The Surprisingly Large Contribution of Small Marine Clouds to Cloud Fraction and Reflectance
Oreopoulos, L., NASA
Clouds with Low Optical [Water] Depths (CLOWD)
Koren, I, L Oreopoulos, G Feingold, LA Remer, and O Altaratz. 2008. "How small is a small cloud?" Atmospheric Chemistry and Physics Journal, in press
Cloud mask for a sparse cumulus cloud field as inferred by using the same threshold at four different spatial resolutions. The upper-left panel is for the original Landsat resolution and the lower-right panel is for a MODIS-equivalent resolution. Note how the small clouds far from other clouds are "dissolved" into the background while small clouds, holes between clouds, and background pixels next to large cloud edges are all classified as clouds.
Resolution effects on the size distributions of a sparse cloud field near Tahiti. Clockwise from top left: log-log cloud size distributions for resolutions ranging from 30 m (blue) to 960 m (yellow); normalized cumulative cloud reflectance distributions; change in apparent clear sky reflectance with resolution; change in cloud fraction with resolution. Note in the top panels how the size and energy distributions shift toward larger clouds as the resolution decreases.
Clouds and aerosols are li
Our paper studies these effects for shallow cumulus cloud fields over the tropical oceans where they often cover large areas. We have at our disposal data from the enhanced thematic mapper plus (ETM+) radiometer aboard the Landsat-7 satellite, which has a resolution of 30 m. Because the ocean surface offers a good dark backdrop, at this resolution we can detect most, but still not all, of these clouds. We find some interesting results. For five fields of shallow marine cumulus clouds, sampled from different geographic locations, we find that the largest number of clouds corresponds to the smallest clouds we can detect with ETM+. Because these clouds are so numerous they also contribute the largest fraction of the total solar radiation reflected by the entire cloud field, even though each individual cloud’s reflectance is rather small. Estimates of the apparent cloud contribution to total reflected radiation that MODIS would give, are half of those at the original resolution (because of dilution of clear skies in our larger pixels that are still however reflective enough to be classified as cloudy). In contrast, the reflectance that comes from apparent cloud-free areas at MODIS resolution appears 25% larger than it actually is (because of cloud contamination in larger pixels that are classified as cloudless).
The importance of the above errors due to instrument resolution artifacts are obvious. When the larger reflectance from apparent cloud-free areas is attributed to aerosol, we reach wrong conclusions about the direct radiative forcing of aerosol in the present climate. Our estimate of the 24-hour forcing error is -0.8±0.2 W/m2 per cloud field, or about 15% to 20% of the total global aerosol effect estimated from MODIS aerosol retrievals. While this may not seem significant if only local, the fact that shallow cumulus cloud fields cover ~ 15% of the global oceans, makes it so.