Characteristics of the Southwest Monsoon

19

CHARACTERISTICS OF THE SOUTHWEST MONSOON AIRl\'1ASS

By p, ,\, 1\[1<:;'\():\

ABSTRACT. TIl{' main featul", of the ,outll\\("t llln!l'O()l1 ailma"" as deriled fronl rodj,,'ollde :I'Cl'lJts of Trh!llldnll11, :\[,"]1:1' ,JrlJ Calclltt" lor the 1<':11' 1()47, al'P 'lIlJ!lllari<;ul

'l'lIt mean kmpl'rall1ll "llIll" "I tIll thlP( ,lati"n' ploitc,j Oll a [('phil'fllm do not SUpp"r!

tilt ptt'\alent vif'\\ th,11 tltt lJ)uTl .... \\i\1I ,tIl IlL\'-., I..,.! hrllll ) 1-'(' I1t 011 .. 1'1l1HlI\ !l1ld Il:J~ .1 "'dhn.dld

,ldiaLatir Japbf" lat(',

Thl: "Oll(hlll~l 1l1tllhllllil 11.1" kVII 0-111(11l11 1)), lalillll:' 1\011\(,1'-. wl(li tltl ..,lldace data :tud ()l'C;t-.j()J1al \1j'1'11 ;\11 klllf'llatlllV data Sim( T{Jj I daily latllOsolHk as('vll1:-. all bC!IJ~ IIlddc ;I( a 1111 111 I>(,t of ~t;[tioll" ill India and It j~

IIIlI\' J.()s..,ihil' t(l ,;lllily tIll' l'll<1la(',cll:,li(''o of lhl' "outh\\,c<;( tllOlhOOtl in grcakl (kwil In 11th llol( tiJl mai1l fl;:l(UICc, (If thi'o ainna"" .1" del i\cd frot1l till' data

ot Tli\.llllliulIl, ;\I.ltll:l~ :l11<ll'aic111l;t f"1 1111' year 1(117 all 'olttll111ali..,cd, 'l'ri".JJldllilll 10. l()(\lk<! Oil 1111' II ^l ,>1 I'()~hl oj TJ1(li,l :lliO\1( thirty 1lliles !r01l1 Capl l.'ttll]()J III TIll" III:l\, III ('OlIO-HIli ('(1 ;t" the tlt:-1 'olatillll 1I1 India t() hc

"tllleh 11\ til, ~()ulll\\l."l 1I1()II"O{)JI afl( J ('IO"'oIIIF the "jlW101 '\1 (hi .... ..,tage lli", 'ool1t!t\\c..,1 111(>1I"IH)1J h llilIlIfllllIII ((1 h\ allV 11;11\ 10\'11 l.md '\!adla.., i"

..,itllat,"d 011 fIll (',I~( ('<1,1"( 01 (lie ill.'lIit1c,lI1.1 a11d ItU1Cl i-. \1.'1\ c,ui(ahlc fOt

"tt1(lYI!1~: 111\ 1'11maclli I..,til"> {'f tIll' 111()lhO()1l ,Ill aj'll il Ii"" l'I""''oll111" Chals

(\Iilll .t..,,,oci,j(l'd ltfttlH' ,111d sllt."i(lvtllt.') ,1lItJ ,II:,,, tla\dlcd O\ll .j0[, mtle ... ot

!:t11t! l'alcl!lta I" 1o('.ll\d :It t1l. l,]:]('l <It .I, Ilcl'tI(JJl Ilj tlil ,,"tlllll\l"( 11 lOll ,,0011 illto a "'()l111H,;l..,t(ll~ ('llI1\'llt UUtlll \11t.: ill111111ICC (If tlw 111011,,(,(111 lbllltH.d1 ()\'U (;all~ct]( plain and \\"111<1 tl111'o l'1\l tilt t'lUJ'lcl,II..,lic'o <If th, dd1ech:t1

111011'00011

~

..

.,

~;~ao

!

III

""' • ~I! 0 ~ -l--t--___ e - . , -

-' o

,."

.,

I:

~ ~~1O / C.

150

^A.

'l'able I gives the variation of tempernlure at various level:., and also the Illean temperature and the average variability. The monsoon at Calcutta i.., warmer at all levels than at 'frivantlrum by 2 to .'(l'. At grouud level Madra..,

i~ wannest and at 850 mb, the temperature b the same as that over Calcutta.

But from 700 mh. onwards the l11ean temperatures at Madras anc11'rivandrul1l Me almost identical. This shows that tIll' effect of the land havel in readllllg Madras is confined only to levels below 700 mbs. and the Trivaudrum eharae- ted:.tics are preserved aloft.

In Fig. I, the mean temperature curve:. for the three ~tations arc plotkd

011 a tephigram. It will bl- ~eell that the lapse rates at both l'rivandrulIl and Calcutta arc dbtinctly greater than the ..,aturated adiabatic. This does not agree with the prevalent view tliat the monsoon air l\Jas~ has a saturated adiabatic lapse rate. At Madras the la]>~l- rate between 1000 and 700 mh:-.. i~

greater than either ot the other two stations but aloft the curve is identical WIth Trivandrul11.

TelUperaturt' (OC)

TAlIU! I (a) Groun(llevd

TrivandnllJ1 Calcuttd

FrequelH'\ No of I FrequellCl

(Oo) O('(3"JOJl.... \n~}

No of Frf'(jucnry No of

oc\a~ion~ {(:o. Lcca~jl)n""

I

~--_.

__

25

2()

31

JJ .H

37 Total No of Observations

~ean Tetnperature ( 'C)

~verage Deviation

, S.i

2() 31.5 2

3·"

49 53·S i ^11.6 3 2.6

54 '7 2<":: ~ 11 95

J I IlJ \2 2 j .~ 129

i ^11.6 33

4 () i ²⁰ 17 2

.3 S ()

I,

I 7 i Ii.o

~.6

---;---1--

.. -_.- I ^!

25.6 27.9 29.6

0.6 1. I 1.6

Characteristics of the Southwest Monsoon Airmass

151

Temperature (OC)

I ' .'

II

II>

J~

19

2.,

21

27

T"tal ]'I." of nh'cn ati, ll~

;\Teall T('llIP< ratUle (OC)

\ v< rage DL Vldl jon

TABU'; I (collld.)

(6) (,50 mb. level

'}'1 iVHlldrul1I Calcutta Madra~

---

'\Ill of Frequelll ~ No of PrequellC) i No of I FrcquellL'

(J('ca~i )JJ~ (/;f) ()l (Hsio1l5 1%1 ()c('a6ion~ 1%1

u9

44

1" I I 1 _J 7 "Y

11 10 , 1 7 3·4

Ih 19·7 33 4 3·4

>() 28 <) ) I 7 I I 95

1,~ 197 Ii ^{I() ()} l.' 1" 4

,2 2 l~ 23 , 19 103

~ I' Ii'}

2,

<) IS ( I IS 12 <)

~ {-. ,I 20 172

I 7 3 ^{2 ()}

3 ^{2 II}

J 7

I i \

- - - -- --- 1---

6" I16

17 I ^{20 b 2"} 0

I ,

J"

152

Tempt'ratUl"t"

(OCI

3

5

(i

7

~

<) I "

I 1 U

1';

14

Tolfll No. ()j

Ubsen'ations Mean TellJperature

1°C)

1\ vcrage DeviatJOIl

P. A. Menof'l

TABLE I I( oil/d.)

(c) 7Ull Illb. level

Tri\"anclrlll11 l'Akutta

N(). of

O(.'CU!-J1Ullf>

.)

f>

I.J

c"'

I I

.

3

8<)

l.A

Frequcnl ~ ,D,d

1.1

.l 1

f)·7

6./

7 'J 2h I)

I~ 1 : 1·'\

7.<) ,)·1

l" 0 01

()ccasion'"

()

11 'I

"

.~S

12.2 I 4

I'requcl,cy

10/ /0

1.\

1.1'-.

I.S

1<). ,1

~·3

'2,; 1

l(l,/1

25 1 .;.()

---~-.---'---

Madra'

No. of

I Fre<juelll"~

uccasions \ CX»)

(1·0

/) 5·S

4 ,l·7

h 5·S

11 12·9

1" ^111.(1

2..: .::!o • ..!

1,-; l/)./)

1'1 ^{J 2 ()}

1.8

(l.r)

1.8 1.S

In')

<)h J ~

Characierrstlcs of the Southwest

A rrmass 153

It IIIperaturt I"C I

()

- I

- (

- )

- Jl -12

-I,

-15

lotal]l.,o of Db,,:! vatl >11'>

'ITean lunperdtlll t (Oe)

"crage DCI Idt 'll

TABII!. J (conid)

Id 5(J0 mb kvLi

TllvandrullI Cuillltta

]I., o)f I J(><jUlIh Y

O( laSlOn., I I

2 2 ,

2 2 S

2 ,

h ;

1/ ^{~ I ,}

1 h ~

'> I

1 , J _

I 2

h 1

No of )ll1Sjollb

3

9 7

I I

7 9

I I equt'lH \

.%

62

2 I Ih I>

J 1 ( 22 ') 11⁰

J" ~

2 I

,

I ,

'\ of

It

O( ('aSlCllI~ ~ 10) _

J !)

3 - I)

.,

<i 79

2') 21'1 5

'4 13·

J I <) 7

') 79

" ""

I I

17

J 9

ell)

I ,

6 ,

J J

154

Temp'!ra til r('

1°C,

P. A. Menon

'fABU( I (contd.) (t) 300 mb. level

'l'rivandrulII Calcutta

No. of Frequllll'Y

(}('l'a&ion~ I (% t

No. of

,,('casiol1~ Frequency

(0/.,)

. _ - - - _ . _ - -

-IY -20

-~l

-22 -2.1 -24 - :z~

-20 -27 -28 -29 -30

-~l

-32 -33 -34 -35 -3"

-37 -311 -31) -4"

-41 -42 -43 1'otlll No of

ObseTvati'1n~

~Iean Temperatult (OCi Average Dedati(lll

I

4 3

; 3

I"

6 7 2 b 4

'i4

- 324

3·7 3 7 3 'i .) i

-" ¹⁸⁵

1 1\ 4 14.1\

7 4 4 111\

5·5

5·5 i 259

5·:; ² i 4

18 5 , i

13 ("I

1I2 3 6 10·9 18 73 3 i 1.1\

1.1\

'3

Madras

-~o

-1~reCJuency

ol'casionl> (%)

1 5

I I S

I 1.5

6 8.9

3⁰ 1 6"

<} 1.~ 4

14 ~o 9

lJ 134

<) 13 1

3 55

5 7 S

1.5

1 5 15

22

Thc temperatures at ground level at 'l'ri,'andrtt1l1 are remarkably uniform, being 011 85% of o(ca~ions either 25 01' 26°('. 1h1s ha!> ~iven rise to the impres!:lion that the monsoon is a very homogeneous current. Tallie I, howevcr, shows that il ^IS /l(ll so. 'fill! tel11pl!rature~ at higher levels are not at all uniform, and "ary het\\cclI fairly wide lilllith. The avt:rage variability (which represent:. a measUlC of the Ilcgree of variatiolJ of tt:lnlleratl1l"l."!) steadily increases from 0.6 at the ground It:vel to 2.1' at 300 11Ih. level. Thlts even at TrivandlU\1J the southwe~t monS0011 is not a hOllJllgel1l."!ou!> airmas!> at upper levels and shows appreciable temperature variations. The avclagc varia- hility for Calcutta is more than that for Trivandnul1 at ground It:vel and 850 mb. but less than Trivandtl111l aloft. 'l'bis shows that the air at Calcutta is less heterogenous than at Trivandrum at uppcr levels. 'fhe average variability at M8dra~ is the highest at ground level and tbe !>ame as at Calcutta al 850 mb. level. At 700 and 500 mb. it i~ nearly the same as at 'I'rivanc1ru1I1

Charackri:slics

0/

which !>upport!> the view arrived at hefore that the air at 1Iladras ahove 700 mho is unmodified hy the intervenlllg land travel.

High relativt! humidity is an important property of the monsoon air mass.

Table II gives the number of occasion~ when the relative humidity at 700 mb.

level was less than 50%.

Station No of a'(,('lIt, that

I rUldll'd 7 '0 1111. 11'\ d

Trh':lIHlrl1l11 ^,',1,

J Il)

Calcntta

No o{ fI('('w.i()n~

"hen Tl'lath ( hl1ll1idit \' at ion IIIh "A' lr~s thAII <;"':0

I'.

Percentage of

()('ca~iol1s II hen R H at iOO mh

"a~ les~ than 50%

, ~. 0

7"/ f.

Though at Trivandrulll thl mOIJ~oon is • fresh' from the sou Tl'l' region of the moisture. the relative humidity there at 70C) mho level is less than 50%

more frequentI~ than eithel at l'IIadra" or Calcutta. This seems to b<.: all the more strauge wll<.:n \ve take into consideration a spec.iaJ f",ature pertaillill~ to the year lQ47. During this year there \\ ere fairly prolonged break in tlit' mOll

:-;oon and as snch Olle would natUl ally expect that 'l'rivalldl urn, being locatt!d at the southernmost point of India. would hl' more frequently undt'1' the sway of genuine soutll\\ est 1110nSOOll currc.nt than Calcutta and henct· :obould sho\\

greatd occasion of high hll1lliditie~ at 700 mb. than at Calcutta. In ... pite of thb we find that on significantly greater number of occasions Tnvandrulll had a lower relative hl1midity at 700 11111 level than Calcutta. As high hUlllidit.>

i~ the chief <.haracten!>tic of the monsoon air mass, it may be concluded that the depth of the 1110nsoon air1l1as~ at T1'ivandrum or l\Jadra~ i~ les~ than at l alcntta.

A1I oft r~pcated statement i:;. that the monsoon air, in clOssing the Western (;hats and owing to any precipitation in its travel over Deccan. i~ denuded of moisture. Table III gives the mixing ratto (gm/kg) at Madra::, and Trivall- dllllll at 850 and 700 mh. levels. Strangely the mixing raho i" greatcl at Madras than over Trivaudrutll. How the intervening land between '1'rivall- drulll and Madras acts as a sonrce for moisture is a matter for fUrtiICr "tudy.

'fABUl; IV

'\ '" rage l1Ii"ing

ratio at S5'1 IIIIl i" 1\11> I No IIf ill\el'~iol1' III Total No. of

I i!>()th~rtnallavers ob~t'n'atioll'

Station

Madras '1'1 ivandrnnJ 22

1'riv811dru111 12 I 7 " ^MadlR~

i

Calcutta

... i

156

A.

The number occasions of occurrence of illversiolls or isothermallaycr::, between ^<.)00 and SOO Illhs. at the three stations is given in Table IV. Thus resistance to cOllvection is more frequent at Trivandrum and Madras than at Calcutta.

Investigation of the monsoon characteristics at other stations and the

si~nificallce of the day tll day variations in temperature and humidity are in progress.

\ C K 1\ () \\' T, H n G E :'II To: N T S

The author wOllld like to expre::,s his very grateful thanks to Ilr. S. Mull alld Mr. Y. P. Rao for suggcstill~ the prohlem and guidance.

Hf.t;W.'iU ':\1111'. CE'iTI<Jo.

K'M:J'nl