Some applications of humidity profiles estimated from INSAT infra red digital cloud imagery data

Transcription

1 Some applcatons of humdty profles estmated from INSAT nfra red dgtal cloud magery data ONKARI PRASAD, SANT PRASAD, KANTI PRASAD and R R KELKAR Inda Meteorologcal Department, New Delh, Inda Mosture profles have been estmated over the regon bounded by the lattudes 40~ and 40~ and longtudes 30~ to 130~ usng INSAT dgtal nfra red cloud magery data. The representatveness of these profles n representng mosture feld assocated wth the development and movement of synoptc scale systems durng the perod September 15th, 1996 to March 31st, 1997 has been examned. It has been shown that the changes n the mosture feld assocated wth the wthdrawal of the southwest and northeast monsoons from the Indan sub-contnent, development and movement of synoptc scale sytems (depressons, tropcal cyclones and waves n easterles) and equatoral troughs n the Indan Ocean could be clearly seen n humdty profles. The ntal development of tropcal systems s frst seen n the humdty feld n the upper troposphere. These profles could be used n montorng the ntal development and subsequent movement of tropcal systems. Further the data on mosture dstrbuton from the data gap regons of the Indan Ocean could be used as an addtonal source of mosture n numercal analyss and prognoss. 1. Introducton The non-avalablty of an adequate number of radosonde observatons from ocean areas causes defcences n the analyss of mosture felds. Accurate humdty felds are requred for useful short and medum range forecast from numercal models. Dgtal nfra red (IR) cloud magery data from geostatonary satelltes are especally suted for estmatng mosture n the atmosphere coverng large areas of tropcs on an operatonal bass at synoptc hours. Estmaton of mosture from dgtal IR cloud magery data s based on the premse that an analyss of the equvalent black body temperature of each cloud pxel, whch s referred to as cloud top temperature (CTT), n an area co-located wth a radosonde ascent could be used to classfy the cloud depth, amount and type n that area, and ths cloud classfcaton cloud be related to the dew pont depresson profle of the radosonde ascent. At numercal predcton dvson of the Japan Meteorologcal Agency (JMA), GMS nfra red cloud magery data have been used for estmaton of humdty profles (Baba 1987). Prasad and Rao (1991) have demonstrated the estmaton of humdty profles from the Indan Ocean regon usng INSAT IR cloud magery data. As they have used only a lmted perod of data for obtanng the profles, many cloud categores are not represented n the profles obtaned by them. A smplfed verson of the JMA scheme has been developed at the Bureau of Meteorologcal Research Centre (BMRC) by Mlls and Davdson (1987). The method s relatvely smple as t uses a total of only nneteen profles for representng mosture n the troposphere. An examnaton of these profles durng the perod May-June 1994 (Prasad et al 1996) showed that the profles are able to clearly delneate the buld-up of mosture n the vertcal and ts northward propagaton assocated wth the onset of the southwest monsoon. In the present paper we have examned these profles n relaton to the development of synoptc scale systems and ther movement over the Indan Ocean regon. The results are brefly dscussed n the followng sectons. The ultmate am of examnng these profles s to use them as an addtonal source of mosture n the numercal analyss for the Inda Meteorologcal Department's Keywords. Humdty profles; nfra red cloud magery data; INSAT; numercal weather predcton (NWP). Proc. Indan Acad. Sc. (Earth Planet. Sc.), 107, No. 1, March 1998, pp Prnted n Inda 5

2 6 Onkar Prasad et al 3O ,0C 6O(} 70C 80(: 90C CLEAR BKN STF M A x (834) SCT STF U A x (50) l j I l l I a, 9,, C 70(: 80( 90r t / ' ~ - / BKN STF 167 l r 9 M~A'X,/ ekn Cu (Or) ~MAqX /! SCT Cu (79) el > 300 6o0 UJ n- 7oc u,) ul 800 uj n- O. 900 l J 9 N STF 18 (79) ~ * A 9 t I / ~ M, I AX SCT STF 139) I I I I * * I,/. " 8KN Cu 10 (,2~), I I I I 9 I J,.. I - SCT Cu ll l ~, ~61 I ' ) (36) 9,, p~, "" S.,,, Soo.), 7- / 6OO 700 eoo 9OO BKN STF 6.,. ~"~ s,o ;s 20 s ~'o S IO t$ BKN Cu t SCT Cu (6) l OEWPOINT OEPRESSION foc) Fgure 1. Dew pont depresson profles for each of the nneteen cloud categores, together wth the type of Cloud (cununulform (Cu) or stratform (STF)) whether greater (BKN) or less (SCT) than 50% cloud cover. Bracketed numbers ndcate the number of matchups n each category, whle '' ndcate the pressure layer n whch the maxmum cloud amount resdes (after Mlls and Davdson 1987). Fgures n the bottom left corner ndcate the profle number.

3 Humdty profles from INSA T cloud magery data 7 Fgure 2. INSAT nfra red cloud magery of 0000 UTC of Lmted Area Model (LAM) and to examne ts mpact on predctng development and movement of synoptc scale weather systems and ranfall over Inda. IMD's LAM s a multlevel prmtve equaton model whch runs n an operatonal mode on Cyber 2000U computer system. 2. Data used and method of analyss Usng the method of Mlls and Davdson (1987) humdty profles (charts of dew pont depresson n K at 850, 700, 500 and 300 hpa) for 0000 UTC have been obtaned for the perod September 15th, 1996 to March 31st, Only a selected number of dew pont depresson contours have been drawn on each chart. The contour values are 4, 6, 8 and 10 at 850 hpa; 6, 11, 16 and 21 at 700 hpa; 6, 13, 20 and 27 at 500 hpa; 10,16, 22 and 28 at 300 hpa. Out of the 19 profles avalable n Mlls and Davdson method (fgure 1), profle numbers 18, 16 and 14 represent the humd profles. Other profles represent relatvely dry atmosphere. At 700 hpa the value of the dew pont depresson relatng to the humd profles are 3, 5 and 5 respectvely. Thus at ths level the humd regons are delneated by the dew pont depresson

4 8 Onkar Prasad et al ~DU ~/-UU : top /% ~t \-_~'Fx~--~_ J - 0 OI!. Fgure 3. Dew pont depresson chart at 700hPa at 0000UTC of contour of 6 K. As we move away from the regons of 6 K dew pont depresson contours the atmosphere becomes relatvely dry. For the purpose of llustraton, the full dsk INSAT IR pcture for 0000 UTC of February 1st, 1997 (fgure 2) and 700 hpa dew pont depresson chart obtaned usng the dgtal IR cloud magery data of ths pcture are reproduced n fgure 3. We have reproduced charts of 700 hpa only (except n some specfc cases) for dscussons under secton 3 as the tropcal systems are best represented at ths level. However, the charts of 500 and 300 hpa level could also be used for the purpose of dscusson as mosture generally extends up to upper troposphere n tropcal regons n assocaton wth synoptc scale systems. The pcture s dfferent at 850 hpa as mosture s generally hgh over larger areas around the synoptc systems. The humdty profles dscussed below nclude the followng stuatons: 9 Wthdrawal of southwest and northeast monsoons Development and movement of synoptc scale system n the Bay of Bengal and Araban Sea durng northeast monsoon: 9 Cyclonc storm n the Araban Sea (22nd-27th October, 1996).

5 ~~ 20 ~ 15 ~ \ Humdty profles from INSA T cloud magery data 9 * ' I I I ~ 90~ 9 Depresson n the Bay of Bengal (27th-29th October, 1996). 9 Severe cyclonc storm n the Bay of Bengal (hth- 7th November, 1996). 9 Severe cyclonc storm n the Bay of Bengal (27/ 11/96 to 6/12/96). 9 Development and movement of Inter-tropcal Convergence Zone (ITCZ)/Equatoral troughs (ETs) (Sth-28th October, 1996). The track of the cyclonc storms/depressons mentoned above s shown n fgure 4. These tracks are prepared by the Inda Meteorologcal Department (IMD) by plottng the storm centres whch are determned by makng use of M1 avalable data related to the system, e.g., pressure, wnd drecton and speed as reported by the shps from the storm feld, storm centre and ntensty as estmated from cloud organzaton as seen n cloud magery, radar observatons etc. 3. Dscussons ~ 15 ~ ~ 80 ~ 85 ~ 90~ Fgure 4. Track of depressons and cyclonc storms Wthdrawal of southwest monsoon The southwest monsoon wthdrew from western parts of west Rajasthan on September 15th. It further wthdrew from remanng parts of west Rajasthan; some parts of Haryana and western parts of Punjab and Jammu and Kashmr on September 17th. It further wthdrew from H.P., west U.P., Saurashtra and Kutch; remanng parts of Rajasthan, of Haryana ncludng Delh, of Punjab and Jammu and Kashmr; some parts of west M.P. by September 25th. Thereafter, the further wthdrawal of SW monsoon was delayed due to the development of a depresson over west central Bay of Bengal off south Andhra Pradesh (A.P.) coast on the mornng of October 1st. It crossed the A.P. coast between Ongole and Kaval n the early nght of the same day. Movng n a north westerly drecton t weakened nto a well marked low pressure area over south Maharashtra on the evenng of October 2nd. The SW monsoon further wthdrew from Maharashtra, northern parts of Karnataka and of A.P., remanng parts of M.P., East U.P., Bhar and most parts of Orssa by October 9th. The SW monsoon wthdrew from the entre country on October llth (fgure 5). Some of the selected dew pont depresson charts pertanng to the perod of wthdrawal of SW monsoon-1996 are reproduced n fgure 6. The wthdrawal s seen n the southward shft of the area covered by 6K dew pont depresson contour. Smultaneously the advance of the dry ar s seen n the southward movement of the regon covered by the dry profles (dew pont depresson 11, 16 and 21). The development of a depresson off A.P. coast, on October 1st, s seen n the westward extenson of the dew pont depresson contour of 6 K over pennsula from where the SW monsoon had wthdrawn earler.

6 10 Onkar Prasad et al 35N 30 I I I I I I 17 SEP. t. ~..----~WITHDRAWAL OF S-W MONSOON.) I ISEP. L-,15,s SE,R..",! l,'~ "% ~ SEP. NORMAL DATES OF C~ 9 Wl TH D RAWAL SEP. ~24 SER ACTUAL DATES OF WITHDRAWAL N 25 SER ~/I OCT.,7 OCT.._/" 9,, / / "-'. V " /ALB,# BHP / / 3 / R?~c ' ~ < 9 c,.s t,-'" 1 -~"10 OCT SEP. "~,,.15 OCT. 1 OCT./ 'SK "" ~ OCT. M~S MDS 0 &PBL 11 OC" 0 7lOE :;. 715 elo SIS E Fgure 5. Dates of wthdrawal of southwest monsoon By October 11th, the day when the SW monsoon had been wthdrawn from the entre country, no part of pennsular Inda s covered by the dew pont depresson contour of 6 K. Subsequent to the wthdrawal of SW monsoon - 96, NE monsoon - 96 (October - December) was declared to have set n over south pennsula on October 1 lth. NE monsoon was actve throughout ts perod. Ths was seen n the development of a number of synoptc systems n north Indan Ocean (Bay of Bengal and Araban Sea). Ths s dscussed n secton 3.2. NE monsoon wthdrew from the southernmost porton of Tamlnadu on December 23rd. By January 15th the area covered by humd profles had moved close to the equator (fgure 6(g)). Thus the southward movement of the humd tropcal ar assocated wth the wthdrawal of SW monsoon and ts replacement by dry contnental ar s clearly seen n the dew pont depresson charts. 3.2 Development of synoptc systems and ther movement Durng the perod of northeast monsoon (October - December) synoptc systems develop n southwest Bay of Bengal. They generally move n a westerly drecton and gve ranfall over south pennsula. The year 1996 wtnessed an actve NE monsoon when a number of synoptc systems developed n the Bay of Bengal and gave ranfall over south pennsula whch was n excess (> 20% of normal) n coastal A.P. (+27%), Rayalaseema (+86%), Tamlnadu and Pondcherry (+51%) and normal (ranfall wthn + or -19% of normal) n south nteror Karnataka (+18%) and Kerala (-9%). The development of ndvdual systems and ther movement as seen n the humdty profles s dscussed below: Severe cyclonc storm n Araban Sea (22nd-27th October, 1996) A well marked low pressure area formed over SW Bay of Bengal and moved westward over south Andhra Pradesh on 19th October. Movng northwestwards t emerged n east central Araban Sea on 21st October. It ntensfed nto a severe cyclonc storm (SCS) and was located about 250km south of Veraval on October 24th. It contnued to move northwards tll the mornng of 25th October when t was centered

7 - - B0OE Humdty profles from INSA T cloud magery data 11 30~ EO 30 ~ N "16 11, 6- ~ 76 I ~ ;22 1' -., 20~ 10~.~,o'.J~ I <~ ~~/ " f c ~.~ EQ k~l - A~- ~, ' %1 "b'~ ~&;~0 ),~. Vt-'X",o,I "- "~Jt/,~2f ~2,~/ \~ /.d~..-,-~.,-~-~,l --=-/ \<"~k ~~176 F~'//"4,~':~ ~--._~.zt~77-~--~--'--~, 0O% "-~/ [,""7 /, "'~ (.~J~_._[ dll--'~ k 1"/ 1" 80~ 100 ~ occuped by humd profles 1~--_ >'..~,'--' "%,~"~,~-XJ o0oe Fgure 6. Dew pont depresson charts at 700 hpa for selected dates durng wthdrawal of southwest monsoon : a: ; b: ; c: ; d: ; e: ; f: ; g: about 100km south of Veraval. It weakened nto a depresson centered about 30 km SSW of Veraval on the mornng of 26th October. Ths system remaned practcally statonary tll the mornng of 27th October. Fnally t moved westwards from Saurashtra coast, weakened nto a low pressure area over Araban Sea and became unmportant. The track of the system from 22nd-27th October s gven n fgure 4(a).

8 12 Onkar Prasad et al 20 N 10 N - S 10 S 20 I [ I 10 - ~ 2 D s E 90E IOOE 20 N ' N <@ S ~o I I 60 E 70 E 80 E 90 E 100 E Fgure 7. Dew pont depresson charts at 500 hpa relatng to the ntal development of severe cyclonc storm n the Araban Sea (16th-21st October, 1996): a: ; b: ; c: ; d: ; e: The development of the system could be seen n the humdty feld earler than that n the surface pressure feld. The ntal development occurred n an actve northern hemspherc equatoral trough (NHET) and was frst seen n upper levels (500 and 300hPa). Events leadng to the development of NHET are dscussed n secton 3.3. The dew pont depresson charts pertanng to the ntal development are reproduced n fgure 7. The ntal development of the system was seen on October 18th as a sem-crcular regon of 6K dew pont depresson over eastern pennsula and parts of west central Bay of Bengal. Subsequent movement of the system northwestward and ts emergence n east Araban Sea on October 21st could be clearly seen n fgure 7. From October 21st onwards, the system could be seen at 700 hpa also. Ths s dscussed below. The dew pont depresson charts from 21st-30th October are reproduced n fgure 8. The area covered by the dew pont depresson contour of 6 K clearly

9 Humdty profles from INSA T cloud magery data 13 30~ 20~ ~- lcu EQ ~,vt-.-- ~,',~ ~'I ""Vll//l"tll,,,, 6 I~ a0on.. -X~/, 6 20~ ~ ~ 10~ 30~ 60 ~ E 80~ 100~ SO ~ ~ 20~ 10~ EQ. lo% ~ jt(/(~.,, 60~ - 80~ 100' 60~ 80~ Fgure 8. Dew pont depresson charts at 700 hpa relatng to the development and movement of the severe cyclonc storm n the Araban Sea (21st-30th October 1996) and depresson n Bay of Bengal (27th-29th October, 1996): a: ; b: ; c: ; d: ; e: ; f: ; g: ; h: ; : ; j:

10 14 Onkar Prasad et al show the system's movement towards north up to 26th October. It remaned close to Saurashtra coast tll the mornng of 27th October. Tll 0000 UTC of 28th October the area occuped by dew pont contour of 6K had already moved southwards off the Maharashtra-Goa coast. Thereafter, the regon moved further southwestwards and the area covered by 6 K dew pont depresson contour also reduced. On 30th October the regon had already moved west of 60~ and the area covered was much reduced. Thus the area covered by dew pont depresson contour of 6 K closely followed the track of the system Deep depresson n the Bay of Bengal (27th-29th October) A depresson formed over west central Bay of Bengal at about 430 km south east of Vsakhapatnam on the mornng of 27th October. The system moved n a north-northeasterly drecton and ntensfed nto a deep depresson located about 400 km southsouthwest of Calcutta on the mornng of 28th October. It crossed Bangladesh coast durng mdnght of 28th October, weakened nto a depresson and lay centered about 230 km northeast of Calcutta on 29th October. It became less mportant on October 30th. The track of the system s shown n fgure 4(b). The dew pont depresson feld from 23rd-30th October s shown n fgure 8. The development of the system on 24th October and ts movement close to A.P. coast on the mornng of 26th October s seen as a nearly crcular area of dew pont depresson contour of 6 K whch covers west central Bay of Bengal and parts of coastal A.P.. Thereafter, the regon covered by dew pont depresson contour of 6K moved northeastwards. Thus the movement of the regon of humd profles closely followed the track of the system Development of a severe cyclonc storm wth a core of hurrcane wnds n the Bay of Bengal (5th-Tth November) The INSAT magery of 0600 UTC of 4th November ndcated the formaton of a vortex near lat. 16.5~ long. 91.0~ It further ntensfed and was classfed as a depresson at 1500 UTC of November 4th and at 0000 UTC of November 5th, t lay centred near lat. 16.0~ 87.0~ about 650km east of Machlpatnam. The system contnued to move n a westerly drecton and ntensfed nto a severe cyclonc storm (SCS) wth a core of hurrcane wnds by the afternoon of November 6th when t lay centred about 220 km ENE of Machlpatnam. It crossed A.P. coast about 50km south of Kaknada durng the nght of November 6th. Movng n a WNWly drecton, the system weakened nto a low pressure area over Telangana by the evenng of November 7th. The track of the system s shown n fgure 4(c). The dew pont depresson charts pertanng to the SCS are reproduced n fgure 9. At 0000UTC of November 4th, the humd regon was confned to the areas of Bay of Bengal to the east of 90~ By 0000 UTC of November 5th, a sem-crcular regon of humd profles developed n west central Bay of Bengal. It came close to A.P. coast on November 6th. By the mornng of November 7th, t was located manly over land. Thereafter, the humd regon moved westward. By the mornng of November 8th, the regon of the humd profles had moved over Telangana and adjonng areas of Maharashtra. Thus the movement of the regon of humd profles closely followed the track of the system Development and movement of cyclonc storm wth core of hurrcane wnds n the Bay of Bengal (27th November to 6th December) A well marked low pressure area formed n the evenng of November 27th over southeast Bay of Bengal wth the central regon near lat. 9.0~ 81.5~ Movng northwestward t concentrated nto a depresson on the mornng of November 28th. Movng north and then northeastward t concentrated nto a deep depresson near lat. ll.5~ 86.5~ on the mornng of November 29th. It then moved northward and south-eastward and weakened nto a depresson on the mornng of December 1st. Thus t took a complete loop over the central Bay of Bengal by the mornng of December 1st and lay centered near lat. 14.5~ 89.0~ at 0300 UTC. The system ntensfed agan and attaned the ntensty of a CS at 0000UTC of December 2nd and lay near lat. 14.0~ 87.0~ Movng westward t further ntensfed nto a SCS at 0300 UTC of December 3rd wth the centre near lat. 14.5~ 84.5~ It ntensfed nto a SCS wth the core of hurrcane wnds by 12 UTC of December 3rd and lay centred near lat. 14.5~ 83.5~ It moved close to the A.P. coast by 1200 UTC of December 4th. At ths pont the system showed weakenng and dsorganzaton n satellte pctures. It appeared that the upper porton of the system moved nland and the lower porton moved southward. It was relocated near lat. 14.5~ 82.0~ as a SCS at 0300UTC of December 5th. It made a loop movng clockwse from 04/1200 to 05/1200UTC. Movng slowly n southwesterly drecton, t came close to Chenna on the evenng of December 6th. It crossed the coast between Chennal and Pondcherry and rapdly weakened nto a low pressure area by the mornng of December 7th. The humdty feld assocated wth ths system showed characterstcs dfferent than the systems dscussed earler: the system was not well defned n the humdty feld at 700 hpa throughout ts lfe hstory. However, t was seen n the humdty feld at 500 and also at 300 hpa. Accordngly the dew pont depresson chart s reproduced for 500 hpa level (fgure 10). On

11 Humdty profles from INSA T cloud magery data 15 SO~ 20~ 21 1,OON lodk?-2, ~ 21 6 'k ~ ~ ~ ~'~,~ ~-"L~ 30~ - / "-~'~ kk q, '6 kv_,4,g _ 60~ 80~ 100~ 60~ 80~ 100~ Fgure 9. Dew pont depresson charts at 700 hpa relatng to the development and movement of severe cyclonc storm n the Bay of Bengal (4th-9th November, 1996): a: ; b: ; c: ; d: ; e: ; f: November 28th t was seen as a sem-crcular regon of 6 K dew pont depresson n central Bay of Bengal wth the centre near 12.0~176 Thereafter t moved north and then east. On the mornng of December 2nd t was seen as a crcular regon of 6 K dew pont depresson coverng west central Bay of Bengal. On December 3rd t showed movement southward. It contnued to move southward tll December 5th, when the regon of 6 K dew pont depresson lay over central regons of south CAP and north Tamlnadu. By ths tme the area occuped by 6 K dew pont depresson contour had also reduced consderably. By the mornng of December 6th a closed contour of dew pont depresson 13 K could be seen over the regon. Here agan the humd regon assocated wth the CS as seen n the dew pont depresson feld closely followed the track of the system. 3.3 Development of Inter-tropcal Convergence Zone (ITVZ)/Equatoral Troughs (ETs) The development of ITCZ was seen between the equator and 10~ on October 8th. On ths date the area covered by ITCZ was confned to the longtudes 60~ and 80~ A weak northern hemspherc equatoral trough (NHET) was also seen between 70~ and 80~ roughly along 10~ By October llth a well marked east-west orented ITCZ developed along 5 deg south. NHET contnued to be present roughly along 10~ By October 12th the ITCZ showed weakenng to the west of 80~ and extended up to 105~ n the east. To the north of the equator NHET became actve between 60~ and 80~ By October 14th the man cloudness shfted to the north of the equator. By October 17th NHET showed strengthenng and extended from 50~ to 100~ Southern hemspherc equatoral trough (SHET) was also seen. The development of a vortex was seen n SHET. SHET weakened consderably by October 19th. A vortex developed n NHET over pennsula. The vortex n SHET became less marked on October 21st. The vortex over the pennsula emerged nto the Araban Sea and subsequently developed nto a tropcal cyclone. SHET remaned weak, n general, durng the subsequent two months,.e., November and December SHET

12 - 16 Onkar Prasad et al 3 o ~ N ' ~ EQI. ( ~ I% ~ % b-..-~-,ut//._z.,,~.~//,j 30~ 20 ~ N f o t~ O~ EQ. 60~ 80~ 80~ lo0~ Fgure 10. Dew pont depresson charts at 500 hpa relatng to the development and movement of severe cyclonc storm n the Bay of Bengal (27th November to 7th December, 1996): a: ; b: ; c: ; d: ; e: ; f: remaned generally actve durng all the weeks of the months of January and February Occasonally NHET also developed. Whle SHET remaned confned to the areas between the equator and 10~ NHET was confned to the areas south of 5~ Both dd not show movement to the north. In the month of March 1997 SHET developed on 4th, 5th and 9th. On the frst two occasons t was confned to the areas west of 70~ On 9th t was confned to the east of 75~ Thereafter, an E-W orented SHET developed roughly along 5~ between 50~ and 90~ on 23rd March. Ths showed movenmnt northwards and on March 31st t was occupyng areas up to 10~ from 75~ to 100~ The development of equatoral troughs (ETs) and ther movement was clearly seen n the dew pont depresson profles. To llustrate ths pont the dew pont depresson feld assocated wth the development of ITCZ/ETs n the month of October s dscussed below. The dew pont depresson feld related to the development of SHET and NHET descrbed above wth the help of the satellte pctures s reproduced n fgure 11. On October 8th a closed area of 6K dew pont depresson was confned to the south of the equator and also some parts of south Bay of Bengal. On 9th, 10th and llth October west to east orented closed areas of 6 K dew pont depresson were seen on ether sde of the equator. On October 12th the area to the north of equator moved over south Araban Sea and the area covered by 6K dew pont depresson south of the equator showed movement to the north and occuped a much larger area extendng up to 100~ By October 16th east-west orented cloudness was manly confned to the north of the equator. To the south of the equator t was confned to the areas east of 80~ only. By October 18th the area covered by 6K dew pont depresson to the south of the equator was confned to a quas-crcular one between the equator and 15~ roughly along 80~ The area covered by 6K dew pont depresson to the north covered parts of south pennsula and parts of east Araban Sea off Kerala coast. By October 21st, the area enclosed by 6K dew pont depresson had emerged nto east Araban Sea. The quas-crcular

13 Humdty profles from INSA T cloud magery data r',,.-,so I -- '= %1, 60E 70E 80E 90E IOOE 60E 70E 80E 90E IOOE Fgure 11. Dew pont depresson charts at 700hPa relatng to the development of Inter-tropcal Convergence Zone (ITCZ)/ Southern Hemsphere Equatoral Trough (SHET) and Northern hemspherc Equatoral Trough (NHET) (8th-20th October, 1996): a: ; b: ; c: ; d: ; e: ; f: ; g: ; h: area of 6 K dew pont depresson n south Indan Ocean had moved southwestward and was located near 16~ 65~ Further movement of the area of 6 K dew pont depresson n the Araban Sea s dscussed n secton In addton to the development of synoptc systems n north Indan Ocean (Bay of Bengal and Araban Sea) and ITCZ/ETs, the development and movement of tropcal and extra-tropcal cyclones over south Indan Ocean and troughs n the zonal westerles, responsble for weather over north Inda durng wnter, were also montored n the dew pont depresson feld. It was noted that they were also tracked n the dew pont depresson feld as was the case of the

14 18 Onkar Prasad et al synoptc systems n the north Indan Ocean. However, the scope of the paper dd not allow us to nclude the dscusson on the movement of the humd regon assocated wth these systems. 4. Conclusons The examnaton of the humdty felds (dew pont charts) obtaned from INSAT nfra red cloud magery data has shown that the followng could be clearly montored n the dew pont depresson feld: 9 Wthdrawal of southwest and northeast monsoons from the Indan sub-contnent. 9 Development, ncludng the ntal development, and movement of tropcal systems (depressons and cyclonc storms). 9 Development of ITCZ. 9 Development and movement of equatoral troughs. 9 Development and movement of troughs n zonal westerles and extratropcal cyclones over south Indan Ocean. In order to examne the mpact on the numercal analyss and forecast, the mosture data have been lnked to the analyss and forecast system of IMD's Lmted Area Model (LAM). The results on the mpact of ths addtonal humdty data on the forecast of development and movement of synoptc scale systems and ranfall over dfferent regons of Inda are beng studed and shall be reported separately. References Baba Atsush 1987 Improvement n the estmaton method of mosture data from satellte cloud soundngs; JMA/NPD Tech. rep. No. 16, pp. 54 Mlls G A and Davdson N E 1987 Tropospherc mosture profles from dgtal IR satellte magery: System descrpton and analyss/forecast mpact; Australa Met. Mag Prasad O and Rao A V R K 1991 Estmaton of relatve humdty profles from INSAT dgtal IR cloud magery data; Mausam Prasad O, Prasad S and Kelkar R R 1996 Humdty profles from INSAT IR dgtal cloud magery data. (to appear n the Bulletn of Indan Meteorologcal Socety: Vayu Mandal).