International Journal of Integrated Sciences & Technology 2 (2016) 55-61

Transcription

1 International Journal of Integrated Sciences & Technology 2 (2016) Changes in Latent Heat Energy and Moist Static Energy Contents of the Atmosphere over Bangladesh and Neighbourhood during the Formation and Movement of Tropical Cyclones: Sidr and Nargis S. Karmakar 1*, U. K. Mitra 2 and A. M. Chowdhury 3 1Bangladesh Centre for Advanced Studies, Dhaka, Bangladesh 2 Department of Physics, Jahangirnagar University, Savar, Dhaka Abstract Available rawinsonde data of 0000 UTC for the standard isobaric surfaces from 1000 hpa to 100 hpa over different stations surrounding the Bay of Bengal such as Madras, Bhubaneshwar, and Dhaka for the cyclone 'SIDR' and Madras, Bhubaneshwar and Bangkok for the cyclone 'NARGIS' for periods November 2007 and 22 April to 05 May 2008 respectively have been used to study the changes energy contents of the troposphere during the formation, movement and landfall of the two cyclones. Different components of energy such as latent heat energy and moist static energy per unit mass of air in the troposphere over the stations have been computed on different dates (as mentioned) before and after the landfall of two severe cyclones. When the cyclone is nearer to the coast, latent heat is found to increase up to 200 hpa or 100 hpa. With the advancement of SIDR towards Bangladesh, the moist static energy over Dhaka was found to increase significantly from surface to 650 hpa on 14 November 2007 and from surface to top of the troposphere on 15 November The moist static energy of the troposphere over Madras decreased on 25 April 2008 in the layer between 1000 hpa and 850 hpa levels during the formative stage of the cyclone NARGIS. With the movement of the cyclone due northwest towards Madras, the moist static energy of the troposphere over Madras was found to increase significantly from surface to about 700 hpa. The energy of the troposphere increases in the direction in which a cyclone moves and decreases in the direction from which the cyclone moves away. Keywords: Cyclone, SIDR, NARGIS, latent heat energy, moist static energy and troposphere. 1. Introduction Because of the special geographical location, Bangladesh is affected by different natural disasters like tropical cyclones and associated storm surges, nor'westers, tornado, floods, etc. Out of these, tropical cyclones are the most devastating natural phenomena which cause huge damage to properties and loss of lives in Bangladesh. The formation, movement and landfall of cyclones are associated with changes of the atmospheric properties and parameters such as energy components of the atmosphere. The study of the energy components of the atmosphere is important in respect of the formation, movement and landfall of tropical cyclones. A lot of works have been done on atmospheric energy, it fluxes, release of the latent heat and kinetic energy and their transport by different Corresponding author: karmakarsamarendra@gmail.com For color version visit: ISSN : authors, such as Hastert (1966), Alestalo and Holocaine (1980), Keshavamurthy (1970), Krishnamurty (1971), Chowdhury and Karmakar (1980) and Karmakar (2003). Comparatively little is known about the moisture, energetic and their fluxes over India-Bangladesh-Pakistan sub-continent, especially with respect to the tropical cyclones in the Bay of Bengal making a landfall either at Bangladesh coast or Indian coast except the study made by Anjaneyulu et al. (1965) in which the latent heat and sensible heat energy have been estimated around the lateral boundary of October 1963 cyclone. Karmakar (1998) studied the verticallyintegrated tropospheric energy and their fluxes over Bangladesh during the landfall of three major cyclones such as cyclone of 25 May 1985, cyclone of 29 November 1988 and cyclone of 29 April Therefore, the scientists are always engaged in finding the secrets of the tropospheric properties and

2 their changes which are related to the future movement of tropical cyclones. The main objective of the present study is to investigate the changes in tropospheric energy during the formation, movement and dissipation of the major cyclones named SIDR and NARGIS which formed in the Bay of Bengal and made landfall at Bangladesh coast and Myanmar coast respectively. 2. Data Used Rawinsonde data of 0000 UTC for the standard isobaric surfaces from 1000 hpa to 100 hpa over Madras, Bhubaneshwar, and Dhaka for cyclone SIDR and Madras, Bhubaneshwar and Bangkok for cyclone NARGIS for periods November 2007 and 22 April to 05 May 2008 respectively have been collected from the Storm Warning Centre (SWC) of Bangladesh Meteorological Department (BMD) and internet. In case of Dhaka station where 00 UTC data is not available, 12 UTC data have been considered. 3. Basic Equation The equations for latent heat energy and moist static energy per unit mass of air are as follows: Latent heat energy, Moist static energy, Where L is the latent heat of evaporation, q is the specific volume, g is the acceleration due to gravity and z is the geopotential height. 4. Tracks and Satellite Imageries of the Cyclones 4.1 Cyclone SIDR E 1 = Lq E = C p + gz + 1 Lq Cyclone SIDR was a tropical cyclone that resulted in one of the worst natural disasters in Bangladesh in the recent past. While in the deep sea, it quickly strengthened to reach peak 1-minute sustained winds of 260 km/hr. The death toll was minimum (about 3447 people) due to timely warning by Bangladesh Meteorological Department and taking proper evacuation measure by the Government of Bangladesh. Its formation along with satellite imagery and tracks are given below: Formed as low over South Andaman Sea and adjoining area in the morning on 09 November Depression on at 06 UTC on 11 November Deep depression on 12 November Cyclonic storm 'SIDR' at 06 UTC on 12 November. Severe Cyclonic Storm at 12 UTC on 12 November. Severe Cyclonic Storm with a core of hurricane winds (ECP 968 hpa) at 03 UTC on 13 November. The Severe Cyclonic Storm 'SIDR' with a core of hurricane winds over the North Bay of Bengal moved S. Karmakar et.al/int. J. Integ. Sci. Tech. 2 (2016) north-north-eastwards; the Centre crossed the coast near Baleshwar River at 15 UTC. At 21 UTC of 15 November, the SIDR completed crossing the coast by moving north-eastwards and was lying over the Southern and Central parts of Bangladesh as a land depression. The satellite imagery and track of SIDR are given in Figs. 4.1 and 4.2 respectively. Fig. 4.1 shows a distinct eye of the cyclone. Fig. 4.1: Satellite imagery of SIDR Fig. 4.2: Track of the cyclone SIDR (BMD, 2007) 4.2 Cyclone NARGIS Cyclone NARGIS was a rare, eastward moving at low-latitude strong tropical cyclone which caused the worst natural disaster in the recorded history of Myanmar. During the movement, NARGIS was a Category 4 cyclone at one point, with sustained winds of 210 kilometers per hour. It caused a huge damage and loss of live in Myanmar. Its formation along with satellite imagery and tracks are given below:

3 Detected as feeble low was detected first on 24 April 2008 over Southeast Bay and adjoining area Intensified into a Well Marked Low on 25 April Depression over the same area and was centred at 00 UTC on 27 April 2008 Deep depression over Southwest Bay and adjoining Southeast Bay at midnight on 27 April Cyclonic storm 'NARGIS' with ECP 993 hpa and was centred at 03 UTC on 28 April Severe cyclonic storm "NARGIS" (with ECP 984 hpa) and then lied over West Central Bay and adjoining Southwest Bay centred at 06 UTC on 29 April 2008 Intensified into a severe cyclonic storm with a core of hurricane winds "NARGIS" (with ECP 972 hpa) and was centred at 06 UTC on 02 May 2008 near Lat 16.0 N & Long 93.5 E. Moved eastwards and was crossing Myanmar coast near Bassein at 12 UTC on 02 May The Myanmar coast crossing severe cyclonic storm with a core of hurricane winds "NARGIS" (with ECP 972 hpa) was still crossing southern coastline of Myanmar (near Bassein) at midnight on 02 May. Crossed Myanmar coast by morning on 03 May The satellite imagery and the track of the NARGIS are given in Figs. 4.3 and 4.4 respectively. Fig. 4.3: Satellite imagery of NARGIS Fig.4.4: Track of the cyclone NARGIS (BMD, 2008) S. Karmakar et.al/int. J. Integ. Sci. Tech. 2 (2016) Results and discussion 5.1 Latent heat energy content of the troposphere The vertical profiles of the latent heat energy for the stations Madras, Bhubaneswar and Dhaka during the formation, movement and landfall of SIDR are shown in Figs. 5.1 (a-b), 5.2 (a-b) and 5.3 (a-b) respectively. Fig. 5.1 shows that the latent heat content of the troposphere over Madras has increasing trend in the lower troposphere when the cyclone moved in a northwesterly direction towards Madras. When the cyclone made landfall at Bangladesh coast, the latent heat content of the troposphere over Madras is seen to decrease significantly in the layer from 1000 hpa to about 700 hpa level. After the landfall of the cyclone, the latent heat is seen to increase over Madras significantly from 1000 hpa to 350 hpa [5.1]. The figures also show that the latent heat energy of the troposphere decreases with height and vanishes at about 400 or 300 hpa and above especially during the formative stage of the cyclone. From the above discussion, it is clear that when the cyclone moved towards a station, latent heat increases in the troposphere over that station. When the cyclone moved away from the station, the latent heat energy decreases significantly on the date of landfall at a far station. After the landfall of 'SIDR', the latent heat energy over Madras is found to increase significantly and extended more upwards in the troposphere. Almost similar pattern is found over Bhubaneshwar where latent heat energy of the troposphere over Bhubaneshwar [Fig. 5.2(a-b)] in case of cyclone 'SIDR'. Fig.5.3 shows that when the cyclone SIDR was in the formative stage on 12 November, the latent heat energy vanishes at about 700 hpa level over Dhaka. This means that the upper air becomes dryer over Dhaka during the formative stage of SIDR. When the cyclone moved towards the Bangladesh coast and made landfall, significant and prominent increase in latent heat energy is found in the lower troposphere and the latent heat is extended upwards up to about 200 hpa in the troposphere over Dhaka [Fig. 5.3]. After about six days of the landfall, the latent heat energy is found to decrease significantly over Dhaka.

4 S. Karmakar et.al/int. J. Integ. Sci. Tech. 2 (2016) Fig. 5.1: Vertical profiles of latent heat energy (J/gm) over Madras during the movement of SIDR at 00 UTC on 8-10 November and November, 2007 Fig. 5.3: Vertical profiles of latent heat energy (J/gm) over Dhaka during the movement of SIDR at 12 UTC on November and. 15, 21 November, 2007 Fig. 5.4: Vertical profiles of latent heat energy over Madras during the movement of NARGIS at 00 UTC on April and 2-4 May, 2008 Fig. 5.2: Vertical profiles of latent heat energy (J/gm) over Bhubaneshwar during the movement of SIDR at 00 UTC on November and November, 2007 The vertical profiles of latent heat energy of the troposphere over Madras, Bhubaneshwar and Bangkok are shown in Figs. 5.4(a-b), Figs. 5.5(a-b), and Figs. 5.6(a-b) respectively for the cyclone 'NARGIS'. When the cyclone 'NARGIS' moved in a northwesterly direction and was relatively nearer to Madras, the latent heat energy content is found to increase in the lower troposphere. With the approach of the cyclone towards the east for ultimately landfall at Myanmar coast, the behavior of the vertical distribution of the latent heat is found to be a little bit erratic and seen to decrease significantly throughout the troposphere after the landfall of NARGIS. The latent heat energy content of the troposphere over Bhubaneshwar is found to increase significantly from surface to 500 hpa when the cyclone 'NARGIS' moved in a northwesterly direction [Fig. 5.5]. As the cyclone 'NARGIS' moved away from Bhubaneshwar, the latent heat energy of the troposphere is found to decrease significantly from surface to about 500 hpa level as shown in Fig After the landfall of the cyclone, the latent heat energy content of the troposphere over Bhubaneshwar is also found to increase significantly from surface to about 400 hpa. The vertical distribution of latent heat energy of the troposphere over Bangkok shows some erratic behavior up to 26 April 2008 [Figs. 5.6 (a-b)]. The latent heat content of the troposphere over Bangkok is found to increase slightly on 28 and 29 April when the cyclone was approaching towards Myanmar coast. 5.2 Moist static energy of the troposphere The vertical distribution of moist static energy of the troposphere over Madras, Bhubaneshwar and Dhaka for the cyclone 'SIDR' are shown in Figs. 5.7 (a-b), Figs. 5.8 (a-b) and Figs. 5.9 (a-b) respectively. The vertical distribution of moist static energy of the troposphere over Madras has increasing trends in the lower troposphere from surface to about 850 hpa level when the cyclone moved in a northwesterly direction over Madras. But the energy has decreasing tendency from 850 hpa to the top of the troposphere [Fig.5.7 ]. When the cyclone moved away from

5 Madras for ultimately landfall at Bangladesh coast, the moist static energy is seen to decrease significantly from surface to 700 hpa level on 15 November 2007 [Fig: 5.7]. After the landfall, weakening and subsequent movement of the cyclone Fig.5.5: Vertical profiles of latent heat energy over Bhubaneshwar during the movement of NARGIS at 00 UTC on April 2008 and 3-5 May 2008 'SIDR', the moist static energy over Madras is seen to increase significantly from surface to about 500 hpa level. When the cyclone 'SIDR' was in the formative stage and far away from Bhubaneshwar moist static energy is seen to decrease significantly from surface to 600 hpa levels. [Fig: 5.8]. As the cyclone moves towards north keeping the station Bhubaneshwar in the left of the track, the moist static energy is seen to decrease between 900 hpa to 700 hpa level over Bhubaneshwar. After the landfall of the cyclone 'SIDR', the moist static energy over Bhubaneshwar is found to increase significantly from surface to about 450 hpa level on 16 November 2007 [Fig: 5.8], and also found to increase onwards. As the cyclone moved towards Bangladesh, the moist static energy over Dhaka is seen to increase significantly from surface to 650 hpa on 14 November 2007 and from surface to top of the troposphere on 15 November 2007 [Fig. 5.9]. After six days of the landfall and movement towards northeast, the moist static energy is seen to decrease significantly from surface to 200 S. Karmakar et.al/int. J. Integ. Sci. Tech. 2 (2016) hpa level [Fig: 5.9]. It is also clear from the figures that the contribution of latent heat energy to moist static energy is prominent. The above discussions substantiate that when the cyclone moved towards a station, the moist static energy in that station increase in the lower troposphere and when it moves away from a station, the moist static energy decrease significantly in the lower troposphere, sometimes up to mid-troposphere. The moist static energy also decreases in the left of the track of the cyclone and increase significantly up to the top of the troposphere along the track of the cyclone. The vertical distribution of moist static energy of the troposphere over Madras, Bhubaneshwar and Bangkok for the cyclone 'NARGIS' are shown in Figs (a-b), Figs (a-b) and Fig (a-b) respectively. The moist static energy of the troposphere over Madras is seen to decrease on 25 April 2008 in the layer between 1000 hpa and 850 hpa levels during the formative stage of the cyclone NARGIS. As the cyclone began to move northwest towards Madras, the moist static energy of the troposphere over Madras is seen to increase significantly from surface to about 700 hpa [Fig: 5.10 ]. There is no definite change of moist energy over Madras when the cyclone made landfall at Myanmar coast. But one or two days after landfall, the moist static energy is seen to increase significantly over Madras [Fig: 5.10 ]. Fig. 5.7: Vertical profile of moist static energy over Madras during the movement of SIDR at 00 UTC on November and November, During the formative stage of NARGIS, the moist static energy over Bhubaneshwar decreases significantly in the northwest of the cyclone position [Fig: 5.11 ]. As the cyclone moved and came relatively nearer to Bhubaneshwar, the moist static energy is found to increase significantly from the surface to the mid-troposphere [Fig: 5.11]. It continues to increase up to 2 May 2008 even after Fig. 5.6: Vertical profile of latent heat energy over Bangkok landfall of the NARGIS (not shown). During the during the movement of NARGIS at 00 UTC on formative stage of 'NARGIS', the moist static energy April and April 2008 of the troposphere over Bangkok has a tendency to

6 S. Karmakar et.al/int. J. Integ. Sci. Tech. 2 (2016) Fig. 5.8: Vertical profile of moist static energy over Fig.5.11: Vertical profile of moist static energy over Bhubaneshwar during the movement of SIDR at 00 UTC Bhubaneshwar during the movement of NARGIS at 00 on November and November, UTC on April and April, 2008 increase from surface to about 450 hpa level [Fig: 5.12 ]. As the cyclone moved away from Bangkok towards northwest, the moist static energy has a slight decreasing trend throughout the troposphere (not shown). When the cyclone began to move eastward, the moist static energy over Bangkok has a slight increasing trend below the mid-troposphere as shown in [Fig: 5.12 ]. Fig. 5.9: Vertical profile of moist static energy over Dhaka during the movement of SIDR at 00 UTC on November and 15, 21 November, Fig. 5.10: Vertical profile of moist static energy over Madras during the movement of NARGIS at 00 UTC on April and 2-4 May, Fig. 5.12: Vertical profiles of moist static energy over Bangkok during the movement of NARGIS at 00 UTC on April and April, The above discussion indicates that the moist static energy decreases in the north-northwest direction of the cyclone. It may also be opined that the moist static energy increases in the direction in which the cyclone moved and decreases in the direction from which the cyclone moves away. 6. Conclusions On the basis of the present study, the following conclusions may be drawn: (i) The latent heat energy of the troposphere decreases with height and vanishes at /above 400 hpa or above. When SIDR moved towards the Bangladesh coast and made landfall, significant and prominent increase in latent heat energy is found in the lower troposphere and the latent heat is extended upwards up to about 200 hpa in the troposphere over Dhaka. (ii) The moist static energy of the troposphere over Madras have increasing trends in the lower troposphere from surface to about 850 hpa level over Madras when the cyclone 'SIDR' moved in a northwesterly direction. (iii) When the cyclone moved away from Madras for ultimately landfall at Bangladesh coast, the moist static energy decreases significantly from surface to 700 hpa level. After the landfall, weakening and subsequent movement of the cyclone 'SIDR', the moist static energy over Madras increases significantly from surface to about 500 hpa level.

7 (iv) When the cyclone 'SIDR' was in the formative stage and for away from Bhubaneshwar moist static energy is seen to decrease significantly from surface to 600 hpa level. In the left of the track of SIDR, the moist static energy is seen to decrease between 900 hpa to 700 hpa level. After the landfall of the cyclone 'SIDR', the moist static energy over Bhubaneshwar is found to increase significantly from surface to about 450 hpa level on 16 November 2007 onwards. And that over Dhaka increases significantly from surface to 650 hpa on 14 November 2007 and from surface to top of the troposphere on 15 November (v) During the formative stage of NARGIS, the moist static energy decreases significantly in the northwest of the cyclone. As the cyclone moved and came relatively nearer to Bhubaneshwar, the moist static energy is found to increase significantly from the surface to the mid-troposphere. It continues to increase up to 2 May 2008 even after landfall of the NARGIS. Initially, the moist static energy of the troposphere over Bangkok has a tendency to increase from surface to about 450 hpa level. (vi) The energy of the troposphere increases in the direction in which a cyclone moves and decreases in the direction from which the cyclone moves away. Acknowledgement The authors are thankful to the Director and officers of Bangladesh Meteorological Department for providing the relevant data for the study. References 1. M. Alestalo, and E. Holopainen,, "Atmospheric energy fluxes over Europe", Tellus, 32, 6, 500 (1980). 2. T.S.S. Anjaneyulu, D.R. Sikka and G. Gurunathm, "Some aspects of Bay of Bengal cyclone of October 1963" Indian J. Meteor. Geophys, 16, 4, pp-539 (1965). 3. M. H. K. Chowdhury, and S. Karmakar, "Some aspects of energetics of the troposphere over the Arabian Sea with advancement of south-west monsoon", FGGE Operations Report: Results of Summer Monex Field Phase Research, 9, Part-A, pp-81 (1980). 4. S.L. Hastenrath, "On general circulation and energy budget in the area of Central American Seas", J. Atm. Sci., 23, 6, pp-694 (1966). S. Karmakar et.al/int. J. Integ. Sci. Tech. 2 (2016) S. Karmakar, "Vertically-integrated tropospheric moisture, energy and their fluxes over Bangladesh during landfall of tropical cyclones", Mausam, 49, 2, pp-187 (1998). 6. S. Karmakar, "Vertically-integrated sensible heat content of the troposphere and its fluxes over Bangladesh during landfall to tropical cyclones", Presented in the Seminar on Tropical Cyclones and SW-monsoon in Bangladesh, arranged by SMRC on December 12, 1995, The Atmosphere, 2, pp-18 (2003). 7. R.N. Keshavamurthy, and S. T. Awade, "On the maintenance of the mean monsoon trough over North India", Mon. Weath. Rev., 98, pp-315 (1970). 8. T.N. Krishnamurthy, "Observational study of the tropical upper tropospheric motion field during the northern hemisphere summer", J. Appl. Met., 10, pp-1066 (1971).