";,*lt+'EiihfftEH|+J F,158fit2 i

51 63lL '+i'iitl +121j

Papers in Nleteorologl and Geophl'sics \rol. li. No. 2, pp 51-63.December l99J

Numerical Simulation of Regional Scale Dispersionof Radioactive Pollutants from the Accident at the
Chernobyl Nuclear Power Plant
byTakehiko Satomura, Fujio Kimurax, Hidetaka Sasaki,
Tomoaki Yoshikawa
Meteorological ResearchInstitute, Tsukuba, Ibaraki, 305 Japatt
andfoshitaka Muraji**International MeteorologicalOceanographicConsultant, Shintomi,
Chuou-ku, Tokyo, 104 Japan(Received\{arch 31,1994; RevisedNovember15,1994)

Abstract
Air concentrationand depositionof radioactivepollutants in Europe after the Chernobl'l
nuclear pou.er plant accident is simulated using a long-rangetransport model developedin
the MRI. This model is composed of the rveather forecasting part u,hich rvas the routine
regional weather forecastingmodel of the JN{A and the Lagrangian advection-diffusionpart.
The source data distributed b5,ths ATN'IES project is used to determine the initial release
rate from the pou'er plant.
Calculatedconcentrationsof Cs-137and I-131in the surfacelevel atmosphereagreervellu,ith observation. Surface deposition of Cs-137has. however, poor correlation r.r,iththeobserr-eddeposition.Bad precipitation forecast of the r'veatherforecastingpart of the modeland the differencebetu'eenthe horizontal scalerepresentedby observationand b1.simrrlation
are consideredto be responsiblefor this poor correlation.

*)**)

Present affiliation: Institute of Geoscience,Univ. of Tsukuba, Tsukuba, Ibaraki, 305 Japan
Present affiliation: Energl' Sharing Co., Ltd., Arakan'aoki-Nishi, Tsuchiura, 300-11Japanel99a by the Meteorological ResearchInstitute52

S a t o m u r a T . , F . K i m u r a , l l . S a s a k i .T . \ ' o s h i k a i r a a n d Y . \ l u r a i i

1. Introduction
Recently, regional and international concern on air pollution has increased,and u'ith itneedsfor tools to evaluatelong-rangetransportof pollutants have augmented.In the Meteorological Research Institute, a Lagrangian air
pollution transport model has beendevelopedto
simulate air pollutants related with acid rainover Eastern Asia. Although databases of
releasedand concentratedpollutants averaged
over the time scales of months are wellestablished(for example,Johnson,1983;Air andEnergy EngineeringResearchLab., 1989),observation of air concentration and deposition ofpollutantsreleasedfrom a u'ell-definedsourceis
hardly available; this kind of data is indispensable to evaluate and improve the longrange transport model.
On April 25, 1986,a seriousaccidentoccurred at the Chernobylnuclearpov,'erplant, USSR,
and a large amount of radioactive substanceswere releasedinto the atmosphere.Becauseof
its serious impact over Europe, studies of the
movement and deposition of radioactive substances have been completed (for example,
Kimura and Yoshikawa, 1988;Hass e/ al., 1990:
Puhakka et al., 1990). These studies haveestablishedthe following: the radioactiveclouds
releasedby the main explosionof the plant u'astransported northward very fast in the u'arm
conveyor belt and deposited over northern
Europe in the first several days after the
explosion. On 30 April, the radioactive cloudn-rarchedto central Europe.At the same time, apart of the pollutants transportedto the middle
troposphere moved eastr,vardat high speed andreachedJapan on 4 May 1986.
Depositedradioactive pollutants have serrouseffectson liveson the surfaceand,therefore,
they should also be analyzed and predictedprecisely. h.r the case of Chernobyl, wet
deposition is believed to be important in thenorthern and central Europeanareas(Hasse1al,
1990; Puhakka et al., 1990). In the United
Kingdom, Clark and Smith (1988)investigateddistribution of dry and wet depositionof I 131
and Cs-137after the accident. The amount ofdeposited Cs 137 correlated rnell with precip-\ - o l . 1 5 .N o . 2

itation. This means that the wet deposition
seemsto be dominant for Cs-i37.On the other
hand, distribution of depositedI-131 rvas quite
different from that of Cs 137,so that the drl'
depositioncan be expectedto be more importantf o r I - 1 3 1O
. n t h e g l o b a ls c a l e P, u d y k i e w i c z( 1 9 9 0 )reported the distribution of the radioactive
pollutant simulatedb-va global scale predictive
tracer model. He calculated the pollutant distribution by both diagnostic mode and fully
predictivemode of the model and comparedthe
resultswith the observation.The model includes
dry and wet deposition, but the amount ofdepositedradioactivit-vrvas not comparedrvith
observation.
The mechanismof transport and removal of
the radioactivepollutantrvouldbe similar to thatof the long-range transport of sulphate andnitrate, w'hich are the main precltrsorsof acidrain. The accidentgave Ltsa chanceto evaluate
the accuracyof the long-rangetransport model
for acid rain such as the model usedhere.
In this study, the N{RI long-rangetransportmodel is evaluatedin the case.of the Chernobyl
accident.The emissionand observationaldata
on air and depositedconcentrationof radioactive pollutants used in this studr' ',r'erethe data
distributed in a project narned ATMES (Joint
IAEA ,/ WN{O CEC Atmospheric Transport
llodel Evaluation Studv). This project wasstarted a ferv months after the accident.and theaim of the project \\'as to ret,ie'nvand intercalibrate atmospheric transport models using
the observationaldata of the Chernobylaccident( K l u g , e l a l . . 1 9 9 1 a )I .n t l . r i sp r o j e c t ,a t i m e s e r i e sof the releasedanrountof radioactivepollutants
u.ith their effective heights and time series of
surface air concentration and of deposited
pollutantsobsen ed overamountsof rzrdioacti,,-eEr-rropeu'ere distributed. Though the amountand the fornr of the emitted substancesare notprecisell'determined(Petro,,',private communtcation) and up to 3 hours ambiguity remains in
the obsen.ation time (Raes e/ a/., 1989,1990),
thesedistributeddata are believedto be the best
data set over Europeon the Chernobylaccident.

Numerical Sinulation of Regional Scale I)ispersion of Radioactive pollutants

2. Numerical

Model

53

1* sir-r60"
ttI

The model consistsof tu'o parts: a weather
forecasting part r,vhichpredicts meteorologicalvariables, and an advection-diffusionpart ofpollutants u,'hichuses meteorologicalvariables
forecastedby the first part as input data.

1 + S 1 np

9is the latitude,
zr: P, -Pt,prprpressureat the model top,
surface pressure,

P:pipa,u

2.1.Weather ForecastingPart
This part of the model is almost the sameas the old version of the routine weather
forecasting model of the Japan Meteorological
Agency (JMA). Details of this model aredescribed in a technical report issued by the
Numerical Prediction Division of JMA (1986).
We modified the topography and physicalparameterization in the original forecastingmodel to apply it to the Chernobylaccident.In
this section,a brief descriptionof the model isg i v e n f o r r c a d e r s 'c o n v e n i e n c e .

q

E

N{

r u/ mand n ui m,respectively,
water vapor mixing ratio,
vertical eddy flux of water
vapor,
vertical eddy flux of sensibleheat,amount of decreasein water
vapor by condensationor
nrpcinifa

tinn

2. 1.1. CouerningEquations

Reynoldsstressby verticalwind shear,
F,, F", FA, FN
horizontal eddy diffusion of u,u, d, and q, respectively,
and the other symbols are conventional.

We use the primitive equations in o-coordinatesand the polar stereographicprojection
at 60'N. The equationsare:

2.1.2. Grid Systemsand DiJference
Schemeso I nod, * , d l;ro \ ftt '\i l \ , , , , ) ' d . r J t t ^ t t ) ' ; r . \ v ^ t to) n' \ , , , r " 1 , , , , r ,t l )/\/ittr ' out\
t'
,).,
d.r')

'

t Jd'
; r 0 d l ' ^' . .
' ",,,
f"
ttr ?,..ti.r'l in'r\,,r.(;;,J.

;: ' - - 'o t :.
('*'' #(',',1,')) --"t)\ oir'tr*';,\ ,:,,',1,,,,
,.,,

I
\'

'

6nt ' dtn\
"
d.r'
dj )-

( iirr;r 66
t r 1 d P ^' ^'o'-ttt 6t'
ttr ,).Y,,,t\jo)^ '-,:'. l r "- ,riio l (\ ) 1 t ' uI\ oi. Y\ t t * o ) oi- \ ' 1 t * u ) -oiO (\ i l't'' " , /u )otH
ct'ottt2P' 60a;' d rtro t)'
6x\tr^q 6:.\,*qt
6n\,,,r't1

o t :r\(lol \ ti,' )'

p
.; ""
,lrr
ln'
m'
6tf 6>* 6/trot
I

o tt

\

a

i, \,,,'/ ;;
A,l'
()o

;;

. . , , d l ) ^- '

LDo '
06

rvhere the symbols are as follows

The calculationdomain and the topography
in the model are shown in Fig. 1. The domain iscoveredhorizontally by Arakawa's B grid (73X
55). The grid size d is \27 km at 60'N. The
domain is divided into 16 layers in the verticaldirection as shown in Fig. 2. Finite difference
schemesin space are based on Arakawa andLamb (Igi7), and time difference scherrre
is Tatsumi's
economical explicit scheme"( T a t s u m i ,1 9 8 3 ) .
2. 1.:1. Bottndary Conditions

{-ll

JI:.
06

;o(,;,;,) r)

T

(3)

fr.,
tttto

n
' o

-

ano u'(5)

At the upper and the lower boundaries,we
assumeri - 0.At the lateral boundaries,we apply
the Hovermaleconditionby addingthe following
term to the right-hand sides of eqs. (1) (5):r<(f)'vlre Ae),
(6)

(8)

q/

S a t o m u r a ' f . , F . K i m r r r a , I l . S a s a k i ,T . \ ' o s h i k a w a a n d Y . N ' l u r a t i

V o l . 1 5 .\ o . 2

Fig.1.Domainof integration.Coastlinesareinclicatedbl'dots.Contolrrforterrainelevation"S". 'Ilu".
"A", "Bi"( s o l i d l i n e )b e g i n s a t 2 0 0n i , a t i n t e r v a l so f 2 0 0m . P o i n t s l a b e l e d
\l",
"Be"
are Stockhohn, Budapest, \lol. Attikis. Bilthoven and llerlin. respectively.
andr.ariable,A is a prescribedu,.hereA is a ph-vsicalboundary value, / is the distance from theboundarl'," s e ct ,K=4x10
L:6d ,
and,12v

In.r,.\',

I.f 'f,.d..\dAr.r,a.:' at- - 1 ,I

r.

/..\

.l

r
d,.)

4rlrr,.r,2. 1.4. Parameterizationof Pkysicul
Processesa) Horizontal and vertical diffusion

J,

-,r

(9)

As the vertical eddy' diffusion, wee m p l o l ' t h e c l o s u r em o d e l o f l e v e l 2 ( M e l l o r
and Yan'rada.197,1).At the lowest layer,
Monin-Obukor-'ssimilarity theory is applied
to determinevertical fluxes from the lorverboundarl'. The surface temperature ispredicted.As the horizontal eddy diffusion,an artif icial ,lth order diffusion is appliedas
follorvs :F - ,-,-

I(r.\'{rV (V ?a)}tr, -K r . V{ ; r V ( V ? b ) } ,Kar"rVlV ,'(V a)}'(10;

u'here rz is horizontal velocity u or r., b isscalar variable 0 or q,

-\unterical Sintulation of Regirnal Scale Dispersion of Radioaclir.e It)llgtants0;=0

o=0.050.1
0 .1 5

0.25

N

? R

0 . 45o.ss--/--'---l
0 . 64o . 72o.79
o . 850.9
o . 940 . 971

Fig.2. \'ertical grid s1'stem in a.coordinate.

2.1.5. Input Datal
.1 t, -./)

l. r.{,.'

r

d

Lr

l

r',.. .

Iih..5x1()rm2lsec.
Ko,.-10x104m2isec.
These tern.rs are ir.rtroducedto dissipate
noise rvith a ver]' short r',,ar.e1ength,andl eaning.
h a v e n o p h 1 ' s i c amb) Conr,ectionand Condensation\Ve use t"vo t1'pesof parameterizati<nfor precipitationprocessin the model : theparameterization
large-scalecondensatior-r
and the moist conr,'ectiveadjustment.Thef o r m e r p r e v e n t s s u p e r s a t u r a t i o rar n d t h e
l a t t e r k e e p s t h e a t n r o s p h e r i cl a p s e r a t e
lreu
r e e n r h e d r r , a d i a b aitc a n d t h e m o i s t
adiabatic lapse rate.

At the time of the Chernob-vlet'ent,a global
objectir,eanal-vsisrnasexecuted everl' 12 hours
b1-lhe JMA. It has 2.5degreesresolutionboth rnlatitude and longitude on 16 pressure levels.
From this objectiveanalysis,\\'e prepare initialand boundarl' data for our model bf interpolation.
Sea sr-rrf
ace temperature is interpolated
from global clirnatologicaldata rvith 1 degree
resolutionin latitude and longitudepreparedby
Alexander and N{oble1'(19i6).

2.2.Advection-DiffusionPart
Advection and diffusion of radioactive
pollutants are described b1- a random-u'alk
model in the same coordinates)'stemsas in ther.veatherforecasting part. Winds, precipitationand diffusion coefficients used in the randomr'valk n-iodel are predicted bl' the 'nveather
forecasting model. The variables are stored at

S a t o m u r a1 ' . . l ' . K i r n u r a , I l . S a s a k i T . Y o s h i k a r i ' aa n d Y . \ l u r a j i

56every onehour integrationof the forecastmodel,
but the data of the first 12 hours are not usedbecausethe so-calledspin-uptime of the f orecast
model is about 12hours.Therefore,w.eintegrate
36 hours in the forecastingpart to create input
d a t a f o r 2 , 1h o u r s i n t e g r a t i o n o f t h e a d v e c l:^l:ff..^:^Ll(rll_url I u)lulr

The equationsof the random rvalk modelare :t d x --rt.
I

;,

o.t

) d

j '

)I ; n
|lo

''

(11)

- a'tt.

l d tu'here X, Y, ar.rdo are positionsof a particle rn
space.A random variable
the three-dimensional
R is defined asrD\ - * r .2\

K l
6/

,,

I;"pdtddep(Li)

I laep2.2.1. GouermngEquationsI

are satisfied: the height of the particle isl o r i e r t h a n a p r e s c r i b e dc r i t i c a l h e i g h t/ l 0.99 in a-coordinate,and a number given
randon.rl-v
for each particle and each time
step is smaller than a value Pat,1,
defined asI^^-+par r.

Vol. t5. \'o.:( 1 2)

rvhere Kz is the verticai diffusion coefficientderived from the forecast model, D/ is the timeinter.u-alin the random r,r'alkmodel,and the signof the right-hand side is chosen randoml-vfor
each particle ar.rdeach time step.
The effect of horizontal subgrid-scalediffusion is not included.The initial plume isbroadened bl' the horizontal and the verticalu'ind shear.

2.2.2. Time DifferenceScheme
A s i m p l i fi e d R u n g e - K u t t a m e t h o d t s
employed for time integration of threedimensionaladvection terms u.ith a long time
step dl :10 min. We enrplol.theEuler-backvn
ard
schemefor random vertical diffusion terms rvith
shorter time step Dr-2 n.rinin order to avoid
artificial convergenceor divergenceof particles
at the la1'erwhere Kz varies abruptl)'.
2. 2.3. Parameterization for Depositiotta) Dr)'deposition
A particle is assumedto be depositedby.
a drv processif the following tu'o conditior.rs

where I/a2is the dry deposition velociry
11a,n-100m.In this study, I/a*,:1 X 10 3 mr s
is usedfor I 131.Basedon the measurementin the United Kingdom, Clark and Smith( 1 9 8 8e) s t i m a t e dV t 1 , - 3 X 1 0 : r m r ' sf o r I - 1 3 1 .
They also shou-ed,hovn'ever,that the valuesof deposition velocities \\''ere lvidelydistributed.Therefore, W,1,-yX10 3mr/sfor
I-131usedin this studf is not an artificiallysmall r,'aluervhile this value is three times
smaller than that thel' estimated. For
Cs 137, Vt1,:5X10 a m,is is used and is thesame as the estimate b1' Clark and Smith(19 8 8 ) .
b) Wet depositior.r
Wet deposition uf the pollutants n-as
estimatedevery one hour. The particlesare
depositedon the surferceu,ith a probabilitl'
defined b1'
Ltuao,

Ca,ro' dl 'RIt.

rvhere Cr.,, is the n.e,t deposition rate, D/ theestimatedduration cif u-et deposition(onehour)a n d R R i s a p r e c i p i t a t i o nf a c t o r .
The deposition rates for pclllutants are
difficult to estimate.since the1.may vary u'itht h e p h a s e, , f t h c n r a t e r i a l a n d m e t e o r o l o g i c a l',conditions.For SO: and SOr n'et depositionr a t e sa r e e s t i n . r a t et do b e i n t h e r a n g eo f 3 - - 5 0 x
o
(
)
.
5
10 s'ancl
2 x 1 0 t s ' ( E l i a s s e n 1, 9 8 0 ) .
Pudl'kien'icz (1990)assumed Ca,.p-3.5X10ss 1
for Chernobl'l radioactive pollr"rtants.In this''s r isstr-rdy',5 x 10
assumed for the u,etd e p o s i t i o nr a t e o f I - 1 3 1a n d C s 1 3 7 .
T h e p r e c i p i t a t i o nf a c t o r R R i s d e f i n e d a s :o for particles at grid points
RR:1 if the predictedprecipitation rateis greater than 0.05mm,,hour,
RR:0 otherl.,,ise,o f or particles betweengrid points
RR is interpolatedhorizontallyusing RRs

1 9 91\unrertcal

arn',ndfho

Simulation of Regionai Scale Dispcrsion oI Radioactiye follutants

narfinla

2.2..1.SourceTermsa) Releaseheight
The release heights described in the
ATMES technical specification document
included in Klug et al. (I991b) are converted
to o ler,'elas :a-0.965300m
o:0.954600m-a:0.853
1500m-_b) Other sourceterm characteristics
We assumethe normal distribution rn'fheboth horizontal and vertical directions.
standard deviation used in this model isI 2 7 k m i 2 : 6 3 . 5 k m i n t h e h o r i z o n t a dl i r e c t i o r r
and 0.05in the a level (vertical directior-r).
Release rates of radioactive aerosol,
specified in the ATN,{ES Protocols, areshown in Fig.3. As a correction ofd i s i n t e g r a t i o n , 8 . 0 2d a 1 ' s i s u s e d f o r a
h a l f - l i f ed a - vo f I 1 3 1 .

; ::;]]' SourceDaral flq1Day
2.0 1A

1 . 5 l 0

I.o

10'

5-0 t00.0 1026 21 2A 2e.ool n

2

3

4

s

6

Fig.3. Time change of Cs-137(black bar)and I-13l(gre-vbar) release(unit in Terra
Bq/da1) b1' the Chernobyl accident. Solidline u'ith asterisk is the effectir,einitial
plume center of mass height (unit in meters).

3. Results
3.1.Movementof pollutants

2. 2. 5. Concentration calctthtiott
The number of releasedparticlesis 20000tn-foe a c h f o r t h e c a s e so f I 1 3 1 a n d f o r C s - 1 3 7 .
obtain the surface air concentration at a'2
location, particles rvithin a box of horizontal
grid lengths1--250km) fronr the location and 0.1
r - e r t i c a l l e n g t h i n a c o o r d i n a t e( - 1 k m ) a r e
coLlntedand the number of particlesis converted
into the concentration.To obtain the depositedconcentrationat a location,the particles lvithin
an area of 2 horizontal grid lengths from thelocation are counted and converted into theamoLrntof deposition.

Figure '1 shows forecasted r,veathermaps
and radioactive clouds.The rnodelpredicts then'ea ther

natt ern

rrell

comna red

rr it htheobjecti,,'eana11,5isof the JMA : centers of
c1-clonesand antic5'clonesare predicted u.ith
errors less than about 500 km and 4--8 hPa
ercept t$'o cases.The trvo exceptionalcasesare
or,er the Adriatic Sea on April 29, u-here themodel predicts a too strong c-vclone,and oversor"rthEngland on NIa.v 3, u.here the model'
predicts a too n eak cyclone (not shown).
, \ l t h o r r g ht h e s t r u r r g( r r ' e a k ct v c l o n eo \ e r t h e
A d r i a t i c 5 e a ( s o r r t hE r r g l a r r dmr a \ a t t r a c t t h eraclioactive ckrud stronger (rieaker) than theobservation,the model is expectedto represent
the or.-erallmovement of the radioactive cloudin good accurac-v.
The radioactivecloud releasedat Chernobl'1
on April 26 moved northr,vestu.ard
and reached
the Srvedishcoast of the Baltic sea in only trvo
days.Thus, the speedof the pollutantsaveraged
in these first two days is about 10 m,,'sec.This
rapid movementindicatesthat a warm conveyor
belt formed b], a lorv pressurezone o\rer \',,estern

58

S a t o m u r a ' l ' . ,F . K i n u r a , H . S a s a k i ,T . \ ' o s h i k a u ' aa n c l\ ' . \ 1 u - a t i

ffi
t\il"r

(a)

!1\
(b)

(c)

l,,''',i:'l(d)

F i g . 4 . F o r e c a s t e ds u r f a c ep r e s s u r em a p ( l e f t c o l u m n ) a n dp o s i t i o no f C s - i 3 7p a r t i c l e s
in the air (right column) at 00 UTC (a) Zfi April ; (b) 1 N,Ial'; (c) .1N{al' : (d) 6 N{at'.
Contorlr interval of surface pressureis I hl,a.

\ ' r r ] .1 r . \ o . l191.1\ r n r e r i c a l S i l t r r l a t i o no f I l e g l o n a iS c a l eD i s p e r s i r r n
o f l l a c l i o a c t i v cl , o l l u t a n t sEurope and the nurth Atlantic Ocean transported the pollutants.This rnovementcoincides
r,viththe trajectory anall.sisby Puhakka e/ a/.
( 19 9 0 ) .
After April 30,the pollutantsreleasedearlyon enteredthe cold section of the c1'cloneoversouthern Europe and thel' u'ere transpurted
southwestnard b1 a' cold conveS.orbelt tti
Austria ar.rd Sli'iss. On the other hand thepollutantsreleasedlater moved in the southerl-v
direction and reachedor,er Greek, Turkey andthe Mid-East countries.
P o l l u l a n l sr e l e a s e da f t e r ) l a 1 ' J m i g r a t e dover Eastern Europe in a fer",.da1'sand graduallrmoved further to the East.
3.2. Comparison with surface
observations of air concentration
Obserr.'ed
data of radioactir,epollutants at
about ninetl' stations for air concentrationand
about thirt].for deposition\\reredistributed b1'
the ATMES steering committee (Raes c/ a1,
1989)and stored in the data bank named RENI
Data Bank (Raesel al., 7990).\Ve selectedfour
stations Stockholm, Mol, I3udapestand Attikrs
from the distributed list of obserr.ationstations
and compared observedpollutants rn'ithcalculated values. \Ve chose the pcrintsbecausethe
locations of thesefour stations ranges oyer the."vholeEuropeanContinent (Fig. I ) and the)' areexpected to be t1'pical in the area over rvhich
dense pollutant cloud flor,r,ed.Figr-rre5 shor','s
observed and calculated surface air concent r a t i o n o f C s - 1 3 7a t S t o c k h o l n r N, { o l ,B u d a p e s t^ . - r A + l : 1 - : ^T L : . r : - - . . - . ) s h o , , l s a g o o d c o r r e l a t i o ndtlu

nLt1Kt5.

r r1r5rrgutc

betrveen simulated concentration and observation : initial arrir.al day of pollutants (except
a t B u d a p e s t )a n d t h e t e n d e n c l ' t o i n c r e a s eo r
decreasein time at each location agrees rvith
observation.Looking int<t the detail. horver,er,
lve notice some discrepanciesbet',veenobservation and the model results,especialll.underestimation of simulated concentrationon N{ay'
2 4 at Stockholm and after \Ia1. 7 at \{ol. We
also notice the delal- of the initial arrival tinre
in the simulatior.tat Budapest as mentioned
abot'e.
The surface air concentration of I-131 rs

shon'nin Fig. 6.Obserr,edconcentrationpatterns
are almost the same as in F-ig. 5, rvhile theconcentrationof I-131 rvas denser than that of
C s - 1 3 7( n o t e t h a t t h e m a x i m u m v a l u e o f t h eordinate in Fig. 6 is one order larger than thatin Fig. 5). Our rnodel simulates the air concer.rtrationof I-131 rvith almost the same accurac.v as that of Cs-137,and the discrepancies
mentionedabor.estill exist.
The effect of r.ertical subgrid"scalediffusion is included in the model b1' employing arandom-'"r'alkmethod as eq. (12).The effect ofhorizontal subgrid-scalediffusion is, hou'ever,not included in the diffusion part nor in ther",'eatherforecasting part. The initial plume isbroadenedonll'b1'thehorizontaland the vertical',vind shear. 'fhe pollutant cloud is, therefore.
much narrolverthan the other investigations(forexample, Puhakka et al., 1990,and Hass e/ al,1990).We sr-rspectedthat the discrepancyof the
air concentration betu'een the model and theobservationmentionedabove resultedfrom thislack of horizontal diffusion. Son-readditional
simulationswere, therefore,performedto studl't h e e f f e c t so l s u b g r i d - s c a lhco r i z o n t a ld i f f u sion. Recause no reliable parameterization
schemefor modelsusing 8x -- 100km is known,u'e used constant horizontal diffusion coeff i c i e n t so f 2 x 1 0 5 ,5 x 1 0 5 ,1 x 1 0 ' r 2, X 1 0 6m 2 i s e c .
Ho',vever the results did not indicate clear
impror-ement: the results '"1'ereimproved at
somepointsbut had no effect at other points(notshor,r,'n).
Probabl-v, more sophisticated pararneterization for the subgrid-scalehorizontal
diffusion is required to improl'e the model
perforniance. A long-range transport experiment performed after careful preparation u'ill
be great help for choosing the scheme of thehorizontal subgrid-scalediffusion.
Trvo other factors also affect the horizontal
rvidth of the pollutant cloud: the initialhorizclntaland r,ertical u.idth of the source andthe number of releasedparticles. If the initialrvidth is broadenedor if the number of particles
is increased, the sir.nulatedpollutant plume'fhis
r.r.ouldbe broadend.
broadening has apotential to dissolr,esome part of the above
discrepanciesin general.In this paper,however.
the effects of r-ariationof theseparametersare

Satomura'l'..

F . I ( i m u r a . I l . S a s a h i .T . Y o s h i k a ua a n d l

V o l .4 i , N o . 2

BUDAPESTI-131 Air C0ncentnlion
to'

to'

tot

to'

too

-

4

)ro-'

d'-'

]ro-'
I

I

i

to-'
to

I

I
I

ro-'

t
to-t

a/2

5/4

5/2

5/4

l\{oL
1-131Arr Concenlration

(b)
1o'

10-

1o'

tot
-

I

i', ^-'

I'I; 1 0 _

10'

E'o-'

I

i
to-o
to

10'

t

1o_5
4/24

4/30

5/2

5/4

5/6

5/8

4/24

4/30

5/6

5/8

Fig.5. Air concentrationof Cs- 137at (a) Stockholm, Su.eden, tUi ft"f . Belgir.rrn:(c) Rudapest, Hungarr. ; (d) Attikis, (]reece. Solid line is the obsen'ation anddashed line is the rnodel result.

1o'

10_

too

too

;,"-r
- 10-'

.to_t

; ro
to'

'

to'

to-t

to-t

ATTlKISa s- I Jl Arr G)nccnlralion
to'

to'

roo

too

;

-to_'

-lo'

I
;10-'

,i

to_'

ro-'
ro'

to-t

to-5
5/2

5/4

5/6

4/20

4/30

5/2

Fig.6. Same as Fig. 5, except for I-131 air concentration

Numericiil -Sjrrulation of Regional Scale Dispersion oi Radioactivt, Pollutants

not discussed
in detailfor the f<tllorvingreasons:thesignifir:anceof source-termspecificationrvould
be lost b1'broadeningthe initial source u'idth,
and the discrepancyfound at Stockholm rvould
not be improved b1-the increaseof the released
particle number as realized in Fig. ,1c.
3.3. Comparison

with observationsofdeposition
Simulatedand observeddaily depositionsof
Cs-i37 at Bilthoven and at Berlin are shou.nrn
Fig. 7a and b, respectivell.. The simulated
deposition does not correspond u'ell w.ith theobservation,although our model sirnulatedu,eli
the air concentration at these tr,vo points asshown in Fig. 7c and d. This discrepancv rs
probablf inducedby the follor,vingtwo factors:
the poor precipitation prediction in the modeland the difference in the horizontal scale
betweer.robsenation and the model. The daily
precipitation at Bilthoven and Berlin predicted
little correlation with theb-vthe model shor,ved
observation as sho\\.nin Fig. 8. Becausemost
deposition of Cs-137occurred in u'et process( l I a s s e / a / . ,1 9 9 0; P u h a k k a e t a L . . 1 9 9 0t )h,i s p o o r
prediction of the precipitation is undoubtedil
one cause of the poor prediction of the depo-sition. On the other hand, the amount of r,ver
depositionof the radioactir,'epollutantsstrongll'
dependson the place as u.el1as precipitation:models must simulate the right position oflocationand time of precipitationand pollutants'
arrival to predict the r,t,etdepositionaccuratell'.
If the precipitationand the lvet depositionhave
a rather small scale, good prediction of rvet
depositionb-vthe mesoscalemodel$.ith ax = 100
km becomesdifficult even in the caservherethe
model predicts averagedprecipitation rvell. In
this sense,observedvaluesdo not representli'etdepositionin the area u'ith the horizontal scaleof model 2 grid lengths(- 250 km).
To improve the precipitation forecast, anerv long-rangetransport model rvhich uses aner,vroutine 'uveatherfrtrecasting n-rodelof the
JMA as the u'eather forecasting part is being
developed.In this model,both the horizontal andthe vertical resolutionsare improt ed to simulate
fine-scalephenomena.

61

4. Summary
As a case studl', u,'e simulated the mor,ements and depositionof radioactive pollutantsreleasedby the Chernobylaccident.The model\\,as composedof the weather forecastingpartthat was the routine regional n'eather forecasting model of the JMA and the Lagrangian
advection-diffusionpart. The source data distributed by the ATMES project was used. In
spite of there beingno horizontaldifiusion in our
diffusion part, the simulated surface air concentration of Cs-137 and I-i31 showed goodcorrespondenceu'ith the observation. The
effectsof subgrid-scale
horizontaldiffusionwere
examinedby extra simulationsand it was foundthat the results r,",'ere
improved at some points
but deterioratedat the other points.
Simulated depositionhad poor correlation
rvith the observeddeposition.We considerpoor
precipitation forecast and the locality of observation are responsiblefor this discrepancy.

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BERLINCs l -1?Air Concentration

Fig.7. Daily deposition of Cs-137at (a) Rilthor.en,Netherlands, (b) Berltn,
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