Optical depth measurements of aerosol, cloud, and water vapor using sun photometers during FIRE Cirrus IFO II

Abstract

Optical depths in the visible to infrared spectral region were obtained from solar extinction measurements with two sun photometers during the First ISCCP Regional Experiment Phase II Cirrus Intensive Field Observation in Kansas. A method is described to correct sun photometry for gaseous absorption and is extended to istimate the water vapor amount. The approcah uses a prior computation of gaseous absorption for the narrowband-pass sun photometry, parameterized with the slant-path absorber amount. These produce correction coefficients for gaseous absorption, as determined by LOWTRAN 7 models. Near-infrared channels were calibrated by modified Langley plots taking account of gaseous absorption. After the correction and calibration, the aerosol optical depths at the wavelengths of 0.4-4 μm were obtained for clear sky conditions. The aerosol optical depth at the wavelength λ = 0.5 μm was 0.1-0.2 during the campaign. The cloud optical depth at λ = 0.5 μm was obtained for cirrus events on 26 November and 5 December 1991 after correction of multiple scattering effects involved in sun photometry. The column amount of water vapor was estimated from the 0.94-μm-channel measurement and compared with results from radiosonde measurements. The comparison has shown a good agreement within a 10% difference during the campaign when the equivalent water vapor amount ranges from 0.3 to 1.2 g cm-2.