Effects of electromagnetic wave interference on observations of the Earth radiation budget

Abstract

This paper investigates conditions necessary to match the irradiance derived by integrating radiances measured by a narrow field of view scanning radiometer with the irradiance measured by a hemispherical radiometer, both placed at a satellite altitude for Earth radiation budget estimates. When all sources are similar and they are spatially distributed randomly, then integrating radiance for the irradiance does not introduce a bias. Although the exact magnitude of the bias in other conditions is unknown, a finite area of the aperture that is much larger than the coherence area of radiation contributing to the Earth radiation budget, and a finite time to take a single measurement that is longer than the coherence time are likely to make the difference of the irradiance integrated from radiances and the irradiance measured by a hemispherical instrument insignificant. This conclusion does not contradict the existence of spatial coherence of light from incoherent sources. Therefore, electromagnetic energy absorbed by Earth is derivable from radiances measured by a scanning radiometer integrated over the Earth-viewing hemisphere and then averaging across all locations on the satellite orbital sphere when combined with solar irradiance measurements. Comparisons made in earlier studies show that the difference is less than 1%. In addition, when surface irradiances computed by a radiative transfer model constrained by top-of-atmosphere irradiances derived from radiance measurements are compared with downward shortwave irradiances taken by combinations of a pyreheliometer and a shaded pyranometer, or pyranometers, and with longwave irradiances taken by pyrgeometers, the biases in monthly mean irradiances are less than the uncertainties in the surface observations.