How small is a small cloud? I. Koren1, L. Oreopoulos2,3, G. Feingold4, L. A. Remer3, and O. Altaratz1 1Department of Environmental Sciences Weizmann Institute, Rehovot 76100, Israel 2Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, Baltimore, Maryland, USA 3Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA 4NOAA Earth System Research Laboratory, Boulder, Colorado, USA
Abstract. The interplay between clouds and aerosols and their contribution to the
radiation budget is one of the largest uncertainties of climate change. Most
work to date has separated cloudy and cloud-free areas in order to evaluate
the individual radiative forcing of aerosols, clouds, and aerosol effects on
Here we examine the size distribution and the optical properties of small,
sparse cumulus clouds and the associated optical properties of what is
considered a cloud-free atmosphere within the cloud field. We show that any
separation between clouds and cloud free atmosphere will incur errors in the
calculated radiative forcing.
The nature of small cumulus cloud size distributions suggests that at any
resolution, a significant fraction of the clouds are missed, and their
optical properties are relegated to the apparent cloud-free optical
properties. At the same time, the cloudy portion incorporates significant
contribution from non-cloudy pixels.
We show that the largest contribution to the total cloud reflectance comes
from the smallest clouds and that the spatial resolution changes the
apparent energy flux of a broken cloudy scene. When changing the resolution
from 30 m to 1 km (Landsat to MODIS) the average "cloud-free" reflectance at
1.65 μm increases from 0.0095 to 0.0115 (>20%), the cloud
reflectance decreases from 0.13 to 0.066 (~50%), and the cloud
coverage doubles, resulting in an important impact on climate forcing
estimations. The apparent aerosol forcing is on the order of 0.5 to 1 Wm−2 per cloud field.
Citation: Koren, I., Oreopoulos, L., Feingold, G., Remer, L. A., and Altaratz, O.: How small is a small cloud?, Atmos. Chem. Phys., 8, 3855-3864, doi:10.5194/acp-8-3855-2008, 2008.