Cotton, W.

Duda, D.

Stephens, G.

Stevens, B.

English

MPG.PuRe

111PostersDevelopment of a Scheme for Parameterizingthe Effects of Cloud Condensation Nucleion Stratocumulus Cloud AlbedoW. R. Cotton, B. Stevens, D. Duda, and G. L. StephensColorado State UniversityDepartment of Atmospheric ScienceFort Collins, Colorado• Drizzle formation is a complicated function of the CCNspectra, especially giant and ultra-giant aerosol, cloudliquid water contents, mixing processes, and the timescales of cloud drafts which are determined by thestrengths and depths of cloud drafts.ApproachOur approach is to produce large eddy simulations (LES)or cloud-resolving models (CRMs) of marine stratocumulusclouds including the use of bin-resolving aerosol and cloudmicrophysics. The model is designed to obtain an internallyconsistent coupling between stratocumulus cloud dynamicsand cloud microphysics.The simulations are performed for “generically” representedinitial conditions for the First ISCCP(a) Regional Experiment(FIRE I) stratus case described by Betts and Boers (1990).The credibility of the simulations is evaluated by comparingthem with observations performed during the case. Thendetailed statistical analyses of model output data, includingtwo-dimensional radiative transfer calculations, areperformed. The control simulation is then perturbed toinvestigate the influence of changing CCN concentrationson cloud microstructure, macrostructure, and cloud albedo.Model DesignThe LES model is based on the nonhydrostatic version ofthe Regional Atmospheric Modeling System (RAMS) asset up for LES (Hadfield et al. 1991, 1992; Walko et al.IntroductionThe goal of this research is to develop a scheme forparameterizing the effects of varying concentrations ofcloud condensation nuclei (CCN) on stratocumulus cloudalbedo for use in global climate models.The challenges in achieving this goal are• CCNs are not a horizontally and vertically uniformspecies, but vary depending upon differential horizontaladvection of differing CCN sources and upon localconditions (e.g., CCN formation by gas-to-particleconversion and cloud removal). CCNs are probably asvariable as water vapor.• The peak supersaturations that determine the activationof CCN and resultant aerosol and cloud dropletdistributions are a function of cloud updraft anddowndrafts on scales of a few hundred meters withmagnitudes of a few meters per second. These scalescannot be represented properly either with large-scalegrid-resolvable ascent rates of the order of 0.01 m/s orby eddy diffusion models.• Cloud albedo is a strong function of the cloudmacrostructure (holes, hills, troughs in cloud structure)and of the liquid water paths and aerosol/hydrometeorspectra.• The aerosol/hydrometeor spectra, in turn, are not onlya function of nucleation of cloud droplets at cloud baseon resident CCN in the atmospheric boundary layer(ABL), but also the entrainment of CCN from above thecapping inversion and the formation of drizzle.(Entrainment is one of the most difficult processes tomodel in the cloud-topped boundary layer [CTBL]). (a) International Satellite Cloud Climatology Project.112ARM Science Meeting1992). The finite difference scheme in the original RAMSwas modified to include a hybrid scheme—Leap Frog—foradvection of winds and buoyancy terms and forwarddifferencing for scalar terms. A six-order flux-correctedtransport algorithm with peak preservers was found to beessential to obtain proper simulations of the transport ofCCN and reasonable values of diagnosed supersaturations,particularly near cloud top and cloud base.Condensation, collection, and evaporation of droplets isbased on the two-moment discrete bin model of Tzivionet al. (1987); this model has been shown to have accuracycomparable to that of a single-moment bin model with twoto three times the number of bins.CCNs are simulated in a six-bin sectional model in whicheach bin is described in terms of the supersaturationactivation of the aerosol. This approach avoids the necessityof describing the chemical properties of the CCN, whichare generally of internally mixed chemical composition andare chemically heterogeneous across the aerosolspectrum.Diagnostic radiation calculations are performed with thespherical harmonic spatial grid method of Evans (1993).RAMS is set up with 25-m grid spacing in the vertical and55-m grid spacing in the horizontal over a 3.025- by 3.025-by 2.25-km domain with cyclic lateral boundary conditions.The Chen and Cotton (1983) cloud radiative param-eterization is used, and a Smagorinsky-type subgridturbulence closure is used with surface fluxes determinedfrom similarity theory using Louis’ (1979) scheme.The model is initialized with a horizontally homogeneousaircraft sounding described by Betts and Boers (1990),and since CCN spectra were not observed on this day,CCN spectra from 6/29/87 obtained by Hudson and Frisbie(1991) for another FIRE case were used.Summary of Resultsand Conclusions• The simulated cellular pattern of the cloud field wasconsistent with satellite observations for the day.• Cloud liquid water paths and mixing ratios wereconsistent with the observed properties of those clouds.• Cloud effective radii ranged from 5-7 microns nearcloud base to 12-13 microns near cloud top.• Droplet concentrations above cloud base in the controlsimulation were about 100/cc, and no drizzle wassimulated• Introduction of a sixfold enhancement of CCNconcentrations above the capping inversion (from largeCCN concentrations of 100/cc to over 600/cc) resultedin a linear increase in subcloud CCN concentrationswith time.• One hour after the enhanced CCN had been introducedabove the inversion, droplet concentrations increasedby 40%, cloud top effective radii decreased by 14%, andalbedo increased by 10%.• Because both the control and the perturbed simulationswere drizzle-free, there was no significant change in thedynamic or macroscopic structure of the cloud due toenhanced CCN concentrations.AcknowledgmentThis research was funded by the U.S. Departmentof Energy’s (DOE) National Institute for Global Envi-ronmental Change (NIGEC) through the National InstituteOffice at the University of California, Davis, under contractNIGEC-91-S01.ReferencesBetts, A. K., and R. Boers. 1990. A cloudiness transition ina marine boundary layer. J. Atmos. Sci. 47:1480-1497.Chen, C., and W. R. Cotton. 1983. Numerical experimentswith a one-dimensional higher order turbulence model:Simulation of the Wangara Day 33 Case. Bound.-LayerMeteorol. 25:375-404.Evans, K. F. 1993. Two-dimensional radiative transfer incloudy atmospheres: The spherical harmonic spatial gridmethod. J. Atmos. Sci. 50:3111-3124.Hadfield, M. G., W. R. Cotton, and R. A. Pielke. 1991.Large-eddy simulations of thermally forced circulations inthe convective boundary layer. Part I: A small-scale circu-lation with zero wind. Bound.-Layer Meteorol. 57:79-114.113PostersHadfield, M. G., W. R. Cotton, and R. A. Pielke. 1992.Large-eddy simulations of thermally forced circulations inthe convective boundary layer. Part II: The effect of changesin wavelength and wind speed. Bound.-Layer Meteorol.58:307-327.Hudson, J. G., and P. R. Frisbie. 1991. Cloud condensationnuclei near marine stratus. J. Geophys. Res. 96:20,795-20,808.Louis, J. F. 1979. A parametric model of verticaleddy fluxes in the atmosphere. Bound.-Layer Meteorol.17:187-202.Tzivion, S., G. Feingold, and Z. Levin. 1987. An efficientnumerical solution to the stochastic collection equation.J. Atmos. Sci. 44:3139-3149.Walko, R. L., W. R. Cotton, and R. A. Pielke. 1992. Largeeddy simulations of the effects of hilly terrain onthe convective boundary layer. Bound.-Layer Meteorol.53:133-150.

Development of a scheme for parameterizing the effects of cloud condensation nuclei on stratocumulus cloud albedo

http://hdl.handle.net/11858/00-001M-0000-0028-8E57-D

Development of a scheme for parameterizing the effects of cloud condensation nuclei on stratocumulus cloud albedo

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