Cloud Phase Determination Using Ground-Based AERI Observations at SHEBA

Turner, D. D., National Oceanic and Atmospheric Administration

General Circulation and Single Column Models/Parameterizations

Cloud Properties

Turner, D.D., S.A. Ackerman, B.A. Baum, H.E. Revercomb, and P. Yang, 2003: "Cloud Phase Determination Using Ground-Based AERI Observations at SHEBA," Journal of Applied Meteorology 42(6):701-715.

The SHEBA experiment in Barrow, Alaska used data collected by the ground-based radiation observations from the Atmospheric Emitted Radiance Interferometer (AERI). (Photo Credit: SHEBA Project Office)

Key Contributors: S.A. Ackerman, B.A. Baum, H.E. Revercomb, P. Yang,

In the frigid environs of the Acrtic, ARM scientists at the North Slope of Alaska/Adjacent Arctic Ocean (NSA/AAO) site are studying cloud and radiative processes at high latitudes. Data from these research efforts are being used to refine climate models and parameterizations as they relate to the Arctic environment. Now, a new algorithm that uses ground-based spectrally resolved infrared observations with thresholds determined from radiative transfer simulations will allow scientists to create the first climatology of cloud phase at the ARM site in Barrow, Alaska.

Determining cloud phase is especially difficult in the Arctic, where the underlying snow-covered surface, persistent temperature inversions, and long periods of polar night make satellite retrievals very difficult. Observations from ground-based polarization sensitive laser instruments can be used to determine cloud phase. However, because there are currently no such instruments at the Barrow site or elsewhere in the Arctic, scientists have had little information about cloud phase there. The new cloud phase determination method uses measurements taken by the Atmospheric Emitted Radiance Interferometer (AERI), an instrument developed for the ARM Program which measures downwelling radiance at high spectral resolution.

As described in the paper, \"Cloud phase determination using ground-based AERI observations at SHEBA,\" the new method takes advantage of the differences in the amount of absorption by liquid water and ice in the spectral region between 11 and 19 um. These differences in absorption properties cause clouds composed of liquid water droplets to emit different amounts of radiation than clouds composed of ice particles.

Knowledge of cloud phase is an important component in correctly modeling cloud microphysical and optical properties. Assuming an incorrect phase can lead to errors up to 100% in particle size and optical thickness, resulting in errors of 5-20% in the amount of modeled downwelling radiation reaching the surface. The ability to correctly identify cloud phase at the Barrow site will greatly improve understanding of the radiative effects of mixed-phase and ice phase clouds in the Arctic, and adds yet another refinement to OBER's overall climate change research.