Mausam, (1989), 40, 2, 137·d2

551 .577: 551.553 .21 : 551.509.31

Main features of the westward-moving low pressure systemswhich form over the Ind ian region during the summermonsoon season and their relat ion to the monsoon rainfall

D. A. MOOLEY and J . SHUK LA

Centre for Oceotl·Lonn.Atmosplzere Interaction,University of Marylallll, College Park , U.S .A.

( Received /9 October / 987)

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_ 'IT''I 'T': '""'I.'I ..n .. '"'" "" >;ffrr.mI ~rir "~, Rf"" R"lll"TlR1ii ( LPS) .,;, *"" 'IT''I if '""'1.'1..n if f<irt .,.,if >ia11Pl 'li,r~ ,~ oj; "':;Hf.r<;f"'" """rom ( LPS) oj; ff;ii ~ *""~ '""'1.'1 ..n(s m'R if.mr.:) ,,'\7 _\flV< '""'1.'1 "'; ( IlIf.!>R if 3W) if Wi".,., ii" 'lfu>! ~ 'l'\7l'1 'l#i ii " '81"'14' 31'1<1 "ITfwr f<mf~ ~ I '1['1 i.'\Tr-t ( LPS) f.r<;f "'" ll"TIR1ii "" '!! 'If>'<'IIfor,!," "'l''''~ '"' "". 'l1"1 '"' 'l1

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AHSTRACT. The main features of the transitory monsoon 10" pressure systems I LPS) over the Indianregion during the period I Jo(g~· 19~J. in respect of their formation, life• . mavcm:nl. intensity and ~ issiJl~tion,as well a" thei r in teran nual va riabilltv are examined b)' utilizing the Information and data co ntained In theIndian Daily H't'otlu'r Report s. The relationship- of these feat ures with the monsoon rain fal l over India/northIndiace ntral India-south India are examined.

The paper discusses a variety of statistics about the low pressure systems. e.e., formation, location. li fe,intensity. movement and di ssipation.

T he number of tp.:) formed i.:; 1101 significantly related to the monsoon rainfall over India except (or aweak rela tionship with central India monsoon rainfall. Number of LPS days during the season is signifi­cantly and directly related 10 the Indian monsoon rainfall (above 5 ;{) and 10 central India monsoon rainfal l(a bove I %) and these relationships generally show good stability. Total westward longitudina l di splace mentof the LPS during the season and the monsoon rainfall oyer India and central India are directly and sig ni­ficantly (above 1%) related and these relat ionships also generally che w good stability.

I . Introduction

During the summer mon soon sea so n (June throu ghSeptember), the syn opt ic sys tems wh ich a ffect the wca­ther over Ind ia and neighbourhood are, th e semi-perma­nent monsoon trough which extends from west Pakistan10 Burma, the closed low pressure systems which fre­quently move in a westerly direction across the centralpa rts of Ind ia lind the westerly trough s which occasion­ally move in an eas ter ly direction and affect the north­western parts of India. The westward-movi ng lowpressure systems either form over the Indian subcont i­nen t, the Bay of Ben gal an d th e Ara bian Sea (hereafter,

Ind ian region) o r develop from the remnants of depres­sio ns/s to rms which strike the Viet Nam coast and movewestward into the Bay of Benga l, hereafter. Bay (Sahaet al. 1981), A large majority of the se low pressure sys­terns (hereafter LPS) form over the Ind ia n region northof 15Q N. As per cri teria of I n d i ~ Meteorological Dcpa,rtment. an LPS ISa depression If the Wind speed asso­ciated with the e~'c1on ie circula tion is 17-33 kt, a cyclonicstorm,. If the :--'Ind speed IS ;;' 34 kt, a severe cyclonicstorm if the Wind speed is ;;, 48 kt and a severe cyclo nicsto rm with core of h~rrieane win ds if the wind speed is;;' 64 kt. T hese cntena a rc generally follo wed fo r idcnt i­fying the systems over the sea and adjoining coasta l a rea.

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