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February 2007

On Thin Ice

Despite advances, retrieval algorithms for ice clouds remain slippery

Results: As part of a science team supported by the U.S. Department of Energy's Atmospheric Radiation Measurement Program, Pacific Northwest National Laboratory researchers specializing in ice cloud properties found that thin clouds (optical depth less then 0.3) are often below the detection level of most remote sensors. Yet, these clouds may play a significant role in the radiative heating in the upper troposphere, particularly in tropical regions where cirrus clouds persist at extremely cold temperatures. This data gap represents one of the key uncertainties in climate model simulations—how ice crystals in cirrus clouds absorb and reflect radiant energy, and in some cases, enhance the amount of radiant energy emitted towards the Earth's surface. The team's recent studies are summarized in the February 2007 issue of the Bulletin of the American Meteorological Society.

Why it Matters: Upper tropospheric clouds, like cirrus clouds, are found at altitudes of about 9 miles above sea level. An important aspect of understanding the radiative feedbacks of upper tropospheric clouds is being able to measure the size, shape, density and other microphysical properties of the ice crystals within the cloud. The microphysical properties of cirrus clouds are difficult to measure and model because they are highly variable in nature and their ice crystal size distribution and habit are not well characterized. However, these clouds are important modulators of the Earth's energy balance and climate because they reflect less incoming solar radiation and absorb more infrared radiation than water clouds, in essence enhancing the "greenhouse effect."

Backscatter charts
Radar reflectivity and lidar backscatter of cirrus clouds observed on March 9, 2000, at the ARM Climate Research Facility's Southern Great Plains site demonstrate the optical depth range of cirrus clouds. Because they can be so thin, it is challenging for sensors to accurately retrieve their microphysical properties. Enlarged View
Comparison of ground-based retrievals of ice water path and visible optical depth
Using satellite (VISST) and aircraft based measurements, a comparison of ground-based retrievals of ice water path (IWP) and visible optical depth obtained on March 9, 2000 indicates the range (blue shading) and average (red line) of the cloud properties retrieved by the various ground-based algorithms. The largest discrepancies are apparent when optical depth is less than 0.3. Enlarged View

Methods: Using measurements obtained from both passive and active remote sensing instruments, scientists rely on a variety of complex mathematical formulas—called "retrieval algorithms"—to estimate the microphysical properties of clouds. As part of an ongoing intercomparison of retrieval algorithms for upper tropospheric ice clouds, the researchers examined the ice water path and optical depth derived from 14 different algorithms. The ground-based algorithms were compared with in situ measurements made by airborne sensors and satellite based algorithms.

Next Steps: Though researchers have made significant progress in the evolution of ice cloud retrieval algorithms in the last decade, the addition of satellite-based lidar and radar capabilities reinforce the need to understand the uncertainties and assumptions that underlie the algorithms that scientists will use to characterize the global distribution of clouds. The science team will use these results to help develop combined retrievals that choose the best algorithm based on atmospheric conditions. Additional challenges facing the improvement of algorithms for upper-tropospheric ice clouds lie in the characterization of the ice crystal size distribution and particle shape.

Research Team: Jennifer Comstock and Sally McFarlane, PNNL; Robert d'Entremont, Atmospheric and Environmental Research, Inc.; Daniel DeSlover and David Turner, University of Wisconsin-Madison; Gerald Mace, University of Utah; Sergey Matrosov and Matthew Shupe, National Oceanic and Atmospheric Administration; Patrick Minnis, National Aeronautics and Space Administration; David Mitchell, Desert Research Institute; Kenneth Sassen, University of Alaska Fairbanks; and Zhien Wang, University of Wyoming, Laramie.

Source: Comstock, J.M., R. d'Entremont, D. DeSlover, G.G. Mace, S.Y. Matrosov, S.A. McFarlane, P. Minnis, D. Mitchell, K. Sassen, M.D. Shupe, D.D. Turner, and Z. Wang, 2007: An Intercomparison of Microphysical Retrieval Algorithms for Upper-Tropospheric Ice Clouds. Bull. Amer. Meteor. Soc., 88, 191-204.

Sponsor: This research was funded by the U.S. Department of Energy, Office of Biological and Environmental Research.

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