THE GENESIS AND MOVEMENT OF THE NOR'WESTERS OF BENGAL*

Transcription

1 THE GENESS AND MOVEMENT OF THE NOR'WESTERS OF BENGAL* BY PRABHAT K SEN GUPTA DSc (Meteorologst Safdarjang Aerodrome New De!h nda) Receved Aprl (Communcated by Dr KR Ramanathan rasc) THE Nor'westers of Bengal are thunderstorms wth whch volent squalls are assocated Frst seen perhaps on an solated day n the month of February n assocaton wth a western dsturbance t becomes a famlar feature of the hot afternoons of Aprl and May Thereafter wth the onset of monsoon ther frequency and volence decrease consderably The down-draft from the thunderstorms whch consttutes the squall frequently strkes from the northwest Whch has gven the Nor'wester ts popular name However a good many of the squalls strke from other drectons also Other well-known features of a thunderstorm vz thunder lghtnng ranfall drop n temperature rse n humdty and rse n pressure are common to Nor'westers also The Nor'wester s shortlved Startng on most days wth almost clear skes n the hot and uncomfortable afternoon the approach development and the fury are practcally over n an hour or two leavng fne and cool weather behnd These thunderstorms seem to develop anywhere over Bengal and the adjonng areas n the Nor'wester season But nvestgatons 15 have shown that there are a number of susceptble spots over the hlly regons or the base of the hlls where some of the earlest thunderstorms form and then the actvty seems to move n some drecton These 'soft' spots and the drectons of movement are llustrated n Fg 1 t s generally beleved s that these thunderstorms are caused by the nteracton of the man ar masses of the season vz a most SE/SW ar n the lower layers wth a dry W-NW ar over t as n the sectonal dagram gven n Fg 2 The thermodynamcal character of the ar masses reveals * Abrdged from a report presented n the Symposum on Physcs of Thunderstorms under the auspces of the ndan Academy of Scences n New Delh on the 28th December 1951 t Locally known as '*Kal-Bashakh or the calamtes of the month of Bashakh whch covers theperod md-aprl to md-may when the destructve volence of the squalls s felt an~l feared ~ost b) the people 303

2 PRABHAT K SEN GUPTA 304 S5 : 90 ~!!~:;';:;;::: - o -- o ~ - : : : J '~ : o ::: ::!::: o ' ~' ~! ~5 ALB ~- 27 la ~~ ~' 0 '~ C~N T O U R S Above llt~ ---~ A6ov~ -5 r m F~o 6 ~w B ENGAL the presence of consderable amount of energy of latent nstablty 6 t~ even after a!lown8 for entranment of ar from the envronment and ths energy

3 The Geness and Movement of the Nor'Westers of Bengal 305 when released manfests tself as a thunderstorm 6 For the release of energy of latent nstablty however t s essental to brng about a condton whch nduces the ascent of ar ether from the surface or from some level hgher up A process whch functons as above s known as 'Trgger Acton' Some of the well-known trggerng processes whch may operate ndvdually or jontly are dscussed below (a) nsolaton 6 we1 known for causng the heat thunderstorms durng the afternoon (Fg 3) The dfferental effect of nsolaton over rocky areas 3 C~ ~ r-~ as compared wth plans makes t more ntense over the Chota Nagpur hlls and the base of the hlls to the north andeast wth the result some of the earlest thunderstorms form there The effect of anabatc wnds of the hll sdes n the development of these thunderstorms n the afternoon s also smlar (b) Cold front assocated wth a western dsturbance travellng from west to east through Bengal may cause Nor'westers at any tme durng the day and nght by frontal acton (Fg 4) The katabate flow 1 11 from the base of Hmalayas to the north base of the Khas hlls and the flow through the Surma Valley also acts lke a cold front (Fg 5) the acton of whch s manly confned to the nght and earler part of the mornng C; ( 'll %'-! co -o ::: : :?: Fq 5

4 306 PRABHAT K SEN GU'XA (c) Acton of cold outflow from another thunderstorm n the neghbourhood operate as follows: () From the surface layers 2 3raThe mass of cold ar left by a downdraft s cold and deep enough (upto ft) to act lke a mnature cold front n nducng more thunderstorms (Fg 6) () From the upper layersz~---as ponted out by the author n 1948 a thunderstorm n the dsspatng stages brngs about a consderable fall n temperature at the upper levels also Ths mass of cold ar n the process of beng carred downwnd and lowerng the upper ar temperatures n the vcnty ncreases nstablty and generates more thunderstorms (Fg 7) ~m 6 { (;s ) ~ 2 3 -/) ) w/ l(tr 6 JJ - 6 Ths aspect was studed further wth the ad of a few specal rado-sonde ascents n May 1951 n Calcutta The changes n temperature durng the progress of the Nor'wester and afterwards are clearly brought out by the tephgrams for a typcal day reproduced n Fgs 8 9 and 10 When the thunderstorm was n progress (mature stage) there was a fall n temperature at 600 mb level wth no sgn of cold pool on the ground But after the -lo o'c 1o *zo 3o 4o'c -~o o'c 1 /f=~ / ~o/ ~ o ~ ;/ '--/7 r / Fly S ^o/ / - Or / 2/5951 n3oo m-s ~~z o 1-40 t ~Jlb - *3o 7 F/ff 9 2- ~~5~ =ch~~ J e'~ T

5 The Geness' and Movement of the Nor'Westers of Bengal 307 -m 0c -To zo -3o 44c t 1 [ / - J/ /o- Ao o AoO-ez ;;; thunderstorm actvty was over the drop n temperature at 700 mb was 7 C and the cold pool on the ground was notceable~: (d) Waves n the convergence fronts--at the dscontnuty between the dry and most ar masses unstable waves develop and are followed by a mnature cyclone whch manfests tself n the form of a thunderstorm MOVEMENT OF 1N~OR'WESTERS The Nor'westers seemngly develop n a haphazard fashon but there are some whch apparently follow some knd of order nasmuch as the actvty progresses n partcular drectons gvng the mpresson of movement wth a tme-sequence Such progressve movement can be traced over a dstance of 100 mles or more wthn several hours As the actvty of a sngle cumulonmbus cell rarely lasts more than an hour obvously t s not the same cell takng part n a dstant Nor'wester a few hours later Actually t s a case of successve thunderstorm formaton xl each thunderstorm nducng the next one n a partcular drecton untl stable regons are reached One of the movements from west to east was found to be correlated wth the upper wnds at 3-4 Kms n Bengal 15 n Amerca smlar features were notced also 2 The dfferent movements wth probable causes are dscussed as follows: () Thunderstorms formng n successve postons of a movng cold front Assocated wth a western dsturbance the progress s from west to east The katabatc flow 1 11 from the base of the hlls and through the Surma valley s responsble for movement from the north to south and from northeast to southwest n January 1952 ssue of the ndan Journal of Meteorology and Geophyscs a case has been reported n whch the rado-sonde balloon apparently entered a thunderstorm TM The exstence of a downdraft at 671 rob level s notceable wth a marked fall n temperature

6 308 PRABHAT K SEN GUPTA () Cold outflow on the ground ~ 3 ~ from a parent thunderstorm progressng n a partcular drecton and nducng more thunderstorms Usually ths movement follows the drecton whch'the man squall takes but the actvty des out wthn a few mles n the face of opposng southerly current n Bengal (Fg 6) () Cold and most ar at the upper levels left by a parent thunderstorm n the dsspatng stage and transported forward under the nfluence of the prevalng wnd nducng more thunderstorms of the hgh alttude varety n the drecton of the upper ar stream vz WNW/ESE ~2 (Fg 7) (v) The top porton of a cumulonmbus advances ahead of the base under the nfluence of stronger upper wnds at hgher levels and as revealed by the formaton of Altocumulus Marnmatus underneath contans downdrafts and pools of rancooled ar (Fg 1 ) These overhangng dark tops ~ 1 are notceable n the Nor'wester season mles away from the man trunk of the cumulonmbus just before the squall starts The arrval of the squall soon after the developments apparently ndcates that these advancng tops behave lke an upper cold front n nducng mere thunderstorms along the lne of flow of upper wnds at those levels The new thunder clouds so generated have generally a hgh base to start wth and the tops are hdden from vew by the dark layer underneath t wll be seen from the above that the acton of process () wll gve rse to a cluster of thunderstorms near each other whle that of () or (v) wll form other groups some dstance ahead leavng nactve regons n between ACKNOWLEDGMENT My thanks are due to the Drector-General of Observatores nda for permsson to publsh ths report am also grateful to Mr J M Sl Deputy Drector-General of Observatores (nstruments) for makng some rado-sonde nstruments avalable to me for specal ascents durng 1951

7 The Geness and Movement of the Nor'Westers of Bengal 309 and to the members of Rado-sonde Secton Alpore Calcutta for ther co-operaton n lettng off specal ascents REFERENCES AND BBLOGRAPHY Banerjee BN Nature of Nor'westers of Bengal and smlarty wth others Betrge Zr Physk der freen Atmosphre 3 Byers HR and Braham R R 3 Chatterj G and Sur N K 4 Das AK 5 Desa B N and Mal S 6 Normand C WB The Thunderstorm- Report of the Thunderstorm Project 1949 Results of soundngs at Jhkargacha (Bengal)n Aprl and May 1929 ndan Met Memors 26 part 9 On the Machansm of Thundersqualls n Bengal Gerlands Betrge Zr Geophsk Thundersqualls of Bengal bd On nstablty from water vapour Quart JR Met Soc Pramank SK Forecastng of Nor'westers n Bengal Proe Nat nst Sc (nda) Roy AK ' Ar Mass Structure and the Mechansm of Thunderstorms n nda durng the Pre-Monsoon and Post-Monsoon seasons bd Roy S C and Chatterj G Orgn of Nor'westers Nature Sen SN ' Mechansm of Bengal Tornadoes n the Nor'wester season bd; Sen Gupta PK Kalbashakhs (Thundersqualls) of Bengal Scence and Culture Nor'westers of Bengal L Met D Colloqua at Poona Feb 1948 (Report) t3 Sohon VV Thunderstorms of Calcutta L Met D Scentfc Notes Temperature changes n Calcutta thunderstorms bd Techncal Note No 10 (L Met D)- Nor'westers of Bengal 16 Venkteswaran SP nterestng features shown by a Rado-Sonde ascent at and Tlakan A RB Poona on 26 Aprl 1950 durng a thunderstorm btdan Journal of Meteorology and Geophyscs A3