Comparison of skylight polarization measurements and MODTRAN-P calculations

Increased use of polarization in optical remote sensing provides motivation for a study of instruments and methods that can be used to test and validate polarized atmospheric radiative transfer codes and simulation tools. An example comparison of measured skylight polarization and calculations from a preliminary version of the polarized MODTRAN radiative transfer code (MODTRAN-P) for cloud-free conditions is presented. The study combines data from an all-sky polarization imager at 452, 491, 532, 632, and 701 nm, a solar radiometer, a zenith-viewing aerosol and cloud lidar, a weather station, and radiosonde profiles of atmospheric temperature and pressure to compare measurements and model calculations of the maximum degree of linear polarization for cloud-free atmospheres. Comparisons for conditions ranging from extremely clear to thick forest fire smoke indicate that the additional data most needed for constraining calculations are aerosol size distributions. Nevertheless, comparisons made with standard aerosol models in version 2.1-alpha of MODTRAN-P with an unpolarized multiple-scattering algorithm illustrate the methodology and provide quantitative information about the range of conditions for which a single-scattering radiative transfer code is useful for predicting skylight polarization. This approach is also warranted because many users simulate atmospheres with the MODTRAN standard aerosol models. The agreement of model calculations with measurements is high for low aerosol optical depth and degrades with increasing optical depth. Agreement between measurements and model results is best for the longer wavelengths.