Growth of cyclone Viyaru and Phailin – a comparative study SDKotal1, S K Bhattacharya1,∗, SKRoyBhowmik1 and P K Kundu2 1India Meteorological Department, NWP Division, New Delhi 110 003, India. 2Department of Mathematics, Jadavpur University, Kolkata 700 032, India. ∗Corresponding author. e-mail: [email protected] The tropical cyclone Viyaru maintained a unique quasi-uniform intensity during its life span. Despite beingincontactwithseasurfacefor>120 hr travelling about 2150 km, the cyclonic storm (CS) intensity, once attained, did not intensify further, hitherto not exhibited by any other system over the Bay of Bengal. On the contrary, the cyclone Phailin over the Bay of Bengal intensified into very severe cyclonic storm (VSCS) within about 48 hr from its formation as depression. The system also experienced rapid intensification phase (intensity increased by 30 kts or more during subsequent 24 hours) during its life time and maximum intensity reached up to 115 kts. In this paper, a comparative study is carried out to explore the evolution of the various thermodynamical parameters and possible reasons for such converse features of the two cyclones. Analysis of thermodynamical parameters shows that the development of the lower tropospheric and upper tropospheric potential vorticity (PV) was low and quasi-static during the lifecycle of the cyclone Viyaru. For the cyclone Phailin, there was continuous development of the lower tropospheric and upper tropospheric PV, which attained a very high value during its lifecycle. Also there was poor and fluctuating diabatic heating in the middle and upper troposphere and cooling in the lower troposphere for Viyaru. On the contrary, the diabatic heating was positive from lower to upper troposphere with continuous development and increase up to 6◦C in the upper troposphere. The analyses of cross sections of diabatic heating, PV, and the 1000–500 hPa geopotential metre (gpm) thickness contours indicate that the cyclone Viyaru was vertically tilted (westward) and lacked axisymmetry in its structure and converse features (axisymmetric and vertical) that occurred for the cyclone Phailin. In addition, there was a penetration of dry air in the middle troposphere of Viyaru, whereas high moisture existed in the middle troposphere of Phailin. The vertical wind shear (5–10 ms−1) near the core of the storm region between 850 and 200 hPa was favourable for both the systems but was higher in the northern region of the cyclone Viyaru. The divergent development of these thermodynamic features conspired to produce converse characteristic of the two cyclones. 1. Introduction few decades efforts have been made to tackle these challenges. Assimilation of enormous satellite data Operational forecasting of tropical cyclones pri- into the sophisticated numerical weather predic- marily involves genesis, track, and intensity predic- tion (NWP) models has enabled the researchers to tions before and after landfall. A major hurdle for look into the intricacies of the structure by study- the study of the structure of tropical cyclones had ing different important variables at various levels been lack of data over the seas. During the last of the atmosphere within and around the systems. Keywords. Cyclogenesis; diabatic heating; potential vorticity; vertical wind shear. J. Earth Syst. Sci. 123, No. 7, October 2014, pp. 1619–1635 c Indian Academy of Sciences 1619 1620 S D Kotal et al. Several studies have been carried out in fore- about 48 hr from its formation as depression. The casting tropical cyclone. A tropical cyclone gen- system also experienced rapid intensification phase esis parameter was defined by Gray (1968)asa (intensity increased by 30 kts or more during sub- product of three thermodynamic parameters, viz., sequent 24 hr) during its life time and maximum sea surface temperature (SST) (>26◦Cuptoa intensity reached up to 115 kts. An insight into depth of 60 m), mid-tropospheric relative humid- different aspects of both the systems is necessary ity, and three dynamical parameters, viz., Cori- to understand the mechanism of the growth of olis parameter, reciprocal of vertical wind shear, the system to cyclonic storm and maintenance of and relative vorticity in the lower levels of the the quasi-uniform intensity for an unusually long troposphere. DeMaria et al. (2001), McBride and duration for cyclone Viyaru and rapid intensifica- Zehr (1981) developed different methods to pre- tion of the cyclone Phailin. Therefore, this study dict cyclogenesis using various dynamical and ther- includes analyses of dynamical and thermodynam- modynamical atmospheric variables. Roy Bhowmik ical parameters to explain the converse feature of (2003) highlighted some threshold values of Genesis the cyclone Viyaru and Phailin. parameter to differentiate the degree of intensifi- The paper is organized as follows: brief lifecycle cation at early genesis stages over the Indian seas. of the cyclone Viyaru and Phailin is described in Kotal et al. (2009) developed a Genesis Potential section 2. The data sample used in this study is Parameter (GPP) consisting of various dynamical described in section 3. A synoptic overview of the and thermodynamical variables to differentiate environmental condition is given in section 4.Anal- between nondeveloping and developing low pres- yses of thermodynamic features are presented in sure systems over the North Indian Ocean. section 5. Summary and conclusions are given in Sanders and Gyakum (1980) defined ‘bomb’ as section 6. an extratropical surface cyclone with central pres- sure falling at the rate of 1 mb h−1 or more for 24 hours. Most of the studies on rapid cyclogene- 2. Cyclonic storms sis and rapid decay have been carried out for sys- tems over the Atlantic Ocean and Pacific Ocean 2.1 Cyclonic storm Viyaru (10–16 May 2013) (Anthes et al. 1983; Sanders 1986; Kuo and Reed A low pressure system that formed over south- 1988;Whitakeret al. 1988;Kuoet al. 1991; Bosart east Bay of Bengal on 8 May 2013 intensified into et al. 1995; Kaplan and DeMaria 2003;Martin depression at 0900 UTC of 10 May 2013 near lat. and Otkin 2004;Kaplanet al. 2009). Kotal et al. 5.0◦N and long. 92.0◦E. Initially it moved north- (2012) carried out a case study of rapid inten- westwards and intensified into a deep depression at sification of cyclone GIRI over the Bay of Ben- 1200 UTC of the same day and further intensified gal. They found the lower-tropospheric latent heat into a cyclonic storm (T.No. 2.5), Viyaru at 0300 release and the resulting increase of positive lower UTC of 11 May 2013. The cyclonic storm recurved PV anomaly by the effect of this latent heat release to north–northeasterly direction on 13 and 14 May played a major role for rapid intensification of and further intensified from T.No. 2.5 to T.No. the cyclone. Recently, Kotal and Roy Bhowmik 3.0 at 0600 UTC of 16 May 2013. Moving north– (2013) developed a statistical-dynamical technique northeastward direction the cyclonic storm crossed for probabilistic forecast of rapid intensification of ◦ ◦ Bangladesh coast near lat. 22.8 N and long. 91.4 E, cyclone. around 0800 UTC of 16 May 2013. After landfall, The average life span of tropical cyclones over it continued to move north–northeastwards and the North Indian Ocean is 4–5 days out of which weakened into a deep depression over Mizoram at 3–4 days are spent over the seas prior to landfall. 1200 UTC and into a depression over Manipur at Best track data of IMD during the period May 1990 1800 UTC of 16 May 2013. It further weakened into to May 2013 show that only nine tropical cyclones a well marked low pressure area over Nagaland at spent their journey, a period of 120 hr or more, over 0000 UTC 17 May 2013. The observed track of the the sea surface, from the stage of depression. Out cyclone Viyaru is shown in figure 1. of which, eight subsequently crossed coasts and one decayed over the seas without making land- fall. Barring cyclone Viyaru, all the eight cyclones 2.2 Very severe cyclonic storm Phailin attained a maximum intensity equal to or more (8–14 October 2013) than severe cyclonic strom (SCS) intensity (> 47 kts). The cyclone Viyaru maintained cyclone inten- A low pressure system that formed over North sity (34–47 kts) for 126 hr travelling about 2150 Andaman Sea on 7 October 2013 intensified into km, neither decayed nor intensified. Whereas, the depression at 0300 UTC of 8 October 2013 near cyclone Phailin over the Bay of Bengal intensi- lat. 12.0◦N and long. 96.0◦E. It moved northwest- fied into very severe cyclonic storm (VSCS) within wards and intensified into a deep depression at Growth of cyclone Viyaru and Phailin 1621 Figure 1. Observed track of the cyclone Viyaru. Figure 2. Observed track of the cyclone Phailin. 0000 UTC of 9 October 2013 and further intensified into a cyclonic storm (T.No. 2.5), Phailin at 1200 horizontal resolutions using IMD GFS-T574L64 UTC of the same day. The cyclonic storm contin- analysis as first guess and forecasts as bound- ued to move in northwesterly direction and intensi- ary conditions. Using analysis and boundary con- fied into severe cyclonic storm (T.No. 3.5) at 0300 ditions from the WRFDA, the WRF (ARW) is UTC of 10 October 2013 and subsequently inten- runfortheforecastupto3dayswithdouble sified into very severe cyclonic storm (T.No. 4.0) nested configuration with horizontal resolution of at 0600 UTC of same day. Moving northwestward 27 km and 9 km and 38 Eta levels in the verti- direction the system further rapidly intensified to cal. WRF at resolution 9 km data have been used T.No. 4.5, T.No. 5.0, and T.No. 5.5 at 1200, 1500, for the analyses of various thermodynamic para- and 2100 UTC of same day (10 October 2013), meters for the study of growth of tropical cyclone respectively.