ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-8-19637-2008 Turbulent dispersion in cloud-topped boundary layers Verzijlbergh R. A. 1 Jonker H. J. J. 1 Heus T. 1 3 Vilà-Guerau de Arellano J. 2 Department of Multi-Scale Physics, Delft University of Technology, Delft, The Netherlands Meteorology and Air Quality Section, Wageningen University, The Netherlands Current affiliation: Royal Netherlands Meteorological Institute, De Bilt, The Netherlands 18 11 2008 8 6 19637 19677 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/8/19637/2008/acpd-8-19637-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/8/19637/2008/acpd-8-19637-2008.pdf

Compared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary layer (for reference), 2) a "smoke" cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer. <br><br> We show that the dispersion characteristics of the smoke cloud boundary layer as well as the stratocumulus situation can be well understood by borrowing concepts from previous studies of dispersion in the dry convective boundary layer. A general result is that the presence of clouds enhances mixing and dispersion – a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress dispersion by diminishing solar irradiance. <br><br> The dispersion characteristics of a cumulus cloud layer turn out to be markedly different from the other three cases and the results can not be explained by only considering the well-known top-hat velocity distribution. To understand the surprising characteristics in the shallow cumulus layer, this case has been examined in more detail by 1) determining the velocity distribution conditioned on the distance to the nearest cloud and 2) accounting for the wavelike behaviour associated with the stratified dry environment.

 References Angevine, W M.: \newblock An integrated turbulence scheme for boundary layers with shallow cumulus applied to pollutant transport. \newblockJ. Appl. Meteorol., 44(9), 1436–1452, 2005. Bretherton, C S., et~al.: \newblock An intercomparison radiatively-driven entrainment and turbulence in a smoke cloud, as simulated by different numerical models, Quart. J. Roy. Meteor. Soc., 125, 391–423, 1999. Briggs, G.: Plume dispersion in the convective boundary layer. part ii: Analyses of condors field experiment data, J. Appl. Meteorol., 32(8), 1388–1425, 1993. Ching, J. K S., Shipley, S T., and Browell, E V.: Evidence for cloud venting of mixed layer ozone and aerosols, Atmos. Environ., 22(2), 225–242, 1988. Cotton, W.: Cloud venting - a review and some new global annual estimates, Earth-Sci. Rev., 39, 169–206, 1995. Csanady, G.: Turbulent diffusion in the environment, D Reidel Publishing company, 170–174, 1973. Cuijpers, J. W. M. and Duynkerke, P G.: Large-eddy simulation of trade wind cumulus clouds, J. Atmos. Sci., 50, 3894–3908, 1993. de~Roode, S. and~Duynkerke, P.: Observed lagrangian transition of stratocumulus into cumulus during astex: mean state and turbulence structure, J. Atmos. Sci., 54, 2157–2173, 1997. Deardorff, J W.: Stratocumulus-capped mixed layers derived from a three-dimensional model, Bound.-Lay. Meteorol., 18(4), 495–527, 1980. Dosio, A. and J Vilà-Guerau~de Arellano: Statistics of absolute and relative dispersion in the atmospheric convective boundary layer: A large-eddy simulation study, J. Atmos. Sci., 63(4), 1253–1272, 2006. Dosio, A.,~Vilà-Guerau~de Arellano, J., Holtslag, A A M., and Builtjes, P J H.: Relating eulerian and lagrangian statistics for the turbulent dispersion in the atmospheric convective boundary layer, J. Atmos. Sci., 62(4), 1175–1191, 2005. Duynkerke, P G. de~Roode,, S R., and coauthors: Observations and numerical simulation of the diurnal cycle of the eurocs stratocumulus case, Quart. J. Roy. Meteor. Soc., 140, 3269–3296, 2004. Heus, T., and Jonker, H J J.: Subsiding shells around shallow cumulus clouds, J. Atmos. Sci., 65(3), 1003–1018, 2008. Heus, T.,~van Dijk, G., Jonker, H J J., and Van~den Akker, H E A.: Mixing in shallow cumulus clouds studied by lagrangian particle tracking, J. Atmos. Sci., 65(8), 2581–2597, 2008. Hundsdorfer, W.,~Koren, B.,~Vanloon, M., and Verwer, J G.: A positive finite-difference advection scheme, J. Comput. Phys., 117(1), 35–46, 1995. Hunt, J.: Diffusion in the stably stratified atmospheric boundary layer, J. Appl. Meteorol., 24(11), 1187–1195, 1985. Jonker, H. J J.,~Heus, T., and Sullivan, P P.: A refined view of vertical mass transport by cumulus convection, Geophys. Res. Lett., 35(7), L07810, doi:10.1029/2007GL032606, 2008. Kemp, J R., and D J. Thomson: Dispersion in stable boundary layers using large-eddy simulation, Atmos Environ, 30(16), 2911–2923, 1996. Lamb, R G.: A numerical simulation of dispersion from an elevated point source in the convective planetary boundary layer, Atmos. Environ., 12(6–7), 1297–1304, 1978. Neggers, R. A J., Jonker, H J J., and Siebesma, A P.: Size statistics of cumulus cloud populations in large-eddy simulations, J. Atmos. Sci., 60(8), 1060–1074, 2003. Nieuwstadt, F. and~de~Valk, J.: A large eddy simulation of buoyant and non-buoyant plume dispersion in the atmospheric boundary layer, Atmos. Environ., 21(12), 2573–2587, 1987. Nieuwstadt, F. T M.: A large-eddy simulation of a line source in a convective atmospheric boundary layer–i. dispersion characteristics, Atmospheric Environment. Part A. General Topics, 26(3), 485–495, 1992. Pasquill, F.: The estimation of the dispersion of windborne material, Meteor. Mag., 90, 33–49, 1961. Siebesma, A P. and Cuijpers, J W M.: Evaluation of parametric assumptions for shallow cumulus convection, J. Atmos. Sci., 52(6), 650–666, 1995. Sorbjan, Z. and~Uliasz, M.: Large-eddy simulation of air pollution dispersion in the nocturnal cloud-topped atmospheric boundary layer, Bound.-Lay. Meteorol., 91(1), 145–157, 1999. Taylor, G.: Diffusion by continuous movements, Proc. London Math. Soc., 20, 196–212, 1921. Thomson, D J.: Criteria for the selection of stochastic models of particle trajectories in turbulent flows, J. Fluid Mech., 180, 529–556, 1987. Vilà-Guerau~de Arellano, J., Kim, S.-W., Barth, M C., and Patton, E G.: Transport and chemical transformations influenced by shallow cumulus over land, Atmos. Chem. Phys., 5(12), 3219–3231, 2005. Weil, J.,~Patton, E., and~Sullivan, P.: Lagrangian modeling of dispersion in the stable boundary layer, 17th Symposium on Boundary Layers and Turbulence, 2006. Weil, J C., Lawson, R P., and Rodi, A R.: Relative dispersion of ice crystals in seeded cumuli, J. Appl. Meteorol., b32(6), 1055–1073, 1993. Weil, J C., Sullivan, P P., and Moeng C H.: The use of large-eddy simulations in lagrangian particle dispersion models, J. Atmos. Sci., 61(23), 2877–2887, 2004. Wicker, L J. and Skamarock, W C.: Time-splitting methods for elastic models using forward time schemes, Mon. Wea. Rev., 130, 2088–2097, 2002. Willis, G E. and Deardorff, J W.: A laboratory study of dispersion from an elevated source within a modeled convective planetary boundary layer, Atmos. Environ., 12(6–7), 1305–1311, 1978.