Larry D. Travis

Contribution of water-leaving radiances to multiangle, multispectral polarimetric observations over the open ocean: bio-optical model results for case 1 waters

Multiangle, multispectral photopolarimetry of atmosphere-ocean systems provides the fullest set of remote sensing information possible on the scattering properties of aerosols and on the color of the ocean. Recent studies have shown that inverting such data allows for the potential of separating the retrieval of aerosol properties from ocean color monitoring in the visible part of the spectrum. However, the data in these studies were limited to those principal plane observations where the polarization of water-leaving radiances could be ignored. Examining similar potentials for off-principal plane observations requires the ability to assess realistic variations in both the reflectance for and bidirectionality of polarized water-leaving radiances for such viewing geometries. We provide hydrosol models for use in underwater light scattering computations to study such variations. The model consists of two components whose refractive indices resemble those of detritus-minerallike and planktonlike particles, whose size distributions are constrained by underwater light linear polarization signatures, and whose mixing ratios change as a function of particulate backscattering efficiency. Multiple scattering computations show that these models are capable of reproducing realistic underwater light albedos for wavelengths ranging from 400 to 600 nm, and for chlorophyll a concentrations ranging from 0.03 to 3.0 mg/m(3). Numerical results for spaceborne observations of the reflectance for total and polarized water-leaving radiances are provided as a function of polar angles, and the change in these reflectances with wavelength, chlorophyll a concentration, and hydrosol model are discussed in detail for case 1 (open ocean) waters.

Galileo Photopolarimeter-Radiometer Observations of Jupiter and the Galilean Satellites

Photopolarimeter-Radiometer (PPR) maps of daytime temperatures on Ganymede at a resolution of 220 kilometers show the expected anticorrelation with albedo, but morning temperatures were about 10 kelvin warmer than expected. Europa had a subsolar temperature of 128 kelvin and a lower effective thermal inertia than either Ganymede or Callisto, and Ios night side was cooler than predicted by recent models, perhaps requiring revision of heat-flow estimates. The lowest 250-millibar temperatures in the Great Red Spot (GRS) generally corresponded to the visually darkest regions. Temperatures remained cold north of the GRS, but they rose by as much as 6 kelvin to the south over the 2800-kilometer PPR resolution. A visually bright region northwest of the GRS was also relatively cold. It is likely that NH3 clouds affected the determination of the 500-millibar temperature field, which appears qualitatively different.

Aerosol polarimetry sensor for the Glory Mission

This paper describes the Glory Mission Aerosol Polarimetry Sensor (APS) being built by Raytheon under contract to NASAs Goddard Space Flight Center. Scheduled for launch in late 2008, the instrument is part of the US Climate Change Research Initiative to determine the global distribution of aerosols and clouds with sufficient accuracy and coverage to establish the aerosol effects on global climate change as well as begin a precise long-term aerosol record. The Glory APS is a polarimeter with nine solar reflectance spectral bands that measure the first three Stokes parameters vector components for a total of 27 unique measurements. In order to improve the reliability and accuracy of the measurements, additional 9 redundant measurements are made, yielding a total of 36 channels. The sensor is designed to acquire spatial, temporal, and spectral measurements simultaneously to minimize instrumental effects and provide extremely accurate Raw Data Records. The APS scans in the direction close to of the spacecraft velocity vector in order to acquire multi-angle samples for each retrieval location so that the Stokes parameters can be measured as functions of view angle.

Remote sensing of aerosols with the earth-observing scanning polarimeter

Tropospheric aerosols have potential climate forcing roles through both the direct effect on solar radiation scattered and absorbed and the indirect effect as condensation nuclei for cloud particles. Present capabilities for characterizing the global aerosol climatology are limited by the existing satellite remote sensing measurements, making a quantitative determination of the climate forcing effects of aerosols rather difficult and uncertain. The Earth Observing Scanning Polarimeter (EOSP) instrument under consideration for Earth Observing System (EOS) mission platforms and for a proposed Climsat Earth Probe mission is intended to provide global mapping with multispectral photopolarimetry, which can be exploited for the more pronounced aerosol signature in the polarization of the scattered light as compared to the radiance. We describe the essential features and performance specifications of the EOSP instrument and discuss approaches for retrieving tropospheric aerosol properties from multispectral photopolarimetry.

Derivation of cloud properties from biwavelength polarization measurements

The influence of the top altitude of terrestrial liquid water clouds on the polarization of sunlight reflected by a cloudy atmosphere is theoretically investigated. It is compared to the influences of other cloud properties and of stratospheric aerosols. Using a typical atmosphere model, we show how accurately the cloud top pressure may be derived from nadir measurements performed by satellites of the polarization at 350 nm. The accuracy of the derived pressure is about 5 mb under favorable conditions, when the accuracy of polarization measurements is 0.1%, and it depends mainly on knowledge of the density of the cloud and of the stratospheric aerosol optical thickness. The stratospheric aerosol optical thickness may be estimated with an accuracy of 0.02 using observations of the polarization at 670 nm having an accuracy of 0.1%.