J. Atmospheric and Oceanic Technology, 16, 2, 251-262, 1999

A Methodology for Measuring Cirrus Cloud Visible-to-Infrared Spectral Optical Depth Ratios

Daniel H. DeSlover and William L. Smith
Cooperative Institude for Meteorological Satellite Studies
University of Wisconsin-Madison, Wisconsin

Paivi K. Piironen and Edwin W. Eloranta
Space Science and Engineering Center
University of Wisconsin-Madison, Madison, Wisconsin.

Abstract

Knowledge of cirrus cloud optical depths is necessary to understand the earth's current climate and to model the radiation impact on future climate. Cirrus clouds, depending on the ratio of their shortwave "visible" to longwave "infrared" otical depth, can act to either cool or warm the planet. In this study, visible-to-infrared cirrus cloud optical depth ratios were meaured using ground-based lidar and Fourier transform spectrometry. A radiosonde temperature profile combined with the 532 nm-high spectral resolution lidar vertical cloud optical depth profile provided an effective weighting to the cloud radiance measured by the interferometer. This allowed evaluation of cirrus cloud optical depths in 18 infrared atmospheric microwindows between water vapor absorption lines within the 800-1200 1/cm infrared atmospheric window. The data analysis was performed near the peak solar and terrestrial emission regions, which represent the effective radiative cloud forcing efficiency of the given cloud sample. Resuslts are also preented that demonstrate the measurment of infrared optical depth using an assummed uniform cloud extinction cross section, which requires generic lidar cloud boundary data. The measured cloud extinction profile provided a more robust solution that would allow analysis of multiple-layer cloudsand removed the uniform cloud extinction cross-section asssumption. Mie calculations for ice particles were used to genrate visible and infrared extinction coeffiecients; these were compared against the measured visible-to-infrared otical depth ratios. The results demonstrate strong particle size and shape sensitivity across the infrared atmospheric window.

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eloranta@lidar.ssec.wisc.edu