The Role of Mid-Level Vortex in the Intensification and Weakening Of

J. Earth Syst. Sci. (2017) 126:94 c Indian Academy of Sciences DOI 10.1007/s12040-017-0879-y

The role of mid-level vortex in the intensiﬁcation and weakening of tropical cyclones

Govindan Kutty* and Kanishk Gohil

Indian Institute of Space Science and Technology, Thiruvananthapuram 695 547, India. *Corresponding author. e-mail: [email protected]

MS received 2 October 2016; revised 18 April 2017; accepted 25 April 2017; published online 6 October 2017

The present study examines the dynamics of mid-tropospheric vortex during cyclogenesis and quantifies the importance of such vortex developments in the intensification of tropical cyclone. The genesis of tropical cyclones are investigated based on two most widely accepted theories that explain the mechanism of cyclone formation namely ‘top-down’ and ‘bottom-up’ dynamics. The Weather Research and Forecast model is employed to generate high resolution dataset required for analysis. The development of the mid-level vortex was analyzed with regard to the evolution of potential vorticity (PV), relative vorticity (RV) and vertical wind shear. Two tropical cyclones which include the developing cyclone, Hudhud and the non-developing cyclone, Helen are considered. Further, Hudhud and Helen, is compared to a deep depression formed over Bay of Bengal to highlight the significance of the mid-level vortex in the genesis of a tropical cyclone. Major results obtained are as follows: stronger positive PV anomalies are noticed over upper and lower levels of troposphere near the storm center for Hudhud as compared to Helen and the depression; Constructive interference in upper and lower level positive PV anomaly maxima resulted in the intensification of Hudhud. For Hudhud, the evolution of RV follows ‘top-down’ dynamics, in which the growth starts from the middle troposphere and then progresses downwards. As for Helen, RV growth seems to follow ‘bottom-up’ mechanism initiating growth from the lower troposphere. Though, the growth of RV for the depression initiates from mid-troposphere, rapid dissipation of mid-level vortex destabilizes the system. It is found that the formation mid-level vortex in the genesis phase is significantly important for the intensification of the storm. Keywords. Tropical cyclone; mid-level vortex; WRF.

1. Introduction (Hendricks et al. 2004; Montgomery et al. 2006) and on a smaller meso-β scale, cyclogenesis is Tropical cyclone formation governed by superfluous observed to be following a ‘top-down’ dynamics number of processes and cyclogenesis is a multi- (Ritchie and Holland 1997; Simpson et al. 1997). stage process which involves interaction from The ‘top-down’ theory suggests that the formation different spatial scales (Alpert et al. 1996; of a vortex approximately near mid-troposphere Tory et al. 2006). Over a broader meso-α scale, acts as a precursor to cyclogenesis. The circulation the genesis of a cyclone has been proposed to further propagates downwards and initiates sur- be taking place through ‘bottom-up’ mechanism face circulation. In ‘bottom-up’ mechanism, surface 1 94 Page 2 of 12 J. Earth Syst. Sci. (2017) 126:94 circulation engenders mid-troposphere vortex. Both (2012) used relative vorticity dynamics during the the approaches have been widely accepted and genesis period to explain the overall strengthening based on the spatial scale in consideration, either of a tropical storm. a ‘wave pouch’ over the lower troposphere (on a Another potential atmospheric factor which meso-α scale) or a mid-level vortex over the mid- affects the intensity of TC is vertical wind shear. troposphere (on a meso-β scale) can be observed. Several studies have illustrated the role of vertical Raymond et al. (2011) suggested that a cold wind shear on the genesis and subsequent inten- core over the lower troposphere and a warm core sification/weakening of tropical cyclone (DeMaria over the mid-troposphere can help in construct- and Kaplan 1999; Corbosiero and Molinari 2003; ing a thermodynamically favourable environment Zeng et al. 2010; Xu and Wang 2013). Black et al. to facilitate ample deep convection at lower levels, (2002) carried out observational studies on the hence putting ‘top-down’ approach into perspec- hurricanes Jimena and Olivia for demonstrating tive. Studies such as Sippel and Zhang (2008) the significance of shear in the intensification of a illustrated the formation of a vortex over mid- tropical cyclone. The main premise was the weak- troposphere that leads to the intensification of ening of Olivia due to an increment in the wind tropical cyclone. Recently, Gjorgjievska and Ray- shear through the west. As a result, the convective mond (2014) hypothesized that the mid-level vor- structure broke apart, which in turn reduced the tex leads to the development of bottom heavy intensity of cyclone intensity. Frank and Ritchie mass flux region within the shallow layer over (2001) found that enhanced vertical wind shear lower troposphere, which ultimately leads to a would generate much stronger asymmetry in the strong updraft at lower levels. Conversely, Haynes storm structure. and McIntyre (1987) proposed convergence and In this study, we discuss the role of mid- vorticity assimilation that exist near the lower tro- tropospheric vortex in the formation, develop- posphere, further leads to the intensification of ment, intensification and/or decay of a tropical a tropical storm. However, the pathway through cyclone, comprehensively. An additional analysis which the genesis of tropical cyclone happens may has been carried out on the influence of vertical vary for different flow regimes and spatial scales wind shear in the intensification of tropical cyclone. (Halverson et al. 2007). The importance of mid-level vortex and vertical Potential vorticity (PV) has been regarded as a wind shear in the tropical cyclone intensification highly efficient tool for analyzing tropical cyclones. is investigated by comparing an intensifying and Studies such as Hoskins et al. (1985) used PV two non-intensifying cyclonic systems formed over to examine the dynamical interactions of upper the Northern Indian Ocean. The objectives of this tropospheric synoptic scale trough and the low study are: (1) To understand the role of probable level protovortex formation. Molinari et al. (1998) mid-level vortex during genesis phase of an intensi- specifically takes PV into consideration because of fying and a non-intensifying storm; (2) commenting its quasi-static characteristic that results from the on the disability of a non-intensifying storm to con- baroclinic instability in the environment. Further- vert into a cyclonic vortex through an analysis of more, PV analysis incorporates thermodynamic storm structures during the genesis phase; (3) to properties of the system and therefore, separate determine the effect of vertical wind shear in the attention for such properties are not required in intensification/decay of the tropical cyclone. the analysis. Delden (2003) suggested that the The paper is organized as follows. Section 2 PV anomaly over the mid-troposphere is stronger describes briefly about the synoptic background of and moves faster than the lower troposphere. the case studies. Section 3 describes the model, con- When the superposition between the two pre- figuration and numerical experiments. The experi- existing anomalies takes place, a stable cyclonic ment results are detailed in section 4 and section 5 vortex structure can be formed. Molinari et al. concludes the paper. (1995) proposed that for a stronger tropical storm, the intensification takes place due to the constructive interference between the PV anoma- 2. Synoptic background lies on upper and lower troposphere. Moreover, the approach of mid-level PV anomaly towards The developing and non-developing cyclone, are the pre-existing low level anomaly contributes to defined based on the Saffir–Simpson scale, in this the intensification of a tropical cyclone. Wang study. The tropical cyclones reaching Category 3 J. Earth Syst. Sci. (2017) 126:94 Page 3 of 12 94 and higher are considered as developing cyclones. analysis. The WRF model is a state-of-the-art The tropical cyclones which fall below Category atmospheric modeling system which is fully com- 3 in Saffir–Simpson scale is considered as non- pressible and non-hydrostatic with advanced developing cyclone. The tropical cyclone Hudhud, parameterization schemes. This study employs is a Category 4 cyclone, while the non-developing WRF single moment (WSM) five-class scheme for cyclone, Helen is Category 1 cyclone. microphysics (Hong et al. 2004), Yonsei State Uni- The intensifying cyclone, Hudhud, started to versity Scheme (Hong et al. 2006) for planetary form on October 6, 2014, over the Bay of Bengal boundary layer parameterization, Unified Noah region of Northern Indian Ocean. By October 8, Land surface model (Tewari et al. 2004), Rapid 2014, the cyclone had entered in the rapid inten- Radiative Transfer model for long-wave radiation sification phase. On October 9, it was declared as (Mlawer et al. 1997) and Dudhia scheme (Dud- a severe cyclonic storm, just before it underwent hia 1989) for shortwave radiation calculations. The rapid deepening followed by intensification. Dur- simulation was carried out in two-way nesting over ing its genesis phase (6–9 October), the central three horizontal domains of 36, 12 and 4 km reso- pressure dropped from 1004 to 990 hPa. Hudhud lution, respectively. The model domain is centered made a landfall on October 12, a few hours over Bay of Bengal region with 35 non-uniform lev- prior to which it attained a minimum central els in vertical. All the analyses shown in this paper pressure of 950 hPa and a maximum wind speed are from 4 km resolution domain. of 175 km hr−1. The initial and boundary conditions for the Helen was a non-intensifying tropical cyclone WRF model are generated from National Cen- that started developing around November 18, 2013, ter for Environmental Prediction (NCEP) – Final over Bay of Bengal. The genesis period for Helen is Analysis (FNL) at 10 x10 resolution (NCEP 2000). estimated to be from 18 November 00 UTC to 19 Topography and land use datasets were obtained November 12 UTC, 2013. The intensification for from United States Geological Survey (USGS) Helen was not as severe as Hudhud, and it was global covers (https://lpdaac.usgs.gov/data access/ categorized as a severe cyclonic storm during its data pool). The tropical cyclone genesis time is intensification phase from 20–23 November. The decided based on India Meteorological Department lowest central pressure attained was 990 hPa, while (IMD) report and location were obtained from the maximum wind speed was about 100 km hr−1. Joint Typhoon Warning Center (JTWC) best track The results from Hudhud and Helen, is com- data (http://www.usno.navy.mil/JTWC/). pared to a deep depression to further reinforce the significance of the mid-level vortex in the genesis of a tropical cyclone. The depression that formed over the Bay of Bengal during the second week of 3.2 Experimental design November 2013 was initially seen as a low pressure area over the eastern Bay of Bengal. The A series of 24 h model integrations were performed low-pressure area intensified to a depression on 12 from the initial conditions generated from NCEP- November, 2013. The depression further intensified FNL over a period covering the genesis phase of into a deep depression and by 13 November, 2013, both the tropical cyclones. The design of exper- it subsequently moved westwards and made a land- iment adopted in this study is demonstrated in fall near Nagapattanam, Tamil Nadu. The genesis figure 1. The purpose of the simulations are to gen- period of depression is estimated to be from 11 to erate dynamically balanced high resolution dataset 13 November, 2013, 00 UTC. for analysis. An analysis based on the potential vorticity and relative vorticity evolution is predominantly carried out to understand the role 3. Data and methods of mid-level vortex in the formation of tropical cyclone. Potential vorticity analysis is performed 3.1 Model configurations using Ertel’s hydrostatic PV equation shown in equation (1), for which all variables were interpo- In this study, Advanced Research Weather lated from isobaric to isentropic coordinates. In this Research Forecast (ARW-WRF) model of version study, we express the potential vorticity values in 3.6.1 (Skamarock et al. 2005) is used to gen- Potential Vorticity Unit (PVU;1 PVU corresponds erate high resolution datasets, required for the to 1 × 10−6 m2 Ks−1 kg−1). 94 Page 4 of 12 J. Earth Syst. Sci. (2017) 126:94

Figure 1. An illustration of the experimental design adopted in this study.

(ζθ + f) 4. Results and discussions Π = −g . (1) ∂p/∂θ 4.1 Potential vorticity Following Wang (2012), equation (2) is used for vorticity budget analysis. Figure 2 illustrates the spatial distribution of PV for first (figure 2a, d and g), second (figure 2b, e and h) and third (figure 2c, f and i) ∂ζ dV = −∇ · (ζV ) −∇· −ωk X + ∇ · R. (2) day of genesis and intensification of Helen (top ∂t dp panel), Hudhud (middle panel) and the depression (bottom panel). The PV values may not vary The first term on the right hand side (RHS) of substantially within the boundary layer due to tur- equation (2) corresponds to advective vorticity that bulence and hence the estimation of PV for all consider the effects of horizontal convergence and the three cases was done above 325 K surface divergence at a fixed level. The second term on (≈850 hPa); the approximate altitude where the RHS is the non-advective vorticity which takes into boundary layer ends. In a similar manner as stated consideration the effects of vertical transport of above, figure 3 demonstrates the evolution of PV in vorticity across isobars, while the third term is the mid-troposphere, over 350 K surface (≈500 hPa). residual term. The examination of midlevel vortex The primary purpose behind separately depict- in a barotropic atmosphere was done by observing ing PV evolution over lower and mid-tropospheric 0 × 0 0 × 0 relative vorticity, over a 2 2 and 6 6 box regions is to highlight the difference in the hori- around the center of the cyclone. zontal extension, intensity and organization of PV Vertical wind shear for deep and shallow tro- at both the levels. During the first day of gen- posphere has been calculated as the vector wind esis, PV anomaly seems to be scattered over a differences between two levels. larger region for all the three cases. However, it can be seen that PV anomalies are widely expanded V850 − V200 Sheardeep = , over time for lower and mid-tropospheric regions. 850 − 200 The magnitude of PV is found to be substan- or tially higher over mid-troposphere as compared to lower-troposphere for all the three cases. Having V850 − V500 said this, the accumulation and organization of Shearshallow = . (3) 850 − 500 PV is substantially different for Helen, Hudhud and the depression. Unlike Helen, cyclone Hudhud In both the above expressions, V corresponds to exhibits a stronger mid-level vortex and organized the net wind speed at a particular altitude (850, PV structure, which indicates the significance of a 500 and 200 hPa in the cases being considered). strong mid-level vortex in the intensification of an J. Earth Syst. Sci. (2017) 126:94 Page 5 of 12 94

Figure 2. Spatial distribution of potential vorticity at 325 K isentropic co-ordinate for first (a, d and g), second (b, e and h)andthird(c, f and i) day of genesis and probable intensification phase of Helen (top panel), Hudhud (middle panel) and the depression (bottom panel). incipient tropical storm. The smaller regional span days, during the genesis and probable intensifi- of PV, as compared to Hudhud, can be attributed cation phase of the storms. The cross section is as a reason for weakening of Helen. The PV centered on estimated eye of the storm with the growth pattern followed by the depression resem- axis extending out from it radially outward for bles somewhat closely to that of Helen, but lacks 165 km in both the directions. In general, the an organized structure at both lower and mid-level PV structure for Helen and the depression is dis- altitudes. This can be thought of as a reason that organized as compared to Hudhud. For Hudhud, it could not intensify into a tropical cyclone, even stronger positive PV anomalies are noticed over after the large span of its existence. Furthermore, upper and lower levels of troposphere near the the movement of the depression was relatively storm center on 00 UTC October 7 (figure 4e). We slower than both the cyclones, which could have speculate that Hudhud has possibly intensified due resulted in the lack of organization in its overall to the constructive interference of upper and lower structure. level positive PV anomaly maxima. The interfer- Figure 4 depicts the evolution of vertical ence results in the growth of perturbation energy structure of PV along the eye for three consecutive at the expense of vertical wind shear (Molinari 94 Page 6 of 12 J. Earth Syst. Sci. (2017) 126:94

Figure 3. Same as that of figure 1, except for 350 K isentropic co-ordinate. et al. 1998), which may subsequently strengthen The reason assumed behind this was the absence the tropical cyclone. The lack of such constructive of a sufficiently warm core mid-level vortex that interference of PV maxima may have led to non- could work towards enhancing the spin up. The intensification of cyclone Helen and the depression. traits displayed by the PV contour at later stages By 00 UTC October 8, a stable and well organized show some resemblance to the trend displayed by PV structure was observed for Hudhud, which Helen. had seemingly started developing from the mid- level region and propagated downwards. Further, the troughs on both sides of strong PV max- 4.2 Relative vorticity ima over the storm center facilitated downdraft to intensify the cyclone, Hudhud. For the depression, Further investigation was performed by analyzing PV structure seemed to be following a develop- vertical structure of relative vorticity (RV), around ment path for that of a tropical cyclone and had the center of all the three cases. Following Wang slight potential for developing into a stronger storm (2012), the evolution of RV structure is exam- and might have even intensified into a cyclone. ined in meso-α and meso-β scales using a 2◦ × However, on later stages of development, the struc- 2◦ box and a 6◦ × 6◦ box, respectively. The ture got completely disfigured and disorganized. positive (negative) RV value denotes convergence J. Earth Syst. Sci. (2017) 126:94 Page 7 of 12 94

Figure 4. Potential vorticity anomaly representation (on a vertical cross-section at a ﬁxed latitude, passing through the eye of the storm) for ﬁrst (a, d and g), second (b, e and h)andthird(c, f and i) day of genesis of Helen (top panel), Hudhud (middle panel) and the depression (bottom panel).

(divergence). For Helen, RV starts intensifying by observed in meso-α scale. It is speculated that a 24 hr over both meso-α and meso-β scales strong mid-level vortex prior to the intensifica- (figure 5). As the time progresses, it is noticed tion of cyclone has instigated intense updrafts from that intensification mostly initiates near the lower lower levels through a suction like effect. This leads levels, which progresses upwards via a ‘bottom- to a supposition that the pre-existing mid-level vor- up’ growth mechanism. However, for Hudhud, the tex in the genesis phase of Helen was not strong RV growth and intensification begins by 40 hr and enough to initiate a strong updraft from the lower the pattern seems to follow ‘top-down’ mechanism, troposphere. As a result, strong spin-up has not initiating from around 500 hPa downwards over developed and therefore, Helen could not inten- meso-α scale (figure 6). Concurrently, in meso-β sify into a stronger cyclone. For the depression, a spatial scale, the RV growth begins slightly after weak and disorganized mid-level vortex was seen 30 h in lower troposphere, which after reaching at 24 hr after the genesis (figure 7). The growth 40 h follows a similar ‘top-down’ mechanism, as of RV for the depression started from the lower 94 Page 8 of 12 J. Earth Syst. Sci. (2017) 126:94

Figure 5. Vertical distribution of relative vorticity for Helen Figure 6. Same as that of figure 4, except for cyclone at (a)2◦ × 2◦ box and (b)6◦ × 6◦ box for 48 hr from genesis Hudhud. time. The abscissa is time (hr) and ordinate is vertical levels in pressure levels (hPa). troposphere, through the mid-troposphere which 42 hr, which later on propagated downwards is then followed by the beginning of the dissi- towards lower troposphere. The advective vortic- pation of RV from the mid-level altitudes. In ity for the depression depicts larger regions of addition to this, the short-lived vertical tower had disorganized divergence zone, which could be a lower expanse and was not self-sustaining so as possible reason for non-intensifying nature of the to facilitate the storm to intensify into a tropical depression (figure 8c). This is, yet again, the rea- cyclone. son behind no-strong updraft, as well. Figure 9 For better understanding of the dynamics respon- depicts the non-advective component of vorticity sible for the development of mid-level and low-level for Helen and Hudhud. Non-advective component vorticity, analysis on individual contributions of corresponds to the contribution to vorticity that advective and non-advective components of vor- comes from the vertical transport of vorticity ticity has been carried out using equation (2). across the isobars. In comparison with advec- The evolution of vorticity budget is calculated in tive term, the non-advective term is stronger over a2◦ × 2◦ box around the center of the cyclone. the upper troposphere and weaker in lower- and The advective component of RV budget equa- mid-troposphere. Stronger values of non-divergent tion accounts for the effect of horizontal diver- vorticity observed near 500 hPa by 24 h, hints gence and vorticity stretching. Figure 8(a) shows that updrafts began from the mid-level altitudes, the evolution of advective component of vortic- for Hudhud. However, in addition to the fact ity for Helen. Convergent vorticity gets stronger that non-divergent vorticity values had low pos- by 42 hr and such higher values of vorticity are itive magnitude, existing downdraft between 700 found dominant over lower troposphere without and 500 hPa led to Helen being a weak tropi- much contribution from any pre-existing strong cal cyclone. This might hinder the precipitation mid-level vortex. At the same time, advective which would have further destabilized the storm vorticity component for Hudhud (figure 8b) has structure. For the depression, evolution of non- started intensifying over mid-level altitudes by divergent vorticity is very different from Helen and J. Earth Syst. Sci. (2017) 126:94 Page 9 of 12 94

Figure 7. Same as that of ﬁgure 4, except for the depression. Figure 8. Vertical distribution of advective component of relative vorticity (a) Helen, (b) Hudhud, (c) the depression for 48 hr from genesis time. The abscissa is time (hr) and ordinate is vertical levels in pressure levels (hPa). Hudhud and found to be stronger than Helen at times. It is observed that the advective term predominantly decided whether or not a storm will convert to a cyclone and that whether or not a cyclone will undergo strong intensiﬁcation. At the same time, the values corresponding to the non- advective term for each of the cases decided the net updraft or downdraft that might take place during the course of cyclogenesis. It is quite understand- able to have this kind of trend because supposedly, there needs to be an overall aggregation of moisture and vorticity from the environment into a concentrated region in a systematic and organized manner, only after which a storm can gain a potential to intensify. Prior to this aggregation, it is very unlikely that self-sustained vertical updrafts would begin, or even if they begin, they would be strong enough to strengthen a storm to an extent of turn- ing it to a strong cyclonic vortex.

4.3 Vertical shear

The vertical wind shear may have a direct impact on the formation and intensiﬁcation of mid-level vortex. To quantify this, we compared the evolution Figure 9. Same as that of ﬁgure 7, except that of non- of deeper layer (850–250 hPa) and shallow layer advective component of vorticity. 94 Page 10 of 12 J. Earth Syst. Sci. (2017) 126:94

Figure 10. Evolution of vertical wind shear for (a) Helen, (b) Hudhud, and (c) the depression for 48 hr from genesis time. The abscissa is time (hr) and ordinate is wind shear values (s−1). The deep (shallow) layer vertical wind shear is represented by blue (red) line. (850–500 hPa) vertical wind shear for an intensi- that a conducive ambience for the formation of fying and a non-intensifying tropical cyclone. For convergent mid-level vortex over mid- troposphere a storm to convert into a strong cyclonic vortex, that capacitates the vortex to strengthen by insti- the vertical shear should be preferably smaller than gating updrafts that persist till the tropopause. critical threshold of vertical wind shear, which is The tendency of evolution of deep layer vertical approximately 7–10 ms−1. Apart from that, it is wind shear for the depression is seen similar to that speculated that the shallow layer vertical wind of Helen up to 36 h and then weakens (figure 10c). shear will substantially degrade the formation of The higher magnitude of shear values as compared mid-level vortex. Figure 10 shows the evolution of to Helen might have resulted in the dissipation vertical shear in deep and shallow layer of tropo- of already developed mid-level vortex of the sphere for Helen and Hudhud during its genesis depression. phase. For Helen (figure 10a), though the initial vertical shear for shallow layer is smaller than the deeper layer, it was found that its tendency was 5. Conclusions monotonically increasing beyond the 12 h mark. This overall increasing tendency of vertical wind The present study examines the role of shear implies stronger tilt and hence, a weaker ten- mid-level vortex in the tropical cyclone intensi- dency to develop into a strong and stable cyclonic fication by comparing an intensifying and two vortex. This could have led to a destabilization and non-intensifying tropical cyclonic systems formed hence decay of the vortex structure. At the same over the northern Indian Ocean. The develop- time, for Hudhud (figure 10b), the vertical shear ment of the mid-level vortex was analyzed with is observed to be decreasing continuously from a regard to the evolution of potential vorticity, rel- value of about 12 s−1 to about 3 s−1. This confirms ative vorticity and vertical wind shear. The PV J. Earth Syst. Sci. (2017) 126:94 Page 11 of 12 94 evolution shows substantial difference in the hor- References izontal extension, intensity and organization over lower and mid-tropospheric regions for an intensi- Alpert P, Krichak S, Krishnamurti T, Stein U and Tsidulko fying and non-intensifying cyclone. The PV growth M 1996 The relative roles of lateral boundaries, initial for Hudhud was highly intense and widespread as conditions, and topography in mesoscale simulations of compared to that of Helen. The evolution of ver- lee cyclogenesis; J. Appl. Meteorol. 35 1091–1099. Black M, Gamache J, Marks Jr F, Samsury C and tical structure of PV depicts that Hudhud has Willoughby H 2002 Eastern Pacific Hurricanes Jimena of intensified due to the constructive interference of 1991 and Olivia of 1994: The effect of vertical shear on upper and lower level positive PV anomaly max- structure and intensity; Mon. Weather Rev. 130 2291– ima. For Hudhud, the evolution of RV shows the 2312. ‘top-down’ dynamics, in which the growth starts Corbosiero K L and Molinari J 2003 The relationship between storm motion, vertical wind shear, and convec- from the middle troposphere and then progresses tive asymmetries in tropical cyclones; J. Atmos. Sci. 60 downwards. As for Helen, the RV growth seems to 366–376. follow ‘bottom-up’ mechanism in which the growth Delden A V 2003 Adjustment to heating, potential vortic- starts from the lower troposphere. The existence of ity and cyclogenesis; Quart. J. Roy. Meteorol. Soc. 129 such a weak mid-level vortex for Helen may have 3305–3322. led to a decay of the storm. For the depression, a DeMaria M and Kaplan J 1999 An updated statistical hurricane intensity prediction scheme (SHIPS) for the Atlantic weak and disorganized mid-level vortex was seen and eastern North Pacific basins; Wea. Forecasting 14 at 24 hr after the genesis. The growth of RV for 326–337. the depression started from the lower troposphere, Dudhia J 1989 Numerical study of convection observed dur- through the middle troposphere which is then fol- ing the winter monsoon experiment using a mesoscale lowed by the beginning of the dissipation of RV two-dimensional model; J. Atmos. Sci. 46 3077–3107. Frank W M and Ritchie E A 2001 Effects of vertical wind from the mid-level altitudes. The ‘top-down’ mech- shear on the intensity and structure of numerically simu- anism does provide sufficient evidences to confirm lated hurricanes; Mon. Weather Rev. 129 2249–2269. the significance of a mid-level vortex in the genesis Gjorgjievska S and Raymond D J 2014 Interaction between phase of a storm. dynamics and thermodynamics during tropical cyclogen- Although, the current work might be able to esis; Atmos. Chem. Phys. 14 3065–3082. provide an insight towards the importance of mid- Halverson J and co-authors 2007 NASA’s tropical cloud systems and processes experiment: Investigating tropical level vortex in the cyclone genesis process, the cyclogenesis and hurricane intensity change; Bull. Am. mechanism of the formation of mid-level vortex Meteorol. Soc. 88 867–882. is still not known. Studies suggest that strong Haynes P and McIntyre M 1987 On the evolution of vorticity advection which occurs over the mid-tropospheric and potential vorticity in the presence of diabatic heating altitudes assimilates excessive moisture in a small and frictional or other forces; J. Atmos. Sci. 44 828–841. Hendricks, E A, Montgomery M T and Davis C A 2004 The region. This eventually develops a localized cir- role of ‘vortical’ hot towers in the formation of tropical culation which initially propagates in the neigh- cyclone Diana (1984); J. Atmos. Sci. 61 1209–1232. bouring regions. When this circulation becomes Hong S-Y, Dudhia J and Chen S-H 2004 A revised approach strong enough to generate a systematically orga- to ice microphysical processes for the bulk parameteriza- nized stratiform precipitation over a large area, the tion of clouds and precipitation; Mon. Weather Rev. 132 extension of the warm core extends downwards cre- 103–120. Hong S-Y, Noh Y and Dudhia J 2006 A new vertical dif- ating a low pressure region and henceforth creating fusion package with an explicit treatment of entrainment a cyclonic vortex. However, no analytical evidences processes; Mon. Weather Rev. 134 2318–2341. are present that would justify this occurrence, thus Hoskins B J, McIntyre M and Robertson A W 1985 On the making them highly feasible for furthering studies use and significance of isentropic potential vorticity maps; in this direction. Quart. J. Roy. Meteorol. Soc. 111 877–946. Mlawer E J, Taubman S J, Brown P D, Iacono M J and Clough S A 1997 Radiative transfer for inhomogeneous atmospheres: RRTM, a validated correlated-k model for Acknowledgements the longwave; J. Geophys. Res. Atmos. 102 16,663– 16,682. This work was supported by funding from Indian Molinari J, Skubis S and Vollaro D 1995 External influences Institute of Space Science and Technology, Thiru- on hurricane intensity. Part III: Potential vorticity structure; J. Atmos. Sci. 52 3593–3606. vananthapuram, India. The authors would like to Molinari J, Skubis S, Vollaro D, Alsheimer F and Willoughby thank Sreejith Nhaloor for providing NCL codes H E 1998 Potential vorticity analysis of tropical cyclone for plotting figures. intensification; J. Atmos. Sci. 55 2632–2644. 94 Page 12 of 12 J. Earth Syst. Sci. (2017) 126:94

Montgomery M, Nicholls M, Cram T and Saunders A 2006 A Skamarock W C, Klemp J B, Dudhia J, Gill D O, Barker vortical hot tower route to tropical cyclogenesis; J. Atmos. D M, Wang W and Powers J G 2005 A description of Sci. 63 355–386. the Advanced Research WRF; version 2, NCAR Technical NCEP 2000 National Centers for Environmental Prediction/ note -468+STR. National Weather Service/NOAA/U.S. Department of Tewari M and Coauthors 2004 Implementation and verifi- Commerce: NCEP FNL Operational Model Global Tro- cation of the unified NOAH land surface model in the pospheric Analyses, continuing from July 1999, Boulder, WRF model; 20th Conference on Weather Analysis and CO, https://doi.org/10.5065/D6M043C6, Research Data Forecasting/16th Conference on Numerical Weather Pre- Archive at the National Center for Atmospheric Research, diction, pp. 11–15. Computational and Information Systems Laboratory. Tory K, Montgomery M and Davidson N 2006 Prediction Raymond D, Sessions S and López Carrillo C 2011 Thermo- and diagnosis of tropical cyclone formation in an NWP dynamics of tropical cyclogenesis in the northwest Pacific; system. Part I: The critical role of vortex enhancement in J. Geophys. Res. Atmos. 116. deep convection; J. Atmos. Sci. 63 3077–3090. Ritchie E A and Holland G J 1997 Scale interactions during Wang Z 2012 Thermodynamic aspects of tropical cyclone the formation of Typhoon Irving; Mon. Weather Rev. 125 formation; J. Atmos. Sci. 69 2433–2451. 1377–1396. Xu Y and Wang Y 2013 On the initial development of asym- Simpson J, Ritchie E, Holland G, Halverson J and Stewart metric vertical motion and horizontal relative flow in a S 1997 Mesoscale interactions in tropical cyclone genesis; mature tropical cyclone embedded in environmental ver- Mon. Weather Rev. 125 2643–2661. tical shear; J. Atmos. Sci. 70 3471–3491. Sippel J A and Zhang F 2008 A probabilistic analysis of Zeng Z, Wang Y and Chen L 2010 A statistical analysis of the dynamics and predictability of tropical cyclogenesis; vertical shear effect on tropical cyclone intensity change J. Atmos. Sci. 65 3440–3459. in the North Atlantic; Geophys. Res. Lett. 37.

Corresponding editor: Kavirajan Rajendran