5.2.3 Computation of Shortwave and Near-Infrared Albedo

The radiance measured by NISTAR can be represented as (1)

where M is the flux leaving the entire Triana-viewed hemisphere in all directions and ADM is the effective anisotropic directional model for the wavelength interval . The value of the ADM depends on the scenes comprising the hemisphere and their angular positions relative to the satellite and Sun as given by the solar zenith o, viewing zenith , and relative azimuth angles. Estimation of the flux from the Triana radiance requires a value of ADM for each measurement because the scene changes as the Earth rotates under the satellite. For simplicity, the shortwave Triana ADM is designated XT, while the longwave ADM is .

The shortwave ADM can be estimated from the EPIC data at time t as , (2)

where Ri is the weighted average reflectance for the 443, 645, 870, and 905 nm channels for EPIC pixel i, A is the pixel area, µo = coso, N is the total number of pixels used, and X is the normalized bi-directional reflectance factor for scene type K that is determined through the cloud screening process and the geography corresponding to the location of pixel i. The values of X will be taken from lookup tables like those used by CERES (e.g., Suttles et al., 1988). Figure 27 shows a Galileo flyby image of Earth, showing sun-glint region. Figure 28 depicts the pattern of X for clear ocean from Minnis and Harrison (1984) simulated for three of the Triana positions viewing the Pacific Ocean. Note, the bright area near the middle of the globe corresponds to the regions most likely to be affected by sun-glint. The reflectance for clear ocean increases toward the limb because of enhanced Rayleigh and aerosol scattering.

Figure 27 Galileo flyby image of Earth, showing sun-glint region.

The albedo for the hemisphere is, (3)

where S is the insolation over the viewed sunlit portion of the Earth. The value of X NIRT can be derived using the average reflectances from the 870 and 905 nm channels to compute Ri from (2). The near-infrared albedo can be estimated from (3) using the measured near-infrared radiance LNIR(t) and XNIRT. Unless there are significant differences in the weightings of Ri for the near infrared and the total shortwave in (2) or if near infrared bi-directional reflectance models become available, then the ratios for the radiances and fluxes will be almost identical. With proper temporal averaging, these albedo values can be compared directly to GCM-derived albedos to verify the climate model calculations.

Figure 28 Variation of normalized bi-directional reflectance factor over clear ocean for simulated Triana views of the Pacific Ocean. Sun-glint area corresponds to the bright region near the center of each image.

The longwave ADM could also be estimated as , (4)

where LIR is the narrowband infrared radiance for some imager pixel i and is the limb- darkening function from some established lookup table (e.g., Suttles et al., 1989). Although the EPIC does not have infrared channels, it may be possible at some point in the Triana program to use infrared radiances from the matched pixels from other satellites as discussed earlier or from CERES as discussed below. In that manner, it would be possible to provide a reasonable estimate of the OLR for the hemisphere viewed by Triana.