A Multichannel Cloud Pyranometer System for Airborne Measurement of Solar Spectral Reflectance by Clouds

Abstract

The design and performance of a spectral radiometer system are described for airborne measurements of solar flux reflectance by clouds. The system consists of a pair of identical multichannel pyranometers: one installed on the top and the other on the bottom of an aircraft fuselage to measure the downward and upward solar irradiances, respectively. This measurement scheme has an advantage in that reflectances derived from ratios between the upward and downward irradiances can avoid the need for absolute radiometric calibrations. The multichannel cloud pyranometer (MCP) system measures near-monochromatic solar irradiances at nine discrete wavelengths between 420 and 1650 nm by using interference filters with very narrow bandwidths. Included among these wavelengths are 760 and 938 nm in the oxygen and water vapor absorption bands, respectively. Solar radiation passing through the filters is instantly detected by a silicon photodiode for wavelength ?<1 µm and by a germanium photodiode for ?>1 µm. Good performance of the MCP system was confirmed through laboratory calibrations and airborne tests. The MCP system is suitable for remote sensing application to retrieve cloud physical parameters of water clouds from airborne spectral reflectance measurements.