Synoptic weather conditions during the pilot study of Bay of Bengal and Monsoon Experiment (BOBMEX)

Transcription

1 Synoptic weather conditions during the pilot study of Bay of Bengal and Monsoon Experiment (BOBMEX) SRKALSI India Meteorological Department, New Delhi, India BOBMEX-Pilot was organised from 23rd October -- 11th November, 1998 when the seasonal trough had already shifted to south Bay of Bengal. The activity during this period was marked by the development of a monsoon depression from 26th--29th October that weakened over the sea; onset of northeast monsoon along the east coast of India on 29th October; a low pressure area that formed on 2nd November over southwest Bay off Sri Lanka -- southtamilnadu coast; and another cyclonic circulation that formed towards the end of the BOBMEX-Pilot period. This paper describes the development of these synoptic systems through synoptic charts and satellite data. 1. Bay of Bengal and Monsoon pilot experiment (BOBMEX-Pilot) The main aim of the experiment was to study air-sea coupling and its high frequency variability over the Bay in the summer monsoon season. It had specific objectives relating to the influence of the ocean on convection, and changes in the upper ocean driven by the air-sea fluxes and fresh water discharge. DOD committed its ship Sagar Kanya in this connection. Intensive observations for the Bay of Bengal convection and the marine boundary layer were conducted. The track of the ship is given in figure 1. The position of the ship is also shown in charts included in this study wherever required. It cruised mostly in the central parts of south Bay of Bengal where the seasonal trough had shifted as discussed in the following paragraphs. 2. Deep depression from 26th -- 28th October 1998 The seasonal shear line in the lower troposphere during the initial phase of the pilot experiment was located over the south Bay of Bengal along and south of 10 N latitude with light winds on either side of the trough on 23rd October. A huge convection lay over Malay peninsula and adjacent areas of the Andaman Sea as seen in the INSAT imagery included in figure 2. The initial disturbance at this stage discerned at about 8 N=101:5 E was highly innocuous as seen in the satellite imagery. The wind strength as seen from the scanty data available from the shear zone was just knots. Outgoing longwave radiation (OLR) minima of 160 watts per square meter associated with this pocket indicated strong convection which shifted westwards and became more compact with time. Figure 3 indicates that even on 25th October OLR minima of 180 watts per square meter was seen over southern parts of the Bay of Bengal. Along with the convection associated with the northern hemispheric equatorial trough (NHET) another band of low OLR was seen due to the southern hemispheric equatorial trough (SHET) (see figure 11). Surface meteorological observations collected on board ORV Sagar Kanya during the pilot phase of the BOBMEX indicate that the equatorial westerlies seem to have strengthened to some extent between these two troughs. The persistent diabetic heating associated with the intense convection in the south Bay of Bengal where scanty ship reports and buoy data reveal warm sea with sea surface temperature more than 29 C gave rise to the formation of a low pressure on 25th October that concentrated into a depression on 26th October over Keywords. BOBMEX; northeast monsoon; satellite imagery; monsoon depression. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 109, No. 2, June 2000, pp # Printed in India 211

2 212 S R Kalsi Figure 1. Pilot BOBMEX cruise track, Sagar Kanya Cruise No: SK 138C. Period: to (Goa -- Tuticorin). southeast Bay. The average daily OLR again reached a low of 160 watts per square meter on 27th October though it was seen in an earlier study (Kalsi et al 1995) that the OLR value may reach as low as 100 watts per square meter during the formation of a depression. Cloud organisation in the satellite imagery is seen on the basis of which it was declared and accepted as a depression, though no characteristic organisation of depression (Kanti Prasad et al 1990; Srinivasan et al 1971) is seen in the visible satellite images of 26th October. It moved initially northwestwards and intensified into a deep depression at 12 UTC of 27th October when it was located near latitude 13:0 N/ long. 87:0 E. Figure 4 shows the track of this depression. The global analysis charts of the National Centre for Medium Range Weather Forecasting (NCMRWF) indicate that there was virtually no slope of this system between 850 and 500 hpa levels (figure 5) at 00 UTC of 27th October This is in conformity with results of Kanti Prasad et al (1990) wherein it has been seen that during the intense stages of monsoon depression, the associated cyclonic circulation becomes vertical. Thereafter as seen through the subsequent satellite images the depression moved first westward up to 12 UTC of 28th October and again took a northwest course. ORV Sagar Kanya came under the influence of the depression only on 28th October when it cruised towards the Bay of Bengal. It reported strengthening of the wind speed to about 34 knots at 00 UTC of 28th October. The winds weakened in the vicinity of this ship around noon of 28th October. Under the influence of a strong mid and upper troposphere westerly/southwesterly current seen over central and northwest India ahead of a westerly trough observed over Pakistan on 29th October far to the northwest of the depression, it apparently weakened over the sea itself and its convection got sheared off northeastwards along and off east coast of India. 3. Onset of northeast monsoon Though the depression discussed above weakened on 29th October, relatively strong ITCZ conditions

3 Synoptic weather conditions during BOBMEX 213 Figure 2. INSAT picture of 23rd October 1998 showing a cloud cluster over Malay Peninsula and neighbourhood.

4 214 S R Kalsi Figure 3. INSAT derived OLR. Units: watts/mtr.sq; Date: ; X -- position of Sagar Kanya ship. Figure 4. continued over the south Bay. The northeast current strengthened on 28th October itself in association with the deep depression along and off the east coast of India as seen in figure 6 that shows deepening of the low at 850 hpa level. After the shearing off process of the depression discussed above, the seasonal trough got somewhat elongated northeastwards. This is supported by the backing of the winds reported by Sagar Kanya from a direction of 280 at 00 UTC of 28th October to 240 at 06 UTC of 29th October when the wind speed also decreased from about 34 knots to 14 knots. Figure 7 shows the sequence of satellite pictures indicating weakening of the depression and the advance of northeast monsoon. Though the tropical circulations are in general insulated from mid-latitude flows by sub-tropical highs, the penetration of troughs in westerlies into tropics impacts the tropical vortices by shearing them off northeastwards. The depression in the west central Bay also got sheared off under similar circumstances. The remnant of the depression came close to the north Tamilnadu and south coastal Andhra Pradesh on 30th October and became unimportant on 31st October after crossing the coast. The weekly rainfall chart of the week ending 4th November 1998 showed excess rainfall in Tamilnadu, coastal Andhra Pradesh and Rayalaseema in association with the arrival of the northeast monsoon.

5 Synoptic weather conditions during BOBMEX 215 Figure 5. NCMRWF analysis of hPa hpa. Figure hpa analysis at 1200 UTC of 28th October 1998.

6 216 S R Kalsi Figure 7. Sequence of INSAT pictures (IR) showing weakening of depression and advancement of NE monsoon.

7 Synoptic weather conditions during BOBMEX 217 Figure hpa analysis at 0000 UTC of 28th October to 31st October X -- Position of Sagar Kanya. 4. Low pressure area in the southwest Bay of Bengal The shear zone continued over southwest Bay even after weakening of the depression. Figure 8 shows a set of satellite images for the period 1st -- 4th November 1998 whereas the corresponding analysis is given in figure 9. A satellite observed cloud disturbance is seen throughout this period that moved westward from 1st November and lay over coastal Tamilnadu and its neighbourhood on 4th November. A low pressure developed off south Tamilnadu and east Sri Lanka on 2nd November as seen in the analysis charts (figure 9). In fact, the cloud cluster had become almost circular and intense on 2nd November in which OLR as low as 160 watts per sq. meter was observed on this day (figure 10). The low pressure persisted over the southwest Bay until 6th November and became less marked on 7th November. It was more marked on 2nd November when the associated upper air circulation at 850 hpa level was seen positioned vertically above the surface low. This circulation split up into two parts on 3rd November and again became flat on 4th November in which the shear line was still visible. This weak scenario continued for the next one week. A circulation was seen embedded again in the shear line on 10th and 11th November 1998.

8 218 S R Kalsi Figure 9. Sequence of INSAT pictures (1st -- 4th November 1998) showing clouds associated with low pressure area over southwest Bay and neighbourhood.

9 Synoptic weather conditions during BOBMEX 219 Figure 10. ship. 850 hpa analysis at 0000 UTC from 1st to 4th November 1998 with shear line ( ). X -- Position of Sagar Kanya 5. Conclusion The following features of circulation were observed during the pilot study period of BOBMEX: A shear line in the south Bay of Bengal and the Nicobar Islands with weak to moderate monsoon conditions. Development of a depression in the southeast Bay on 26th October, its further intensification on 27th, movement towards westnorthwest and eventual dissipation over west central Bay. Establishment of northeast monsoon on 28th October. Formation of a low pressure area in the southwest Bay off Sri Lanka, south Tamilnadu coast. Development of another cyclonic circulation in the shear line over south Bay on 10th November.

10 220 S R Kalsi Figure 11. INSAT derived OLR on with 850 hpa shear line ( ) X -- Position of Sagar Kanya ship. References Kalsi S R, Rao A V R K, Misra D K, Jain R K and Rao V R, 1995 Structural variability of tropical disturbances in the Bay of Bengal, Advances in Tropical Meteorology; (ed) R K Dutta, (Concept Publishing Company). Kanti Prasad, Kalsi S R and Dutta R K 1990 Wind and cloud structure of monsoon depressions; MAUSAM, Srinivasan V, Raman S and Ramakrishnan A R 1971 Monsoon depression as seen in the satellite pictures; IJMH&G,