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(, ^^SlATHEMATiCAL MODEL OF ZONALLY UNIFORM ATMOSPHERIC C^ CIRCULATION <C by N. P. Fofonoff / ?v> / Technical Report 3 Office of Naval Research Contract NR 083-050 June 1953 Department of Oceanography Brown University Providence 12, R. I„

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Page 1: ^^SlATHEMATiCAL MODEL OF ZONALLY UNIFORM ATMOSPHERIC CIRCULATION · 2018-11-08 · A MATHEMATICAL MODEL OF ZONALLY UNIFORM ATMOSPHERIC CIRCULATION by N. P. Fofonoff S Technical Report

(,

^^SlATHEMATiCAL MODEL OF ZONALLY UNIFORM ATMOSPHERIC

C^ CIRCULATION

<C

by

N. P. Fofonoff

/ ?v> /

Technical Report 3

Office of Naval Research

Contract NR 083-050

June 1953

Department of Oceanography

Brown University

Providence 12, R. I„

Page 2: ^^SlATHEMATiCAL MODEL OF ZONALLY UNIFORM ATMOSPHERIC CIRCULATION · 2018-11-08 · A MATHEMATICAL MODEL OF ZONALLY UNIFORM ATMOSPHERIC CIRCULATION by N. P. Fofonoff S Technical Report

A MATHEMATICAL MODEL OF ZONALLY UNIFORM ATMOSPHERIC

CIRCULATION

by

N. P. Fofonoff

S Technical Report 3

Office of Naval Research

Contract N7onr -35811

NR -083 -050

June 1953

Dipanmtnt of Oounof raphy

Brown Unlrtriity

Prorldtnet 18, R. X.

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-1-

Abs.tr act

A mathematical model of a simplified atmosphere is described

which is suitable for considering certain processes of heat exchange.

The model is set up for an alnost incompressible atmosphere, that

is, the variations in densjty are neglected except in the term

representing gravitational force. Only the dynamics, not the thermo-

dynamics, of the problem is considered. The resulting equations of

motion are simplified by perturbation methods regarding the ratio

of atmospheric thickness to the radius of the earth as the perturb-

ation parameter. Given a density distribution in the atmosphere as

an arbitrary function of latitude and height, the model determines

the zonal velocity component and the meridional circulation. A

simple numerical example is given to illustrate the type of flow to

be expected fro-: the model.

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-i.i-

Table of Contents Pa 50

1. Introduction I

2. Simplifying assumptions 3

3. Equations of motion *••

k. Dimensional analysis 7

5. Perturbation equations 9

6. Solutions of the zero-orcer equations 11

7. Surface torque I1*

8. A simple numerical cxaranle 15

9. Discussion 16

10. Plans for future work 17

Aclcnovlodgment 18

References 19

Illustrations

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V

}

i

-1-

1. Intjodjuctiqn

Although much effort has been devoied to constructing

mathematical and iicchanical r.odels of the atmosphere with the aim

of explaining the general features of the atmospheric circulation,

there is at present no satisfactory manner of linking the observed

behavior of the atmosphere to the fundamental sources of energy-

responsible for that behavior. The ultimate source of energy for

atmospheric circulation must be solar radiation. The motion in the

atmosphere derives its energy from the unequal distribution of heat

produced by the different races at which heat energy is absorbed

from and radiated into outer space at different parts of the atmos-

phere. The motion in turn furnishes processes of heat exchange

which tend to produce a uniforr: distribution of heat.

The circulation is observe'1, to be near a quasi-equilibrium

state at all times so that large surpluses or deficits of heat

energy in the atmosphere do not occur. There are, however, signif- t

icant departures of the circulation from the mean state which do not

seem to depend unon a particular distribution of heat energy in the

atmosphere but rather suggest a dyna-.lc instability of the basic

zonal circulation in at least some of its modes.

The problem of the stability of zonal flow is of great im-

portance and has received a good deal of attention in the past few

years. Kuo (1950), for exanple, has considered the stability problem

in considerable detail and has discussed a possible role which I •

stability plays in maintaining and regulating the general circulation, is

Studies of the stability problem, however, do not give very much in-

sight into how the general circulation is maintained since the

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I conversion of heat .Into kinetic and potential energy is not con-

sidered.

Much of the difficult-/ in constructing a theory of the

I general circulation Is encountered in choosing a sir.pie, meaningful

Rodel in which the thermodynamics of the atmosphere may be included.

The mathematical models whic'i have been developed for the study of

dynamic stability of zonal circulation are not suitable for the

thermodynamic problem because most of them either do not explicitly It

include the effects of vertical structure in the atniospnere or are

applicable only when the thermodynamic processes are negligible. I

The classical Hadley model is l^ore suitable but Is difficult to I

construct mathematically.

In the classical Hadley model it can be shown by qualitative

reasoning that at least three meridional circulation cells must II

exist to correspond apprcx?' atciy to the observed zonal winds. The

subtropic and arctic cells are driven thermally while the middle

cell is driven indirectly by the other two cells through lateral

friction. The arguments leading to this picture of the circulation

consisting of three cells and the dynamics of the middle cell have

been given by Rossby. A semi-empirical model of a three cell type

has been constructed by Dorodnitsyn, Izvekov, and Schwetz (1939)

from the observed temperature distribution in the atmosphere and

the surface pressure distribution, using a method developed by

Kochin (1935). Rossby (19*+D gave an excellent discussion of the

Hadley model and pointed out some of the difficulties encountered

in attempting to explain certain features of the circulation with

this model.

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-3-

A model of a simplified atmosphere is constructed here in

order V at scne processes of heat exchange car. b-i studied analytical-

ly, The purpose of this model i.-; to ^ive the distribution of zonal

and meridional velocity components corresponding tc a given density

field and representing a balance of pressure-gradient, Coriolis,

gravitational, and frictional forces. Assuming the density field

to be known, the velocity field is determined. Using the energy

equation and the equation of state, the distribution of heat sources

and sinks necessary to maintain the particular density field in the

steady state can he calculated. "he thermodynamics of the model

can be considered as a separate problem and, therefore, is net taken

up in detail here.

2. Simplifying Assumptions

The equations of mass end momentum conservation serve as the

basis for the model considered in the following analysis. It is

necessary to make simplifying assumptions at the outset in order

to obtain a model sufficiently simple for analytical treatment.

The atmosphere is considered to be an almost incompressible

• fluid covering a smooth earth to a mean depth h v/hich is very small

in comparison with the earth' s mean radius a , The motion of the

atmosphere is considered in the steady state and is assumed to be

driven by a known density distribution. It is assumed that the

density variations are small enough to be sufficiently accurately

represented in their effect on the fluid motion by considering a

variable gravitational force. Frictional forces are assumed to

arise from Reynolds stresses and are ta)"en into account by introducing i

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§

a constant, isotrcpic kinematic eddy viscosity. In adopting an

eddy viscosity, it is irplieitly assunri that t/.c steed" .-notions

arc arrived at bv an. averting proces".. The non-linear terms arc

neglected to siir.rlify the mathematical analysis so tnnt conclusions

based on this model would be valid only v/hen these terms are rela-

tively small. The motion is assumed to be independent of longitude

since otherwise the motion docs not appear to have any obvious

physical significance unless surface irregularities of the earth are

is I*. taken into account.

The coordinctcs of a point in the atmosphere may be specified

by giving the distance z of the point from the geoid measured along

the local vertical direction, the latitude 9, and the longitude \

at the geoid. It is assumed that this coordinate system can be

sufficiently well approximated by a spherical coordinate system

(\, 9, r) in which gravitational forces act in the radial direction.

Here, \ is the longitude measured eastward from a reference meridian,

9 the latitude measured from the equatorial plane'and r measured

from the origin at the center of the earth. The radial distance r

of a point at height z above the geoid is approximated by a + z,

where a is the mean radius of the earth. The velocity components

(u, v, w) are chosen so that u is the eastward component, v the

northward conronent, and w the upward component of velocity,

3 • Equations _of Motion

On the basis of the assumptions above, the equations of

motion can be written in the following form

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*

s

2Q [ :OJ 9 w - sin cp v ] = K[ S]2^ - ^ u *- 1 (1)

2Qsin 9 u = - -A d£ J- K[ y V - pr ocp

v ±. £H ] (2) cp r~ 89

2 cos 9 u ... I U - G(9,r) + K [y2v - 2jj . ^ 9v p cr r 89

+ 2 tan cp v -1 r2

r cos 9 9 cos 9 v , 1 , 9r v

99 J? I~L 0

(3)

r" 9 r

where Q is the earth's angular velocity, p is the density and is

considered constant while j(9, r ), the unit-mass gravitational force,

is modified to include the effects of small variations of the density.

The pressure is given by p, and the 'rinomatic eddy viscosity by }'.

The Laplacian operator in tills coordinate system is

V2 = 1 SL C cos 9 M + -t -2- [ r2 ± ] rr-cos 9 99 99 r^ 9r 9r

A stream fraction \|> is introduced for the meridional cir-

culation so that

v = r c

1 9J os 9 ^r

M r cos 9 39

(5)

(6)

The equations can be e;.pressed more compactly by introducing 0

the quantities M, x, D , and Cr, where

M = r cos 9 u

x = sin 9

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-6-

3x'

j = xSL + (l - y'J -2. dr r 9x

a

end M is the unit-mass relative angular momentum of the zonal flow,

and D , G are commutative differential operators. The equations

become

+ 2QxiJ = r

- 2 QG(\|») = X D2(H)

- (1 - x2) |2+K1 [D2nlO] pr 9x r dr

(7)

(8)

p p3r j? dx (9)

I

Eliminating the pressure from (8) and (9) yields

2QG(M) = (1 - x2) ££ + K D\\|0 ax (10)

i

The boundary conditions at the earth's surface are that

normal and tangential velocity components are zero. At the upper

surface the components of torque are zero. The condition of con-

tinuity requires zero net volume transport in the steady state a-

cross all latitude circles. In terns of V. and ^ , these boundary

conditions become

ib = SSt = 0 dr L

f at r = a,

h = 0

i

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-?nd

-7-

4i = .a. d 3r -i,- All

r< 9r

3r " M 1 _7J

;tt r = a + h,

0

1 |

The mathematical problem is specified once g is given as

a function of x and r.

h. Dimensional Analysis

Unfortunately, it is difficult to solve equations (7) and

(10) regardlers of the choice of g. Further simplification of the

equations is achieved by dimensional analysis. The equations are

converted into non-dime.nnicnal form by introducing the non-dimen-

sional quantities a , z', M1 , v}1' , g1, and p', where

aQ = h/a

z1 = z/h or r = a (1 + aQz' )

M' = M/aU

\J>' = \J)/P

g' = s/?>

P1 = p/pg h

and U, *P, g are characteristic values of the zonal velocity component,

stream function, and gravity respectively.

By choosing

<P = K* AQ ~

U = aQ/7 /2Q

e = K/2 Q h2

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-fi-

i

6 • I

equations (7) and (10) o-.n be written in the following non-dimen-

sional form, the primes now '--eing omitted

- :':(4) - T^C;;)

G(h) = (1 - x2) |£ + e2Df(*)

(3D

(12)

where the differential onerators in non-dimensional form are

D = -1— + a ~ o

- (l - x2) a2

dz' (1 + o.-z) ax'

a az

PL a - x2) A ^ TTT^TzT ax

The order of magnitude of the various terms can be estir.-rted

by choosing representative values of the dimensional constants. For

example, the values

a = 6371 km

h = 10 Ian

K = ic5 2 -1 cm sec x

g = 103 p cm sec

P = 10-3 g cm"3

Q = 7.3xlO"5 sec"-1-

yield the following characteristic constants

u = 110 m sec

<p = 5x10° nrsec

ao = 0.0016

e = C.C007

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5

i

-9-

5. Ferturbation Equations

The inver.ti?i?.tton of equations (11) and (1?) can be simpli-

fied considerably by regarding a as a perturbation parameter and

expanding the solutions and the differential equation as perturba-

tion series in ao.

The functions M, ab, and g are expanded as power series in

ao

f! » ; o + ao Ml + »o M2 +

* « \|»0 + a0 ^ • aQ ^2 +

2 g = 20 + a0 g]L + aQ g2 •

These series are substituted into equations (11) and (12) and all

terms are collected as coefficients of terms in a power series in

a . Considering a as an arbitrary parameter, the power series can

have a sum function identically zero only if all the coefficients

of powers of a are zero. This condition leads to an infinite set

of differential equations for M. and i|> . In the following analysis

only the equations corresponding to the zero and first order terms

in a are considered.

The equations of zero-order in aQ are

e2 **2-x.2!° = - (1 -x2)i£2 (13)

dz 9z 6x

1%+ X*t° =0 (11*) dz2 dz

with the boundary conditions

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and

-10-

*0 - atj»0/az = o ^ at z - 0,

:i0 * C

^o = a"Vaz2 = °

3MQ/3z = 0 j

at z - 1,

The equations of first-order in aQ are

E2 *k . „ !Hi . . (1 - x2

dz* dz

32M.

9Mo + a*l

a?/- a- ax

(15)

(16)

!

with the boundary conditions

T|,1 = ty /dz = o] I at z = 0,

!»'! = oj

and ^ = o, e^/az" = 2 a^0/az'

> at z = 1.

3MX/ 3z = 2M0 j

The perturbation equations for the pressure can be obtained

in a similar manner from equations (8) and (9) after converting them

into non-dimensional form.

5o ~3z = - Zi

(1 - JP,

x > "fiT = - xM„ + e' a3^o 3z-

(17)

(18)

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!

ar-i

IE'

-11-

'1-o - 81

(I - x'-) ep1 "ST = - X

2 &*\h ;, + £<- _i 8ZJ

(19)

(20)

6. Solutions jof .the.Zero-order, fija.uatIons.

Solutions to the zero-order equations can be obtained in

closed form for the special case that 9g0/dx is a .function of x

only; so for the purposes of studying sinrole flows predicted by the

present model, this special case is considered in detail, "he more

general case where 6g0/9x is a function of both z and x can be

solved in terms of an integral over a Green's function or by bound-

ary-layer methods.

The solutions to the zero-orde" equations may bo written

in the form

where

40 = - .CL- xtl*** 0 (Z,k) x^ 9x

o x ox '

0(z,k) = 1 - T. A* exp (\,z) 3=1 J J

* (z,!0 = / 0<i,k) d-c

(21)

(22)

(23)

(2»f) co

and X, (j = 1,2,3,^), represent the four roots of the equation

V? = - x2/e2 = - k k\

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-12-

The roots are

X1 = - k(l - i)

where

w2 = - k(l + * \

1/2

X = + k(l 4 i)

\ = + k(l - i)

i = (-1) 1/2

L

The function 0(z,k) can be found by solving the four

simultaneous algebraic equations arising from the boundary condi-

tions for the coefficients A... The function is found to be

0(z,k) = 1 - ,—--,———£—.—-_ 7 I [cos k cosh k(l-z) sin kz I sinh 2k - sin 2k I

- cosh k sinh kz cos k (1-z)]

+ [cosh k cos k - sinh k sin k ] [ cosh k(l-z) sin k(l-z) 1

1-z) 1J (2?)

- [cosh k cos k + sinh !: sin k ] [ sinh k(l-z) cos k(:

I

There are two cases for which this expression is closely approximated

by a much simpler form:

Case I. k> 5

0(z,k)~l - (2)'^ exp(-kz) cos (kz ~nA) - e;:p(-k(l-z)) cos k(l-z)

Case II. k<l

0(z,k) - (kz) (l-z)Q-2z)

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-13-

Frcfiles of ®(z.k) and 0(z,k) rre given for soveral values of k in

fig. 1 and fig. ? respectively.

The zero-order pressure distribution can be calculated from

equations (17) and (10). If 9g0/3x is a function of x only,

g (z,x) nay bo written as

g0(z,x) = gol(z) + go2(x)

and from equation (17), the pressure p , as

p0(z,x) = Pol(z) + go2M Cl + n(x) - z ] (26)

The equation for T)(X) IS obtained by substituting from (21), (22)

and (26), the expressions for ^0, MQ, and pQ respectively, into

equation (18). The resulting equation is

Qgo2n a^—

9g aT

o2 1 - cosh 2k + cos ?k - 2 cosh k cos h; k(sinh 2k - sin 2k)

(27)

!

The boundary condition for n(x) is

1

J rj(x) dx = 0.

The function rj(x) represents the departure of the upper surface

from the surface z = 1 necessary to bring the pressure-gradient

forces into equilibrium with Coriolis and frictional forces.

The horizontal pressure gradient can be expressed as follows

8P -5T * "I Cgo2(i* n- 2)1

[ ^02 ax

cosh 2k + cos 2k k (sinh 2k

2 cosh k cos k sin 2k) - z (28)

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-llf-

It is clear that the horizontal comr.onont of the pressure gradient

must change sign in the lowei' frictional layer and will be zero

along the surface

z = cosh ?k + cos 2k - 2 cosh k cos k (29) v (Sinh 2k - sin 2k)

for any choice of 9gQ/9x which depends only on x. Along this sur-

face the Coriolis and frictional forces are of equal magnitude.

?• Surface Torque

It is of interest to note that equation (iS) can be integrated

with respect to z directly. The resulting equation is

92° + ^o = ° (30)

This implies that the meridional transport at a given latitude is

directly proportional to the vertical gradient of the zonal angular

momentum. Since 8MQ/az is also proportional to the torque exerted

by the zonal flow, equation (30) may be interpreted to mean that the

volume transport per unit time in the meridional direction below a

certain height is proportional to the torque exerted by the zonal

flow above that height. Since there is no volume transport below

the surface z = 0, the zonal flow given by the zero-order equations

cannot exert any torque on the earth's surface in the steady state.

The solutions to the first-order equations can be obtained

in terms of a Green's function, but as these solutions cannot be

expressed in the sir-pie form found from the zoro-order solutions,

they are not considered in detail here. However, it is possible

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1

1

-15-

to draw some conclusions about the torque on the earth.' s surface

from the first-order differential equations. Integrating equation

(16) with respect to z throughout the atmosphere ^ives the result

that the surface torque must be

Te = - -a- s 3x

(1 - x2) M ' X

~iz = 1 <31>

Since M (l,x) is zero at the noles and at the equator, the torque

must change si^n in the interval 0 < |x | <1. This, in turn implies

that the zonal flow must reverse itself in some region between the

poles and the equator. As this reversal of the zonal flow is con-

tributed by the first-order equations, and consequently is small

relative to the zero-order torms, the region of reversal will not

extend beyond the lower layer of frictional influence.

8. A. Simple Numerical Sxampjle

Most of the features of this model can be illustrated by

considering a numerical example in which 3g0/6x is assumed to be a

simple polynomial in x. Since g0 is functionally dependent on

density in the approximation adopted in this model, assuming ag0/8x

to be known is equivalent to assuming the distribution of the hori-

zontal density gradient throughout the atmosphere to be known. The

density gradient is chosen to be anti-symmetrical about the equator,

to vanish at poles, and to be proportional to

j|2 = px3(l - x2) (32)

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-16-

The effects of friction are measured by the dimensionless constant

e, which in this example is chosen to hove the value e = lO"-5 to

simplify numerical calculations.

"'he corresponding zero-order solutions are

^0 = - px(l - x2)2 0(z,k) (33)

M0 = p3r(l - x2)2 *(z,k) (3^)

g0 = f(z) 4 ^ xk(3 - 2x2) v35>

where p is a dimensionless constant, and f(z) is an arbitrary func-

tion of z.

Meridional cross-sections have been drawn shoving isotachs

of the zonal velocity component in figure 3» and streamlines of

the meridional circulation in figure *+, The normalisation factor

for the zonal velocity component is umax = 19t0 {3 m sec"1, and for

the stream function, «P = 6.85xlCr 0 ra^sec**1. Tho maximum values of

the velocity components obtained in the numerical example aro»

"max • 1C"° P m soc~l

v„_ = 0.59 0 m sec in tho.upper friotional layor

vmax = °»38 P m sec"1 in the lower frictional layer

wmax s °»00ll+ P m sec .

9. Discussion

The simple example of zonal and meridional flow given above

illustrates the type of flow to be expected from this modol. The

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flow pattern is essentially different from the flew to ce expected

on the basis of the classical Hadley model since the transport of

relative angular rovc-ntum by the meridional circulation is not

taken .into account. The easterly v/inds near the equator predicted

by the model pre not due to horizontal meridional currents as sug-

jested by the Hadley model but arc due tc a net vertical motion

throughout the atmosphere in the equatorial regions. The easterly

1 winds have an appreciable horizontal extent only in the case that

meridional density gradients are very small in the equatorial regions.

Thus, a density gradient varying as x or x in the vicinity of the

equator would require very strong east winds within 2 or 3 degrees I

of the equator. Since the derivation of the perturbation equations

implicitly assumes that no such rapid changes in velocity occur,

the density gradient in the example treated above is chosen to vary

as x^ near the equator.

The example given above is not intended to resemble the

actual circulation in the atmosphere. It would not be difficult

to choose a density distribution which would give a more realistic

zonal velocity profile; however, such a choice could not be justi- I

fled from the dynamics of the model alone and would require the

investigation of the thermodynamic processes associated with the

notion for its justification.

10. Plans for Future Work

The basic purpose of pursuing this analysis was to construct

a model in which heat exchanges could Vie considered analytically.

n i

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•SQ -xO

The model described here is considered as the first approximation

to a more realistic model and would have to be generalised accord-

ing to the problem under consideration. Many generalisations can

be suggested; for example,, it may be necessary to take into account

the compressibility of the atmosphere before significant therrao-

dynamic processes can be considered. Also the effects of lateral

stresses can be taken into account by considering a nonisotropic

eddy viscosity. Both of these generalizations make the mathematics

of the model more complicated but, it is hoped, not impractical.

The effects of the non-linear terras would have to be considered

seriously as they are not always relatively small and may invalidate

conclusions made from the linear models. It is not proposed to

study models with longitudinal dependence because in such models

time dependent terms would have to bo considered and the analysis

would be much more complicated.

The immediate plan is to apply this model to the investiga-

tion of the dynamics and thermodynamics of the Antarctic Circum-

polar Current because only minor changes of the present model are

necessary in applying it to the ocean. In particular it would be

necessary to compute the solutions to tho zero-order equations cor-

responding to a given distribution of surface torque exerted by the

prevailing winds.

Acknowledgment

The writer wishes to thank Dr, G. F, Carrier, Gordon McKay

Professor of Engineering, Harvard University, for suggesing the

problem and discussing many of the questions which appeared during

the subsequent investigation,

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J7

Doro^nitsyn, A.A., D.I. Isvokov, and M.E. Schvetz, 1939i

A mathematical theory of the general circulation (in

Russian). Meteorolcgiia i Girdrologiia, Ho, ht

roehin, N.E., 1935: On the simplification of the hydrodynamic

equations for the case of the general circulation of the

atmosphere (Ju Russian). Trudi G.G.O., No. h. Also

included in Sobranie Soehinenii, Tom I, (Collected Works,

Vol. I). Moscow, Izdatel'stvo Akademii Nauk, SSSR, 19^9

Kuo H.L. 1950s Dynamic aspects of the general circulation anu

the stability of zonal flow. Report No. £ General Circula-

tion Project, Contract AF 19-122-153, Cambridge, Mass. Inst.

Tech.

Rossby C.G., 1938: Fluid mechanics applied to the study of at-

mospheric circulations. A. On the maintenance of westerlie s

south of the polar front. Papers in Physical Occanogre^y

and Meteorology. Vol. VII, No. 1.

191+U The scientific basis of modern meteorlogy. Yearbook

of Agriculture, Climate end Man. pp. 599-655.

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o „o

0.8 1.0 •<«.*>

Fig. 1. The function $(z,k), defined hv eq. (25), is plotted for several values of k. For the numeric- al example discussed, this function is proportional to the zonal velocity component along a vertical line through the atmosphere.

II

Plf. 2. The function #(s,lc), defined by eq. (26), ia plotted for several values of k. For the numeric- al example discussed, this function Is proportional to the stream function of the meridional circulation

._ vuiv . - - — -^i~ m*?*

_«_ i.

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I .Or

0.8r

0.61-

04h

OEJ-

1

80*

Pig. 3.

70* 60» 30- 40* 30* muuiuimuumuumm 20* 10* «

rig. i. A meridional cross-section shoving laotachs of the sonal velocity component (solid lines) normalised to unit msjtl.n»un speed st s • 1 The BUT face along which the horlsontal pressure gradient vanishes Is narked b» « broken line. The hatched region Indicates the horizontal extant «r »k. .... erly winds as plven by the first-order stations. *"' °f th* 8"t-

a*

0*

041-

O*

Mg. U. A meridional cross-section showing atfMiallnM of ^ „.,•!,!!onAl slrculatlon normalised to unit maxlnua value of %h« stream function st t - O.t.

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'ISTRT ;-:"?I 0* I ;?-Cf>3-c>;>

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