Revised for thecd Bulletin of the American Meteorological So

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Dynamics and Chemistry of Marine Stratocumulus–DYCOMS-II

Bjorn Stevens12Donald H.Lenschow,Gabor Vali,Hermann Gerber,A.Bandy,B.Blomquist, J.-L.Brenguier,C.S.Bretherton,F.Burnet,,T.Campos,S.Chai,I.Faloona,D.Friesen,

S.Haimov,http://doc.guandang.netursen,D.K.Lilly,S.M.Loehrer,Szymon P.Malinowski,B.Morely,

M.D.Petters,D.C.Rogers,L.Russell,V.Savic-Jovcic,J.R.Snider,D.Straub,Marcin

J.Szumowski,H.Takagi,D.C.Thorton,M.Tschudi,C.Twohy,M.Wetzel,M.C.van Zanten

Revised for thecd Bulletin of the American Meteorological Society

2nd November2002

ABSTRACT

The second Dynamics and Chemistry of Marine Stratocumulus(DYCOMS-II)?eld study is described. The?eld program consisted of nine?ights in marine stratocumulus West-Southwest of San Diego California. The objective of the program was to better understand the physics and dynamics of marine stratocumulus. Toward this end special?ight strategies,including predominantly nocturnal?ights,were employed to opti-mize estimates of entrainment velocities at cloud top,large-scale pergence within the boundary layer,driz-zle processes in the cloud,cloud microstructure,and aerosol-cloud interactions.Cloud conditions during DYCOMS-II were excellent with almost every?ight having uniformly overcast clouds topping a well-mixed boundary layer.Although the emphasis of the manuscript is on the goals and methodologies of DYCOMS-II,some preliminary?ndings are also presented—the most signi?cant being that the cloud layers appear to entrain less and drizzle more than previous theoretical work led investigators to expect.

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The stratocumulus topped boundary layer(hereafter the STBL),which prevails in the subtropics in

regions where the underlying ocean is much colder than the overlying atmosphere,is thought to be an important component of the climate system.Perhaps most striking is its impact on the radiative balance at

the top of the atmosphere.The seasonally averaged net cloud radiative forcing from the STBL has been

estimated to be as large as70Wm(Stephens and Greenwald,1991),more than an order of magnitude larger than the radiative forcing associated with a doubling of atmospheric CO.This means that even rather

subtle sensitivities of the STBL to changes in the properties of the atmospheric aerosol(cf.,Twomey,1974;

Albrecht,1989;Brenguier et al.,2000b),or the large-scale environment(Rodwell and Hoskins,2001),can

still project signi?cantly onto the overall radiative budget.In addition,the effect of the STBL on the surface

energy budget and thus the overall climatology of the tropics is also thought to be signi?cant(cf.,Mechoso

et al.,1995;Ma et al.,1996;Philander et al.,1996).However,attempts to quantify these,and other,effects

are frustrated by our inability to quantify,let alone understand,key elements of stratocumulus physics.

Two questions stand out:First,how ef?ciently do stratocumulus entrain(incorporate through turbulent mixing)air from the warm,dry,quasi-laminar,free troposphere,into the cool,moist,turbulent boundary layer?Second,how important is drizzle?The two processes are,of course,related.Both act directly to re-duce the amount of water in the cloud layer,and indirectly to modify the heat budget,thereby impacting the dynamics.Moreover,because drizzle is thought to suppress entrainment(Stevens et al.,1998),and because entrainment is thought to suppress drizzle,the relative interplay between the processes may be subtle,which could make them dif?cult to untangle.Nonetheless,recent advances in observational technology have intro-duced new possibilities for understanding entrainment,drizzle,their interplay,and external processes(such as factors regulating cloud microstructure,and cloud-aerosol interactions)which may regulate this inter-play.This combination of re?ned theoretical questions,and advances in observational technologies,helped to motivate a recent?eld program,DYCOMS-II(Dynamics and Chemistry of Marine Stratocumulus)which this paper aims to describe.

Entrainment

To help one understand why entrainment is so important it helps to think in terms of the mixed layer theory of Lilly(1968),wherein the STBL is identi?ed as a distinct layer of the atmosphere whose properties are largely determined by exchanges with the underlying surface on the one hand,and dilution through the incorporation of air from the free-troposphere(i.e.,entrainment)on the other.The entrainment velocity, can be de?ned in terms of an equation for the depth,of the STBL:

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cooling in the vicinity of the cloud top interface leads to systematic biases in estimates of entrainment.More

uni?ed treatments of physical processes have helped reduce the discrepancies among models,but(perhaps because of poorly understood numerical sensitivities)signi?cant differences persist.In a survey of recent

work Stevens(2002)shows that different entrainment rules derived from state-of-the-art simulations can

still differ by more than a factor of two.Moreover,when these parameterizations are incorporated into a

mixed-layer model,the equilibrium solutions for typical climatological conditions have equilibrium sensible

and latent heat?uxes that vary by as much as40Wm and cloud liquid water paths that vary by factors of two or more.With this degree of discord,one might think that observations could usefully arbitrate disputes

posed by models.However,estimating entrainment from real data has also proven to be challenging.

Fundamentally there are two different techniques for inferring from data,we call these the pergence and tracer method respectively.The pergence method evaluates from(1)as:

(3) where

can be estimated independently in two ways(either directly via eddy-correlation,or as a residual of the budget of over the STBL as a whole),to the extent that the numerator in(3)is the sour …… 此处隐藏：41641字，全部文档内容请下载后查看。喜欢就下载吧 ……