- In a first step, the algorithm deals separately with each viewing direction. Spectral albedos are derived from POLDER narrowband reflectances (previously corrected from gaseous absorption). For cloudy pixels, we make use of the same plane-parallel model as for the derivation of cloud optical thickness. For clear pixels, we use a relation between albedo and bidirectional reflectance established from simulations of light reflected by turbid atmospheres.
Ideally the retrieved values of albedo are the same whatever the viewing direction is. However, differences are observed notably because of the limits of the plane-parallel assumption (Buriez et al., 2001). The retrieved values of albedo are thus averaged using angle-weighting functions. In this way, the most reliable viewing directions are taken into account. The angle-weighting process is based on the statistical analysis of a large set of ADEOS 1 - POLDER data and should compensate the induced theoretical approximations used in the algorithm.
These mean spectral albedos A are calculated at 443 nm, 670 nm and 865 nm. However, note that only the values at 670 nm over land and 865 nm over ocean are preserved in the POLDER "ERB, WV & clouds" products. As the cloud optical thickness is derived by two different ways (retrieving pixel by pixel then averaging over 3 x 3 pixels, or averaging then retrieving), the spectral albedos are also calculated by using these two ways. However, only the results of the first method are used in the following.
- The broadband or shortwave albedo ASW is then derived as a function of the previous spectral albedos A443, A670 and A865. While the spectral albedos are estimated without gaseous absorption, the shortwave albedo derivation takes into account the atmospheric absorption; in particular, the solar water vapor absorption is estimated from the ratio of the POLDER reflectances at 865 nm and 910 nm.
- ASW which is an instantaneous albedo at the local time of observation tobs (around 10:30 am) is not equivalent to the daily average. The albedo at the other times of day is estimated from ASW(tobs) by using diurnal interpolation and extrapolation procedures (Viollier et al., 2002). The daily reflected flux Frefl (expressed in W/m²) is then deduced. The monthly average reflected flux M(Frefl) as well as the monthly average incident flux M(Finc) are provided in the POLDER "ERB, WV & clouds" level-3 products. Therefore anyone can derive a monthly albedo M(Frefl) / M(Finc).
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