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

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 microm with 0.05-microm resolution. Polarization at wavelengths longer than approximately 4 microm generally is negative (p, or vertical) and increases with incidence angle up to approximately 75 degrees, beyond which the horizontally polarized reflected atmospheric radiance begins to dominate the surface emission. The highest p polarization (approximately 4-10%) is found in the atmospheric window regions of approximately 4-5 and 8-14 microm. In the 3-5-microm spectral band, especially between 3 and 4 microm, 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 degrees.