Tuesday, 4 June 2002: 9:30 AM
Effect of a spherical Atmosphere in the Irradiance Estimate from Radiances Measured by Satellite-based Instruments
Since the Earth is spherical, the view zenith angle of satellite-based instruments depends on the reference altitude used to determine the angle. When the view zenith angle referenced at the surface is used to compute the irradiance, both shortwave and longwave broadband irradiances are under estimated more than 1 W m-2 because radiances at the view zenith angle greater than the tangent angle to the surface of the Earth are not included. The reference altitude needs to be large enough so that radiances near the view zenith angle of 90 degree at the reference altitude have negligible contribution to the irradiance. In order to compute the scene dependent irradiance from radiances measured by the Clouds and the Earth's Radiant Energy System (CERES) instrument including radiances at the view zenith angle greater than the tangent angle to the surface, MODTRAN is used to estimate the radiance at these large view zenith angles because of the difficulty to identify the scene type over a CERES footprint at these angles. Even though MODTRAN-derived radiances have known errors at these large view zenith angles, the error in the resulting irradiance is approximately less than 0.5 W m-2 for all wavelength regions, shortwave, longwave and window. The estimate of errors is based on a comparison between MODTRAN-derived radiances computed for clear-sky, thin cirrus and thick clouds and radiances measured by the CERES instrument averaged over all scene types.