Extended Abstract (588K)P4.28
Estimating thin cloud properties using an improved CO2 slicing approach
Hong Zhang, CIMSS/Univ. of Wisconsin, Madison, WI; and W. P. Menzel
CO2 slicing has been generally accepted as a useful algorithm for determining cloud top pressure (CTP) and effective cloud amount (ECA) for tropospheric clouds above 600 hPa. It has been used to estimate high cloud cover trends with HIRS data for the past twelve years, to estimate thin cloud tracer heights for cloud motion vector estimation, and to supplement the Automated Surface Observing System (ASOS) with high cloud cover determinations. To date, the technique has assumed that the surface emissivity is that of a blackbody in the long wavelength infrared radiances and that the cloud emissivity of spectrally close bands are approximately equal.
This paper investigates the effects of these two assumptions. Surface emissivity is adjusted according to the surface types. The ratio of cloud emissivities in spectrally close bands is adjusted away from unity according to radiative transfer calculations. The modified CO2 slicing algorithm is examined with MODIS (Moderate Resolution Imaging Spectroradiometer) Airborne Simulator (MAS) CO2 band radiance measurements over thin clouds and validated against Cloud Lidar System (CLS) measurements of the same clouds. The modified CO2 slicing algorithm is applied to Geostationary Operational Environmental Satellite (GOES) Sounder data to study the overall impact on cloud property determinations. For high thin clouds an improved product emerges, while for thick and opaque clouds there is little change. For very thin clouds, the bias of differences is about 10 - 20 hPa and RMS difference is approximately 50 hPa; for thin clouds, there is about 10 hPa bias and RMS difference is approximately 30 hPa. The new CO2 slicing algorithm places the clouds lower in the troposphere.
Poster Session 4, Radiances, Clouds, and Retrievals
Wednesday, 17 October 2001, 9:15 AM-11:00 AM
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