Current uncertainties in the total solar irradiance (TSI) and aerosol radiative forcings of climate are so large that they
limit quantitative evaluation of climate models against global temperature change. Reducing these uncertainties is the
objective of the NASA Glory mission scheduled for launch in November 2010 as part of the NASA A-Train. Glory is
intended to meet the following scientific objectives: Improve the quantification of solar variability by continuing the
uninterrupted 32-year satellite measurement record of TSI, facilitate the quantification of the aerosol direct and indirect
forcings of climate, and provide better aerosol representations for use by other operational satellite instruments.
KEYWORDS: Sensors, Polarization, Polarimetry, Aerosols, Signal to noise ratio, Telescopes, Calibration, Short wave infrared radiation, Error analysis, Space telescopes
The Aerosol Polarimetry Sensor (APS) is the primary Earth observing instrument of the Glory Mission. It is expected to
launch into space in the 4th quarter of 2010. This paper summarizes results from the APS ground-testing, completed in
2009. Ground testing established that the instrument meets or exceeds performance requirements: SNR, dynamic range,
radiometric accuracy, polarimetric accuracy, response vs. scan angle, boresight co-alignment, and calibration sources
accuracy. The APS demonstrated excellent performance stability during sensor and spacecraft level testing over a wide
range of environmental conditions. The APS will be a significant improvement over existing sensors that measure
aerosols from space. It will provide the scientific community with new information about the distribution and properties
of aerosols around the globe. Scientists will use APS data to estimate the radiative forcing imposed on the Earth by
aerosols, to assess the effects of aerosols on the Earth's climate.
This paper describes the Glory Mission Aerosol Polarimetry Sensor (APS) being built by Raytheon under contract to
NASA's 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.
Tropospheric aerosols play a crucial role in climate and can cause a climate forcing directly by absorbing and reflecting sunlight, thereby cooling or heating the atmosphere, and indirectly by modifying cloud properties. The indirect aerosol effect may include increased cloud brightness, as aerosols lead to a larger number of smaller cloud droplets (the so-called Twomey effect), and increased cloud cover, as smaller droplets inhibit rainfall and increase cloud lifetime. Both forcings are poorly understood and may represent the largest source of uncertainty about future climate change. In this paper we present results from various field experiments demonstrating the contribution that the multi-angle multi-spectral photopolarimetric remote sensing measurements of the NASA Glory mission will make to the determination of the direct and indirect radiative effects of aerosols.
The extensive set of measurements performed during the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) experiment provides a unique opportunity to evaluate aerosol retrievals over the ocean from multiangle, multispectral photometric and polarimetric remote sensing observations by the airborne Research Scanning Polarimeter (RSP) instrument. Previous studies have shown the feasibility of retrieving particle size distributions and real refractive indices from such observations for visible wavelengths without prior knowledge of the ocean color. This work evaluates the fidelity of the aerosol retrievals using RSP measurements during the CLAMS experiment against aerosol properties derived from in-situ measurements, sky radiance observations and sunphotometer measurements, and further extends the scope of the RSP retrievals by using a priori information about the ocean color to constrain the aerosol absorption. Satisfying agreement is found for all aerosol products.
Ice clouds have been identified as one of the most uncertain components in atmospheric research. In recent years, the atmospheric radiative transfer and remote sensing community has made a concerted effort to improve the characterization of cirrus clouds. A number of airborne and balloon-borne observations have demonstrated that cirrus clouds are essentially composed of nonspherical ice crystals with various habits (or shapes) and sizes. In this paper, we report on some recent progresses towards the computation of the single-scattering properties nonspherical ice crystals and the relevant applications to remote sensing and radiative transfer simulations. Specifically, we have developed a database of the optical properties of ice crystals at the infrared wavelengths. In conjunction with the application of the scattering database, we also developed a fast infrared transfer model under cirrus cloudy condition, which is applied to the retrieval of ice clouds from satellite-based infrared measurements.
We present an improved version of GACP (Global Aerosol Climatology Project) algorithm which uses channel 1 and 2 radiances of the Advanced Very High Resolution Radiometer (AVHRR) to retrieve aerosol optical thickness and Angstrom exponent over the ocean. We specifically discuss recent changes in the algorithm as well as the results of a sensitivity study analyzing the effect of several sources of retrieval errors not addressed previously. Uncertainties in the AVHRR radiance calibration (particularly in the deep- space count value) may be among the major factors potentially limiting the retrieval accuracy. On the other hand, the performance of two-channel algorithms weakly depends on a specific choice of the aerosol size distribution function. The updated algorithm is applied to a 10-year period of observations (July 1983 - Aug 1994), which includes data from NOAA-7, NOAA-9 (February 1985 - November 1988),and NOAA-11 satellites. The results are posted on the world wide web at http:gacp.giss.nasa.gov/retrievals. The NOAA-9 record shows no discernable long-term trends in the global and hemisphere averages of the optical thickness and Angstrom exponent. On the other hand, there is a discontinuity in the Angstrom exponent values derived from NOAA-9 and NOAA-11 data and a significant temporal trend in the NOAA-11 record. The latter are unlikely to be related to the Pinatubo eruption and may be indicative of a serious calibration problem.
We discuss the methodology of interpreting channel 1 and 2 AVHRR radiance data to retrieve tropospheric aerosol properties over the ocean and describe a detailed analysis of the sensitivity of monthly average retrievals to the assumed aerosol models. We use real AVHRR data and accurate numerical techniques for computing single and multiple scattering and spectral absorption of light in the vertically inhomogeneous atmosphere-ocean system. Our analysis shows that two-channel algorithms can provide significantly more accurate retrievals of the aerosol optical thickness than one-channel algorithms and that imperfect cloud screening is the largest source of errors in the retrieved optical thickness. Both underestimating and overestimating aerosol absorption as well as strong variability of the aerosol refractive index may lead to regional and/or seasonal biases in optical thickness retrievals. The Angstrom exponent appears to be the most invariant aerosol size characteristic and should be retrieved along with optical thickness as the second aerosol parameter.
In this paper we discuss the possible effect of nonsphericity of solid tropospheric aerosols on the accuracy of aerosol thickness retrievals from reflectance measurements over the ocean surface. To model light-scattering properties of nonspherical aerosols, we use a shape mixture of moderately aspherical, randomly oriented polydisperse spheroids. We assume that the size distribution and refractive index of aerosols are known and use the aerosol optical thickness 0.2 to computer the reflectivity for an atmosphere-ocean model similar to that used in the AVHRR aerosol retrieval algorithms. We then use analogous computations for volume- equivalent spherical aerosols with varying optical thickness to invert the simulated nonspherical reflectance. Our computations demonstrate that the use of the spherical model to retrieve the optical thickness of actually nonspherical aerosols can result in errors which, depending on the scattering geometry, can well exceed 100%.
In this paper we discuss the application of the T-matrix approach to rigorously compute light scattering by polydisperse, randomly oriented nonspherical particles with sizes comparable to and larger than wavelengths of observation. First, we describe an efficient method for suppressing the numerical instability of the regular T-matrix approach in computations for spheroids and finite cylinders with size parameters larger than about 25. Second, we describe how the T-matrix approach can be used to rigorously compute the scattering of light by randomly oriented two-sphere aggregates (bispheres). Both methods are extremely efficient, are applicable to scatterers with equivalent-sphere size parameters exceeding 50, and are especially suitable in computations for polydisperse particles. We report results of numerical computations that demonstrate the capabilities of the two methods and are used to discuss the effects of particle nonsphericity on light scattering.
In this paper, we study theoretically light scattering by polydisperse, randomly oriented, rotationally symmetric particles of size comparable to the wavelength of radiation. In our computations, we use the T-matrix approach, as extended recently to randomly oriented particles [M. I. Mishchenko, J. Opt. Soc. Am. A 8, 871 (1991)]. Results of extensive numerical calculations for particles of different shape are presented. The influence of particle size distribution, shape, and refractive index on the scattering patterns is examined and implications for remote sensing of nonspherical aerosols are discussed.
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