an efficient and effective explicit damping time

48
U.S. DEPARTMENT OF COMMERCE NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION NATIONAL WEATHER SERVICE NATIONAL METEOROLOGICAL CENTER OFFICE NOTE 249 An Efficient and Effective Explicit Damping Time Integration Scheme Clifford H. Dey Development Division DECEMBER 1981 This is an unreviewed manuscript, primarily intended for informal exchange of information among NMC staff members. . i

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Page 1: An Efficient and Effective Explicit Damping Time

U.S. DEPARTMENT OF COMMERCENATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION

NATIONAL WEATHER SERVICENATIONAL METEOROLOGICAL CENTER

OFFICE NOTE 249

An Efficient and Effective ExplicitDamping Time Integration Scheme

Clifford H. DeyDevelopment Division

DECEMBER 1981

This is an unreviewed manuscript, primarilyintended for informal exchange of informationamong NMC staff members.

. i

Page 2: An Efficient and Effective Explicit Damping Time

An Efficient and Effective Explicit DampingTime Integration SchemeS .w~ ; -~~~by Clifford Dey

I. Introduction

There are two general classes of solutions permitted by the set of

primitive equations we use to describe atmospheric motions -- meteorological

modes and gravity-inertia modes. The meteorological modes most closely

;describe the atmospheric motions that produce most of our everyday weather

and are therefore the ones we seek to predict. Gravity-inertia motions,

although present in the atmosphere, are generally of much less importance.

By - In numerical prediction models, however, the amplitudes of the gravity-

inertia modes can become abnormally large and interfere with the prediction

of the more important synoptic-scale meteorological motions. One method

of dealing with this problem has been to develop time integration schemes

that reduce the amplitudes of the gravity-inertia motions while leaving

_ 00 the meteorological motions relatively untouched.

There are two general types of selectively damping time integration

schemes. One type are those whose damping is frequency-dependent. They

rely on the fact that many of the most offensive gravity-inertia motions

have much higher frequencies than do the meteorological modes in order

to achieve their selectivity. Examples of frequency-dependent damping

integration schemes are the Euler-backward method (Matsuno, 1966) and the

Robert time filter (Robert, 1966). The second type of damping time inte-

gration schemes achieve their selectivity by treating the linear terms

of the model equations, which are most closely associated with gravity-

inertia motion, differently from the nonlinear advection terms. The

best-known scheme of this type is the semi-implicit backward method

Page 3: An Efficient and Effective Explicit Damping Time

2

(Kurihara, 1965). The only explicit technique in this class is a vari-

ation of the Euler-backward scheme described by Kurihara and Tripoli

(1976).

All of the explicit damping time integration schemes require more

computer time than the nondamping methods. This seems a rather high

price to pay for noise control. In this paper, an explicit, selectively

damping time integratfon scheme is described that requires substantially

less computer time than simple centered differences. The computational

economy is achieved by incorporating into the centered time integration

method a damping procedure originally conceived by Okamura (see Nitta,

1969) and later generalized by Grant (1974). The next section describes

the basic concept of incorporating the generalized Okamura technique

into the centered time difference scheme. The integration method itself,

the Semi-Interative Method, is presented and analysed in Section III.

W~ 0Simple numerical tests of the Semi-Iterative Method are described in the

fourth section. Section V contains some concluding remarks.

II. Application of the generalized Okamura procedure to centered dif-

ferences in time

The basic idea behind the semi-iterative method, which will be

discussed in the next section, is to incorporate the generalized Okamura

technique into the centered difference time integration scheme in a

particular way. Before considering the semi-iterative scheme in its

entirety, however, it will be instructive to consider the effects of

introducing the generalized Okamura technique into centered time dif-

ferences with a simple example.

Consider the oscillation equation

(1) a i k b

where is any variable, t is time, k the wave number, and c is

Page 4: An Efficient and Effective Explicit Damping Time

3

the phase speed. A centered difference representation of this

equation is

(2) - I 2Lcec t

where CtGkCt ,/4tis the time step, and tAtis the time level. The

generalized Okamura technique applied to is represented by the equa-

tions A

c4) = c4) -0

A A A* (3) ( 4) = V .4 LC -

0 Qt = t14 d+ ) ¢t_ fQ Elimination of reference to ) in (3) gives

(4)

" can be positive number. The first way of incorporating this potr

damping device into (2) is to replace ~ with its damped counterpart.

Using (4) one then obtains

, 0- R t t e~~~~~~It -1 (5) = - L _o. B

in which B = 1-oa C, (note that this can be done n times. Then

B= (i - *CC) . In this note, only n=l will be considered).

In order to analyze the effect of this substitution, let ) be

represented by

Let , the time-dependent part of , be given by

- -L - e Cq At

where c* is the numerical phase velocity. Thus,t~~~~~-i [kC~At

where X = . Furthermore,

(6) L~~~

ent

,j ·V

I -, I I~f:: :r: I

Page 5: An Efficient and Effective Explicit Damping Time

.. .,. - V ,_ .; .' - ,- ? , . .. £ ,, ,,_. . . " ,, , ! --- * . , -.; i .V.-.,I .i, :..r

4

In general, X is a complex number (+ +r+- L) . The magnitude

of the time-dependent part of the solution is

(7) V =

while the numerical phase angle (e) is given by

However, it will be more convenient to refer to the ratio (R) of the

numerical to the analytic (-a) phase angle. R is given by

(8) R =Substituting (6) into (5), multiplying through by , and rearranging

terms results in

(9) X LZ -I = O

The roots of (9) are given by /

(10) X - CO -{--

When _-C i t'L'O , at least one root of (9) will always be

unstable. However, when - + t C- , the magnitude of

both roots is unity. Recalling the definition of B, the stability

requirement becomes

considering the equality and expanding in terms of A, one finds

CILc -Z C4.a((11) H iZ2. 2.i+t AD

The two solutions for c( in (11) are:

(12) i = 2{1+ &4Graphs of a4_ and d_-are given in Fig. 1. The i root reaches a maximum

somewhere around a=3/2, then decreases. The 4 root, on the other hand,

decreases monotonically, reaching the maximum value of d- around a=3.

To fix these values with more assurance, note that

co5 -: SC)_ OT GY 0~~0 ~L

Page 6: An Efficient and Effective Explicit Damping Time

5

Since d;/. is zero when a=3/2, the maximum value ofc_ is indeed reached

at that point. From (12), = / when a=3/2. Since, also from

12), d+ 7 when a=3, the maximum time step in this case is

three times as long as the usual centered difference time step. Every-

where in the stable region, the amplitude response is neutral. Graphs

of l and R are given as a function of a for O< = 4/27 in Figs. 2 and

3, respectively. 7.

Another way to incorporate the genralized Okamura technique into centered

time differences is to replace A with its damped counterpart. Then, (2) becomes

where, as before, = 1- h. Proceeding as in the previous case, (13)

becomes

(14) AZ 4 2L c - SC 0

The roots of (14) are

(15) .A _o- * t -ca i '/2:2.Here, stability will always occur for - . Then,

(16) lxi =

The magnitude can be made less than 1 for any value of a by an appropriate

choice of i. The question is for what values of a will the inequality

hold. Utilizing the definition of B, the inequality can be written as-C + t -door

(17) -

Since o must be positive, the maximum value of a for which (17) will

hold is 1. As -d increases, damping increases but the maximum allowable

value of a decreases. The region for which- C+ 20 is shown in Fig. 4.

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6

However, the scheme is not always unstable when-0-i-5<O. In the narrow

strip between the solid and dashed lines in Fig. 4, the scheme is stable

even though-&-e Q<. Graphs of 1\\ and R are given as a function of a

for C =1.0 in Figs. 5 and 6, respectively.

The third way of incorporating the generalized Okamura technique

into centered time differences is to replace both 4 and 4 with their

damped counterparts. Here, (2) becomes

(18) 4 13 _ 2aJ3 .a

where B is defined as before. Proceeding as in the previous cases, (18)

becomes

(19) - z22ci % --

The roots of (19) are

(20) X = - - +

In this case, stability always occurrs for-15 6 . Then, the magnitude

of both roots is given by

: :E i htxl = { -d

which is the same as in the previous case. Once again, the question is

for what values of a will the inequality hold. Recalling the definition

of B, the requirement becomes

::: ( dt< CL-G ) o:

Proceeding as with the first case, consider now the equality and expand

in terms of d. Then,

(21) ? (Z + (a-Z - <) -+ = 0The two solutions for c in (21) are

(22) c+ = - ; _ = -

Graphs of C+ and ~ are given in Figure 7. (As in Fig. 4, in the narrow

strip between the solid and dashed lines, the scheme is stable even though

Page 8: An Efficient and Effective Explicit Damping Time

7

--.. a I : ~ 0- ). Note the similarity between Figs. 7 and 1. In

contrast with the first case, however, this scheme is a damping one

everywhere within the stable region. In order to determine the maximum

allowable time step, note that

+ 4

The maximum value of i_ occurrs for Q= t , at which point c_( . The

value of a for L=i is 2, or twice that of the usual centered difference

formulation (actually because of the stable region for - +[ 3 , 0

the maximum value of a when is nearly 2.2). Graphs of i} and R are

given as a function of a for = LJin Figs. 8 and 9, respectively.

To summarize the results of this section, there are three basic

ways of incorporating the generalized Okamura technique into a centered

time difference scheme. If the technique is applied only to thetAttime

level, the time step may be increased by up to a factor of three, but no

damping results. If the technique is applied only to theCt-- 6t time level,

strong damping results, but the maximum allowable time step is reduced.

If the scheme is applied to both the tht and (M-)Attime levels, on the other

hand, the maximum allowable time step is increased by over a factor of 2

yet strong damping is still achieved. In the latter two cases, the

amount of damping may be selected by varying the parameter i

Although each of these three choices results in a scheme with interesting

characteristics, none is very practical. First, all the tendency terms

must be computed an extra two times for each iteration of the generalized

Okamura technique. Second, the damping is frequency-dependent and therefore

not very selective. Third, an extra set of fields is required. In each

case, however, the generalized Okamura technique can be applied to centered

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time differences in such a way that the first two objections are removed.

The result, referred to as the semi-iterative scheme, features both

computational efficiency and highly selective damping. The next section

describes and analyzes the semi-iterative scheme and suggests that

it is a very attractive alternative to existing explicit time integration

schemes.

III. The Semi-Iterative Method

In order to illustrate the semi-iterative method, consider the

following system of linear equations:

(23) - +

(24) E + , +

(25) + _ + = 0in which U is a constant zonal current, h is the mean depth of the

fluid varying only in the y direction such that

(26) -_

and u,v, and h are pertubation wind components and height, respectively.

Assume the solutions to (23)-(25) are of the form

(27) = o e :

where c is the phase speed, k is the wave number (k= Zt-, L=wavelength), and

uo,vo, and ho are unknown amplitudes. Substitution of (27) into

(23)-(25) and requiring the determinant of the resulting system of linear

equations to vanish produces an equation for the phase speed c;

(28) (U-c) ( + C - ) + W- ' = oThere are three real roots of (28), given by

(29) Ci = + 3 CS-

(30)= -L A- CA D +c[ + - j

(31). C3 U +2 t 4

".. - 'I �, - - I., -�- $ 1 � f% - ; I- �.'- � � � - , ' � �� wl�'�v . c �� �':� '�::_ ;. . 1; . -�'m .�-� - , . .4 ; 11-1,�A." "'..'�'..: _� - ;�-. , ... . � � % ml . -,, -_

Page 10: An Efficient and Effective Explicit Damping Time

9

where

(32) CL = [- =cu

C1, C2 , and C3 denote the phase velocity of a meteorological wave,

an eastward moving external gravity-inertia wave, and a westword moving

external gravity-inertia wave, respectively.

Referring to C1, C2, and C3 as Cj, j=l,3 and expressing uo and vo 0

in terms of ho, the solutions to (25) may be written as

-3 ~~~~~Z'3(33) ' .-& C)X

4-I V j _c) 7

(35) =j-I

It can be shown (Kurihara, 1965, op. cit) that with substitution of

~ ~(33)-(35) into the original system, any one of (23)-(25) can be written

:-* in the form

"~ +4 K~h-+L L.j-)h=o,,¼:.]h(36) Am ~ D"=

where hj represents uj,vj, or j. This fact allows the analysis of

the semi-iterative method to proceed by considering only one equation of

the form of (27) rather than to the original system of three equations.

Using centered differences in time, equation (36) may be written as

0 hj = h/ -21.< CZ At h -ZLK ¢ C A-F)Athj )

or letting CL - K J t Cak b- K (C -) t , as

(37) 1- - J '

_ .- The semi-iterative method consists of applying the generalized Okamura

technique to the terms associated with gravitational oscillations (those

. . - .. I r � - I - . - ' 1 � , 11 �_ -, I ' . ' I . .1 .1 . 6' ".. , _1 . , - - . . .. , , :1 � .� � : - . - I . . - .� . . � � � . .�� . , .- - r �� , - .I - - - I - , 4 , - .- , � 4 - ; -.1 � 1, .� � -- . - , 1- . ---- '-'.� 11 � '; �-"- I � -

Page 11: An Efficient and Effective Explicit Damping Time

10

involving b), then using the resulting damped fields for the tendency

~* calculations and/or to step from when making a centered time step. In

this case, the generalized Okamura technique applied to the fields at the

time level is written as

a 4-ib%:

::0ff ~ ~ ~ k -;0 :o0I : : i-- h 7;b0Jor;0 ~ ~ 04i i0t-t=O fL J - a t = -b ) J :. :

If this proces is iterated n times, then one finds

(38)

where it is entirely permissible for i to be different (but always positive)

on each iteration. In a similar manner, m generalized Okamura iterations

on the -(i fields results in

-T-~ ~ ~ ~ ~ tI(39) =

Once again, p may differ on each iteration. Replacing j and hj in (37)

by their damped counterparts given by (38) and (39) produces the semi-

iterative method. The result is

(40) PK = J- ) s - WThe analysis of (40) proceeds as in the previous section. Thus

(40) becomes

(41) -X + ' a, b5'- 7)'>- (!-i b) - o

However, rather than evaluating the roots of (41) as in the previous

section, a computer program was used. There are many combinations of

, m, and n that could be examined. By way of example two categories

of combinations will be described in this section. They are:

Category 1: m=l,cK= 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, n=O, =O.

Category 2: m=n-l,o( i = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5.

Category 1 is one iteration of the generalized Okamura procedure

applied only to j , using six different values of c( (when o =0

Page 12: An Efficient and Effective Explicit Damping Time

11

in category 1, the system reduces to simple centered differences in

time). Using the results of the previous section as a guide, we would

expect this to provide an increased time step but no damping. Category

2 is one generalized Okamura iteration applied to both V and using

six different values for o and (when o= P Oin category 2, the system

reduces to simple centered differences in time). By setting = , a

separate iteration of the generalized Okamura procedure would not need

to be performed on the %- fields in actual practice. Again using the

previous section as a guide, we might expect the combinations in this

category to result in an increased time step (although less than in

category 1) and strong damping. As before, the effect of finite differences

in space is not considered in this simple analysis. In all the graphs

shown in this section, both the amplitude of X and the ratio of the numerical

to the analytical phase speed are given as functions of a (CO4CAt)

where the wave length is assumed to be 2000 km.

The six category 1 configurations are described in Figures 10-15. The

behavior of the meteorological mode is shown in Figures 10 and 11. The most

important thing to note here is that the graphs are the same for all

values of i. The effect of the semi-iterative method for category 1

configurations is thus restricted to the gravity-inertia modes, which are

described in figures 12-15.

The gravity-inertia modes are shown for various values of (, with a

mean geopotential of 8x104 m2s- 2 in figures 12 fnd 13. Note there is no

damping in the stable region for any choice of o<c. Wheno( =0.1, a large

unstable region appears at a=l.0 and continues to a=3.1. Then there is

another stable region that extends to a=4.1. However, little or no

Page 13: An Efficient and Effective Explicit Damping Time

12

increase in the time step is allowed until the intermediate instability area

is eliminated. As a increases, the unstable region separating the two

stable regions shrinks, and the limit (in terms of a) on the second stable

region shrinks as well. By the time c =0.3, the intermediate unstable

region disappears, and the stability limit is at a=2.5. The comparison

to the results of Section II can be seen by recalling Figure 1. There,

the Q curve rose above zero for a >1.0. This is the same (intermediate)

unstable region that can be seen in Figure 12. In both figures, the

intermediate unstable area was avoided by making d sufficiently large.

Figure 13 shows that whenci=0.3, the gravity-inertia wave phase speed is

reduced for all values of a below the stability limit. The graphs terminate

when the physical and computational mode curves meet, for the computer

program is unable to distinguish one from the other thereafter. This

should be of little importance, however, because this point is very

close to the stability limit.

Figures 14 and 15 describe the effect of reducing the mean geopotential

while holding o(fixed at 0.25. At first, as the mean geopotential is

reduced, the stability limit increases. However, by the time 4 becomes,2,% 1 l m~ - , the intermediate instability area noted in Figure 12

appears, substantially reducing the maximum allowable time step. The

intermediate instability area for small values of ) can be eliminated

by increasing o< . We note from Figure 12 that this reduces the maximum

allowable time step somewhat. In effect, the maximum allowable time

step for this use of the semi-iterative method will be determined by the

internal gravity-inertia modes, for they will require the largest values

of d to eliminate the intermediate instability area. As expected, there

is no damping when the semi-interative method is applied only to theZfields.

� -�' %�� �' �' , �wz� " ; ;- , : -- , , -�v��: .. ..' I a- - �: a' � ... - '� �;'% : �� 1_ 11 I I :J� 11. .� � - p -- �` :�� - � jc -: , ; -, .. , -� �; I '�' - � : I � '� 'f :� ". .� - ' 1."t : -".1 " -- � ..

Page 14: An Efficient and Effective Explicit Damping Time

13

The behavior of six category 2 configurations are described in Figures

16-21. The meteorological modes are shown in Figures 16 and 17. There is

no effect on the meteorolgical modes when the semi-iterative method is applied

to both the L and -0Ifields. One of the goals of this use of the semi-

iterative method has therefore been attained-there is neither damping

nor phase speed changes of the meteorological modes.

Figures 18 and 19 describe the behavior of the gravity-inertia

modes for various values of c( , where C is held constant at 8x104 m2 s-2.

In this case, there is strong damping of the gravity-inertia modes throughout

the stable regions. These graphs are similar to those in Figure 12

in that the intermediate instability area is once again present. It is

eliminated when d and are >O©J(not the actual minimum value needed).

When oand = 0.4, very strong damping of the gravity-inertia modes

occurrs for a < 1.86. Thereafter, the computational mode quickly becomes

unstable while the physical mode continues to be damped. Therefore,

the other two goals of this use of the semi-iterative method are also

achieved - strong damping of the gravity-inertia modes and an increased

time step. As before, the phase speeds of the gravity-inertia modes are

reduced everywhere in the stable region for ' of (Figure 19).

Figures 20 and 21 illustrate the effect of reducing the mean geopotential

while keeping d and fixed at 0.35. As in the category 1 configuration,

the intermediate instability area appears for small values of

Here, the values of a and ( needed to eliminate the undersired behavior

is larger than the value a needed in the category 1 use. This is also

in accordance with the results of Section II, for the requirement was ( 7

for category 1 type use (Figure 1) whiled=>> Ywas needed for category

2 type use (Figure 7). Application of the generalized Okamura procedure

Page 15: An Efficient and Effective Explicit Damping Time

14

to both 7 and' O fields thus appears to be an attractive choice, for

strong, highly selective damping achieved while the time step may increased.

Both uses of the semi-iterative method are testing using a free-surface

barotropic model in the next section.

IV. Tests of the Semi-Iterative Method in a Free-Surface Barotropic

Model

1. Experimental-Design

The semi-iterative method was tested using a free-surface barotropic

model. The finite difference equations are

- - -L&~Y :~Ky77Y

(42) U>- 1 - y x,~ ~ Yx(43) V t 4 _____I - - X 7 IV V -__ + ~ ~ - - x¥ --___

(44) k + h yI - h -

where u and v are the horizontal wind components, h is the height, f is the

coriolis parameter, and g is gravity. During the generalized Okamura iteration

of the semi-iterative method, the right-hand side of (42)-(44) are set to zero.

In two of the experiments, the Robert time filter was used. The time filter is of

the form

(45) S -i -

where S is u, v, or h. The model was integrated on a 65x65 point Northern

Hemisphere polar stereographic grid true at 60° north latitude. The

grid interval was 381km.

A case was selected in which a closed cyclonic circulation developed

over the western United States during a 5-day period. The initial time

of the case was 1200 GMTt, 17 May, 1981. The initial analyses of u,v,

and h were provided by the Hough global spectral analysis (Flattery,

1971). As a consequence, the initial mass and motion fields were related

quasi-geostrophically and the initial divergence was nearly zero.

Page 16: An Efficient and Effective Explicit Damping Time

15

Initial imbalances between the mass and motion fields lead to generation

of gravity-inertia wave noise. The various schemes were examined with

respect to their comparative handling of both gravity-inertia wave noise

and synoptic-scale meteorological scale motions.

Five 120 hour forecasts were made, differing only in the time integration

method used. The five different time schemes were

Control: centered differences, At =10 min.

Comparison: centered differences, time filter with

H=0.115) t= 8 min.

Experiment 1: Semi-Iterative Method, m=l, ( =0.30, n=0,P?=0,Xt=30 min.

Experiment 2: Semi-Iterative Method, m=l, o( =0.30, n=O0, =0,

time filter with O =0.90, t=30 min.

Experiment 3: Semi-Iterative Method, m=n=l,o:P= 0.45, b = 20 min.

2. Experimental Results

Consider first the graphs of RMS divergence and absolute vorticity

shown in Figures 22 and 23. In the control run, the RMS divergence

maintains a level of from 4x10- 6 to 5x10-6 S- 1 throughout the five

days of the forecast. The use of the time filter with a strongly damping

coefficient of 0.15 in the comparison run very effectively controls the

gravity-inertia wave noise that was present in the control run. In Exp.

1, the RMS divergence decreases at first, then increases slowly. This

is somewhat puzzeling, since the analysis of Section III indicated no

damping for this choice. As will be seen shortly, the five day forecast

maps produced by Exp. 1 were extremely noisy in appearance, much more so

than in the control run. For this reason, the forecast was rerun using

the time filter with a coefficient of 0.9. This forecast is designated

Page 17: An Efficient and Effective Explicit Damping Time

16

S_ Exp. 2. Note that this did not necessitate a reduction of the 30 min.

time step of Exp. 1. It did, however, control the noise. In Exp. 3,

the noise was controlled slightly better than even in the comparison

run. The RMS absolute vorticity decreased in Exps. 2 and 3 and the

comparison run more than in either the control run or Exp. 1, although

the amount was small. The increased reduction of RMS absolute vorticity

was to be expected in the comparison run and Exp. 2 due to use of the

time filter, which is not a very selective noise damper. It was not

expected in Exp. 3.

The forecast 500mb height maps are shown in Figures (24)-(29) (for

a portion of the grid). Here, we note a noisy map from the control run

(figure 24) and an extremely noisy map from Exp. I (Figure 26). The

noise is completely eliminated by use of the time filter in Exp. 2 (Figure

27). The maps from the comparison run and Exp. 3 are also noisefree.

The central depth of the closed circulation is lowest in Exp. 2 and

highest in Exp. 1. It is disappointing to note that the low is not as

deep in Exp. 3 (Figure 28) as in the control run or even the comparison

run (Figure 25). The maps of wind speed and absolute vorticity (not

shown) confirm a slight but consistent loss of amplitude in the five day

forecast of the synoptic-scale features in Exp. 3. This is both puzzeling

and disturbing, for it seems to disagree with the analysis in Section

III. The maximum allowable time step, however, was determined experimentally

to agree quite well with the indications in Section III in all five

forecasts.

3. Assessment of the Experimental Results

Use of the semi-iterative method can provide either an increased

B .~time step with no damping or a smaller increase time step with strong

Page 18: An Efficient and Effective Explicit Damping Time

1. - . ~ 1..1 . I - 1- . - . , :~" .. 'I fl ;IT I - - q '> .V A.-. .. . *A ' , r..

17

damping. The computer time (CPU) required for each 120 hour forecast

was:

Control Run 96 sec.

Comparison Run 121 sec.

Experiment 1 65 sec.

Experiment 2 65 sec.

Experiment 3 99 sec.

Thus, the Semi-Iterative Method proves to be quite efficient. The

saving of computer time would be even greater in a complex baroclinic

model, for the physics would only have to be computed one-half (Exp. 3)

or one-third (Exps. 1 and 2) as often as normally. The damping of the

Semi-Iterative Method is also substantial, whether provided by the time

filter, as in Exp. 2, or by the choice of parameters, as in Exp. 3. In

this sense, the Semi-Iterative Method indeed proves to be an efficient

and effective noise control device, as suggested in Section II.

Several notes of caution must, in fairness, be added to this assessment.

First, the damping in Exp. 3 does not seem to be as selective as was

hoped at the outset and indicated by the linear analysis, for there was

a disturbing amount of damping of the synoptic-scale meteorological

motions. Furthermore, although use of the time filter in Exp. 2 did

control the extreme amount of noise noted in Exp. 1, it must be remembered

that this is only a barotropic model. The noise apparently present in Exp.

1 could therefore be easily controlled by the time filter. It- is possible

that application of this form of the Semi-Iterative Method to a baroclinic

model would result in internal gravity-inertia wave noise that could not

be so easily controlled with the time filter. One more factor must

also betpointed out. The Semi-Iterative Method requires an extra set

Page 19: An Efficient and Effective Explicit Damping Time

18

of fields. This would increase the storage requirements of a model unless

w ~some appropriate reading in and writing out of fields was used.

V. Concluding Remarks

The generalized Okamura procedures is a potent damping device.

This paper described a methodd of incorporating this device into centered

time differences. The result is an increased time step and strong damping.

One drawback lies-in increased storage requirements, although creative

programming might at least alleviate this. A more serious problem lies

in the selectivity of the damping. The results of Experiments 1, 2, and

3 suggest to me that the source of the problem might lie in an imprecise

identification of the terms driving the gravity-inertia wave motions to

which the generalized Okamura technique is applied. If future research

could eliminate this drawback, the Semi-Iterative Method would become a

very attractive candidate for numerical weather prediction models.

Page 20: An Efficient and Effective Explicit Damping Time

19

REFERENCES

Flattery, T., 1971: Spectral models for global analysis and forecasting:

Proc. Sixth AWS Technical Exchange Conference, U.S. Naval Academy,

Annapolis, MD., 21-24 September 1970. Air Weather Service Tech Re.

242, 42-54.

Grant, W.F.K., 1974: On initialization of primative equation models,

M.S. Thesis, Mass. Institute of Technology, 67pp.

Kurihara, Y., 1965: On the use of implicit and iterative methods for

the time integration of the wave equation, Mon. Wea Rev., 93, 33-46.

, and G. J. Tripoli, 1976: An iterative time intergration

scheme designed to preserve a low-frequency wave, Mon. Wea. Rev.,

104, 761-764.

Matsuno, 1966: Numerical integrations of the primitive equations by a

simulated backward difference method, J. Meteor. Soc Japan, 44,

76-84.

Nitta, T., 1969: Initialization and analysis for the primative equation

model. Proc. Int. WMO/IUGG Symp Numerical Weather Prediction,

Tokyo, Meteor. Soc. Japan, pp. VI-11 to VI-20.

Robert, A., 1966: The integration of a low order spectral form of the

primitive meteorological equations, J. Meteor. Soc. Japan, 44, No. 5.

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Page 46: An Efficient and Effective Explicit Damping Time

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Page 47: An Efficient and Effective Explicit Damping Time

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Page 48: An Efficient and Effective Explicit Damping Time

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