Multiangle Photopolarimetric Measurements Research Articles

Retrieval of volcanic and man-made stratospheric aerosols from orbital polarimetric measurements.

Stratospheric aerosols that are caused by a major volcanic eruption can serve as a valuable test of global climate models, as well as severely complicate tropospheric-aerosol monitoring from space. In either case, it is highly desirable to have accurate global information on the optical thickness, size, and composition of volcanic aerosols. We report sensitivity study results, which reveal the implications of making precise multi-angle photopolarimetric measurements in a 1.378-μm spectral channel residing within a strong water-vapor absorption band. We demonstrate that, under favorable conditions, such measurements would enable near-perfect retrievals of the optical thickness, effective radius, and refractive index of stratospheric aerosols. Besides enabling accurate retrievals of volcanic aerosols, such measurements can also be used to monitor man-made particulates injected in the stratosphere for geoengineering purposes.

Multiangle photopolarimetric aerosol retrievals in the vicinity of clouds: Synthetic study based on a large eddy simulation

AbstractWe investigate the effect of cloud contamination and 3‐D radiative transfer effects on aerosol retrievals from multiangle photopolarimetric measurements in the vicinity of clouds. To this end multiangle, multiwavelength photopolarimetric observations are simulated using a 3‐D radiative transfer model for scenes with realistic cloud properties, based on a large eddy simulation. Spatial resolutions of 2 × 2, 4 × 4, and 6 × 6 km2 have been considered. It is found that a goodness‐of‐fit criterion efficiently filters out cloud contamination. However, it does not filter out all scenes that are affected by 3‐D radiative effects, resulting in small biases in the retrieved aerosol optical thickness (AOT) and single‐scattering albedo (SSA). We also found that measurements at higher spatial resolution (2 × 2 km2) do not result in retrievals closer to clouds compared to measurements at coarser spatial resolutions (4 × 4 and 6 × 6 km2). If cloud parameters are fitted simultaneously with aerosol parameters using a 1‐D radiative transfer model and the Independent Pixel Approximation, more successful retrievals are obtained in partially cloudy scenes and in the vicinity of clouds. This effect is most apparent at 6 × 6 km2 and only marginal at 2 × 2 km2 resolution. The retrieved aerosol AOT and SSA from the simultaneous aerosol and cloud retrievals still have a small bias, like the aerosol‐only retrievals. We conclude that in order to substantially improve aerosol retrievals in the vicinity of clouds, a retrieval algorithm is needed that takes into account 3‐D radiative transfer effects.

Aerosol retrieval from multiangle, multispectral photopolarimetric measurements: importance of spectral range and angular resolution

Abstract. We investigated the importance of spectral range and angular resolution for aerosol retrieval from multiangle photopolarimetric measurements over land. For this purpose, we use an extensive set of simulated measurements for different spectral ranges and angular resolutions and subsets of real measurements of the airborne Research Scanning Polarimeter (RSP) carried out during the PODEX and SEAC4RS campaigns over the continental USA. Aerosol retrievals performed from RSP measurements show good agreement with ground-based AERONET measurements for aerosol optical depth (AOD), single scattering albedo (SSA) and refractive index. Furthermore, we found that inclusion of shortwave infrared bands (1590 and/or 2250 nm) significantly improves the retrieval of AOD, SSA and coarse mode microphysical properties. However, accuracies of the retrieved aerosol properties do not improve significantly when more than five viewing angles are used in the retrieval.

Sensitivity of PARASOL multi-angle photopolarimetric aerosol retrievals to cloud contamination

Abstract. An important problem in satellite remote sensing of aerosols is related to the need to perform an adequate cloud screening. If a cloud screening is applied that is not strict enough, the ground scene has the probability of residual cloud cover which causes large errors on the retrieved aerosol parameters. On the other hand, if the cloud-screening procedure is too strict, too many clear sky cases, especially near-cloud scenes, will falsely be flagged cloudy. The detrimental effects of cloud contamination as well as the importance of aerosol cloud interactions that can be studied in these near-cloud scenes call for new approaches to cloud screening. Multi-angle multi-wavelength photopolarimetric measurements have a unique capability to distinguish between scattering by (liquid) cloud droplets and aerosol particles. In this paper the sensitivity of aerosol retrievals from multi-angle photopolarimetric measurements to cloud contamination is investigated and the ability to intrinsically filter the cloud-contaminated scenes based on a goodness-of-fit criteria is evaluated. Hereto, an aerosol retrieval algorithm is applied to a partially clouded over-ocean synthetic data set as well as non-cloud-screened over-ocean POLDER-3/PARASOL observations. It is found that a goodness-of-fit filter, together with a filter on the coarse mode refractive index (mrcoarse > 1.335) and a cirrus screening, adequately rejects the cloud-contaminated scenes. No bias or larger SD are found in the retrieved parameters for this intrinsic cloud filter compared to the parameters retrieved in a priori cloud-screened data set (using MODIS/AQUA cloud masks) of PARASOL observations. Moreover, less high-aerosol load scenes are misinterpreted as cloud contaminated. The retrieved aerosol optical thickness, single scattering albedo and Ångström exponent show good agreement with AERONET observations. Furthermore, the synthetic retrievals give confidence in the ability of the algorithm to correctly retrieve the micro-physical aerosol parameters.

Models for surface reflection of radiance and polarized radiance: Comparison with airborne multi-angle photopolarimetric measurements and implications for modeling top-of-atmosphere measurements

In this paper, we investigate the surface–atmosphere radiative interaction in application to the problem of aerosol satellite remote sensing over land. First, we test different models of the Bidirectional Reflectance and Polarization Distribution Function (BRDF and BPDF) for bare soil and vegetation surfaces using multi-angle, multi-spectral photopolarimetric airborne measurements of the Research Scanning Polarimeter (RSP). Then, we investigate the performance of different models of BRDF and BPDF for modeling top-of-atmosphere measurements. We have found that different BRDF models can describe the RSP measurements equally well. However, for soil surfaces, the different BRDF models show a different dependence on illumination geometry (solar zenith and azimuth angles), as well as a different dependence on viewing angle outside the range of RSP measurements. This implies that different models describe the surface–atmosphere interaction differently, leading for soil surfaces to differences in the top-of-atmosphere reflectance up to 4–5%, whereas at surface level the models agree within 2% for RSP illumination and measurement geometry. For vegetation, the different BRDF models show more similar dependence on illumination geometry, meaning that, in general, the differences in top-of-atmosphere reflectances are smaller than the differences in surface total reflectances. For the BPDF, we compare the empirical model of Nadal and Breon (1999) and the model developed by Maignan et al. (2009) with a newly developed model. The latter model compares better with RSP measurements. It was shown that, though all models have essentially different angular profiles at different illumination and viewing geometries, the difference of the top-of-atmosphere degree of linear polarization is less or is of the same order as the degree of linear polarization difference at the surface level taken at RSP illumination and measurement geometry. For the considered models, it can be up to 0.015 but is mostly below 0.005.

Capability of multi-viewing-angle photo-polarimetric measurements for the simultaneous retrieval of aerosol and cloud properties

Abstract. An important new challenge in the field of multi-angle photo-polarimetric satellite remote sensing is the retrieval of aerosol properties under cloudy conditions. In this paper the possibility has been explored to perform a simultaneous retrieval of aerosol and cloud properties for partly cloudy scenes and for fully cloudy scenes where the aerosol layer is located above the cloud, using multi-angle photo-polarimetric measurements. Also, for clear sky conditions a review is given of the capabilities of multi-angle photo-polarimetric measurements in comparison with other measurement types. It is shown that already for clear sky conditions polarization measurements are highly important for the retrieval of aerosol optical and microphysical properties over land surfaces with unknown reflection properties. Furthermore, it is shown that multi-angle photo-polarimetric measurements have the capability to distinguish between aerosols and clouds, and thus facilitate a simultaneous retrieval of aerosol and cloud properties. High accuracy (0.002–0.004) of the polarimetric measurements plays an essential role here.

Accurate Monitoring of Terrestrial Aerosols and Total Solar Irradiance: Introducing the Glory Mission

The NASA Glory mission is intended to facilitate and improve upon long-term monitoring of two key forcings influencing global climate. One of the mission's principal objectives is to determine the global distribution of detailed aerosol and cloud properties with unprecedented accuracy, thereby facilitating the quantification of the aerosol direct and indirect radiative forcings. The other is to continue the 28-yr record of satellite-based measurements of total solar irradiance from which the effect of solar variability on the Earth's climate is quantified. These objectives will be met by flying two state-of-the-art science instruments on an Earth-orbiting platform. Based on a proven technique demonstrated with an aircraft-based prototype, the Aerosol Polarimetry Sensor (APS) will collect accurate multiangle photopolarimetric measurements of the Earth along the satellite ground track within a wide spectral range extending from the visible to the shortwave infrared. The Total Irradiance Monitor (TIM) is an improved version of an instrument currently flying on the Solar Radiation and Climate Experiment (SORCE) and will provide accurate and precise measurements of spectrally integrated sunlight illuminating the Earth. Because Glory is expected to fly as part of the A-Train constellation of Earth-orbiting spacecraft, the APS data will also be used to improve retrievals of aerosol climate forcing parameters and global aerosol assessments with other A-Train instruments. In this paper, we detail the scientific rationale and objectives of the Glory mission, explain how these scientific objectives dictate the specific measurement strategy, describe how the measurement strategy will be implemented by the APS and TIM, and briefly outline the overall structure of the mission. It is expected that the Glory results will be used extensively by members of the climate, solar, atmospheric, oceanic, and environmental research communities as well as in education and outreach activities.

Comments on: Retrieval of aerosol properties over the ocean using multispectral and multiangle photopolarimetric measurements from the research scanning polarimeter

A recent paper [Chowdhary et al., 2001] (hereafter CCM&T) presents an aerosol retrieval case study using airborne RSP (Research Scanning Polarimeter) data over dark water to demonstrate the contribution polarization measurements can make to passive aerosol remote sensing. The paper also suggests that instruments such as MODIS (Moderate Resolution Imaging Spectroradiometer) [Barnes et al., 1998], MISR (Multi-angle Imaging SpectroRadiometer) [Diner et al., 1998], and POLDER (Polarization and Directionality of Earth Reflectances) [Deschamps et al., 1994], provide only a subset of the retrieval capabilities that would be available from EOSP (Earth Observing Scanning Polarimeter) [Travis, 1993], a satellite version of RSP. In this response, we argue that EOSP is not a replacement but a valuable complement to other measurement approaches. We also contend that MISR is significantly more capable than CCM&T imply. very tight constraints on this parameter Kahn et al. [2001]. The other metrics, which include spectral and angular ratios, provide additional sensitivity to particle microphysical properties because their accuracy over dark targets such as clear ocean (• 2%) is much better than the absolute radiometric accuracy (• 6%). Simulations show that MISR is sensitive to the difference between mono-modal and bimodal

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