The Canadian Climate Center (CCC) GCM has been modified to allow its use for studies in atmospheric chemistry. The initial experiments reported here have been run to test and allow sensitivity studies of the new transport module. The impact of different types of parameterization for the convective mixing have been studied based on the large scale evolution of Rn-222 and Pb-210. Preliminary results have shown that the use of a scheme, which mixes unstable columns over a very short time scale, produces a global distribution of lead that agrees in some aspects with observations. The local impact of different mixing schemes on a short lived tracer like the radon is very important

Beagley, Stephen R.

Degrandpre, Jean

Laprise, Rene

Mcconnell, John C.

Mcfarlane, Norman

English

NASA Technical Reports Server

N95- 10608
A GLOBAL NUMERICAL STUDY OF RADON 222 AND LEAD 21°
IN THE ATMOSPHERE USING THE AES AND YORK UNIVERSITY
CDT GENERAL CIRCULATION MODEL, (AYCG).
Stephen R. Beagley t, Jean de Grandprg l, John C. McConnell 1, Ren6 Laprise 2, Norman McFarlane n.
Department of Earth and Atmospheric sciences, York University, North York, Ont., Canada M3J 1P3. t
D6partement de physique, Universit6 du Qu6bec h Montr6al, Montr6al, Qu6bec, HC3 3P8. z
Canadian Climate Centre,The Atmospheric Environment Service, North York, Ontario, Canada. s
Abstract.
The Canadian Climate Centre (CCC) GCM has been
modified to allow its use for studies in atmospheric chem-
istry. The initial experiments reported here have been run
to test and allow sensitivity studies of the new transport
module. The impact of different types of parameterization
for the convective mixing have been studied based on the
large scale evolution of Rn 222 and Pb 21°. Preliminary re-
suits have shown that the use of a scheme, which mixes
unstable columns over a very short time scale, produces a
global distribution of lead that agrees in some aspects with
observations. The local impact of different mixing schemes
on a short lived tracer like the radon is very important.
1. Introduction.
Recently the modeling of chemistry has moved to the global
scale, permitting the assessment of the multitude of chem-
ical reactions within a global atmospheric model. The
model described herein, the AES and York University Chem-
istry and General Circulation model, is a version of the
Canadian Climate Centre General Circulation Model, (GCM).
The model used is the operational version of tile CCC
spectral GCM (McFarlane et at, 1992). Initial experiments
were aimed at examining the use of the spectral method for
tracer transport and assessing the transport module. This
module includes, multi-tracer advection, convective and
boundary layer mixing, wet deposition, vertical and hori-
zontal diffusion. This work describes some experiments oil
radon and lead tracers chosen to assess the part. played by
convective mixing in the transport of species within the
GCM.
The current results are for T32 resolution and the model
time step is 20 minutes. At this resolution the model re-
solves synoptic and planetary scale motions, however sub-
grid meteorology which impacts on these scales are absent.
We have chosen Rn 2_2 as being representative of species
with short chemical lifetimes such as isoprene and propane,
and Pb _1° as being representative of a longer lived Slmcies,
78
e.g. CO and 03. Very simplistically we are equating Rn 222
with a precursor chemical like NO= or hydrocarbons which
have short lifetimes, while the lead is equated to the longer
lived chemical products, whose presence in the upper tro-
posphere can only be achieved if its precursors are trans-
ported rapidly from their surface source regions. In ad-
dition, radon tests the impact of a simple surface source
while the Pb 21° production is distributed throughout the
troposphere.
2. The Radon Experiment.
As a first test of the model and transport module, a num-
ber of paired Rn 22_ and Pb 2m like tracers were used. This
allowed several convective mixing schemes to be considered
and a general assessment to be made of the overall use of
the spectral GCM as an online tracer advection model. A
test of the mixing, rainout and spectral advection plus a
first check on mass conservation and other numerical prob-
lems associated with sharp tracer gradients modeled in a
spectral model was thus possible.
Initially, radon was distributed uniformly in the hor-
izontal, with a fixed vertical profile, decreasing exponen-
tially with height. The ground level value of 100 pCi/m 3
STP was chosen based upon typical observed surface radon
measurements. Lead was set as a low backgrom_d which
would be dominated rapidly by the radon decay source.
I)ue to its short lifetime, the radon rapidly reached a global
balance between the mo(h!l source and decay within the
first month and was largely unaffected t)y the initial state
thereafter. For the lead, a global mass e(luilit)rium state
was not ext)ected within the first several months due to its
longer tropospheric lifetime. Since the radon surface tlux
is confined to the continent, the sharp boundaries and the
hemispheric gradient, due to the source distribution, pro-
vide test, problems for the model transport and analysis.
The radon source was modeled by assuming a constanl
surface ttux of 1 aloln/scc/cnl 2 from all non ice covered
land on the glol)(!. The radon sink duo to radioactive (h'-
ca?,, was el)rained using a tendency based on the tracers
5.5 day e-folding tiIne. Thus excluding the transport, the
bu(lgel for Ih, =_ is dcscl'ib,',i
https://ntrs.nasa.gov/search.jsp?R=19950004196 2020-06-16T11:46:49+00:00Z
inequation1,
_X, X,
- +F_ (:)
6t r
where X,= Rn 222 tracer mass mixing ratio, r= radon e-
folding time, and Fg= surface flux of radon. The Pb 21° was
assumed as the only decay product of tile radon so that
the sink of radon in molecules was the same as tile source
of Pb 21° in molecules. Its sink was modeled using a simple
rainout scheme, proportional to the local column rainfall
amount and the local tracer mixing ratio (Mahlman and
Moxim, 1978). Thus the budget for Pb 21° is;
6Xv _ )&Mp Xv
-- (2)
6t rM, r,o
where kp= Pb _1° tracer mass mixing ratio, Mp = molecular
mass of Pb _m, M, = molecular mass of ]{n 2'_ and r_o =
rail:out e-fohting lifethne as defined bvlow as;
r.,(_'). < 1' > (a)
Tro -- f)
where r_,= wet deposition liflq.ilne (as a fumtion of height,
k,), P = local average precipitation amount (mm._-l),
and < P > = the global average precilfitation anlonnt
Convective mixing of tracer fioln the boundary layer
into the free troposphere has been i(h'nti/ied as an impor-
tant component in a complete transport description of the
troposphere. Tracer ,nixing schemes that have been used
for radon include those of ,Jacob and Prather (1990), Fe-
ichter and (h'utzen (1990) and Brost and (?hal/ichl (1989).
These schemes vary in comt)lexity wilh a balance being
struck between delailed model description and expense.
However due to the large scale of Ihc (',(!M's horizontal
grid, parameterizations are, at best, highly approxinlate.
,Such a position poses a serious diilicult.v in global model-
ing and rcqtlires careful ass(,HSlllOlll iilld observational basis
before h_lerpret at ion can I>_,considered valid.
It was decided lo lt.sl l;vo st'hclucs ill pa,'alld using tim-
damenlally ditE'renl Iransl)ort assumlH ions. 'lhe lwo basic
schemes used are; a) a direct wlocalion scheme and h) a
diffusive scheme. 'l'hc dirv(t velot'athm scheme is based
on the scheme descril),'d in lhalher el al.(1987), which as-
sunn's that within a model grid box a fraction of tra('er
mass is diwctly advected from the convection base to Ihc
top of the conw'clivcly unstal>h! ('o]uuln. ('onlhluity lhpn
r(!(lllir_?s all illStalllan(!ous Sll])sid(,ll('l' to nlov(' ll/l{'¢'l" frolll
the top down to the m'xl lower box, throughout the col-
umn. Thus total mass is conserved ill Ihc cohluln whih,
redislril,ul i::g tra('('l'. Thl, diffusive scheme was based Ill),.)ll
the convective adjtlslmellt us('(t in the (;(!M for convectiw"
parameterization of heat and moisture and in lhis sense, it
is internally COllSiSl(_lll wilh lhe dym,,dcs. The diffusive
scheme is (litl'er('nt fi(,n, that of th(" dii(,(l rd(,calion, in
that it tests a(ljacem l>oims il_ a colutlln ;tile| mixes be-
tween them if an instabilily exisls. This protcss is done
from the ground up so il is possible for l l';lccr to slowly
'diffuse' up through the column. The two schemes are
fundamentally different and will have significantly differ-
ent impacts upon the transport of tracers with different
lifetimes.
3. Results.
Due to its relatively short life time, the radon is on av-
erage mostly concentrated over the land area. Figure 1
shows the mass mixing ratio of radon at the surface, aver-
aged over the second month, on which can be seen many
features associated with the large scale circulation. In the
tropics, zonal transport produced by the trade winds is ev-
ident as well as the impact of the tropical circulation due
to the presence of the high pressure belt in the southern
hemisphere. The persistence of such an anticyclonic circu-
lation over the tropical ocean produces surface winds fol-
lowing tlle west coasl of Africa and south America causing
a strong gradient on the edge of holh continents. Average
stlrface concentrations of radou obtained oww most of the
northern conlinenls range from 100-300 pCi/m a which fall
in lhe range of what is currently observed (Liu el a[.,1984).
However, spatial and temporal variability of radon is such
that comparison between observation and model results
over specific regions would necessitate further analysis (Fe-
ichter, 1988; .Jacob. 1990).
At mid-and high-lalilude in the winwr season, the im-
pact of collvect i',:omixing is weak no matler which scheme
is used. Tropical regions are however strongly affected
by tlw choice of t)aranwterizalion and show highly local-
ized spalial and temporal variations. To illustrate the
dilfevem'es Iwtwcen lh,' schcuws, Figures 2t) and c show
time snap-shots of lhe radon dislributions for the two dif-
f('rellI parameterizations, for 71°West at model day 33,
00:00 GMT. Figure 2a shows, for comparison, the result
achieved when no con\ct't ire mixing is applied. The 'diffu-
SiV(" _llld IlOll-(-o|lvt,cliv(, mixing varianls are very similar
wilh only a slighl spread of ll'acer (lilt" to diffusion from the
Ira('t'r plumes gencrah'd by synot){ i(" scale ascent. This in-
di('alcs lllat the 'diffusive' technique spreads tracer slowly
to high altitudes I)ul has lillh' power to it:.i,.'('t tla('¢'r higher
(hw to the relatively shorl lifclimc of radon. The 'roloca-
lion" s('lwme however is slHkhlgly dil[(,rent l>roducing a
high level Iraccr maxima in l}u" tlopics righ! u I) to the
Iropol)aUse and lower sll'alosI)hcr¢'. [:Iaddhion, without
('oll\'ccliv,P mixing th(' lllodc[ tracer dHc Io till' SOlll'('o _t[
lhe ground) builds u I) slrong gi'adie:Hs in ltw lowesl lev-
('Is (l:iguve 2a). producillg m,Hu.vical twohlclnS a: land-sea
l>OUli(laricsand al_ovc<'oIHilwi:ls.
Tiw mean wsidc:.c li::wof h'a,l i:t ti,c slralOsl)lwrc
is considcrahly hight| Ihan in 1t.' *v_q>-'-I)hc:*'. wh,'w lhe
lead is washed oul by pr_,cipilalion, l'i,.vious similes have
shown lhat llw quanlilv of lead in the stratosphere is a
signiticant I'raclion. of the total {Moore. 1973) and could
corrc>.l,O:,d Io ::I:_;V of *h,' gh,l>al inas._ ,,f h'ad in the at
i:lOSl>hCl',. {I.;mll,cl'l. 1!}'.2:). ()ur rcsuhs show lhal such a
vcscivoir c;m Iw ;_i>i>1_,achcd I>v using Ihc relocation tech-
,Jiq:w
7q
LL
4, /2 1 .0 _
Figure 1 : Radon =2 surface mass mixing ratios, (pci/m a
at STP), for Radon variant ii) 'Diffusive' convective mix-
ing, Monthly averaged data for February.
L
which gives a stratospheric mass fraction of 25% at quasi-
steady state. The fraction given by the diffusive mixing
scheme is of the order of 5-10% which is significantly lower.
In spite of the slowness of the diffusive s(heme, it still
contributes to tile transport of species into the higher tro-
posphere. Budget analyses show in fact that close to 30%
of the transport of radon to the tropopause could be at-
tributed to this process. To allow the tracer to go further
into the stratosphere, an additional type of parameteriza-
tion could be needed in that case to represent the exchange
of tracer that occurs across the tropopause.
Part of the aim of these experiments was to determine
the capabilities and limitations of the spectral technique.
Initial model experiments reveal the model conserves mass
well, giving a maximum global mass variation of 0.1 per
cent in 2 months. I{unning al T32 resolution lhe radon and
lead tracers together with lhe convective nlixixlg schemes
give rise 'co slr,:ulg gra(li(,nls locally, regiolmlly and glob-
all3', l)espite the noisy nature of the convection and the
i)orlIlalielll Mrong land s_.a gradien*s which arise the to-
tal global magnitude of lhp mmwri<'al errors remain small,
less tha_l 0.25 percent of the global mass being used to
renlovt' n{'gativ(' val/ws ill 1he |I'?I('CI" fi('ld, l:or tla(lon 1t,,
cOlll})]Iw¢l ilnl)aCl ,,[" llw grolln(I Slml'¢{, and t}lo convective
mixillg is a r,'lativcly _,,v_,r,, _'_,sl on thv use of the spectral
advecl it'{' sctwnw. ( ?()llS,_',:illonl 1',," sharI) gradients and low
ba('kgi'oulld valm's will b(' pr_.huc,'(t anld numerical pn'ob
lenls t,(. more likdv..\ surprising bonus of Ill(' convectiv('
mixing has b_','n i*> an,eIioraling effecl upon lhe need for
hole-filling in _hp model. ,.\s the I'('slih of aclive convec
lion, lh(' gradients of I/n 2'e are no h)ng('r so sllarl) and
lrat)i,ing ill th(. Iow{.r nlodc,I levels is t,'(hu'{'d.
A iluli1},ct .f liac_,rs in f*llui(' sludic, will Im like tin z2e
iu Ilavila_ a s,,_m(' r(',lri_l('d to ov,,r ]all,1. This _ i'('atcs a
tracer distribution with steep horizontal gradients at the
continental boundaries. Such a distribution must be mod-
eled by the advection scheme, maintaining the gradients,
to he accurate. Figure 1 illustrates the spectral model's
description of tin 222 near the surface, with strong gradi-
ents in the horizontal, with the tracer mass predominantly
'clinging' to the land surfaces. Future tracers will also have
detailed geographical source distributions, e.g. dependent
on terrain characteristics. The impact of similar sources
varying on wwy small model scales (sometimes below the
models 3 -,lAx limit) will be partially tested as a result
of the 'noisy' nature of the convective mixing which to the
upper troposl)here is acting like a highly variable source
function in both space and time. Witt, these types of ad-
ditional requirement for adverting chemical tracers, the
.,,l)ectral technique is one among many allernative schemes
proposed. GCMs using the sl)ectral technique at low reso-
lution cannot re.solve strong gradients. Other alternatives
exist including semi Lagrangian and second-order moment
techniques which can provide more accurate adveetion but
normally at the expense of more computational effort and
SOllletinles poor(,r inass conservat ion [)ropert it's.
The radon distributions exhibit strong lransl)ort of trac-
ers inlo the Ul)l)er troposphere l)arlicularly in the tropics
bolh via synoptic and sul)grid motions, rl'h,., consequences
of this transp()rt art' imporlanl in any study of tracers
whose source is the trol)osphm'e but whose sink lies in
the st|'atosl)her('. \Vilh this type of transport scenario
in mind the radon and h'ad tracers have been examined
'_O View h()w the model moves t,'acers to the upper tro-
l)(}Sl)her(' and across lh(' Ir()l)ol)ausv into the lower slrato
:_l)hvl*'. Thl' inlpac! of Ihe 'rc[ocalion _onveclb.'e mixing
and larg(, sym)i)lic as('enl upon the r(.dislrii)ulion of radon
from ilw I),.mndar\" la:vr to lh,.' Ulq',('r t f,.)l,osph('_,.' is con
sid('rat)l('. It could hay<' also an ilnpt)l'lalll ('([('c{, ill Ill('
long term, Ul),m lh(' cross {'(luatorial lransl)c, rl of long lived
1[ rlt ('e i".
8(/
a) te
16(
255
4311
653
8O2
b/ 90
-.00_ -.005 _Hr .000
O07L L 15_ L L
.0|6 006__ -001
T _tilltde
L
000
'It60
* I
2_ 1 -._03
H L _D_II IH7
.008141
1
l ,_LTu,L,
l"igur(' '2 : Radon 2e2 mass mixing ratio conslant lon-
gitude (-ross-s('('tious. Iougitudo 71"\VcM, for variants:
a) No COlIVOCtiVt_"tracer mixing, h) 'Diffusive" conw:c-
tire Ira(or ulixillg, and (') 'direct l_clocation' convective
_racer mixing, for l)ay 33, 00:00 (',MT. (Scale<l by 1.0* t*,)
Vertical axis shows data on model hyl)ri(l 1.vols. where
_t = f(P' ¢y )
4. (:onclu'don_
A lev,,I I) online Irac,q-model i_ now collll,iel(" and test-
ing hMicatc_, that mas_ (Oli_,crva tiC'II _ti/(] ll(,_atives are not
signifi.ant I,r(>},l('m_ }mr that a b.,..ttcr adv_'('tion sche,ne
Ilia>' I)o needed if S}larl> gradients produced dynamically
and c}lelni('ally are to he ulainlained acc,u'alely, llowever
the _peclral torhniquo with suftich'nt resolution is still a
pow(,rflll and viable aIH,l'nal ix'('. St'nsii iviiy st ndh's reveal
the importance of the parameterization of convective mix-
ing on tropospheric tracer modeling. The model's convec-
tive mixing has maximum impact over the equatorial con-
tinents, with very smMl effect in the winter hemisphere
higher latitudes. In terms of global impact, the param-
eterization of convective mixing based on the relocation
technique appears to be more efficient in representing ad-
equately the transport of long lived tracers into the lower
stratosphere. However, the local impact of both schemes
over specific regions still needs to be assessed with more
analysis. It has been also noted that convective mixing
acts to reduce spectral problems due to its diffusive effect
on tracer gradients. Strong synoptic vertical motions pro-
duce important transport of radon into the tropical up-
per troposphere, greatly increasing the potential pool of
tracer available for transport into tile lower s_ratosphere.
The convective mixing parameterizations used are rela-
tively simple and questions on the degree of entrainment
throughout the column, fraction of mass 'relocated' and
the height of the ensemble average convection within a
GCM grid box rcmah_ outstanding. Future modeling is
intended using alternative and more cornplex schemes in
order to assess and improve our ability to parameterize
and interpret subgrid mixing within global models.
Acknowh'dgcments: We would like to thank the Natu-
ral Sciences and Engineering Research Council of Canada
for their supl)ort.
l(eferences
Brost,R.A. and ChatfiehI,R.IL, Transport of Radon in a 3D
subhenfisl)heric nlodel, .l. (;eopltys. Hes.. 94, 5095-5119,
19N9.
Feirhter,,J. ;tilt] P..J .( :rllt zel[, Parameterizat ion of vertical tracer
tra, liSl)ort flue to dee 1) culnuhls convection in a global
transporl model _utd its evaluation with l{;tdoI| 222 lll(!a-
surements, Tr:lht,_,42B, 100 117, 1990.
.Jacot),l).,l. and M.,l.Prather, Radon 222 as a It's( of t:onvec-
tive transl)ort, THlus, 42B, 118-13,1, 1990.
l,ambert ,(',., Poliaa,G., Sanak,J., Ardouhl,ll.. lhfisson,B., .le-
gou,A., and Roulley,.l.C., Cych' du Radon et tie ses de-
sc0ndants: application h l'(tutle ties/changes troposphere-
stralosl>here. Ann. (;[OldZ.Vs., 38, ,197-531, 1982.
Liu,S.(!., J.ll.MrAfee, and R..l.(!iterone, lladon 222 and tro-
pospheric w,rlical transport, J. (;_ optqp. 1_(._., 89, 7291-
72!17, 1.9_.1.
Mahhnall,.].l)., and Moxiln,W.,l.,'l'rat'('r silnulati(m using a
(;(:Nl: r,,sulls froln a Mid-latitude insiantaneous source
exporhitellt, J. Atlno._. ,S'ci., 35, 1310 1374, 197N.
M('l"arlaILe,N.A. (;..l.lh)er,,l.-1). Blanchel, and M.l,azare, The
('anadian (:lhnalo ('entre second K('ll('I'itti()II (;('X1 an(t
its Equilibrium (:lima(e, d. Clim,t., to be published,
1992.
MooreAI.E., S.E.Pt)el. and E.A.Martell, Rn 2_, Pb 21°, Bi 2m,
l>o 21° Profiles and aerosol residence times versus Mti-
lu(h'..1, (;_Ol)/Ul.,, lhs.,78, 7065-7075, 1973.
Prat her,M..l., M. M ct':h'oy, S.V¢ofsy, (;.Russell, I).Rind, Chem-
ist ry of the gh)hal trol)osI)here: Fluorocarbons as tracers
()fair motion,,l. (;_ophys. Re s, 92, 6579-6613, 1987.


A global numerical study of radon-222 and lead-210 in the atmosphere using the AES and York University CDT General Circulation Model (AYCG)

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server