Search Results

You are looking at 1 - 4 of 4 items for

  • Author or Editor: Michel Capderou x
  • Refine by Access: All Content x
Clear All Modify Search
Michel Capderou

Abstract

Determination of directional albedo and bidirectional anisotropic function are necessary to the radiance-to-flux conversion. The most recent model, used for the Earth Radiation Budget Experiment data, is based on data from the sun-synchronous Nimbus-7 mission that is observed close to local noon. Because the desert zones are restricted in latitude (20°–35° in each hemisphere), only a small range of solar zenith angles was sampled. Here, the author considers for clear-sky desert regions the improvements that can be made using non-sun-synchronous satellite data, precessing in local time.

This study made from ScaRaB data (Scanner for Radiation Budget, on board the Meteor-3-07 satellite) concerns the clear-sky deserts. The four great deserts on the earth (the Sahara, Arabian Desert, Namib-Kalahari, and Australian Desert) have been considered with 1.25° latitude × 1.25° longitude study areas. A normalization method, explained here, makes it possible to combine the values of bidirectionel anisotropic reflectances of all of these areas. Only one characteristic function is obtained for each desert. It is shown how the characteristic function of the Sahara may be extended to the bright deserts (the Sahara and the Arabian Desert) and how the characteristic function of the Australian Desert may be extended to the dark deserts (the Australian Desert and the Namib-Kalahari). This study concludes with a comparison of the albedo and anisotropic functions obtained from different models.

Full access
Michel Capderou and Michel Viollier

Abstract

Multiangle approaches for radiance-to-flux conversion require accurate coregistration between the observations from nadir- and oblique-viewing directions. The along-track mode of Earth Radiation Budget (ERB) scanning instruments, such as the Clouds and the Earth’s Radiant Energy System (CERES), provides some multiangular observations with almost the same target observed from nadir, aft, and fore directions. To improve the overlaps of multiangle observations, this study explains how to introduce a yaw steering angle in the along-track scan mode so as to reduce the residual collocations errors. The implementation of this correction to the CERES/Terra along-track mode shows that the distances between the nadir and the oblique (55°) observations are reduced from about 40 to 2 km. Both oblique radiances are shown to be equal with small rms differences: 3.9% (all scenes) and 1.8% (homogeneous scenes), compared, respectively, to 7.0% and 3.5% before the scan adjustment.

Full access
Michel Capderou and Robert Kandel

Abstract

Determination of planetary albedo on the basis of satellite observations of reflected shortwave (SW) radiances requires taking into account the anisotropic (non-Lambertian) reflectance properties of the earth–atmosphere system, depending both on the cloud cover and the nature of the underlying, surface. One approach frequently used has been to represent these properties by a limited set of normalized bidirectional reflectance functions (BDRF) for different scene types. The construction of the normalized BDRFs used to process the Earth Radiation Budget Experiment (ERBE) measurements was based mostly on data from the sun-synchronous Nimbus-7 mission, observing close to local noon. Consequently, because desert zones are fairly restricted in latitude, only a small range of solar zenith angles was sampled. Here the authors consider, for clear-sky desert areas, the improvements that can be made using data from the geostationary satellite Meteosat, which samples all solar zenith angles that occur.

The authors define BDRF ratios (between two instants on the same day for the same area) that depend on viewing geometry (five angles for a geostationary satellite) and that together with infrared window radiance measurements allow to distinguish clear and cloudy desert scenes. Using three to five Meteosat images per day over the year 1985, and considering 42 areas in desert zones, the authors compute roughly 12 500 clear-sky BDRF ratios (representing 4.5 million B2 pixels), and sort these into bins in five-dimensional angular space. Values of the BDRF ratio are well defined and stable in each of these bins. Application of the Helmholtz reciprocity principle yields data for angular bins not directly observed. After spectral corrections and normalizations, the authors obtain a completely defined SW angular model (i.e., normalized anisotropic function and directional albedo) for clear-sky desert scenes. This model is quite different from that used in the ERBE analyses. The authors discuss the improvements obtained.

Full access
Michel Capderou and Robert Kandel

Abstract

In order to use the diurnal variation of Meteosat visible channel data to help determine the bidirectional reflectance function for desert areas, criteria must be established for identifying cloud-free scenes. The authors consider Meteosat observations, obtained in different seasons over four years, of desert and arid land areas of Africa and the Arabian peninsula. Normalizing apparent albedo to new-midday values, the authors show that clear and cloudy scenes can be distinguished using relative spatial heterogeneity parameters of this normalized albedo on the one hand, and of the midday infrared window radiance on the other. Further, it is shown that cloud cover estimates based on these parameter are in good agreement with the C1 results of the International Satellite Cloud Climatology Project. Difficulties related to Saharan dust clouds or to homogeneous overcast cases can be resolved by comparing midday IR window radiance with the maximum value over a 10 day period.

Full access