Wavelength dependence of the degree of polarization in cloud-free skies: simulations of real environments.

The visible and NIR maximum degree of polarization (DoP) of cloud-free skylight depends on many factors, including wavelength, sun zenith angle, surface reflectance, and aerosol properties. For clear-sky environments, radiative transfer models accurately estimate the sky DoP when each of these properties is well constrained. (The model used here was recently compared with full-sky polarization measurements with excellent agreement.) Using coincident Hyperion satellite observations and AERONET retrievals to provide model inputs, we simulate the maximum sky DoP for a variety of locations. Results show large variations in the wavelength dependence of sky polarization across different Earth environments. Therefore, accurate modeling of the sky DoP depends largely upon proper representation of the surface and aerosols in the model. Simple models which do not incorporate accurate aerosol and surface information have limited utility for simulating cloud-free sky DoP.

Icy wave-cloud lunar corona and cirrus iridescence.

Dual-polarization lidar data and radiosonde data are used to determine that iridescence in cirrus and a lunar corona in a thin wave cloud were caused by tiny ice crystals, not droplets of liquid water. The size of the corona diffraction rings recorded in photographs is used to estimate the mean diameter of the diffracting particles to be 14.6 µm, much smaller than conventional ice crystals. The iridescent cloud was located at the tropopause [~11-13.6 km above mean sea level (ASL)] with temperature near -70 °C, while the more optically pure corona was located at approximately 9.5 km ASL with temperature nearing -60 °C. Lidar cross-polarization ratios of 0.5 and 0.4 confirm that ice formed both the iridescence and the corona, respectively.

Effects of surface reflectance on skylight polarization measurements at the Mauna Loa Observatory.

An all-sky imaging polarimeter was deployed in summer 2008 to the Mauna Loa Observatory in Hawaii to study clear-sky atmospheric skylight polarization. The imager operates in five wavebands in the visible and near infrared spectrum and has a fisheye lens for all-sky viewing. This paper describes the deployment and presents comparisons of the degree of skylight polarization observed to similar data observed by Coulson with a principal-plane scanning polarimeter in the late 1970s. In general, the results compared favorably to those of Coulson. In addition, we present quantitative results correlating a variation of the maximum degree of polarization over a range of 70-85% to fluctuation in underlying surface reflectance and upwelling radiance data from the GOES satellite.

Comparison of full-sky polarization and radiance observations to radiative transfer simulations which employ AERONET products.

Visible-band and near infrared polarization and radiance images measured with a ground-based full-sky polarimeter are compared against a successive orders of scattering (SOS) radiative transfer model for 2009 summer cloud-free days in Bozeman, Montana, USA. The polarimeter measures radiance and polarization in 10-nm bands centered at 450 nm, 490 nm, 530 nm, 630 nm, and 700 nm. AERONET products are used to represent aerosols in the SOS model, while MISR satellite BRF products are used for the surface reflectance. While model results generally agree well with observation, the simulated degree of polarization is typically higher than observed data. Potential sources of this difference may include cloud contamination and/or underestimation of the AERONET-retrieved aerosol real refractive index. Problems with the retrieved parameters are not unexpected given the low aerosol optical depth range (0.025 to 0.17 at 500 nm) during the study and the corresponding difficulties that these conditions pose to the AERONET inversion algorithm.

Digital all-sky polarization imaging of partly cloudy skies.

Clouds reduce the degree of linear polarization (DOLP) of skylight relative to that of a clear sky. Even thin subvisual clouds in the "twilight zone" between clouds and aerosols produce a drop in skylight DOLP long before clouds become visible in the sky. In contrast, the angle of polarization (AOP) of light scattered by a cloud in a partly cloudy sky remains the same as in the clear sky for most cases. In unique instances, though, select clouds display AOP signatures that are oriented 90 degrees from the clear-sky AOP. For these clouds, scattered light oriented parallel to the scattering plane dominates the perpendicularly polarized Rayleigh-scattered light between the instrument and the cloud. For liquid clouds, this effect may assist cloud particle size identification because it occurs only over a relatively limited range of particle radii that will scatter parallel polarized light. Images are shown from a digital all-sky-polarization imager to illustrate these effects. Images are also shown that provide validation of previously published theories for weak (approximately 2%) polarization parallel to the scattering plane for a 22 degrees halo.

Dual-field imaging polarimeter using liquid crystal variable retarders.

An imaging Stokes-vector polarimeter using liquid crystal variable retarders (LCVRs) has been built and calibrated. Operating in five bands from 450 to 700 nm, the polarimeter can be changed quickly between narrow (12 degrees ) and wide (approximately 160 degrees) fields of view. The instrument is designed for studying the effects of differing sky polarization upon the measured polarization of ground-based objects. LCVRs exhibit variations in retardance with ray incidence angle and ray position in the aperture. Therefore LCVR-based Stokes polarimeters exhibit unique calibration challenges not found in other systems. Careful design and calibration of the instrument has achieved errors within +/-1.5%. Clear-sky measurements agree well with previously published data and cloudy data provide opportunities to explore spatial and spectral variations in sky polarization.

Radiometric cloud imaging with an uncooled microbolometer thermal infrared camera.

An uncooled microbolometer-array thermal infrared camera has been incorporated into a remote sensing system for radiometric sky imaging. The radiometric calibration is validated and improved through direct comparison with spectrally integrated data from the Atmospheric Emitted Radiance Interferometer (AERI). With the improved calibration, the Infrared Cloud Imager (ICI) system routinely obtains sky images with radiometric uncertainty less than 0.5 W/(m(2 )sr) for extended deployments in challenging field environments. We demonstrate the infrared cloud imaging technique with still and time-lapse imagery of clear and cloudy skies, including stratus, cirrus, and wave clouds.