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

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16 Southwest MonsoonFig. 2.9 Upper air isotherms ( o C) April
<strong>Met</strong>eorological <strong>Monograph</strong> : Synoptic <strong>Met</strong>eorology No. 1/1976 172.6.5 In July (Fig. 2.10), at the height of the southwest monsoon, a thermal high lies over Iran,Iraq and central parts of Arabia at 850 mb and a thermal ridge runs from it to Lat. 35 o N to thenorth of India. Two thermal troughs run – one along the west coast of the Indian Peninsula and theother along the Burma coast – while a thermal ridge is present over the west Bay andneighbourhood. The thermal pattern at 700 mb is nearly similar except that the troughs and ridgesare absent to the south of about 20 o N and the thermal gradient over India is less. The thermal highover north America is at a little higher latitude and over west Africa at a lower latitude than overwest Asia. At 500 and 300 mb, a thermal ridge runs along 25° – 30 o N with appreciabletemperature gradient to the south at 300 mb. At 200 mb, the thermal ridge is along 30° – 35° Nwith decrease in temperature to the south. Aloft, this thermal ridge disappears. At 150 and 100 mb,the temperature increases from the south to the north, from the Equator to the Pole.Standard deviation of temperature is about 2° C and 700 and 500 mb, increasing to 2° to5° C aloft. Standard deviations are higher in the north than in the south.Tropopause is highest between 25 o and 30 o N, where it is between 100 and 95 mb. Thepressure at tropopause increases to the south, being 120 mb at Port Blair and 115 mb atTrivendrum. To the north of 30° N also, pressure at the tropopause increases. The temperature atthe tropopause is uniform upto 30 o N, being about 198°A.Monsoon air mass prevails over most of the country where lapse rates are near saturatedadiabatic. Between 850 and 700 mb the lowest values are along the west coast and the highest nearMadras which has less rain. They are 5° to 6° C Km –l between 700 and 500 mb and increase withheight becoming a maximum at 300–200 mb or 200–150 mb where values of 7° to 8° C Km –1 arereached. Aloft, they decrease. Between 150 and 100 mb, higher values are to the north, with5.7 ° C Km –1 at Delhi and only 2.1° C Km –l at Trivendrum.2.6.6 The warmest region shifts markedly in the monsoon period, more so in the uppertroposphere. The thermal ridge at 200 mb over Tibetan region is an important feature.2.6.7 Banerjee and Sharma (1967) find from harmonic analysis of monthly upper airtemperatures over India, that in the first harmonic, there are two oscillations, one at the surface andthe other in the upper troposphere. The first, due to ground heating, is confined to below 700 mband is in phase with the movement of the Sun. The other is between 600 and 150 mb withmaximum amplitude at 350 mb. The maximum occurs from mid–July to mid–August. Itsamplitude decreases at 300 mb to one–eighth from 30° N to 10° N. This oscillation is apparentlydue to a different physical process.2.7 Upper Winds2.7.1 The sub–tropical high pressure belt passes over and near India throughout the year. It ismasked in the lower troposphere in southwest monsoon season by the effect of continental summerheating. In the middle and upper troposphere, it is prominently seen and defines the winddistribution.2.7.2 In April (Fig. 2.11 a and b), a trough line runs along 77° E at 900 mb over the Peninsulaas at sea level. Weak high cells probably exist over the central Bay and the Arabian Sea. The sub–tropical ridge appears over land near about 18° N at 850 mb and persists at 700 mb. Aloft, itgradually shifts southwards to 8° N at 200 mb. At this level, the anticyclone from the east extendsupto Sri Lanka. A trough is also present over northeast India between 900 and 800 mb with axisalong 87 o E. Westerlies increase with height over northern India and the adjoining Peninsula.Maximum speeds are about 40 kt near 25° N at 300 mb and 50–60 kt between 25° and 30 o Nat 200mb, decreasing slowly aloft.
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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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13.3 RadiationMete
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Met.Monograph on SW Monsoon - (IMD), Pune

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