Frederic Parol

Information Content of AVHRR Channels 4 and 5 with Respect to the Effective Radius of Cirrus Cloud Particles

Abstract This paper investigates the important difference in the relationship between brightness temperatures between the 11-μm and the 12-μn AVHRR data and the microphysical properties of the semitransparent cirrus clouds. In the nonscattering approximation, the emittance for channels 4 and 5 are related through the absorption coefficient ratio that is the key parameter giving access to the size of cloud particles. The observed mean value of this parameter corresponds to effective radius of 18 μm for polydisperse spheres and 12 μm for polydisperse infinitely long ice cylinders. Taking the multiple scattering into account, the brightness temperature difference enhances much more for cylinders than for spheres owing to the fact that the forward peak of scattering is less large for cylinders. To obtain the size of cloud particles, the method developed in the nonscattering case is still applicable if one makes use of the effective emittance that implicitly includes the effects of mattering. Thus, an effectiv...

Results of POLDER in-flight calibration

POLDER is a CNES instrument on board NASDAs ADEOS polar orbiting satellite, which was successfully launched in August 1996. On October 30, 1996, POLDER entered its nominal acquisition phase and worked perfectly until ADEOSs early end of service on June 30, 1997. POLDER is a multispectral imaging radiometer/polarimeter designed to collect global and repetitive observations of the solar radiation reflected by the Earth/atmosphere system, with a wide field of view (2400 km) and a moderate geometric resolution (6 km). The instrument concept is based on telecentric optics, on a rotating wheel carrying 15 spectral filters and polarizers, and on a bidimensional charge coupled device (CCD) detector array. In addition to the classical measurement and mapping characteristics of a narrow-band imaging radiometer, POLDER has a unique ability to measure polarized reflectances using three polarizers (for three of its eight spectral bands, 443 to 910 nm) and to observe target reflectances from 13 different viewing directions during a single satellite pass. One of POLDERs original features is that its in-flight radiometric calibration does not rely on any on-board device. Many calibration methods using well-characterized calibration targets have been developed to achieve a very high calibration accuracy. This paper presents the various methods implemented in the in-flight calibration plan and the results obtained during the instrument calibration phase: absolute calibration over molecular scattering, interband calibration over sunglint and clouds, multiangular calibration over deserts and clouds, intercalibration with Ocean Color and Temperature Scanner (OCTS), and water vapor channels calibration over sunglint using meteorological analysis. A brief description of the algorithm and of the performances of each method is given.

Apparent pressure derived from ADEOS-POLDER observations in the oxygen A-band over ocean

The POLDER radiometer was on board the ADEOS satellite from August 1996 to June 1997. This instrument measures radiances in eight narrow spectral bands of the visible and near infrared spectrum. Two of them are centered on the O2 A-band in order to infer cloud pressure. By assuming the atmosphere behaves as a pure absorbing medium overlying a perfect reflector, an ”apparent” pressure Papp is derived from POLDER data. For validation purposes, Papp is first compared to the sea-surface pressure Ps for clear-sky conditions; Papp is found to be close to Ps (within ∼30 hPa) for measurements in the sunglint region. For overcast conditions, Papp differs from the cloud-top pressure mainly because of multiple scattering inside the cloud. When Papp is compared to the cloud pressure determined from brightness temperature measurements, large differences are observed (typically 180 hPa).

Large-scale analysis of cirrus clouds from AVHRR data : Assessment of both a microphysical index and the cloud-top temperature

Abstract An algorithm that allows an automatic analysis of cirrus properties from Advanced Very High Resolution Radiometer (AVHRR) observations is presented. Further investigations of the information content and physical meaning of the brightness temperature differences (BTD) between channels 4 (11 μm) and 5 (12 μm) of the radiometer have led to the development of an automatic procedure to provide global estimates both of the cirrus cloud temperature and of the ratio of the equivalent absorption coefficients in the two channels, accounting for scattering effects. The ratio is useful since its variations are related to differences in microphysical properties. Assuming that cirrus clouds are composed of ice spheres, the effective diameter of the particle size distribution can be deduced from this microphysical index. The automatic procedure includes first, a cloud classification and a selection of the pixels corresponding to the envelope of the BTD diagram observed at a scale of typically 100 × 100 pixels. ...

Review of capabilities of multi-angle and polarization cloud measurements from POLDER

Abstract Polarization and directionality of the Earth’s reflectances (POLDER) is a multispectral imaging radiometer–polarimeter with a wide field-of-view, a moderate spatial resolution, and a multi-angle viewing capability. It functioned nominally aboard ADEOS1 from November 1996 to June 1997. When the satellite passes over a target, POLDER allows to observe it under up to 14 different viewing directions and in several narrow spectral bands of the visible and near-infrared spectrum (443–910 nm). This new type of multi-angle instruments offers new opportunity for deriving cloud parameters at global scale. The aim of this short overview paper is to point out the main contributions of such an instrument for cloud study through its original instrumental capabilities (multidirectionality, multipolarization, and multispectrality). This is mainly illustrated by using ADEOS 1-POLDER derived cloud parameters which are operationally processed by CNES and are available since the beginning of 1999.

Top-of-Atmosphere Albedo Estimation from Angular Distribution Models Using Scene Identification from Satellite Cloud Property Retrievals

Abstract The next generation of earth radiation budget satellite instruments will routinely merge estimates of global top-of-atmosphere radiative fluxes with cloud properties. This information will offer many new opportunities for validating radiative transfer models and cloud parameterizations in climate models. In this study, five months of Polarization and Directionality of the Earth’s Reflectances 670-nm radiance measurements are considered in order to examine how satellite cloud property retrievals can be used to define empirical angular distribution models (ADMs) for estimating top-of-atmosphere albedo. ADMs are defined for 19 scene types defined by satellite retrievals of cloud fraction and cloud optical depth. Two approaches are used to define the ADM scene types. The first assumes there are no biases in the retrieved cloud properties and defines ADMs for fixed discrete intervals of cloud fraction and cloud optical depth (fixed-τ approach). The second approach involves the same cloud fraction inte...

Angular Variability of the Liquid Water Cloud Optical Thickness Retrieved from ADEOS–POLDER

Abstract The usual procedure for retrieving the optical thickness of liquid water clouds from satellite-measured radiances is based on the assumption of plane-parallel layers composed of liquid water droplets. This study investigates the validity of this assumption from Advanced Earth Orbiting Satellite–Polarization and Directionality of the Earths Reflectances (ADEOS–POLDER) observations. To do that, the authors take advantage of the multidirectional viewing capability of the POLDER instrument, which functioned nominally aboard ADEOS from November 1996 to June 1997. The usual plane-parallel cloud model composed of water droplets with an effective radius of 10 μm provides a reasonable approximation of the angular dependence in scattering at visible wavelengths from overcast liquid water clouds for moderate solar zenith angles. However, significant differences between model and observations appear in the rainbow direction and for the smallest observable values of scattering angle (Θ < 90°). A better overa...

Toward New Inferences about Cloud Structures from Multidirectional Measurements in the Oxygen A Band: Middle-of-Cloud Pressure and Cloud Geometrical Thickness from POLDER-3/PARASOL

Abstract New evidence from collocated measurements, with support from theory and numerical simulations, that multidirectional measurements in the oxygen A band from the third Polarization and Directionality of the Earth’s Reflectances (POLDER-3) instrument on the Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) satellite platform within the “A-Train” can help to characterize the vertical structure of clouds is presented. In the case of monolayered clouds, the standard POLDER cloud oxygen pressure product PO2 is shown to be sensitive to the cloud geometrical thickness H in two complementary ways: 1) PO2 is, on average, close to the pressure at the geometrical middle of the cloud layer (MCP) and methods are proposed for reducing the pressure difference PO2 − MCP and 2) the angular standard deviation of PO2 and the cloud geometrical thickness H are tightly correlated for liquid clouds. Accounting for cloud phase, there is thus the potential...

Examination of POLDER/PARASOL and MODIS/Aqua Cloud Fractions and Properties Representativeness

AbstractThe Polarization and Anisotropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar (PARASOL) and Aqua are two satellites on sun-synchronous orbits in the A-Train constellation. Aboard these two platforms, the Polarization and Directionality of Earth Reflectances (POLDER) and Moderate Resolution Imaging Spectroradiometer (MODIS) provide quasi simultaneous and coincident observations of cloud properties. The similar orbits but different detecting characteristics of these two sensors call for a comparison between the derived datasets to identify and quantify potential uncertainties in retrieved cloud properties.To focus on the differences due to different sensor spatial resolution and coverage, while minimizing sampling and weighting issues, the authors have recomputed monthly statistics directly from the respective official level-2 products. The authors have developed a joint dataset that contains both POLDER and MODIS level-2 cloud products collocated on a common sinuso...

Validation of IIR/CALIPSO Level 1 Measurements by Comparison with Collocated Airborne Observations during CIRCLE-2 and Biscay ‘08 Campaigns

AbstractIn the frame of validation of the spatial observations from the radiometer IIR on board CALIPSO, the two airborne campaigns Cirrus Cloud Experiment (CIRCLE)-2 and Biscay ‘08 took place in 2007 and 2008 in the western part of France, over the Atlantic Ocean. During these experiments, remote sensing measurements were made over cirrus clouds, right under the track of Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) in space and time collocation. For this purpose, a Falcon-20 aircraft was equipped with the Lidar pour l’Etude des Interactions Aerosols Nuages Dynamique Rayonnement et du Cycle de l’Eau (LEANDRE)-New Generation (NG) and the thermal infrared radiometer Conveyable Low-Noise Infrared Radiometer for Measurements of Atmosphere and Ground Surface Targets (CLIMAT)-Airborne Version (AV), whose spectral characteristics are strongly similar to those of the infrared imaging radiometer (IIR). In situ measurements were also taken in cirrus clouds during CIRCLE-2. After comp...