ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-9771-2011 Cloud optical thickness and liquid water path – does the <i>k</i> coefficient vary with droplet concentration? Brenguier J.-L. 1 Burnet F. 1 Geoffroy O. 2 CNRM/GAME, Météo-France/CNRS, URA1357, Toulouse, France Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands 21 09 2011 11 18 9771 9786 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/11/9771/2011/acp-11-9771-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/9771/2011/acp-11-9771-2011.pdf

Cloud radiative transfer calculations in general circulation models involve a link between cloud microphysical and optical properties. Indeed, the liquid water content expresses as a function of the mean volume droplet radius, while the light extinction is a function of their mean surface radius. There is a small difference between these two parameters because of the droplet spectrum width. This issue has been addressed by introducing an empirical multiplying correction factor to the droplet concentration. Analysis of in situ sampled data, however, revealed that the correction factor decreases when the concentration increases, hence partially mitigating the aerosol indirect effect. <br><br> Five field experiments are reanalyzed here, in which standard and upgraded versions of the droplet spectrometer were used to document shallow cumulus and stratocumulus topped boundary layers. They suggest that the standard probe noticeably underestimates the correction factor compared to the upgraded versions. The analysis is further refined to demonstrate that the value of the correction factor derived by averaging values calculated locally along the flight path overestimates the value derived from liquid water path and optical thickness of a cloudy column, and that there is no detectable relationship between the correction factor and the droplet concentration. It is also shown that the droplet concentration dilution by entrainment-mixing after CCN activation is significantly stronger in shallow cumuli than in stratocumulus layers. These various effects are finally combined to produce the today best estimate of the correction factor to use in general circulation models.

 References % vor jede Referenz Abdul-Razzak, H. and Ghan, S. J.: A parameterization of aerosol activation: 2. Multiple aerosol types, J. Geophys. Res., 105, 6837–6844, 2000. Baumgardner, D., Strapp, W. J., and Dye, J. E.: Evaluation of the Forward Scattering Spectrometer Probe. Part II: Corrections for coincidence and dead-time losses. J. Atmos. Oceanic Technol., 2, 626-632, 1985. Bower, K. N., and Choularton, T. W.: A parameterization of the effective radius of ice free clouds for use in global climate models, Atmos. Res., 27, 305–339, 1992. Brenguier, J.-L.: Coincidence and dead-time corrections for particle counters. Part II: High concentration measurements with an FSSP, J. Atmos. Oceanic Technol., 6, 585–598, 1989. Brenguier, J. L.: Parameterization of the Condensation Process: A Theoretical Approach, J. Atmos. Sci., 48, 264–282, 1991. Brenguier, J. L., Baumgardner, D., and Baker, B.: A review and discussion of processing algorithms for FSSP concentration measurements, J. Atmos. Oceanic. Technol., 11, 1409–1414, 1994. Brenguier, J. L., Bourrianne, T., Coelho, A., Isbert, J., Peytavi, R., Trevarin, D., and Wechsler, P.: Improvements of droplet size distribution measurements with the Fast-FSSP, J. Atmos. Oceanic. Technol., 15, 1077–1090, 1998. Brenguier, J. L., Chuang, P. Y., Fouquart, Y., Johnson, D. W., Parol, F., Pawlowska, H., Pelon, J., Schüller, L., Schröder, F. and Snider, J. R.: An overview of the ACE-2 CLOUDYCOLUMN Closure Experiment, Tellus, 52B, 814–826, 2000a. Brenguier, J. L., Pawlowska, H; Schüller, L., Preusker, R., Fischer, J., and Fouquart, Y.: Radiative properties of boundary layer clouds: droplet effective radius versus number concentration, J. Atmos. Sci., 57, 803–821, 2000b. Burnet, F. and Brenguier, J. L.: Validation of droplet spectra and liquid water content measurements, Phys. Chem. Earth (B), 24, 249–254, 1999. Burnet, F. and Brenguier, J. L.: Comparison between standard and modified forward scattering spectrometer probes during the small cumulus microphysics study, J. Atmos. Ocean. Tech., 19, 1561–1931, 2002. Burnet, F. and Brenguier, J. L.: Observational study of the entrainment-mixing process in warm convective clouds, J. Atmos. Sci., 64, 1995–2011, 2007. Burnet, F. and Brenguier, J. L.: The onset of precipitation in warm convective clouds: A case study from SCMS, Q. J. Roy. Meteor. Soc., 36, 374–381, http://dx.doi.org/10.1002/qj.552doi:10.1002/qj.552, 2010. Chen, W.-T., Nenes, A., Liao, H., Adams, P. J., Li J.-L. F., and Seinfeld, J. H.: Global climate response to anthropogenic aerosol indirect effects: Present day and year 2100, J. Geophys. Res., 115, D12207, http://dx.doi.org/10.1029/2008JD011619doi:10.1029/2008JD011619, 2010. Coakley Jr., J. A., Bernstein, R. L., and Durkee, P. A., Effect of Ship-Stack Effluents on Cloud Reflectivity, Science, 237, 1020–1022, 1987. Durkee, P. A., Noone, K. J., and Bluth, R. T.: The Monterey Area Ship Track Experiment, J. Atmos. Sci., 57, 2523–2541, 2000. Dye, J. E. and Baumgardner, D.: Evaluation of the Forward Scattering Spectrometer Probe. Part I: Electronic and optical studies, J. Atmos. Ocean. Technol., 1, 329–344, 1984. Göke, S., Ochs III, H. T., and Rauber, R. M.: Radar analysis of precipitation in maritime versus continental clouds near the Florida coast: Inferences concerning the role of CCN and giant nuclei, J. Atmos. Sci., 64, 3695–3707, 2007. Geoffroy, O., Brenguier, J.-L., and Burnet, F.: Parametric representation of the cloud droplet spectra for LES warm bulk microphysical schemes, Atmos. Chem. Phys., 10, 4835–4848, http://dx.doi.org/10.5194/acp-10-4835-2010doi:10.5194/acp-10-4835-2010, 2010. Hansen, J. E. and Travis, L. D.: Light scattering in planetary atmospheres, Space Sci. Rev., 16, 527–610, http://dx.doi.org/10.1007/BF00168069doi:10.1007/BF00168069, 1974. Hoose, C., Kristjànsson, J. E., Iversen, T., Kirkevag, A., Seland, Ø., and Gettelman, A.: Constraining cloud droplet number concentration in GCMs suppresses the aerosol indirect effect, Geophys. Res. Lett., 36, L12807, http://dx.doi.org/10.1029/2009GL038568doi:10.1029/2009GL038568, 2009. Hudson, J. G. and Yum, S. S.: Maritime-continental drizzle contrasts in small cumuli, J. Atmos. Sci., 58, 915–926, 2001. Jones, A., Roberts, D. L., Woodage, M. J., and Johnson, C. E.: Indirect sulphate aerosol forcing in a climate model with an interactive sulphur cycle, J. Geophys. Res., 106(D17), 20293–20310, 2001. Knight, C. A. and Miller, L. J.: Early radar echoes from small, warm cumulus: Bragg and hydrometeor scattering, J. Atmos. Sci., 55, 2974–2992, 1998. Kulmala, M., Asmi, A., Lappalainen, H. K., Carslaw, K. S., Pöschl, U., Baltensperger, U., Hov, Ø., Brenquier, J.-L., Pandis, S. N., Facchini, M. C., Hansson, H.-C., Wiedensohler, A., and O'Dowd, C. D.: Introduction: European Integrated Project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) – integrating aerosol research from nano to global scales, Atmos. Chem. Phys., 9, 2825–2841, http://dx.doi.org/10.5194/acp-9-2825-2009doi:10.5194/acp-9-2825-2009, 2009. Liu, Y. and Daum, P. H.: Indirect warming effect from dispersion forcing, Nature, 419, 580–581, 2002. Liu, Y., Daum, P. H., Guo, H., and Peng, Y.: Dispersion bias, dispersion effect and the aerosol-cloud conundrum, Environ. Res. Lett., 3, 045021, http://dx.doi.org/10.1088/1748-9326/3/4/045021doi:10.1088/1748-9326/3/4/045021, 2008. Martin, G. M., Johnson, D. W., and Spice, A.: The measurement and parameterization of effective radius of droplets in warm stratocumulus clouds, J. Atmos. Sci., 51, 1823–1842, 1994. McFarquhar, G. M. and Heymsfield A. J.: Parameterizations of INDOEX microphysical measurements and calculations of cloud susceptibility: Applications for climate studies, J. Geophys. Res., 106(D22), 28675–28698, 2001. Ming, Y., Ramaswamy, V., Donner, L. J., Phillips, V. T. J., Klein, S. A., Ginoux, P. A., and Horowitz, L. W.: Modeling the Interactions between Aerosols and Liquid Water Clouds with a Self-Consistent Cloud Scheme in a General Circulation Model, J. Atmos. Sci., 64, 1189–1209, 2007. Pawlowska, H. and Brenguier, J.-L.: Microphysical properties of stratocumulus clouds during ACE-2, Tellus, 52B, 867–886, 2000. Pawlowska, H. and Brenguier, J.-L.: An Observational Study of Drizzle Formation in Stratocumulus Clouds during ACE-2 for GCM parameterizations PACE Topical Issue, J. Geophys. Res., 108(D15), 8630, http://dx.doi.org/10.1029/2002JD002679doi:10.1029/2002JD002679, 2003. Pawlowska, H., Grabowski, W. W., and Brenguier, J-.L.: Observations of the width of cloud droplet spectra in stratocumulus, Geophys. Res. Lett., 33, L19810, http://dx.doi.org/10.1029/2006GL026841doi:10.1029/2006GL026841, 2006. Peng, Y. and Lohmann, U.: Sensitivity study of the spectral dispersion of the cloud droplet size distribution on the indirect aerosol effect, Geophys. Res. Lett., 30, 1507, http://dx.doi.org/10.1029/2003GL017192doi:10.1029/2003GL017192, 2003. Raes, F., Bates, T., McGovern, F., and Van Liederkerke, M.: The 2nd Aerosol Characterization Experiment (ACE-2): general overview and main results, Tellus, 52B, 111–125, 2000. Rauber, R. M., Stevens, B., Ochs III, H. T., Knight, C., Albrecht, B. A., Blyth, A. M., Fairall, C.W., Jensen, J. B., Lasher-Trapp, S. G., Mayol-Bracero, O. L., Vali, G., Anderson, J. R., Baker, B. A., Bandy, A. R., Burnet, F., Brenguier, J-L., Brewer, W. A., Brown, P. R. A., Chuang, P., Cotton, W. R., Di Girolamo, L., Geerts, B., Gerber, H., Göke, S,. Gomes, L., Heikes, B. G., Hudson, J. G., Kollias, P., Lawson, R. P., Krueger, S. K., Lenschow, D. H., Nuijens, L., O'Sullivan, D. W., Rilling, R. A., Rogers, D. C., Siebesma, A. P., Snodgrass, E., Stith, J. L., Thornton, D. C., Tucker, S., Twohy, C. H., and Zuidema, P.: Rain in (Shallow) Cumulus over the Ocean – The RICO Campaign, B. Am. Meteor. Soc., 88, 1912–1928, 2007. Rotstayn, L. D. and Liu, Y.: Sensitivity of the first indirect aerosol effect to an increase of cloud droplet spectral dispersion with droplet number concentration, J. Climate, 16, 3476–3481, 2003. Stephens, G. L.: Radiation profiles in extended water clouds. II: Parameterization schemes, J. Atmos. Sci., 35, 2123–2132,1978. Stevens B., Lenschow, D. H., Vali, G., Gerber, H., Bandy, A., Blomquist, B., Brenguier, J-L., Bretherton, C. S., Burnet, F., Campos, T., Chai, S., Faloona, I., Haimov, S., Friesen, D., Laursen, K, Lilly, D. K., Loehrer, S., Malinowski, S. P., Petters, M. D., Rivera, M., Rogers, D. C., Russell, L., Savic-Jovcic, V., Snider, J., Straub, D., Szumowski, M. J., Takagi, H., Thorton, D. C., Tschudi, M, Twohy, C., Wetzel, M., and van Zanten, M. C.: Dynamics and chemistry of marine stratocumulus – DYCOMS-II, B. Am. Meteorol. Soc., 84, 579–593, 2003. Twomey, S.: Pollution and the planetary albedo, Atmos. Environ., 8, 1251–1256, 1974. Twomey, S.: The influence of pollution on the shortwave albedo of clouds, J. Atmos. Sci., 34, 1149–1152, 1977. van de Hulst, H. C: Light Scattering by Small Particles, John Wiley and Sons, New York, 470~pp., 1957. Warner, J.: The Microstructure of Cumulus Cloud. Part I. General Features of the Droplet Spectrum, J. Atmos. Sci., 26, 1049–1059, 1969. Wood, R.: Parameterization of the effect of drizzle upon the droplet effective radius in stratocumulus clouds, Q. J. Roy. Meteor. Soc., 126, 3309–3324, 2000.