Met.Monograph on SW Monsoon - (IMD), Pune

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24 Southwest MonsoonFig.2.13 - Vertical wind cross section using rawin data only, along 80°E for 13 July1962, 00 GMT.(The line of dots shows the approximate line of separation between theeasterlies and westerlies. The approximate position of the level of themaximum wind, as obtained from the isotach analysis is marked by thethick line ).
<strong>Met</strong>eorological <strong>Monograph</strong> : Synoptic <strong>Met</strong>eorology No. 1/1976 252 . 7.8 In the middle troposphere, winds are very unsteady in July and August but the uppertropospheric zonal easterlies and lower tropospheric westerlies are steady south of 25 o N.2.7.9 To sum up, the chief features of the upper air flow in the season are the monsoontrough sloping southwards with height and the ridges over northern India, Peninsula has lowertropospheric westerlies and upper tropospheric easterlies. The depth of the westerlies increases fromthe southern tip of the country upto 13° N and decreases further north (Krishna Rao and Ganesan,1953). The flow from the Arabian Sea across the Peninsula is usually referred to as the 'ArabianSea monsoon' and that to the north of the trough after some travel over the Bay as the 'Bay monsooncurrent'. The ridge in the upper troposphere near 30 o N and east of 75° E is referred to as the'Tibetan High'.2.8 Easterly Jet Stream2 . 8 . l Near 100 mb, strong easterlies blow to the south of 25 o N in the southwest monsoonperiod, which concentrate into a core of high winds known as the 'Easterly Jet Stream'. Krishna Rao(1952) first pointed out that Nagercoil shows easterly winds of 50 m sec –1 at about 75 mb (19 km)which constitute a part of the easterly jet stream over south India between 7 o and 18 o N,Koteswaram (1958) studied the easterly jet stream over India in detail. These strong easterly windsnear the tropopause are a characteristic feature of Asia and Africa in summer and are not to befound over the Atlantic or the Pacific. In July, winds are easterly 10 to 20 kt at 300 mb south of27 o N and westerly to the north of 32 o N with transition in between. As the temperatures decreaseto the south from 25 o N, easterlies strengthen over the Peninsula to 40 to 50 kt by 200 mb„ At 200mb, the temperature decrease southwards starts at 30 o N itself. At 150 and 100 mb, the temperaturegradient south of 20 o N is very flat but the southward decrease persists to the north. This northwardincrease of temperature extends almost upto the Pole. This is a global feature. However, the steepgradient is between 30 o and 50 N . The easterlies increase with height upto about 150/100 mb overthe Peninsula, at which level they are a maximum. At Madras, the mean wind is 85 /70 kt at 16 km,while at 14 km it is 85°/60 kt and at 18.0 km 85°/50 kt. From the available scanty data, the core ofthe easterly jet over India appears to be near about the latitude of Madras (13 N) at 100 mb.However, at Trivendrum (08°30' N) the maximum speed is at 14.0 km (150 mb) , being 80°/65 kt,higher than that at Madras at that level. Aloft, the speed is less than at Madras. At Gan Island (l°S), the maximum speed of 55 kt is at 150 mb as at Trivendrum and the speed decreases sharply to25 kt by 100 mb. At Bombay (l9° N), speeds at 14, 16 and 18 km are respectively 55, 70 and 60 kt.The speed at 18 km is higher than that at Madras. Thus while the core of the easterly jet is near13° N at 100 mb, the highest speeds at 150 mb are further to the south and above 100 mb further tothe north.2.8.2 Even at 100 mb, strong temperature gradient prevails over northern India. Henceeasterly winds increase with height even above 100 mb. New Delhi (28 30' N) shows an increaseof speed from 10 kt at 14 km to 35 kt at 24 km. The easterlies may not be extending to north of 30°N in the upper troposphere, as the few available winds at Ambala (30° 30' N) show westerliesbetween 14 and 18 km. The easterlies above 18 km over Delhi may be a part of the globalstratospheric easterlies in summer.2.8 . 3 The easterly jet stream over the Peninsula is the effect of the southward decrease oftemperature in the entire troposphere. This thermal gradient is effective in reversing themoderately strong westerlies of the lower troposphere and building speeds of 80 kt by 100 mb .2.8.4 The horizontal shear at 100 mb to the south of the jet between Madras and Trivendrumis 0.09 hr –1 . To the north, between Madras and Visakhapatnam it may be of the order 0.07 hr –1 .The vertical shear between 9 and 16 km varies between 15 and 20 hr –1 , the maximum shear being inthe layer 5 km below the core. Above the core, between 16 and 18 km the shear is about 22 hr -1 .
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Page 7 and 8: VP R E F A C EThe Southwest Monsoon
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Page 11 and 12: VCONTENTSFrontispiece : Mean Wind F
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CHAPTER 9OTHER SYNOPTIC SYSTEMS9.1
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CHAPTER 10OROGRAPHIC EFFECTS10.1 Or
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CHAPTER 12CLOUD AND RAINFALL CHARAC
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CHAPTER 13BALANCE OF MASS, RADIATIO
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Met.Monograph on SW Monsoon - (IMD), Pune

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