Extended Abstract (116K)P4.26
Retrieval of cirrus ice-water path, particle size, and optical thickness using thermal infrared satellite data
Robert P. d'Entremont, AER, Lexington, MA; and D. L. Mitchell
Retrievals are presented of cirrus cloud ice water path (IWP), effective diameter (Deff) and optical depth (tau) using two newly developed cirrus analysis methods. Proof-of-concept retrievals are performed with satellite and in-situ data from two ARM cirrus IOPs during March 2000, using GOES-8 thermal infrared data and ground-based radar data. The retrievals coincide with in-situ measurements over the SGP ARM CART site, which is used to test retrieval performance.
The two new cirrus analysis methods are newly developed by AER and DRI. The first is a thermal infrared (TIR) technique that retrieves the cirrus effective emissivity and temperature that simultaneously predict observed radiances in two or more TIR channels. Since the emissivity depends on absorption efficiency, IWP, and Deff, a knowledge of the ice-crystal size distribution shape N(D) as a function of cirrus environmental temperature is required.
The second is a radar backscatter technique that retrieves ice water content (IWC) as a function of the radar reflectivity factor, given information about the N(D) shape and dispersion, and power-law relations between crystal mass and size. Using ground-based radar backscatter returns it will be possible to integrate the IWC profile from cirrus base to cirrus top to obtain IWP, and compare the result with coterminous TIR retrievals of IWP and Deff.
Common to both approaches is an explicit dependence of the retrievables not just on IWP and ice-crystal effective size Deff but also on N(D) shape, including the slope parameter and dispersion of bi-modal modified gamma ice-crystal size distributions. These attributes have been related to temperature for tropical and mid-latitude cirrus using ARM and CEPEX in-situ aircraft observations. Comparisons between the ground- and satellite-based retrievals employing the same physics and methodology will reveal the sensitivity of IWP and Deff on the vertical structure of cirrus cloud microphysics. In evaluating these differences, it may be possible to expand our retrieval confidence from small scales (e.g., using ground-based and aircraft data) to larger scales (e.g., satellite-based) for cloud radiative-forcing in climate models.
Suggested Area of Interest: (4) Retrievals of Atmospheric Profiles and Constituents; or (2) Climatology and Long-Term Satellite Data Studies
Poster Session 4, Radiances, Clouds, and Retrievals
Wednesday, 17 October 2001, 9:15 AM-11:00 AM
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