ACCURACY OF THE INDEPENDENT PIXEL APPROXIMATION FOR SATELLITE ESTIMATES OF OCEANIC BOUNDARY-LAYER CLOUD OPTICAL DEPTH

A theoretical study has been conducted on the effects of cloud horizon tal inhomogeneity on the retrieval of optical depth by remote sensing of visible reflectance. Forty-five Landsat scenes of oceanic boundary layer clouds provide a sampling of real cloud fields, including trade cumulus, open and closed cell broken stratocumulus, and solid stratocu mulus. The spherical harmonic discrete ordinate method (SHDOM) radiati ve transfer model is used to calculate two-dimensional reflectances fr om subsampled cloud strips representing the Landsat scenes. The indepe ndent pixel approximation (IPA) is used to retrieve optical depth for comparison to the original input. Results for tau(IPA) versus tau(ref) are presented on scales from the Landsat pixel scale (28.5 m) to an i mager pixel scale (6 km) to near mesoscale (60 km). The random error d ecreases as the averaging scale increases, but error due to inhomogene ity remains. At the 60 km scale the average error is about 6% for high Sun, 2% for low Sun. Individual scenes, however, have retrieved optic al depth errors as high as 45% due to horizontal radiative transport. The ability to retrieve higher statistical moments of the frequency di stribution of optical depth is also assessed. Sigma, (sigma), the stan dard deviation of tau, is retrieved quite well up to a point, then is underestimated due to the smoothing effect of horizontal radiative tra nsport. The gamma function parameter nu, another measure of the width of the tau frequency distribution, is retrieved quite well over a wide range but with a systematic bias which varies with solar zenith angle , again due to horizontal radiative transport. A method is sought to r educe the optical depth retrieval error using a simple correction base d on remotely sensed cloud properties. Of those considered, cloud phys ical aspect ratio (computed here from one possible relation which depe nds on properties obtainable from remote sensing) is found to be the m ost effective correction parameter. The aspect ratio correction reduce s the retrieved optical depth bias error by 50 to 100% and the RMS err or by 20 to 50%. Correction coefficients are presented at three solar zenith angles. This work is limited by its consideration of only singl e-level marine boundary layer clouds, assumptions of conservative scat tering, constant cloud droplet size, no gas absorption or surface refl ectance, and restriction to two-dimensional radiative transport. Futur e work will attempt to remove some of these limitations. The Landsat d ata used are also limited due to radiative smoothing.