a study ofactive monsoon over india using satellite...

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/"dia't J . Md . Gcop 1l ys. (1971), 22 , .J01 .4110

. 551. 55'\. 21:551.511 : 551. 507. ~6~ . 2

A study of active monsoon over Indiausing satellite radiation data

V. SIUNIVASAN

Mdem'ologi<:al O.1Ji<x, Poona

ABSTRACT. 'Ttroe VII radiation data in the 8-12 micron chan nel Ior tho periotl 5.R.Iuly 1063 hl\.8 boen analysed.This was 0. period or good monsoon activity in tho Peninsula and central India. The LR. anRolysis is compared withsurfa ce synoptio end UPI)('C air chert s. Tho reeulte indicate that tho genera l height or tops of monsoon clouda is about7-8 km and theeo clouds presumably ell) not ext end much iut) tho upper tropospheric ceetc rlice. However, in the areasoflocefleod heavy monsoon ect.ivity, tho clouds extend .lot least "':l high as 12 km . Tho average elzc or euch thick cloudCIUlStc~ is of the order or 3 to 5 dcg1'OO8 of 14~t./Lollg. ecroee ",rid in some cases even more.

I ndiaJuly

1. l~tro;luctl !)n

Some of t he sat ellite » in the TIltOS ser iescarried a medium resolut ion infrared radiometer(MRm ) which had five channe ls. One of t hecha nnels (Channel S) was ill the R~ 1 2 micron range.The at mosphere is almost transparent to radiationin this spectral range; and hence this range iscalled tho 'atmospheric window'. The amount ofradiation from the earth awl its nt mosphero which 'reaches the satellite radi ometer in this frequ encyinterval Call be converted into tho equivalentblack body temperat ure (TIJn) t hrough Planck'slaw on th e as sumption that the eart h and its cloudcover emit as a black body (in this frequency 'range), Though Tnn is not to be interpret ed aethe actual t.emperature of the viewed surface, formany purposes it is a good approximation to theactual temperaturc- pnrt.iclIlnrly when we con­eider only the relative TnB distribution over largeareas.

The instantaneous field of view of TIROS VIIradiometer (called t he soan spot ) ha s a diameterof 55 km when viewing vertically downw ard: withlarge nadir angles th e scan spot becomes elonga tedand may cover more than ten t imes th is area. Thoradiation tha t reaches the satellite at any instantis the value averaged over the scan spot . Hencet he satellite measured TUB values can he mean­illgful only when a homogeneous overcast layer"or prefeetly cleur sky fills t he view of the radio­meter. Rut if the sky is par tly covered or if thesky I even though overcast, has cloud s with widelyvarying tops or transparency , t he Tun valuescannot he interpreted as porh"ining to any parti­cular cloud layer .

2. Data used

The basic data for the presen t study was fromTIROS VII and was obtained in the form of com ­pu ter-produeed grid prints of Tnn values, with aaqnare mesh of! ·25° longitude interval on a 1:10 msca le Mercator Projection map. The populat ionchar t (giving t he number of value on which eachgrid value was baaed) was also available. Thedata refer to single open mode (where geographiclocating has high accuracy), The floor s ide andwall s ide read ings are mixed. Only readings withless tha n 72° zenith angle are used, as beyond th is,space contami nation ma kes the Tnn reading.unrealistic.

'nnos VII waa Imnched o~ 19 June 1963 andthe data st udied refer to 5 to 7 July ; by thisdate , .uo degradation of the sensors had set. in.Channel 2 data has been verified to be qu ite relia­ble. The estimated absolute accuracy of Channel 2Tll n values is ± 5°K and the short relative te rmaccurncy ± 2°K (S'.atI ~[embcrs , NASA 1964).

Tlms whi le the temperature observations usedare coarse in the sense that they are averages overaizable areas, these values are sufficiently, reliable.As extreme values are smeared out in the averagingover each scan spot , a relatively high or low va lueof Tnn when it appears in the gr id points, shouldbe given sufficient wcightage as it represen ts themean value over an area. More than the absolutevalu es, the relative values presented on thechar t ar e significa nt as truly delineat ing the varia­t ions in t he heigh ts of cloud tOI"·

Tho analysis of t he Channel 2 data overnnd the neighhouring area, for 5, 6 and 7

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