Thin Liquid Water Clouds: Their Importance and Our Challenge

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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

CORRESPONDING AUTHOR: Dr. David D. Turner, University of Wisconsin—Madison, 1225 West Dayton Street, Madison, W l 53706, E-mail: dturner@ssec.wisc.edu

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

CORRESPONDING AUTHOR: Dr. David D. Turner, University of Wisconsin—Madison, 1225 West Dayton Street, Madison, W l 53706, E-mail: dturner@ssec.wisc.edu
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