ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-7235-2011 A spectral method for retrieving cloud optical thickness and effective radius from surface-based transmittance measurements McBride P. J. 1 2 Schmidt K. S. 1 Pilewskie P. 1 2 Kittelman A. S. 2 Wolfe D. E. 3 Laboratory for Atmospheric and Space Physics, University of Colorado, Campus Box 392, Boulder, Colorado 80309-0392, USA Department of Atmospheric and Oceanic Sciences, University of Colorado, Campus Box 311, Boulder, CO 80309-0311, USA NOAA Earth Systems Research Laboratory, Physical Science Division, Weather and Climate Physics Branch, 325 Broadway, Boulder, CO 80305, USA 25 07 2011 11 14 7235 7252 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/7235/2011/acp-11-7235-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/7235/2011/acp-11-7235-2011.pdf

We introduce a new spectral method for the retrieval of optical thickness and effective radius from cloud transmittance that relies on the spectral slope of the normalized transmittance between 1565 nm and 1634 nm, and on cloud transmittance at a visible wavelength. The standard dual-wavelength technique, which is traditionally used in reflectance-based retrievals, is ill-suited for transmittance because it lacks sensitivity to effective radius, especially for optically thin clouds. Using the spectral slope rather than the transmittance itself enhances the sensitivity of transmittance observations with respect to the effective radius. This is demonstrated by applying it to the moderate spectral resolution observations from the Solar Spectral Flux Radiometer (SSFR) and Shortwave Spectroradiometer (SWS), and by examining the retrieval uncertainties of the standard and the spectral method for data from the DOE ARM Southern Great Plains (SGP) site and a NOAA ship cruise (ICEALOT). The liquid water path (LWP) is derived from the retrieved optical thickness and effective radius, based on two different assumptions about the cloud vertical profile, and compared to the simultaneous observations from a microwave radiometer. Optical thickness and effective radius is also compared to MODIS retrievals. In general, the effective radius uncertainties were much larger for the standard retrieval than for the spectral retrieval, particularly for thin clouds. When defining 2 μm as upper limit for the tolerable uncertainty of the effective radius, the standard method returned only very few valid retrievals for clouds with an optical thickness below 25. For the analyzed ICEALOT data (mean optical thickness 23), the spectral method provided valid retrievals for 84 % of the data (24 % for the standard method). For the SGP data (mean optical thickness 44), both methods provided a high return of 90 % for the spectral method and 78 % for the standard method.

 References Barker, H. W. and Marshak, A.: Inferring optical depth of broken clouds above green vegetation using surface solar radiometric measurements, J. Atmos. Sci., 58, 2989–3006, 2001. Bergstrom, R. W., Pilewskie, P., Schmid, B., and Russell, P. B.: Estimates of the spectral aerosol single scattering albedo and aerosol radiative effects during SAFARI 2000, J. Geophys. Res., 108, 8474, http://dx.doi.org/10.1029/2002JD002435doi:10.1029/2002JD002435, 2003. Chiu, J. C., Huang, C., Marshak, A., Slutsker, I., Giles, D. M., Holben, B. N., Knyazikhin, Y., and Wiscombe, W. J.: Cloud optical depth retrievals from the Aerosol Robotic Network (AERONET) cloud mode observations, J. Geophys. Res., 115, D14202, http://dx.doi.org/10.1029/2009JD013121doi:10.1029/2009JD013121, 2010. Ch\'ylek, P. and Ramaswamy, V.: Simple approximation for infrared emissivity of water clouds, J. Atmos. Sci., 39, 171–177, 1982. Clark, R. N., Swayze, G. A., Wise, R., Livo, E., Hoefen, T., Kokaly, R., and Sutley, S. J.: USGS digital spectral library splib06a: US Geological Survey, Digital Data Series 231, 2007. Coddington, O. M., Schmidt, K. S., Pilewskie, P., Gore, W. J., Bergstrom, R. W., Román, M., Redemann, J., Russell, P. B., Liu, J., and Schaaf, C. C.: Aircraft measurements of spectral surface albedo and its consistency with ground-based and space-borne observations, J. Geophys. Res.-Atmos., 113, http://dx.doi:10.1029/2008JD010089doi:10.1029/2008JD010089, 2008. Dutton, E. G., Farhadi, A., Stone, R. S., Long, C. N., and Nelson, D. W.: Long-term variations in the occurrence and effective solar transmission of clouds as determined from surface-based total irradiance observations, J. Geophys. Res.- Atmos., 109, D03204, http://dx.doi.org/10.1029/2003JD003568doi:10.1029/2003JD003568, 2004. Ehrlich, A., Bierwirth, E., Wendisch, M., Gayet, J.-F., Mioche, G., Lampert, A., and Heintzenberg, J.: Cloud phase identification of Arctic boundary-layer clouds from airborne spectral reflection measurements: test of three approaches, Atmos. Chem. Phys., 8, 7493–7505, http://dx.doi.org/10.5194/acp-8-7493-2008doi:10.5194/acp-8-7493-2008, 2008. Evans, F. K.: The spherical harmonic discrete ordinate method for three-dimensional atmospheric radiative transfer, J. Atmos. Sci., 55, 429-446, 1998. Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., Fahey, D. W., Haywood, J., Lean, J., Lowe, D. C., Myhre, G., Nganga, J., Prinn, R., Raga, G., Schulz, M., and Van Dorland, R.: Changes in Atmospheric Constituents and in Radiative Forcing, in: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., and Miller, H. L., Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 131–217, 2007. Hansen, J. E. and Hovenier, J. W.: Interpretation of the polarization of Venus, J. Atmos. Sci., 31, 1137–1160, 1974. Hansen, J. E. and Pollack, J. B.: Near-infrared light scattering by terrestrial clouds, J. Atmos. Sci., 27, 265–281, 1970. Harrison, L., Michalsky, J., and Berndt, J.: Automated multifilter rotating shadow-band radiometer: an instrument for optical depth and radiation measurements, Appl. Optics, 33, 5118–5125, 1994. Holben, B.: AERONET: A federated instrument network and data archive for aerosol characterization, Remote Sens. Environ., 66, 1–16, 1998. Kikuchi, N., Nakajima, T., Kumagai, H., Kuroiwa, H., Kamei, A., Nakamura, R., and Nakajima, T. Y.: Cloud optical thickness and effective particle radius derived from transmitted solar radiation measurements: Comparison with cloud radar observations, J. Geophys. Res., 111, D07205, http://dx.doi.org/10.1029/2005JD006363doi:10.1029/2005JD006363, 2006. King, M. D., Menzel, P. W., Grant, P. S., Myers, J. S., Arnold, T. G., Platnick, S. E., Gumley, L. E., Tsay, S., Moeller, C. C., Fitzgerald, M., Brown, K. S., and Osterwisch, F. G.: Airborne Scanning Spectrometer for remote sensing of cloud, aerosol, water vapor, and surface properties, J. Atmos. Ocean. Tech., 13, 777–794, 1996. Leontyeva, E. and Stamnes, K.: Estimations of cloud optical thickness from ground-based measurements of incoming solar radiation in the Arctic, J. Climate, 7, 566–578, 1994. Liljegren, J. C. and Lesht, B. M.: Measurements of integrated water vapor and cloud liquid water from microwave radiometers at the DOE ARM Cloud and Radiation Testbed in the US Southern Great Plains., in: IEEE International Geosciences and Remote Sensing Symposium (IGARSS), Lincoln, NE, 21-26 May, 1996, 1675–1677, 1996. Marshak, A., Knyazikhin, Y., Evans, K. D., and Wiscombe, W. J.: The RED versus NIR plane to retrieve broken-cloud optical depth from ground-based measurements, J. Atmos. Sci., 61, 1911–1925, 2004. Nakajima, T. and King, M. D.: Determination of the optical thickness and effective particle radius of clouds from reflected solar radiation measurements. Part I: Theory, J. Atmos. Sci., 47, 1878–1893, 1990. Pilewskie, P. and Twomey, S.: Cloud phase discrimination by reflectance measurements near 1.6 and 2.2 μm, J. Atmos. Sci., 44, 3419-3420, 1987. Pilewskie, P., Pommier, J., Bergstrom, R., Gore, W., Howard, S., Rabbette, M., Schmid, B., Hobbs, P. V. and Tsay, S. C.: Solar spectral radiative forcing during the Southern African Regional Science Initiative, J. Geophys. Res., 108, 8486, http://dx.doi.org/10.1029/2002JD002411doi:10.1029/2002JD002411, 2003. Platnick, S.: Vertical photon transport in cloud remote sensing problems, J. Geophys. Res., 105, 22919–22935, 2000. Platnick, S., King, M. D., Ackerman, S. A., Menzel, W. P., Baum, B. A., Riédi, J. C., and Frey, R. A.: The MODIS Cloud Products: Algorithms and Examples From Terra, IEEE T. Geosci. Remote, 41, 459–473, 2003. Rawlins, F. and Foot, J. S.: Remotely Sensed Measurements of Stratocumulus Properties during FIRE Using the C130 Aircraft Multi-channel Radiometer, J. Atmos. Sci., 47, 2488–2504, 1990. Schiffer, R. A. and Rossow, W. B.: The International Satellite Cloud Climatology Project (ISCCP): The First Project of the World Climate Research Programme., B. Am. Meteorol. Soc., 64, 779–784, 1983. Stokes, G. M. and Schwartz, S. E.: The Atmospheric Radiation Measurement (ARM) Program: Programmatic Background and Design of the Cloud and Radiation Test Bed, B. Am. Meteorol. Soc., 75, 201–1222, 1994. Taylor, J. R.: An Introduction to Error Analysis: The study of uncertainties in physical measurements, 2nd, Sausalito, California, University Science Books, 1996. Turner, D. D., Vogelmann, A. M., Austin, R. T., Barnard, J. C., Cady-Pereira, K., Chiu, J. C., Clough, S. A., Flynn, C., Khaiyer, M. M., Liljegren, J., Johnson, K., Lin, B., Long, C., Marshak, A., Matrosov, S. Y., McFarlane, S. A., Miler, M., Min, Q., Minnis, P., O'Hirok, W., Wang, Z., and Iscombe, W.: Thin liquid water clouds: Their importance and our challenge, B. Am. Meteorol. Soc., 88, 177–190, 2007. Twomey, S. and Bohren, C. F.: Simple approximations for calculations of absorption in clouds, J. Atmos. Sci., 37, 2086–2095, 1980. Twomey, S. and Cocks, T.: Remote sensing of cloud parameters from spectral reflectance in the near-infrared, Beitr. Phys. Atmos., 62, 172–179, 1989. Vukicevic, T., Coddington, O,. and Pilewskie, P.: Characterizing the retrieval of cloud properties from optical remote sensing, J. Geophys. Res., 115, D20211, http://dx.doi.org/10.1029/2009JD012830doi:10.1029/2009JD012830, 2010. Warren, S. G.: Optical Constants of Ice from the Ultraviolet optical thickness the Microwave, Appl. Optics, 23, 1206–1225, 1984. Westwater, E. R.: Ground-Based microwave remote sensing of meteorological variables, atmospheric remote sensing by microwave radiometry, Janssen, M. A., Wiley, New York, NY, 145–207, 1993. Westwater, E. R., Han, Y., Shupe, M. D., and Matrosov, S. Y.: Analysis of integrated cloud liquid and precipitable water vapor retrievals from microwave radiometers during the Surface Heat Budget of the Arctic Ocean project, J. Geophys. Res., 106, 32019–32030, 2001. Wood, R. and Hartmann, D. L.: Spatial variability of liquid water path in marine low cloud: The importance of mesoscale cellular convection, J. Climate, 19, 1748–1764, 2006.