Viewing zenith angle dependence of cloudiness determined from coincident GOES East and GOES West data

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Abstract
The dependence of observed cloudiness on the satellite or viewing zenith angle (VZA) is examined, using a combination of two cloud amount data sets derived from nearly simultaneous, collocated Geostationary Operational Environmental Satellite (GOES) West and GOES East radiances over the Pacific Ocean during May 1979 and July 1983. A third data set, GOES Prime, is used to estimate the amount of cloud variation due to changes in sensor resolution with VZA. The data are analyzed for single‐layer and total cloudiness, as derived with a hybrid bispectral threshold method. The frequencies of occurrence for low, middle, and high clouds, 96%, 47%, and 28%, respectively, are typical of oceanic cloud populations. Cloud fractions increased with increasing VZA for almost all cases. Low clouds showed the greatest increases for small cloud amounts around 0.1, while the greatest increases for high clouds were found for cloud amounts around 0.5. Midlevel clouds showed only a slight dependence on VZA. Total cloudiness increased the most, reflecting its predominantly low‐cloud composition. Uncertainties in retrieved cloud fraction also increase with the VZA. Resolution changes with VZA were estimated to cause up to 28% of the increase in cloudiness for certain low‐cloud amounts. Very little of the high‐cloud increases were the result of resolution changes. Simple geometrical formulas were used to model the results. A single‐layer cumulus model was found to be the best fit to the layer cloud data. A formulation combining the single‐layer models was used to describe the variation of total cloud cover with VZA. The regression fit of these data to the multilayer model reduced the mean bias errors due to VZA effects from ≈0.058 to ≈0.004. It is concluded that the effects of the VZA should be incorporated in the construction of satellite‐derived climatologies and other Earth‐observing applications.