OPTICAL DEPTH MEASUREMENTS OF AEROSOL, CLOUD, AND WATER-VAPOR USING SUN PHOTOMETERS DURING FIRE CIRRUS IFO-II

Optical depths in the visible to infrared spectral region were obtaine d from solar extinction measurements with two sun photometers during t he First ISCCP Regional Experiment Phase II Cirrus Intensive Field Obs ervation in Kansas. A method is described to correct sun photometry fo r gaseous absorption and is extended to estimate the water vapor amoun t. The approach uses a prior computation of gaseous absorption for the narrowband-pass sun photometry, parameterized with the slant-path abs orber amount. These produce correction coefficients for gaseous absorp tion, as determined by LOWTRAN 7 models. Near-infrared channels were c alibrated by modified Langley plots taking account of gaseous absorpti on. After the correction and calibration, the aerosol optical depths a t the wavelengths of 0.4-4 mu m were obtained for clear sky conditions . The aerosol optical depth at the wavelength lambda = 0.5 mu m was 0. 1-0.2 during the campaign. The cloud optical depth at lambda = 0.5 mu m was obtained for cirrus events on 26 November and 5 December 1991 af ter correction of multiple scattering effects involved in sun photomet ry. The column amount of water vapor was estimated from the 0.94-mu m- channel measurement and compared with results from radiosonde measurem ents. The comparison has shown a good agreement within a 10% differenc e during the campaign when the equivalent water vapor amount ranges fr om 0.3 to 1.2 g cm(-2).