Physical decoupling of the GOES daytime 3.9 µ m channel thermal emission and solar reflection components using total solar eclipse data

Abstract

The spectra of solar and terrestrial emissions are considered largely as being decoupled from one another, such that shortwave (reeectance) and longwave (emissive) radiative properties are often discussed as entirely separate regimes. Instruments having detector pass-bands situated in the region of cross- over near the 3.9mm atmosphericwindow (where solar and thermal emissions are of comparable magnitude), however, detect both solar reeection and thermal emission simultaneously during daytime operation. This poses a problem to daytime cloud retrievals which seek to exploit the unique optical properties of liquid water at these near-infrared wavelengths but require measurementof either the solar or the thermal component exclusively. Without a priori knowledge of these components,empirical relationships or iterative processes must be applied, often without a practical means of quantifying the errors implicit to them. Here, total solar eclipses are proposed as a physical mechanism to decoupling the two radiative components by eŒectively removing the reeected solar radiation from the 3.9mm scene during the eclipse. Considerations for this problem are discussed and comparisons to previous approximations presented.