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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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Graham Feingold and Allison McComiskey

). Tradeoffs include the fact that optical measurements are more relevant to radiation than radar reflectivity (sixth moment of the drop size distribution) but can only be measured during daylight, whereas radar reflectivity is measurable at any time of the day but is highly sensitive to the largest drops, the presence of which biases r e retrievals. Aerosol microphysical properties such as aerosol number concentration (typically at diameters >60 nm), CCN concentrations at specific supersaturation set

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Sonia M. Kreidenweis, Markus Petters, and Ulrike Lohmann

gravity, changes in mass can be inferred as the relative humidity within the trap is modified ( Cohen et al. 1987 ). In a series of influential papers, Tang and coworkers (e.g., Tang 1997 ) applied this technique to a large range of single- and multicomponent particles composed of species that may be present in the ambient aerosol. More recently, Reid and coworkers showed how optical tweezers can be used to trap single particles ( Hopkins et al. 2004 ) and measure not only hygroscopic growth, but

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V. Ramaswamy, W. Collins, J. Haywood, J. Lean, N. Mahowald, G. Myhre, V. Naik, K. P. Shine, B. Soden, G. Stenchikov, and T. Storelvmo

the global models could be challenged. The first satellite retrievals of τ aer (at midvisible wavelength) appeared, based on the reflectance from a single visible spectral channel from the AVHRR satellite sensor ( Husar et al. 1997 ; Fig. 14-16 ). Fig . 14-16. The equivalent aerosol optical thickness (EAOT) derived from a single channel (algorithm of the AVHRR satellite sensor). [Reproduced from Husar et al. (1997) .] These retrievals were restricted to cloud-free regions over ocean owing to

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J. H. Mather, D. D. Turner, and T. P. Ackerman

ARM scientists began to demonstrate the value of the long-term operational nature of ARM’s observations to develop robust multiyear analyses. For example, Mace and Benson (2008) developed a multiyear distribution of cloud properties and radiative heating rate profiles at the SGP, Michalsky et al. (2010) constructed a climatology of aerosol optical thickness and Angstrom exponent, Gero and Turner (2011) investigated the trends in downwelling infrared radiance at the SGP site over a 14-yr

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D. D. Turner, J. E. M. Goldsmith, and R. A. Ferrare

measurements: A review . Opt. Eng. , 30 , 40 – 48 , doi: 10.1117/12.55772 . Ismail , S. , and E. V. Browell , 1989 : Airborne and spaceborne lidar measurements of water vapor profiles: A sensitivity analysis . Appl. Opt. , 28 , 3603 – 3615 , doi: 10.1364/AO.28.003603 . Kato , S. , and Coauthors , 2000 : A comparison of the aerosol optical thickness derived from ground-based and airborne measurements . J. Geophys. Res. , 105 , 14 701 – 14 717 , doi: 10.1029/2000JD900013 . Leonard , D. A

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M. A. Miller, K. Nitschke, T. P. Ackerman, W. R. Ferrell, N. Hickmon, and M. Ivey

measure the radiative fluxes and aerosol optical thickness above marine boundary layer clouds and thereby provide a constraint to be used in conjunction with surface radiation measurements from the AMF1 at Graciosa to directly measure the cloud optical thickness in broken cloud fields. There was no road to the top of Pico, only a narrow, steep trail. So the AMF1 technician, Carlos Sousa, carried a small radiation platform on his back to the peak and installed it during the summer of 2010. This system

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

and optical properties of atmospheric ice crystals at South Pole Station . J. Appl. Meteor. Climatol. , 45 , 1505 – 1524 , doi: 10.1175/JAM2421.1 . 10.1175/JAM2421.1 Lax , J. N. , and W. Schwerdtfeger , 1976 : Terrain-induced vertical motion and occurrence of ice crystal fall at South Pole in summer . Antarct. J. US , 9 , 144 – 145 . Lindeman , J. , Z. Boybeyi , and I. Gultepe , 2011 : An examination of the aerosol semi-direct effect for a polluted case of the ISDAC field

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M. Haeffelin, S. Crewell, A. J. Illingworth, G. Pappalardo, H. Russchenberg, M. Chiriaco, K. Ebell, R. J. Hogan, and F. Madonna

ceilometers (e.g., Flentje et al. 2010 ). About 30 microwave profilers are also available, and the potential for low-cost continuous-emission cloud radar networks to develop is high. Hence research developments now also focus on assessing the performance of the low-cost instruments and developing specific retrieval algorithms. 1) Aerosol profile retrievals Detailed knowledge of optical, microphysical, and radiative properties of aerosol particles is required to understand their role in atmospheric

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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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