SATELLITE ESTIMATION OF INCIDENT PHOTOSYNTHETICALLY ACTIVE RADIATION USING ULTRAVIOLET REFLECTANCE

A new satellite remote sensing method for estimating the amount of photosynthetically active radiation (PAR, 400-700 nm) incident at the earth's surface is described and tested. Potential incident PAR for clear sky conditions is computed from an existing spectral model. Actual incident PAR is estimated by accounting for the attenuation of PAR due to reflectance by clouds. PAR reflectance from clouds is inferred from ultraviolet (UV) reflectance at 370 nm, based on the evidently constant reflectivity of clouds across UV and PAR wavebands. A major advantage of the UV approach over existing visible band approaches to estimating insolation is the improved ability to discriminate clouds from high-albedo background surfaces. UV spectral reflectance data from the Total Ozone Mapping Spectrometer (TOMS) were used to test the approach for three climatically distinct, midlatitude locations. Estimates of monthly total incident PAR from the satellite technique differed from values computed from ground-based pyranometer measurements by less than 6%. This UV remote sensing method can be applied to estimate PAR insolation over ocean and land surfaces which are free of ice and snow.