à partir de mesures passives dans le visible et dans le moyen infrarouge. Comparée aux méthodes précédemment développées au laboratoire, cette méthode permet d'obtenir des précisions similaires sur les paramètres nuageux déterminés. Toutefois, elle offre l'avantage d'exploiter naturellement les mesures multidirectionnelles de POLDER ainsi que d'autres informations ou données auxiliaires. De part sa modularité, cette méthode pourrait permettre d'exploiter en synergie les mesures de l'A-train.LILLE1-BU (590092102) / SudocSudocFranceF

Modélisation des propriétés optiques et radiatives des cirrus dans l'infrarouge (validation à l'aide des mesures effectuées lors de l'expéience FRENCH/DIRAC 2001)

LILLE1-BU (590092102) / SudocSudocFranceF

Interband calibration over clouds for POLDER space sensor

International audienceThe Polarization and Directionality of the Earth's Reflectance (POLDER) spatial polarimeter was onboard the Advanced Earth Observation Satellite (ADEOS) satellite that flew from August 1996 to June 30, 1997. POLDER measured both multidirectional reflectance and polarization in visible and near-infrared spectral bands with a very wide field of view. An accurate absolute radiometric calibration is essential for the scientific exploitation of radiance measurements of the Earth. POLDER inflight radiometric calibration has been performed at the Centre National d'Etudes Spatiales (CNES), French National Space Studies Center, from measurements taken only on well-characterized targets. This paper presents the results of the POLDER in-flight radiometric interband calibration over clouds for channels 443 and 490 nm. The method is based on the comparison of measurements to simulations. Selected measurements correspond to observations over oceans for high, thick convective cumulonimbus and for low, thick stratocumulus. Simulations are calculated using the discrete ordinate computing method. An error budget considers the sensitivity of this calibration method to cloud microphysics, to cloud top altitude, and to aerosols and gaseous loading. Calibration results are discussed for different simulated cloud model

Validation of CALIPSO Infrared Imaging Radiometer (IIR) during the Cirrus Cloud Experiment (CIRCLE-2) campaign

International audienceCirrus clouds are now considered of having a major, but still badly determined influence on global climate and radiative phenomena in the atmosphere. Numerous studies have been performed during the last decades in order to enhance our knowledge of their global impact, through for instance a better understanding of their microphysical structure. However due to the high altitude and relative low optical thickness of these clouds, precise observations are still seldom, especially using aircrafts. Consequently, the use of datas from spatial missions dedicated to climate studies such as CALIPSO (part of NASA's A-Train) occurs to be of primal importance. During summer 2006, the CIRCLE-2 campaign was performed with the intention of studying cirrus clouds microphysical properties (optical thickness, ice crystal shape and effective diameter), but also took place as the first CALIPSO validation campaign. In this study we used radiative measurements made by the two aircrafts in order to compare them with CALIPSO datas. Aircrafts were equipped with a lidar, an infrared radiometer, and in situ and thermodynamical measurements instruments. The lidar and radiometer specifications were highly similar to the ones onboard CALIPSO. The comparisons are made in terms of brightness temperature, or of brightness temperature difference using the Split-Window method. Afterwards analogies are obtained using simulations by the FASDOM fast radiative transfer code. Look-up Table methods using linear interpolations were first used to retrieve microphysical properties out of aircrafts and satellite measurements, with the help of several ice crystals models. Good correlations were found between aircrafts and CALIPSO measurements. Furthermore by the use of inversion technics on these datas, we have retrieved optical thickness and crystal effective diameter close from in situ observations. First studies were made within the thermal infrared spectrum, according to the range of the CALIPSO radiometer. Nevertheless, similar studies within visible spectrum are being attended, through the use of POLDER or CloudSat measurements to retrieve similar results, first of all along the CIRCLE-2 campaign tracks to comfort validations

Modeling total and polarized reflectances of ice clouds: evaluation by means of POLDER and ATSR-2 measurements

International audienceFour ice-crystal models are tested by use of ice-cloud reflectances derived from Along Track Scanning Radiometer-2 (ATSR-2) and Polarization and Directionality of Earth's Reflectances (POLDER) radiance measurements. The analysis is based on dual-view ATSR-2 total reflectances of tropical cirrus and POLDER global-scale total and polarized reflectances of ice clouds at as many as 14 viewing directions. Adequate simulations of ATSR-2 total reflectances at 0.865 µm are obtained with model clouds consisting of moderately distorted imperfect hexagonal monocrystals (IMPs). The optically thickest clouds (tau > ~16) in the selected case tend to be better simulated by use of pure hexagonal monocrystals (PHMs). POLDER total reflectances at 0.670 µm are best simulated with columnar or platelike IMPs or columnar inhomogeneous hexagonal monocrystals (IHMs). Less-favorable simulations are obtained for platelike IHMs and polycrystals (POLYs). Inadequate simulations of POLDER total and polarized reflectances are obtained for model clouds consisting of PHMs. Better simulations of the POLDER polarized reflectances at 0.865 µm are obtained with IMPs, IHMs, or POLYs, although POLYs produce polarized reflectances that are systematically lower than most of the measurements. The best simulations of the polarized reflectance for the ice-crystal models assumed in this study are obtained for model clouds consisting of columnar IMPs or IHMs

Validation of CALIPSO Infrared Imaging Radiometer (IIR) during the Cirrus Cloud Experiment (CIRCLE-2) campaign

International audienceCirrus clouds are now considered of having a major, but still badly determined influence on global climate and radiative phenomena in the atmosphere. Numerous studies have been performed during the last decades in order to enhance our knowledge of their global impact, through for instance a better understanding of their microphysical structure. However due to the high altitude and relative low optical thickness of these clouds, precise observations are still seldom, especially using aircrafts. Consequently, the use of datas from spatial missions dedicated to climate studies such as CALIPSO (part of NASA's A-Train) occurs to be of primal importance. During summer 2006, the CIRCLE-2 campaign was performed with the intention of studying cirrus clouds microphysical properties (optical thickness, ice crystal shape and effective diameter), but also took place as the first CALIPSO validation campaign. In this study we used radiative measurements made by the two aircrafts in order to compare them with CALIPSO datas. Aircrafts were equipped with a lidar, an infrared radiometer, and in situ and thermodynamical measurements instruments. The lidar and radiometer specifications were highly similar to the ones onboard CALIPSO. The comparisons are made in terms of brightness temperature, or of brightness temperature difference using the Split-Window method. Afterwards analogies are obtained using simulations by the FASDOM fast radiative transfer code. Look-up Table methods using linear interpolations were first used to retrieve microphysical properties out of aircrafts and satellite measurements, with the help of several ice crystals models. Good correlations were found between aircrafts and CALIPSO measurements. Furthermore by the use of inversion technics on these datas, we have retrieved optical thickness and crystal effective diameter close from in situ observations. First studies were made within the thermal infrared spectrum, according to the range of the CALIPSO radiometer. Nevertheless, similar studies within visible spectrum are being attended, through the use of POLDER or CloudSat measurements to retrieve similar results, first of all along the CIRCLE-2 campaign tracks to comfort validations

Validation of CALIPSO Infrared Imaging Radiometer (IIR) during the Cirrus Cloud Experiment (CIRCLE-2) campaign

International audienceCirrus clouds are now considered of having a major, but still badly determined influence on global climate and radiative phenomena in the atmosphere. Numerous studies have been performed during the last decades in order to enhance our knowledge of their global impact, through for instance a better understanding of their microphysical structure. However due to the high altitude and relative low optical thickness of these clouds, precise observations are still seldom, especially using aircrafts. Consequently, the use of datas from spatial missions dedicated to climate studies such as CALIPSO (part of NASA's A-Train) occurs to be of primal importance. During summer 2006, the CIRCLE-2 campaign was performed with the intention of studying cirrus clouds microphysical properties (optical thickness, ice crystal shape and effective diameter), but also took place as the first CALIPSO validation campaign. In this study we used radiative measurements made by the two aircrafts in order to compare them with CALIPSO datas. Aircrafts were equipped with a lidar, an infrared radiometer, and in situ and thermodynamical measurements instruments. The lidar and radiometer specifications were highly similar to the ones onboard CALIPSO. The comparisons are made in terms of brightness temperature, or of brightness temperature difference using the Split-Window method. Afterwards analogies are obtained using simulations by the FASDOM fast radiative transfer code. Look-up Table methods using linear interpolations were first used to retrieve microphysical properties out of aircrafts and satellite measurements, with the help of several ice crystals models. Good correlations were found between aircrafts and CALIPSO measurements. Furthermore by the use of inversion technics on these datas, we have retrieved optical thickness and crystal effective diameter close from in situ observations. First studies were made within the thermal infrared spectrum, according to the range of the CALIPSO radiometer. Nevertheless, similar studies within visible spectrum are being attended, through the use of POLDER or CloudSat measurements to retrieve similar results, first of all along the CIRCLE-2 campaign tracks to comfort validations

A methodology for simultaneous retrieval of ice and liquid water cloud properties. Part 2: Near-global retrievals and evaluation against A-Train products

International audienc