ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-9-25853-2009 The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree? Uchida J. 1 Bretherton C. S. 2 Blossey P. N. 2 Department of Applied Mathematics, University of Washington, Seattle, Washington, USA Department of Atmospheric Science, University of Washington, Seattle, Washington, USA 02 12 2009 9 6 25853 25883 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-discuss.net/9/25853/2009/acpd-9-25853-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/9/25853/2009/acpd-9-25853-2009.pdf

The sensitivity of a stratocumulus-capped mixed layer to a change in cloud droplet concentration is evaluated with a large-eddy simulation (LES) and a mixed layer model (MLM), to see if the two model types agree on the strength of the second aerosol indirect effect. Good agreement can be obtained if the MLM entrainment closure explicitly reduces entrainment efficiency proportional to the rate of cloud droplet sedimentation at cloud top for cases in which the LES-simulated boundary layer remains well mixed, with a single peak in the vertical profile of vertical velocity variance. <br><br> To achieve this agreement, the MLM entrainment closure and the drizzle parameterization must be modified from their observationally-based defaults. This is because the LES advection scheme and microphysical parameterization significantly bias the entrainment rate and precipitation profile compared to observational best guesses. Before this modification, the MLM simulates more liquid water path and much more drizzle at a given droplet concentration than the LES and is more sensitive to droplet concentration, even undergoing a drizzle-induced boundary layer collapse at low droplet concentrations. After this modification, both models predict a similar decrease of cloud liquid water path as droplet concentration increases, cancelling 30–50% of the Twomey effect for our case. The agreement breaks down at the lowest simulated droplet concentrations, for which the boundary layer in the LES is not well mixed. <br><br> Our results highlight issues with both types of model. Potential LES biases due to inadequate resolution, subgrid mixing and microphysics must be carefully considered when trying to make a quantitative inference of the second indirect effect from an LES of a stratocumulus-topped boundary layer. On the other hand, even slight internal decoupling of the boundary layer invalidates MLM-predicted sensitivity to droplet concentrations.

 References Ackerman, A., Toon, O. B., Stevens, D. E., and Coakley Jr, J. A.: Enhancement of cloud cover and suppression of nocturnal drizzle in stratocumulus polluted by haze, Geophys. Res. Lett., 30(7), 1381, doi:10.1029/2002GL016634, 2003. Ackerman, A., Kirkpatrick, M. P., Stevens, D. E., and Toon, O. B.: The impact of humidity above stratiform clouds on indirect aerosol climate forcing, Nature, 432, 1014–1017, 2004. Ackerman, A. S., vanZanten, M. C., Stevens, B., and coauthors: Large-eddy simulations of a drizzling, stratocumulus-topped marine boundary layer, Mon. Weather Rev., 137, 1083–1110, 2009. Baker, M. B. and Charlson, R.: Bistability of CCN concentrations and thermodynamics in the cloud-topped boundary layer, Nature, 345, 142–145, 1990. Bretherton, C. S. and Wyant, M. C.: Moisture transport, lower tropospheric stability and decoupling of cloud-topped boundary layers, J. Atmos. Sci., 54, 148–167, 1997. Bretherton, C. S., Uttal, T., Fairall, C. W., Yuter, S., Weller, R., Baumgardner, D., Comstock, K., and Wood, R.: The EPIC~2001 stratocumulus study, B. Am. Meteorol. Soc., 85, 967–977, 2004. Bretherton, C. S., Blossey, P. N., and Uchida, J.: Cloud droplet sedimentation, entrainment efficiency, and subtropical stratocumulus albedo, Geophys. Res. Lett., 34, L03813, doi:10.1029/2006GL027648, 2007. Caldwell, P., Bretherton, C. S., and Wood, R.: Mixed-layer budget analysis of the diurnal cycle of entrainment in Southeast Pacific stratocumulus, J. Atmos. Sci., 62, 3775–3791, 2005. Caldwell, P. and Bretherton, C. S.: Response of a subtropical stratocumulus-capped mixed layer to climate and aerosol changes, J. Climate, 22, 20–38, 2009. Coakley Jr, J. A. and Walsh, C. D.: Limits to the aerosol indirect radiative effect derived from observations of ship tracks, J. Atmos. Sci., 59, 668–680, 2002. Comstock, K. K.,Bretherton, C. S., and Yuter, S. E.: Mesoscale variability and drizzle in southeast Pacific stratocumulus, J. Atmos. Sci., 62, 3792–3807, 2005. Gasso, S.: Satellite observations of the impact of weak volcanic activity on marine clouds, J. Geophys. Res., 113, D14S19, doi:10.1029/2007JD009106, 2008. Geoffroy, O., Brenguier, J.-L., and Sandu, I.: Relationship between drizzle rate, liquid water path and droplet concentration at the scale of a stratocumulus cloud system, Atmos. Chem. Phys., 8, 4641–4654, 2008. Nicholls, S. and Turton, J. D.: An observational study of the structure of stratiform cloud layers: Part~II, Entrainment, Q. J. Roy. Meteorol. Soc., 112, 461–480, 1986. Khairoutdinov, M. F. and Kogan, Y. L.: A new cloud physics parameterization in a large-eddy simulation model of marine stratocumulus, Mon. Weather Rev., 128, 229–243, 2000. Khairoutdinov, M. F. and Randall, D. A.: Cloud resolving modeling of the ARM Summer~1997 IOP: Model formulation, results, uncertainties, and sensitivities, J. Atmos. Sci., 60, 607–625, 2003. Larson, V. E. and Kotenberg, K. E.: An analytic longwave radiation formula for liquid layer clouds, Mon. Weather Rev., 135, 688–699, 2007. Lilly, D. K. and Stevens, B.: Validation of a mixed-layer closure. Part I: Theoretical tests, Q. J. Roy. Meteorol. Soc., 134, 47–55, 2008. Petters, M. D., Snider, J. R., Stevens, B., Vali, G., Faloona, I., and Russell, L. M.: Accumulation mode aerosol, pockets of open cells, and particle nucleation in the remote subtropical Pacific marine boundary layer, J. Geophys. Res., 111, D02206, doi:10.1029/2004JD005694, 2006. Pincus, R. and Baker, M. B.: Effect of precipitation on the albedo susceptibility of marine boundary layer clouds, Nature, 372, 250–252, 1994. Sandu, I., Brenguier, J.-L., Thouron, O., and Stevens, B.: How important is the vertical structure for the representation of aerosol impacts on the diurnal cycle of marine stratocumulus?, Atmos. Chem. Phys., 9, 4039–4052, 2009. Savic-Jovcic, V. and Stevens, B.: The structure and mesoscale organization of precipitating stratocumulus, J. Atmos. Sci., 65, 1587–1605, 2008. Stevens, B.: Cloud transitions and decoupling in shear-free stratocumulus-topped boundary layers, Geophys. Res. Lett., 27, 2557–2560, 2000. Stevens, B., Cotton, W. R., Feingold, G., and Moeng, C.-H.: Large-eddy simulations of strongly precipitating, shallow, stratocumulus-topped boundary layers, J. Atmos. Sci., 55, 3616–3638, 1998. Stevens, B., Lenschow, D. H., Faloona, I., and coauthors: On entrainment rates in nocturnal marine stratocumulus, Q. J. Roy. Meteorol. Soc., 129, 3469–3493, 2003. Stevens, B., Vali, G., Comstock, K., Wood, R., van Zanten, M. C., Austin, P. H., Bretherton, C. S., and Lenschow, D. H.: Pockets of open cells (POCs) and drizzle in marine stratocumulus, B. Am. Meteorol. Soc., 86, 51–57, 2005a. Stevens, B., Moeng,, C.-H., Ackerman, A. S., and coauthors: Evaluation of large-eddy simulations via observations of nocturnal marine stratocumulus, Mon. Weather Rev., 133, 1443–1462, 2005b. Stull, R. B.: The energetics of entrainment across a density interface, J. Atmos. Sci., 33, 1260–1267, 1976. Wood, R.: Cancellation of aerosol indirect effects in marine stratocumulus through cloud thinning, J. Atmos. Sci., 64, 2657–2669, 2007. Wood, R., Comstock, K. K., Bretherton, C. S., Cornish, C., Tomlinson, J., Collins, D. R., and Fairall, C.: Open cellular structure in marine stratocumulus cloud sheets, J. Geophys. Res., 113, D12207, doi:10.1029/2007JD009371, 2008. Xue, H., Feingold, G., and Stevens, B.: The role of precipitating cells in organizing shallow convection, J. Atmos. Sci., 65, 392–406, 2008. Wyant, M. Bretherton, C. S., Chlond, A., and coauthors: A single column model intercomparison of a heavily drizzling stratocumulus topped boundary layer, J. Geophys. Res., 112, D24204, doi:10.1029/2007JD008536, 2007. Zhu, P., Bretherton, C. S., Koehler, M., and coauthors: Intercomparison and interpretation of single column model simulations of a nocturnal stratocumulus topped marine boundary layer, Mon. Weather Rev., 133, 2741–2758, 2005.