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Burlington, MA, United States

This paper describes an improvement in the diffraction approximation used to retrieve the size distribution of atmospheric particles from solar aureole radiance measurements. Normalization using total optical thickness based on measurement of the solar disk radiance is replaced with one based on the aureole profile radiance itself. Retrievals involving model calculations for power-law distributions of water droplets show significant improvement using the new algorithm. Tests involving two empirical particle size distributions, one for cirrus and another for aerosols, also show improvement using the new normalization algorithm. Comparisons of the diffraction approximation algorithms with a numerical inversion algorithm found that the accuracy of the latter was higher for two different bimodal aerosol distributions. The role envisioned for the diffraction approximation is in estimating the size distribution of large particles in clouds and especially cirrus. © 2011 American Meteorological Society. Source

Agency: Department of Energy | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 149.90K | Year: 2011

Atmospheric particles act to cool the Earth by reflecting incoming solar radiation if they are small (e.g., aerosols), and they can either warm or cool the earth through thermal absorption and emission if they are large (e.g., cirrus) depending upon their altitude. Climate change monitoring and modeling have improved significantly from advances attributable to the AERONET global network of ground-based sun photometers measuring the properties of aerosols. The climate impact of cirrus cloud particles is much less certain because they occur high in the atmosphere and are more difficult to monitor. Recent work pioneered by Visidyne has employed measurements of the solar aureole profiles caused by cirrus particles to retrieve their size distributions during the daytime. We propose a novel technique to extend this work to nighttime and to the larger, more thermally significant, particles. The basic idea is to determine the profile of aureole scattering patterns around stars at small angles (_1500 to 1_), and to invert this profile to determine the ice particle size distribution over the range D & apos; 50 mm to 10 mm, where D is the effective size of the ice particle. Such measurements are of intrinsic interest to cloud scientists and to climatologists, alike. Our approach utilizes only a good camera lens and a medium-quality astronomical CCD camera. We have carried out some preliminary observations to demonstrate how well, and under what conditions it will yield the desired results. Our approach has the advantages over in-situ measurements in that (i) it can be carried out on virtually any night when thin cirrus clouds are visible, (ii) it is relatively inexpensive to implement, and (iii) the measurements do not disturb the cloud environment or the particles themselves. Finally, we show how such stellar aureole measurements can be run autonomously to enhance existing, ground-based climate monitoring networks with instruments designed to measure stellar aureoles, thereby filling a gap in the information on cirrus clouds necessary for assessing and monitoring their climate impact. Our ultimate goal is long-term monitoring of cirrus ice-crystal size distributions as a function of altitude, season, and geographic latitude

Agency: Department of Defense | Branch: Missile Defense Agency | Program: SBIR | Phase: Phase II | Award Amount: 450.00K | Year: 2006

Visidyne, Incorporated, proposes to complete development of the Efficient Radiation Transport Testbed (ERTT), which combines the strengths of state-of-the-art programmable graphical processing units (GPUs) with standard correlated-k (CK) and line-by-line (LBL) methods for computing atmospheric radiation transport. During Phase I, Visidyne showed that overall performance enhancements (decreased computation time) approaching two orders of magnitude are possible with GPU technology. Visidyne also proposes to develop an ERT API library suitable for incorporation into other atmospheric radiation transport codes, such as SAMM2, and to collaborate with Spectral Sciences Incorporated on incorporating the ERT API into SAMM2. Finally, Visidyne proposes to investigate additional strategies for leveraging the power of GPUs beyond those developed in Phase I, concentrating on new functionality available in recently-released and emerging GPU hardware, and in particular on the application of GPU technology to real-time atmospheric-background scene generation.

Agency: Department of Defense | Branch: Missile Defense Agency | Program: SBIR | Phase: Phase II | Award Amount: 1.25M | Year: 2006

In Phase 1 Visidyne demonstrated the utility of its breadboard, ground-based, Sun and Aureole Measurement (SAMNET) sensor coupled with its state-of-the-science models for particle scattering for measuring and deriving the transmission and scattering properties of clouds. Knowledge of these properties is critical to the correct interpretation of missile launch signals viewed through clouds by space-based, electro-optical sensors. In Phase 2 Visidyne proposes further applications of its SAMNET instruments and analysis software to develop the approach and algorithms as they relate to MDA operational mission requirements for early launch detection through clouds. Visidyne proposes to (1) collect statistics on field test-site measurements of cloud scattering properties, including parameters to correct overhead observations of signals transmitted through clouds, (2) analyze the variability of the measurements of cloud scattering properties affecting overhead observations, and (3) collect data to test approaches for coupling SAMNET measurements with meteorological satellite observations to provide propagated (interpolated) solutions for operational use in areas of interest (AOIs).

Devore J.G.,Visidyne, Inc. | Kristl J.A.,Visidyne, Inc. | Rappaport S.A.,Massachusetts Institute of Technology
Journal of Geophysical Research: Atmospheres | Year: 2013

The aureoles around stars caused by thin cirrus limit nighttime measurement opportunities for ground-based astronomy, but can provide information on high-altitude ice crystals for climate research. In this paper we attempt to demonstrate quantitatively how this works. Aureole profiles can be followed out to ∼0.2°from stars and ∼0.5°from Jupiter. Interpretation of diffracted starlight is similar to that for sunlight, but emphasizes larger particles. Stellar diffraction profiles are very distinctive, typically being approximately flat out to a critical angle followed by gradually steepening power-law falloff with slope less steep than -3. Using the relationship between the phase function for diffraction and the average Fourier transform of the projected area of complex ice crystals, we show that defining particle size in terms of average projected area normal to the propagation direction of the starlight leads to a simple, analytic approximation representing large-particle diffraction that is nearly independent of crystal habit. A similar analytic approximation for the diffraction aureole allows it to be separated from the point spread function and the sky background. Multiple scattering is deconvolved using the Hankel transform leading to the diffraction phase function. Application of constrained numerical inversion to the phase function then yields a solution for the particle size distribution in the range between ∼50 μm and ∼400 μm. Stellar aureole measurements can provide one of the very few, as well as least expensive, methods for retrieving cirrus microphysical properties from ground-based observations. © 2013. American Geophysical Union. All Rights Reserved. Source

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