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Larry D. Travis

Abstract

Mariner 10 Venus images are analyzed into power spectra of the brightness field for planetary zonal wavenumbers n≥3 and for latitudes from 55°–25°N. These spectra are examined for significant features and latitudinal variation as well as compared with cloud brightness distribution spectra similarly determined for the Earth. The Venus image spectra are found to have a systematic variation as a function of latitude. In the equatorial region the average power spectrum lm an approximately −2.7 power law behavior for n>5, while in midlatitudes the average spectrum is characterized by a slope of about −1.8 for n≥3. Although a flattening of the equatorial region spectrum for wavenumbers less than 5 is noted, no obvious peaks are superimposed on the general power law behavior of the Venus spectra. This is in contrast to the result for the Earth cloud brightness spectra, which exhibit a noticeable peak near wavenumber 5 or 6 for the midlatitude region.

The Venus image spectra are further interpreted under the assumption that the UV cloud features serve as markers of large-scale dynamical processes and thus can reveal the characteristics of the eddy kinetic energy spectrum. Support for this assumption is provided by the results of a comparison of the cloud brightness spectra for the Earth with observed eddy kinetic energy spectra. The characteristics of the Venus spectra at low latitudes suggest that the observed clouds are in a region of high static stability in which the large-scale turbulence is essentially two-dimensional, and that the important energy sources are at the largest scales (n<5), with the spectrum exhibiting a slope close to the theoretically expected −3 value for an inertial subrange at larger wavenumbers. It is concluded that the less steep slope observed for the midlatitude spectrum is quite possibly the manifestation of the importance of barotropic eddy disturbances in that region.

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Jacek Chowdhary, Brian Cairns, and Larry D. Travis

Abstract

To evaluate the global effects of aerosols on the direct radiative balance, tropospheric chemistry, and cloud properties of the earth's atmosphere requires high-precision remote sensing that is sensitive to the aerosol optical thickness, size distribution, refractive index, and number density. This study uses the multiangle 0.41-, 0.55-, 0.865-, and 2.25-μm channel data from the airborne Research Scanning Polarimeter to retrieve aerosol properties over the Pacific Ocean. It is shown that such photopolarimetric data are highly sensitive to the size distribution and refractive index of aerosol particles, which reduces the nonuniqueness in aerosol retrievals using such data as compared with less comprehensive datasets. Moreover, it is found that polarized reflectances obtained at the shorter wavelengths (0.41 and 0.55 μm) are significantly less sensitive to the contribution of the ocean's upwelling light than total reflectance measurements, providing a natural tool for the separation between the estimation of oceanic and atmospheric scattering properties.

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Michael I. Mishchenko and Larry D. Travis

The year 2008 marks the centenary of the seminal paper by Gustav Mie on electromagnetic scattering by homogeneous spherical particles. Having been cited in almost 4,000 journal articles since 1955 (according to the Science Citation Index Expanded database), Mie s paper has been among the more influential scientific publications of the twentieth century. It has affected profoundly the development of a great variety of natural science disciplines including atmospheric radiation, meteorological optics, remote sensing, aerosol physics, astrophysics, and biomedical optics. Mies paper represented a fundamental advancement over the earlier publications by Ludvig Lorenz in that it was explicitly based on the Maxwell equations, gave the final solution in a convenient form suitable for practical computations, and imparted physical reality to the abstract concept of electromagnetic scattering. The Mie solution anticipated such general concepts as far-field scattering and the Sommerfeld-Silver-Müller boundary conditions at infinity as well as paved the way to such important extensions as the separation of variables method for spheroids and the T-matrix method. Key ingredients of the Mie theory are quite prominent in the superposition T-matrix method for clusters of particles and even in the recent microphysical derivation of the radiative transfer equation. Among the most illustrative uses of the Mie solution have been the explanation of the spectacular optical displays caused by cloud and rain droplets, the identification of sulfuric acid particles in the atmosphere of Venus from Earth-based polarimetry, and optical particle characterization based on measurements of morphology-dependent resonances. Yet it is clear that the full practical potential of the Mie theory is still to be revealed.

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Michael I. Mishchenko, Brian Cairns, Greg Kopp, Carl F. Schueler, Bryan A. Fafaul, James E. Hansen, Ronald J. Hooker, Tom Itchkawich, Hal B. Maring, and Larry D. Travis

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.

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David J. Diner, Robert T. Menzies, Ralph A. Kahn, Theodore L. Anderson, Jens Bösenberg, Robert J. Charlson, Brent N. Holben, Chris A. Hostetler, Mark A. Miller, John A. Ogren, Graeme L. Stephens, Omar Torres, Bruce A. Wielicki, Philip J. Rasch, Larry D. Travis, and William D. Collins

A comprehensive and cohesive aerosol measurement record with consistent, well-understood uncertainties is a prerequisite to understanding aerosol impacts on long-term climate and environmental variability. Objectives to attaining such an understanding include improving upon the current state-of-the-art sensor calibration and developing systematic validation methods for remotely sensed microphysical properties. While advances in active and passive remote sensors will lead to needed improvements in retrieval accuracies and capabilities, ongoing validation is essential so that the changing sensor characteristics do not mask atmospheric trends. Surface-based radiometer, chemical, and lidar networks have critical roles within an integrated observing system, yet they currently undersample key geographic regions, have limitations in certain measurement capabilities, and lack stable funding. In situ aircraft observations of size-resolved aerosol chemical composition are necessary to provide important linkages between active and passive remote sensing. A planned, systematic approach toward a global aerosol observing network, involving multiple sponsoring agencies and surface-based, suborbital, and spaceborne sensors, is required to prioritize trade-offs regarding capabilities and costs. This strategy is a key ingredient of the Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) framework. A set of recommendations is presented.

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David J. Diner, Thomas P. Ackerman, Theodore L. Anderson, Jens Bösenberg, Amy J. Braverman, Robert J. Charlson, William D. Collins, Roger Davies, Brent N. Holben, Chris A . Hostetler, Ralph A. Kahn, John V. Martonchik, Robert T. Menzies, Mark A. Miller, John A. Ogren, Joyce E. Penner, Philip J. Rasch, Stephen E. Schwartz, John H. Seinfeld, Graeme L. Stephens, Omar Torres, Larry D. Travis, Bruce A . Wielicki, and Bin Yu

Aerosols exert myriad influences on the earth's environment and climate, and on human health. The complexity of aerosol-related processes requires that information gathered to improve our understanding of climate change must originate from multiple sources, and that effective strategies for data integration need to be established. While a vast array of observed and modeled data are becoming available, the aerosol research community currently lacks the necessary tools and infrastructure to reap maximum scientific benefit from these data. Spatial and temporal sampling differences among a diverse set of sensors, nonuniform data qualities, aerosol mesoscale variabilities, and difficulties in separating cloud effects are some of the challenges that need to be addressed. Maximizing the longterm benefit from these data also requires maintaining consistently well-understood accuracies as measurement approaches evolve and improve. Achieving a comprehensive understanding of how aerosol physical, chemical, and radiative processes impact the earth system can be achieved only through a multidisciplinary, interagency, and international initiative capable of dealing with these issues. A systematic approach, capitalizing on modern measurement and modeling techniques, geospatial statistics methodologies, and high-performance information technologies, can provide the necessary machinery to support this objective. We outline a framework for integrating and interpreting observations and models, and establishing an accurate, consistent, and cohesive long-term record, following a strategy whereby information and tools of progressively greater sophistication are incorporated as problems of increasing complexity are tackled. This concept is named the Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON). To encompass the breadth of the effort required, we present a set of recommendations dealing with data interoperability; measurement and model integration; multisensor synergy; data summarization and mining; model evaluation; calibration and validation; augmentation of surface and in situ measurements; advances in passive and active remote sensing; and design of satellite missions. Without an initiative of this nature, the scientific and policy communities will continue to struggle with understanding the quantitative impact of complex aerosol processes on regional and global climate change and air quality.

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