Degree of linear polarization in spectral radiances from water-viewing infrared radiometers

Abstract

Infrared radiances from water become partially polarized at oblique viewing angles through both emission and reflection. I describe computer simulations that show how the state of polarization for water varies with environmental conditions over a wavelength range of 3–15 µm with 0.05-µm resolution. Polarization at wavelengths longer than approximately 4 µm generally is negative (p, or vertical) and increases with incidence angle up to approximately 75°, beyond which the horizontally polarized reflected atmospheric radiance begins to dominate the surface emission. The highest p polarization (∼4–10%) is found in the atmospheric window regions of approximately 4–5 and 8–14 µm. In the 3–5-µm spectral band, especially between 3 and 4 µm, reflected atmospheric radiance usually is greater than surface emission, resulting in a net s polarization (horizontal). The results of these simulations agree well with broadband measurements of the degree of polarization for a water surface viewed at nadir angles of 0–75°.