Many of the clouds important to the Earth's energy balance, from the Tropics to the Arctic, contain small amounts of liquid water. Longwave and shortwave radiative fluxes are very sensitive to small perturbations of the cloud liquid water path (LWP), when the LWP is small (i.e., < 100 g m−2; clouds with LWP less than this threshold will be referred to as “thin”). Thus, the radiative properties of these thin liquid water clouds must be well understood to capture them correctly in climate models. We review the importance of these thin clouds to the Earth's energy balance, and explain the difficulties in observing them. In particular, because these clouds are thin, potentially mixed phase, and often broken (i.e., have large 3D variability), it is challenging to retrieve their microphysical properties accurately. We describe a retrieval algorithm intercomparison that was conducted to evaluate the issues involved. The intercomparison used data collected at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site and included 18 different algorithms to evaluate their retrieved LWP, optical depth, and effective radii. Surprisingly, evaluation of the simplest case, a single-layer overcast stratocumulus, revealed that huge discrepancies exist among the various techniques, even among different algorithms that are in the same general classification. This suggests that, despite considerable advances that have occurred in the field, much more work must be done, and we discuss potential avenues for future research.)
University of Wisconsin—Madison, Madison, Wisconsin
Brookhaven National Laboratory, Upton, New York
Colorado State University, Fort Collins, Colorado
Pacific Northwest National Laboratory, Richland, Washington
Atmospheric and Environmental Research, Inc., Lexington, Massachusetts
University of Maryland, Baltimore County, Baltimore, Maryland
AS&M, Hampton, Virginia
Argonne National Laboratory, Argonne, Illinois
NASA Langley Research Center, Hampton, Virginia
NASA Goddard Space Flight Center, Greenbelt, Maryland
CIRES, University of Colorado, and NOAA/Earth System Research Laboratory, Boulder, Colorado
State University of New York at Albany, Albany, New York
University of California, Santa Barbara, Santa Barbara, California
University of Wyoming, Laramie, Wyoming
Brookhaven National Laboratory, Upton, New York, and NASA Goddard Space Flight Center, Greenbelt, Maryland
