Monsoon circulation from observations of natural radon

EARTH AND PLANETARYSCIENCE LETTERS 6 (1969) 56-60. NORTH-HOLLANDPUBLISHINGCOMP., AMSTERDAM

MONSOON CIRCULATION FROM OBSERVATIONS OF NATURAL RADON RAMA Tata Institute o f Fundamental Research, Bombay 5 Received 28 March 1969 Radon concentrationshave been measured in the surface air over the Indian Ocean and the Arabian Sea. The concentrations in the equatorial maritime air over the Indian Ocean are found to be low (1-4 dpm/ma). Those in the monsoon air over the West Arabian Sea are similar but gradually increase to higher values (20-30 dpm/ma) as the m o n s o o n approaches the west coast of India, indicating a gradual mixing between the lower maritime air and the continental air aloft. There appears to be a significant contribution of moisture to the monsoon current by evaporation over the east Arabian Sea-

1. INTRODUCTION During the months of June, July, August and September, a moist homogenous south westerly current of air (south west monsoon), about 5 km thick, passes over the west coast of India. Early investigations [ 1] led to the belief that this moist current comes from the southern latitudes of the Indian Ocean, i.e., it is a part of the southeast trades which, on crossing the equator, gets deflected into south westerly to westerly current. But, the meteorological observations made during the recent International Indian Ocean Expedition cast serious doubts on this view. These observations indicate that over the west Arabian Sea the monsoon current consists of two very distinct air masses; the lower maritime moist air mass; from the surface to about I km level, coming from the southern hemisphere (deflected south east trades); and the upper relatively dry continental air mass (from 1 km to about 5 km level) coming from north Africa and Saudi Arabia [2,3], with a temperature inversion between the two. As the two masses approach the west coast of India, the temperature inversion between them is destroyed and the entire 5 - 6 km thick layer appears as one homogenous moist air mass. The moisture content of this 5 - 6 km thick layer over the west coast of India exceeds 1.5 times that of its two constituent layers over the west Arabian Sea as can be seen from table 1 (the moisture contents are computed from data referred to by I)¢sai [4] ). The

extra moisture in the current near the coast is of prime importance from the point of view of rainfall over the subcontinent. Two views have been advanced for the origin of this extra moisture: (1) The moisture is introduced into the air current during its traverse over the Arabian Sea. As the two constituent air masses move eastward, there is a gradual mixing between the two and the inversion level gets lifted from 1 km upwards, till at some distahoe from the Indian coast the inversion completely disappears and the whole 5 km thick layer appears as single homogenous air mass. Moisture evaporated from the Arabian Sea is transported upwards as a result of mixing between the two air masses [2,5,6]. (2) The contribution of moisture by evaporation from the Arabian Sea in negligible; the moisture is fed upwards from the lower moist current itself near the west coast of India. This can result when the lower moist current is forced upwards by orography through obstruction by west coast hills. The increase is therefore only apparent [4]. It should be possible to establish the validity of either of these views if sufficient meteorological data were available to permit a study of mass balance of air and moisture in the two components of the current. A simple check is, however, possible just from a study of the interaction between the two components. According to the first view, the mixing between the two components is necessary to transport moisture from the sea surface upwards; this would obviously

MONSOONCIRCULATION

57

Table 1 Moisture content of monsoon current over the Arabian Sea West Arabian Sea

West coast of India (East Arabian Sea)

Lower Layer Surface to 900 mb

Upper Layer 900 mb to 600 mb

HomogenousLayer Surface to 900 nab 900 nab to 600 mb

1.5 g/cm2

1.4 g/cm2

1.7 g/cm2

result in bringing continental air from upper layer to lower layer. According to the second view, the air from lower layer moves upwards into the upper layer and increases its moisture content; but prominent intrusions of upper dry hot air into the lower layer are not allowed since such intrusions would reduce the water content of the lower layer in the absence of contribution of moisture from the sea surface. Thus the two views lead to quite different predictions as to the amount of continental air in the low level air over the east Arabian Sea and near the west coast of India. Our preliminary investigation [7] revealed that the radon content of equatorial maritime air over the Indian ocean is low ( 1 - 4 dpm/m 3) while that of recent continental air such as is supposed to constitute the upper layer can be expected to be 10 to 20 times as

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much. Therefore, any mixing of this air downwards can be detected in an increase in the radon content of the lower layer. A study of vertical radon prof'des should thus be diagnostic in studying the interaction and the consequential moisture transport between the air masses. As a first step, a study of radon content of the surface air alone, which can be carried out far more easily, should provide fair indication of the phenomenon. With this end in view, the radon contents of the surface air over the Arabian Sea and the IndianOcean were measured during the summer monsoon of 1967 and are reported here. 2. EXPERIMENTAL For measuring the concentrations of radon in air, a

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Fig. 1. Concentration of radon in surface air over the Arabian Sea and the Indian Ocean. Units (dpm/m3).

Fig. 2. Concentration of radon in surface air over the Arabian Sea and the Indian Ocean. Units (dpm/m3).

58

RAMA

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Fig. 4. Concentration of radon in surface air over the Arabian Sea and the Indian Ocean. Units (dpm/m3).

Fig. 6. Concentration of radon in surface air over the Arabian Sea and the Indian Ocean. Units (dpm/m3).

simple and convenient method was employed. The air was faltered at constant speed (using a positive displacement pump) through a glass fibre filter for a few hours, and the decay products of radon thus collected on the fdter were assayed on an end window Geiger counter. This technique was previously compared with the method o f direct extraction and of counting of

radon and its decay products inside a hollow scintillation counter as adopted by Moses et al. [8]. The measurements were made at deck level aboard 'State of Haryana' and 'Oceanographer' during several cruises over the Arabian Sea and the Indian Ocean. The accuracy of most measurements is better than ten percent, except in cases where the activities are ex-

MONSOONCIRCULATION

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4. DISCUSSION The observations made during the monsoon period (fil~. 2 to 7) show that the radon concentration in low level air over the equatorial Indian Ocean and West Arabian Sea were very low ( 1 - 4 dpm/m3), indicating the presence of pure maritime air mass over these regions. The low values were occasionally observed over the central Arabian Sea also but rarely over the eastern Arabian Sea. The radon concentration usually started increasing at about 65°E and reached a value of 2 0 - 3 0 dpm/m 3 near the west coast of India. The increase of radon in the lower current implies the introduction of continental air in it probably as a consequence of mixing from above. The component of continental air in the lower current can be correctly assessed if the radon content of the continental air in the upper layer is accurately known. The measurements at high altitudes

60

RAMA

have not yet been made, but we may obtain an indirect approximation as follows. The air from the Indian subcontinent is known to pervade over the Arabian Sea during winter months. Its radon content in the month of November ranged from 25 to 65 dpm/m 3 (figs. 8 and 9) and averaged around 40 dpm/m 3. It may likewise be expected that the radon content o f the air coming from the African continent would be about 40 dpm/m 3 at the surface and somewhat lower at higher altitudes. The radon contents o f the surface monsoon air over the east Arabian Sea range between 5 and 30 dpm/m 3. The values near the western coast of India average around 20 dpm/m 3. Comparing these with a value of "~ 40 dpm/m 3 deduced for continental air in the upper current, one arrives at the conclusion that the component of the continental air in the surface air exceeds 50 percent near the coast, pointing to vigorous vertical mixing between the two air masses. In spite o f a large component of continental air at the lower levels, the moisture content (and humidity mixing ratio) of the lower current does not decrease (see table 1). This is evidently possible if an adequate amount o f water is introduced by evaporation from the sea surface. The data show that at least about forty percent of the extra moisture observed in the monsoon current near the coast originates from evaporation over the Arabian Sea. This appears to be appropriate interpretation of the data in terms o f two-air-mass picture o f the monsoon current which however needs to be verified by measuring radon in vertical prof'des. Fig. 1 shows the radon concentrations measured in a west-east traverse across the Arabian Sea just before the onset of monsoon rains, and off the Saurashtra coast just after the onset. The high concentrations in the latter arise as a consequence of mixing of continental air from above; the continental air originating from North Africa or possible in the mid-tropospheric circulation [9].

5. CONCLUSIONS The radon measurements in the monsoon current provide a convenient method for studying the interaction and moisture transport between its two con-

stituent air masses i.e. maritime equatorial air mass and continental African air mass. The limited data indicate that a significant amount of moisture is introduced into the current by evaporation over the east Arabian Sea.

ACKNOWLEDGEMENTS I am grateful to Prof. M.G.K.Menon, Prof. D.Lal and Dr. F.Ostapoff for their interest in this experiment. The skillful assistance of Mr. S.G.Patil and Mr. B.I. Shaikh is greatly appreciated. I take this opportunity to express m y sincere thanks for the cooperation of the captain, crew and Management of S.S.Haryana and U.S.C. and G.S.S. Oceanographer.

REFERENCES [1] G.C.Simpson, The south west monsoon, Quart. J. Roy. Met. See. 47 (1921) 152. [2] J.A.Colon, On the low level t~ermal stratification of. monsoon air over the A~rabianSea and its connection to the water temperature field, Technical Note 9-SAIL-I, U.S.Dept. of Commerce (1965) p. 209. [3] C.R.V.Raman, Cyclonic vortices on either side of the equator and their implications, Prec. Syrup. Met. Remits of International Indian Ocean Expedition, Bombay (1965) p. 155. [4] B.N.Desai, Relation between moisture transported across the equator and the west coast of the peninsula during the south west monsoon season, Indian J. Met. Geophys. 17 (1966) 559. [5] A.F.Bunker, Interaction of summer monsoon air with the Arabian sea, Prec. Symp. Met. Results of International Indian Ocean Expedition, Bombay (1965) p. 3. [6] P.R.Pisharoty, Evaporation from the Arabian Sea and the Indian Southwest monsoon, Prec. Symp. Met. Results of International Indian Ocean Expedition, Bombay (1965) p. 43. [7] Rama, An attempt to trace the monsoon flow using natural radon, Indian J. Met. Geophy~ 19 (1968) 167. [8] H.Mosses, A.Stehney and H.F.Lucas Jr., The effect of meteorological variables upon the vertical and temporal distribution of radon, J. Geophys. Res. 65 (1960) 1223. [9] F.R.Miller and R.N.Keshavmurty, The Arabian sea summer monsoon, Prec. Symp. Met. Results of International Indian Ocean Expedition, Bombay (1965) p. 337.