Desai, B. N.

Rao, Y. P.

English

Publications of the IAS Fellows

LETTER TO THE EDITOR
On the instantaneous distribution of vertical velocity in the
monsoon field and structure of the monsoon circulation.
By Y. P. RAO, Meteorological Office, Lodi Road, New Delhi-3 and B. N. DESAI, 173, Swami
Vivekananda Road, Vile Parle (Weat), Bombay-56, India
(Manuscript received October 20, 1970)
1. Introduction
In a paper published in this J oumal on the
subject, Saha (1968) has made various state-
ments which would not appear tenable in the
light of facts of topography and climatology as
well as of observations on 7 July, 1963, the
day selected by him for his study. It is proposed
to draw attention in this note to some of the
important points.
2. Discussion
I. Figa. 2a, b and c of distribution of vertical
velocity
(a) There was widespread orographic rain
on the west coast of the Peninsula. Yet ascend-
ing motion is seen only at 850 mb (Fig. 2 a);
there is little ascending motion at 700. mb
(Fig. 2 b) and there was actually descending
motion over the coast south of 17° N at 500 mb
(Fig. 2 c).
(b) There were soundings over the northeast
Arabian Sea on 7 July, 1963 near 23° N, 68° E
and 20° N, 70° E (Desai, 1967). Over these
two locations ascending motion is seen both
at 850 (Fig. 2 a) and 500 mb (Fig. 2 c) levels
and descending motion at 700 mb level (Fig.
2 b).
The sounding data at 23° N, 68° E showed
an inversion from 942 to 850 mb; there was
dry air with adiabatic lapse from 850 to 586
mb and dry air with nearly saturation adia-
batic lapse from 586 to 500 mb. Thus ascending
motion at 850 and 500 mb (Fig. 2 a and e)
would not be supported by the sounding data.
The sounding data at 20° N, 70° E showed
(i) lapse rate between dry and saturation adia-
batic with high humidity from surface to 716
Tellus XXIII (1971), 3
mb, (ii) nearly isothermal layer with 90 to
100% humidity from 716 to 629 mb. (iii)
nearly dry adiabatic lapse with high humidity
from 629 to 574 mb, (iv) nearly saturation
adiabatic lapse with humidity decreasing with
height from 574 to 531 mb, and (v) isothermal
layer with high humidity from 536 to 500 mb.
In view of the above descending motion at
700 mb (Fig. 2 b) was unlikely. The synoptic
charts show that there was ascending motion
at 20° N, 700E due to convergence zones nearby.
Saha's view is that the low-level inversion
over the Arabian Sea is due to subsidence.
It is considered that the same is an airmass
one (Desai, 1970 a, b) as over other parts of
the Arabian Sea.
At 20° N, 70° E there was also the same type
of airmass stratification as at 23° N, 68° E
i.e. cool moist air in the surface layers and dry
unstable continental air above, but the same
was disturbed due to presence of convergence
zones nearby at different levels and the low-
level inversion was destroyed. The dry air
was cooled and humidified by rain falling
through it; Veraval near 20° N, 70° E had 4
em rain between the 7th and 8th mornings.
From the above discussion it would appear
that vertical velocity patterns of Saha are not
suppported by the rainfall over the west coast
and available soundings over the northeast
Arabian Sea in all the cases.
II. Fig. 8 regarding distribution of absolute
vorticity at 850 mb and 24 hr rainfall between
7th and 8th mornings and Fig. 9 of July normal
rainfall
From a study of synoptic charts during
the monsoon season, it is seen that patterns of
circulation and associated rainfall are signi-
264 Y. P. BAO AND B. N. DESAI
ficantly different from day to day. The normal
July rainfall given in Fig. 9 is a mean based on
days of little rain and copious rain besides
rainfall of days when it was about the same as
normal. The authors have given in their papers
under publication in the Indian Journal of
Meteorology and Geophysics contour patterns at
500 mb level from 26 June to 10 July, 1963
and for 850 and 500 mb levels for 26 June, and
1 and 10 July, 1963, 4 and 5 July, 1962 and 12
and 13 August, 1964 with 24 hour rainfall on
850 mb level contour chart; it is seen from those
charts that circulation patterns and associated
rainfall varied significantly from day to day.
Miller & Keshavamurthy (1967) have given
divergence patterns for 700 and 500 mb level
respectively in their Figs. 43 Band C for July,
Figs. 31 Band C for 2 July, 1963, and compos-
ited Figs. 43 Band D for 2, 4 and 7 July, 1963.
Examination of their divergence patterns shows
how different they are when taken for a single
day, for 3 days together and for the whole
month; the associated rainfall distribution for
the cases was also different as seen from our
study.
It would thus appear that it is not advisable
to draw conclusions about :monsoon circulation
from analysis for a single day as done by Saha,
III. Transport of air across the equator
The transport of air across the equator in
the western Indian Ocean during the northern
summer monsoon iv very large. It is seen from
the computed values of meridional transport
across the equator between longitudes 35 and
75° E in the layer surface to 600 mb during
July given in Table 1 of Findlater (1969 b)
that 7.68 x IOU metric tons of air is transported
per day, this value being nearly half of the
total transport across the whole equator; the
bulk of the transport takes place between longi-
tudes 38 and 45° E as seen from Fig. 2 of
Findlater (1969 b), the maximum being between
950 and 750 mb or in the layer 0.6 to 2.4 km
in which the low-level southerly jet occurs
(Findlater, 1969 a, Figs. 10 and 11).
The cross-equatorial flow west of about 60° E
constitutes the Arabian Sea monsoon current
(Desai, 1967, 1968; Rao & Desai, 1969, 1970)
and its depth increases to about 6.0 km over
the Peninsula. The moist air which crosses the
Peninsula constitutes the bulk of the Bay
monsoon current north of about 10° N, only a
small portion of the southern hemisphere air
entering the Bay south of 10° N either from
the south Arabian Sea side or directly across
the equator east of 60° E.
IV. The westerly and easterly jet-streams
The westerly and easterly jet streams would
not appear a part of the monsoon circulation
below about 500 mb (Desai, 1969; Rao & Desai,
1970). The warm anticyclone over Tibet in the
upper levels develops independently of the
trough over the Gangetic Valley in the lower
levels and which is of dynamic origin although
the flow of moist air from across the equator
is influenced by the heat-low over northwest
of the Indian subcontinent. In fact one would
be justified in considering 600-500 mb layer
as transition layer in the mean between the
lower monsoon and the upper easterly circula-
tions as far as the Indian summer monsoon
area is concerned.
3. Concluding remarks
Fro:m the foregoing discussion it would ap·
pear that it is not correct to base model of
monsoon circulation on a single day's data
as there are ordinarily significant day-to-day
variations in the circulation patterns and the
associated rainfall over the subcontinent. With
an open network of stations reporting upper
air data over land and only a few reports from
the sea areas, the vertical velocity computa-
tions will not help to explain or forecast rainfall
satisfactorily over different areas. The low-
level inversion over the Arabian Sea is an
airmass one and not due to subsidence. The
monsoon circulation upto about 600 mb and
the easterly circulation above 500 mb can
develop independently, there being no cause-
effect relation between the two. The massive
flow of air across the equator between about
38 and 45° E in the levels upto about 600 mb
is considered vital for setting up the monsoon
circulation over the Indian subcontinent. The
monsoon trough over the Gangetic Valley is
produced as a result of influence of topogra-
phical features of the subcontinent on the air
flow, the Indian summer monsoon being pecul-
iar in this respect when compared with mon-
soons in other parts of the world.
Tellus XXIII (1971), 3
VERTICAL VELOCITY IN THE MONSOON FIELD 265
REFERENCES
Desai, B. N. 1967. Airmasses over the Arabian Sea -- 1969 b. Interhemispherio transport of air in
during the summer monsoon season. Proc. Symp. the lower troposphere over the western Indian
on Indian Ocean, New Delhi, March, 1967, Ocean. Quart. J. Royal Met. Soc. 95, 400-403.
Bulletin No. 38, National Lnetituie of Sciences of Miller, F. R. & Keshavamurthy, R. N. 1967.
India, pp. 963-981. Structure of an Arabian Sea summer monsoon
-- 1968. Interaotion of the summer monsoon system. International Indian Ocean Expedition
ourrent with water-surface over the Arabian Sea. Meteorological Monograph No.1. Contribution No.
Ind. J. Meteor. Geophsj«. 19, 159-166. 196 from the Hawaii Lnetiuue of GeophyBicB,
-- 1970 a. Discussion of upper air data of 2 and Univ. Hawaii, pp. 1-94.
4 July, 1963 over the Arabian Sea from the point Rao, Y. P. & Desai, B. N. 1969. Origin of the sum-
of presence of airmasses and structure of the mer monsoon ourrent over the Arabian Sea. Curro
oyolonio vortex off the Bombay coast. Ind. J. Sci. (India) 38, 507-508.
Meteor. Geophy8. 21, 71-78. -- 1970. The Indian summer monsoon. Proe,
1970 b. Nature of the low-level inversion Syrup. on Tropioal Meteorology, 2-11 June, 1970,
over the Arabian Sea and role of the Western Honolulu, Hawaii. Bu::. Amer. Met. Soc. 51,
Ghats in modifying airmass stratifioation within No.3, p. 297.
500 km of the west coast of the Peninsula. Ind. J. Saha, K. R. 1968. On the instantaneous distribution
Meteor. GeophY8. 21, No.4 (Ootober). of vertical velooity in the monsoon field and
Findlater, J. 1969 a. A major low-level air ourrent struoture of the monsoon circulation. Tellua 20,
near the Indian Ocean during the northern 601-620.
summer. Quart. J. Royal Met. Soc. 95, 362-380.
Tellus XXIII (1971), 3


On the instantaneous distribution of vertical velocity in the monsoon field and structure of the monsoon circulation

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On the instantaneous distribution of vertical velocity in the monsoon field and structure of the monsoon circulation

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