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Allison McComiskey and Richard A. Ferrare

that it is a primarily light-scattering aerosol. Despite the recognition that there would be an attendant effect from absorbing aerosol, no estimate could be made for lack of knowledge of their mass, optical properties, and distribution. While containing substantial uncertainty, these estimates of cooling by aerosol are of a magnitude that could offset the warming caused by increases in greenhouse gases over the industrial period. The tenet of the ARM Program at the outset was to improve the

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Ulrich Schumann and Andrew J. Heymsfield

wavelength determine the optical extinction efficiency Q ext, i and other microphysical and optical properties of each particle ( Hansen and Travis 1974 ). Integral properties peculiar to contrails are the geometrical cross-section area A c , and the total crystal number N ice , total extinction EA , and total ice mass I per flight distance: Here, the local and the volume-mean ice particle concentrations n i and n ice , extinction ε , and ice water content IWC occur, with mean extinction

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D. Baumgardner, S. J. Abel, D. Axisa, R. Cotton, J. Crosier, P. Field, C. Gurganus, A. Heymsfield, A. Korolev, M. Krämer, P. Lawson, G. McFarquhar, Z. Ulanowski, and J. Um

water droplets from ice crystals as a function of size and shape complexity. Fig . 9-7. The colored markers indicate in which regions of water droplet (dark blue), ice crystal (light blue), or ash (black) measurements were made (figure courtesy of J. Dorsey, University of Manchester). 5. Direct measurement of cloud particle optical properties The optical properties of ice particles—such as extinction coefficient B ext , asymmetry factor g , and scattering phase function P θ —impact how clouds

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Ismail Gultepe, Andrew J. Heymsfield, Martin Gallagher, Luisa Ickes, and Darrel Baumgardner

accurately represent the temporal or spatial formation of fog or its microphysical and optical properties. Our understanding of how ice fog forms, evolves, dissipates, and impacts our environment is complicated by the same factors that have challenged our general understanding of all clouds with ice, that is, insufficient information on 1) sources and properties of ice nuclei (IN); 2) atmospheric cooling rates and the dynamics that drive them; 3) morphology and terminal velocities of ice crystals

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underlying surface? How do radiative processes interact with dynamical and hydrologic processes to produce cloud feedbacks that regulate climate change? The programmatic objectives of ARM call for measurements suitable for testing parameterizations over a sufficiently wide variety of situations so as to span the range of climatologically relevant possibilities. To accomplish this, highly detailed measurements of radiation and optical properties are needed both at Earth’s surface and inside the

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Roger Marchand

one might apply surface and/or aerosol property retrievals). The cloud fraction (i.e., the number of cloudy pixels relative to total pixels) is perhaps the cloud property used most frequently in the evaluation of climate models. Thus, understanding how well and under what conditions clouds can be identified is critical. Typically, the most difficult clouds to identify are optically thin high-altitude cirrus clouds and small-scale (often subimager resolution) cumulus clouds, because both of these

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Matthew D. Shupe, Jennifer M. Comstock, David D. Turner, and Gerald G. Mace

robustness and allow for scanning ( Mather and Voyles 2013 ). Lidars that operate at visible or near-visible wavelengths (e.g., Campbell et al. 2002 ; Turner et al. 2016 , chapter 18) are also critically important as they measure backscatter and depolarization ratio, which together contain information on cloud optical properties, particle shape, and hydrometeor phase, among others. ARM lidars include the Micropulse lidar (MPL), Raman lidar, and more recently the High Spectral Resolution lidar (HSRL

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Andrew J. Heymsfield, Martina Krämer, Anna Luebke, Phil Brown, Daniel J. Cziczo, Charmaine Franklin, Paul Lawson, Ulrike Lohmann, Greg McFarquhar, Zbigniew Ulanowski, and Kristof Van Tricht

also Figs. 2-2 – 2-4 ), cirrus modulate the amount of solar radiative energy received by the climate system, reflecting a portion of the incident sunlight back to outer space. They also control the loss of energy to space by their effect on outgoing infrared radiation emanating from Earth’s surface and lower atmosphere. Important feedbacks involving cirrus, their water content, and optical properties and their influences on climate have been proposed ( Ramanathan and Collins 1991 ; IPCC 2013

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Eli J. Mlawer, Michael J. Iacono, Robert Pincus, Howard W. Barker, Lazaros Oreopoulos, and David L. Mitchell

application of this code (and other fast RT codes) to climate or weather problems also must consider cloudy conditions. As a result, the ARM Program also has given rise to major accomplishments in cloudy-sky radiative transfer within GCMs. This includes a development of the ice optical property parameterization ( Mitchell 2002 ) integrated in RRTMG for use in CESM1. ARM support also led to the Monte Carlo Independent Column Approximation (McICA; Pincus et al. 2003 ; Barker et al. 2008 ), a method to

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Greg M. McFarquhar, Darrel Baumgardner, and Andrew J. Heymsfield

.-N. Liou , 1989 : Solar radiative transfer in cirrus clouds. Part I: Single-scattering and optical properties of hexagonal ice crystals . J. Atmos. Sci. , 46 , 3 – 19 , doi: 10.1175/1520-0469(1989)046<0003:SRTICC>2.0.CO;2 . 10.1175/1520-0469(1989)046<0003:SRTICC>2.0.CO;2 Um , J. , and G. M. McFarquhar , 2007 : Single-scattering properties of aggregates of bullet rosettes in cirrus . J. Appl. Meteor. Climatol. , 46 , 757 – 775 , doi: 10.1175/JAM2501.1 . 10.1175/JAM2501.1 Um , J. , and G

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