INFERENCE OF MARINE STRATUS CLOUD OPTICAL DEPTHS FROM SATELLITE MEASUREMENTS - DOES 1D THEORY APPLY

The validity of plane-parallel (1D) radiative transfer theory for clou dy atmospheres is examined by directly comparing calculated and observ ed visible reflectances for one month of Global Area Coverage Advanced Very High Resolution Radiometer satellite observations of marine stra tus cloud layers off the coasts of California, Peru, and Angola. Marin e stratus are an excellent testbed, as they arguably are the closest t o plane-parallel found in nature. Optical depths in a 1D radiative tra nsfer model are adjusted so that 1D model reflectances match those obs erved at nadir on a pixel-by-pixel basis. The 1D cloud optical depth d istributions are then used in the plane-parallel model to generate ref lectance distributions for different sun-earth-satellite viewing geome tries. These reflectance distributions are directly compared with the observations. Separate analyses are performed for overcast and broken cloud layers as identified by the spatial coherence method. When 1D re flectances are directly compared with observations at different view a ngles, relative differences are generally small (less than or similar to 10%) in the backscattering direction for solar zenith angles less t han or similar to 60 degrees and show no systematic view angle depende nce. In contrast, 1D reflectances increase much more rapidly with view angle than the observed reflectances in the forward-scattering direct ion. Relative differences in the forward-scattering direction are appr oximate to 2-3 times larger than in the backscattering direction. At s olar zenith angles less than or similar to 60 degrees, the 1D model un derestimates observed reflectances at nadir by 20%-30% and overestimat es reflectances at the most oblique view angles in the forward scatter ing direction by 15%-20%. Consequently, when inferred on a pixel-by-pi xel basis, nadir-derived cloud optical depths show a systematic increa se with solar zenith angle, both for overcast and broken cloud layers, and cloud optical depths decrease with view angle in the forward scat tering direction. Interestingly, in the case of broken marine stratocu mulus, the common practice of assuming that pixels are overcast when t hey are not mitigates this bias to some extent, thereby confounding it s detection. But even for broken clouds, the bias remains. Because of the nonlinear dependence of cloud albedo on cloud optical depth, error s in cloud optical depth lead to large errors in cloud albedo-and ther efore energy budget calculations-regardless of whether cloud layers ar e overcast or broken. These findings suggest that as a minimum require ment, direct application of the plane-parallel model approximation sho uld be restricted to moderate-high sun elevations and to view angles i n the backscattering direction. Based on Monte Carlo simulations, the likely reason for the discrepancies between observed radiances and rad iances calculated on the basis of 1D theory is because real clouds hav e inhomogeneous (i.e., bumpy) tops.