Abstract
PARASOL is a satellite which provided multidirectional and polarized observation of the Earth reflectances for the visible and near infrared spectral range for nearly 9-years, from 2005 to 2013. A rich archive of top-of-atmosphere reflectances have been collected over deep convective clouds (DCC) according a devoted data selection. This selection is able to guarantee observation very suitable for calibration purpose as well as a consistent time series (regarding morphology, geometry, radiometry). These selection criteria are described in Fougnie and Bach (IEEE, 2009).
DCC are used for a long while for calibration purposes. First, they were used for interband calibration in the visible range (e.g. for POLDER and Végétation sensors). DCC were very powerful for an accurate monitoring or validation of the sensor degradation with time (e.g. PARASOL, MERIS). DCC were also used to derive the sensor degradation within the entire field-of-view (e.g. PARASOL). Finally, DCC are intensively used for cross-calibration of LEO and GEO sensors in the GSICS framework.
It is known that DCC are very white targets (or with minor variations with respect to the white behaviour) on the visible and near infrared spectral domain. They are also characterized by a moderate bidirectional signature.
PARASOL data collected over the entire archive were used to derive the bidirectional function (BRDF) of the DCC reflectance. For every spectral band, all acquisitions (more than 1 million) were assumed to be generated by the same “mean” cloud. Five ranges of 10° solar zenith angles were built, and on which acquisitions were geometrically binned into 2°x2° boxes in viewing zenith and relative azimuth angles. Results for 490, 670 and 865 will be presented. BRDF effects are found to be very similar for the visible range, about 15%, and are a little bite smaller for the near infrared band, about 10%. It appears that the most isotropic part of the BRDF corresponds to the backscattering geometries.
Computation using radiative transfer code based on discrete ordinate (Lafrance et al., IEEE, 2002) were built. The top of DCC were described as pure hexagonal crystals (PHM), but also Inhomogeneous Hexagonal Monocrystals (RHM). Top of atmosphere reflectances were generated for all geometries and for visible to near infrared bands. Comparison with PARASOL observation, and highlighting a good agreement, will be presented.
Finally, results will be confronted to the directional model from Hu et al. (2004) used as reference in the framework of GSICS for intercalibration of sensors.
The general conclusion will try to recommend geometries for which the cross-calibration will be optimized in term of uncertainties.
Bidirectional Reflectance Distribution Function (BRDF) of Deep Convective Clouds (DCC) Derived from PARASOL Measurements and Compared to Radiative Transfer Computation and Model
PARASOL is a satellite which provided multidirectional and polarized observation of the Earth reflectances for the visible and near infrared spectral range for nearly 9-years, from 2005 to 2013. A rich archive of top-of-atmosphere reflectances have been collected over deep convective clouds (DCC) according a devoted data selection. This selection is able to guarantee observation very suitable for calibration purpose as well as a consistent time series (regarding morphology, geometry, radiometry). These selection criteria are described in Fougnie and Bach (IEEE, 2009).
DCC are used for a long while for calibration purposes. First, they were used for interband calibration in the visible range (e.g. for POLDER and Végétation sensors). DCC were very powerful for an accurate monitoring or validation of the sensor degradation with time (e.g. PARASOL, MERIS). DCC were also used to derive the sensor degradation within the entire field-of-view (e.g. PARASOL). Finally, DCC are intensively used for cross-calibration of LEO and GEO sensors in the GSICS framework.
It is known that DCC are very white targets (or with minor variations with respect to the white behaviour) on the visible and near infrared spectral domain. They are also characterized by a moderate bidirectional signature.
PARASOL data collected over the entire archive were used to derive the bidirectional function (BRDF) of the DCC reflectance. For every spectral band, all acquisitions (more than 1 million) were assumed to be generated by the same “mean” cloud. Five ranges of 10° solar zenith angles were built, and on which acquisitions were geometrically binned into 2°x2° boxes in viewing zenith and relative azimuth angles. Results for 490, 670 and 865 will be presented. BRDF effects are found to be very similar for the visible range, about 15%, and are a little bite smaller for the near infrared band, about 10%. It appears that the most isotropic part of the BRDF corresponds to the backscattering geometries.
Computation using radiative transfer code based on discrete ordinate (Lafrance et al., IEEE, 2002) were built. The top of DCC were described as pure hexagonal crystals (PHM), but also Inhomogeneous Hexagonal Monocrystals (RHM). Top of atmosphere reflectances were generated for all geometries and for visible to near infrared bands. Comparison with PARASOL observation, and highlighting a good agreement, will be presented.
Finally, results will be confronted to the directional model from Hu et al. (2004) used as reference in the framework of GSICS for intercalibration of sensors.
The general conclusion will try to recommend geometries for which the cross-calibration will be optimized in term of uncertainties.