Partly cloudy pixel retrievals of cloud properties and MOD06 cloud products for marine stratocumulus

Retrievals that account for the partial filling of pixels by cloud have been applied to 1-km MODIS observations of low-level, single-layered, water clouds over oceans—marine stratocumulus. As expected from theory, assuming that the pixel is overcast when it is only partially cloud covered, as is done to derive the MODIS MOD06 cloud products, leads to cloud optical depths that are too small and droplet effective radii that are too large. Even though the fraction of 1-km pixels that are partially cloud covered in marine stratocumulus systems is relatively small, ~30%, the average MOD06 cloud optical depth derived for 50-km scale regions that contain no other cloud than the marine layer is 1 to 2 units smaller than that obtained from a retrieval scheme that accounts for the partial coverage. Likewise, the droplet effective radius in the MOD06 product is 1 to 2 microns larger. Interestingly, these differences in optical depth and droplet radius tend to cancel in estimates of the cloud liquid water column amounts. While liquid water amount is underestimated in the MOD06 product, the difference with the value derived using the partly cloudy pixel retrieval is relatively small compared with the differences obtained for the optical depth and droplet effective radius. Case studies are presented in which the retrievals of regional scale fractional cloud cover and visible optical depth using MODIS 250-m resolution data are compared with those derived using the partly cloudy pixel retrieval scheme applied to 1-km pixels.