On the nature and extent of optically thin marine low clouds

Macrophysical properties of optically thin marine low clouds over the nonpolar oceans (60 degrees S-60 degrees N) are measured using 2 years of full-resolution nighttime data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). Optically thin clouds, defined as the subset of marine low clouds that do not fully attenuate the lidar signal, comprise almost half of the low clouds over the marine domain. Regionally, the fraction of low clouds that are optically thin (f(thin,cld)) exhibits a strong inverse relationship with the low-cloud cover, with maxima in the tropical trades (f(thin,cld) > 0.8) and minima in regions of persistent marine stratocumulus and in midlatitudes (f(thin,cld) < 0.3). Domain-wide, a power law fit describes the cloud length distribution, with exponent beta = 2.03 +/- 0.06 (+/- 95% confidence interval). On average, the fraction of a cloud that is optically thin decreases from similar to 1 for clouds smaller than 2 km to <0.3 for clouds larger than 30 km. This relationship is found to be independent of region, so that geographical variations in the cloud length distribution explain three quarters of the variance in f(thin,cld). Comparing collocated trade cumulus observations from CALIOP and the airborne High Spectral Resolution Lidar reveals that clouds with lengths smaller than are resolvable with CALIOP contribute approximately half of the low clouds in the region sampled. A bounded cascade model is constructed to match the observations from the trades. The model shows that the observed optically thin cloud behavior is consistent with a power law scaling of cloud optical depth and suggests that most optically thin clouds only partially fill the CALIOP footprint.