EIGHTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES

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1 WMO/CAS/WWW EIGHTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES 2.1.1: Cyclogenesis Rapporteur: Anantha Aiyyer Department of Marine, Earth and Atmospheric Sciences North Carolina State University Campus Box 8208, Raleigh, NC USA Phone: Working group members: Zhuo Wang (Univ. Of Illinois, USA), Ron McTaggart- Cowan (Environment Canada), S. K. Roy Bhowmik (SAARC Meteorological Research Centre) Abstract: This report is a survey of theory of tropical cyclone formation. Recent studies have proposed aggregation of vorticity produced by deep convection as a viable pathway for establishing the near-surface cyclonic vortex and inflow that are two hallmarks of tropical cyclone formation. We review some of these studies and place the results in context of previous work on this topic Introduction The formation of a tropical cyclone from preceding cloud clusters is a complex phenomenon, and involves atmospheric and oceanic processes on multiple scales. Only a fraction of cloud clusters that exist over the tropical oceans at any given time eventually transform into a tropical storm and this makes for a particularly intriguing problem to solve. While much has been learned about tropical cyclogenesis over the past decades, many open questions still remain Large scale condition for tropical cyclone formation Tropical cyclogenesisis generally deemed to have occurred when the sustained winds in the developing vortex exceed 17 m/s and a warm core structure is present (e.g., Ritchie and Holland 1999). Several environmental factors influence tropical cyclone formation. Some key factors are summarized below. (a) Ocean heat content Sea surface temperature (SST) and sub-surface stratification are known to control the amount of enthalpy transfer to the air from the ocean to the incipient cyclone (e.g., Vincent et al. 2014). Palmen (1948) showed that hurricanes form over regions where SSTs are greater than 26 C and this is the generally accepted

2 threshold for tropical cyclogenesis (e.g., Gray 1968). Dare and McBride (2011) found that SSTs exceeded 25.5 C for nearly 98% of all tropical cyclones that formed across the globe during and they recommend a value between 25.5 and 26.5 C as a suitable threshold. (b) Vertical wind shear This parameter is typically defined as the magnitude of the vector wind difference between upper and lower levels of the atmosphere. Usually, these levels are taken to be, respectively, 200 hpa and 850 hpa. Large vertical wind shear is generally detrimental to tropical cyclogenesis and intensification (e.g., Gray 1968; Tuleya and Kurihara 1981; DeMaria and Kaplan 1994; Frank and Ritchie 2001; Riemer et al. 2010). Extremely large shear can ventilate the incipient tropical cyclone with low entropy air and suppress deep convection necessary for genesis (e.g., Simpson and Riehl 1958; Tang and Emanuel 2012). Tang and Emanuel (2012) describe an index of ventilation that incorporates vertical shear, mid-level entropy deficit and potential intensity. Fig. 1 shows that tropical cyclogenesis in both hemispheres preferentially occurs in areas of low ventilation index. Fig 1. Average Ventilation index (shaded) over and tropical cyclogenesis locations (black dots) for the same period shown for (a) Northern Hemisphere July-October; and (b) Southern Hemisphere December-March. Tang and Emanuel (2012) The composite study of McBride and Zehr (1981) found zero vertical wind shear directly over the pre-tropical cyclone location and large westerly shear poleward. While climatological studies clearly show that tropical cyclones form preferentially in areas of low shear, a clear threshold that separates developing and nondeveloping tropical cyclone is not easy to determine. For instance, Zehr (1992) did not find a significantly different mean shear for developing and non-developing tropical cyclones over the western Pacific. In practice, values of shear above m/s are typically considered as being large enough to preclude tropical cyclone formation

3 The precise role of the shear in organizing convection within the incipient tropical cyclone, especially when placed in context of other environmental factors such as mid-level moisture and interaction of the shear with the parent synoptic scale precursor, still needs to be better understood. The question whether zero shear is the most favourable environment for tropical cyclogenesis still remains to be settled and recent studies have begun to re-examine this. There is growing evidence that low to moderate wind shear may favour tropical cyclogenesis by organizing convection that helps to develop the low level circulation (e.g., Nolan and McGauley 2012). (c) Pre-existing cyclonic circulation Fig. 2. Percentage of storms attributed to each wave type using thresholds of 2 mm day 1 (white bars) and 4 mm day 1 (gray bars). Red lines indicate the 99% significance levels (From Schreck et al. 2012). It was recognized early on that tropical cyclones are invariably linked to precursor disturbances that have low-level cyclonic vorticity (e.g., Gray 1968). Some common precursors to tropical cyclones include tropical easterly waves, equatorial waves such as equatorial Rossby and mixed-rossby gravity waves, the Madden-Julian Oscillation (MJO), and monsoon gyres. Upper-level troughs of midlatitude origin can also induce lowlevel cyclones that can transform into tropical cyclones (e.g., Bosart and Bartlo 1981; McTaggart-Cowan et al. 2008). Schreck et al examined tropical cyclogenesis in association with equatorial waves. They used estimated rainfall data from the Tropical Rainfall Measuring Mission (TRMM) to detect prominent equatorial waves using the filtering method of Wheeler and Kiladis (1999). The two main categories of equatorial waves most often associated with tropical cyclones were easterly waves and equatorial Rossby waves. (Fig. 2) Over the north Indian, south Indian, and western North Pacific basins, the MJO was also connected to tropical cyclone formation

4 Fig. 3. Simulated outgoing longwave radiation (OLR) at (a) day 10 and (b) day 80 in radiative convective equilibrium. From Wing and Emanuel (2014) While tropical cyclogenesis is typically associated with a preexisting precursor disturbance, one question that has arisen in recent years is whether it could also occur spontaneously. Recent studies have examined the self-aggregation of moist convection (e.g., Tompkins 2001; Bretherton et al. 2005; Nolan et al. 2007; Tobin et al. 2012; Emanuel et al. 2013; Wing and Emanuel 2014). The findings of these studies suggest that when the SST is sufficiently high and vertical shear is low, moist convection can self-aggregate even in the absence of background rotation (e.g. Fig. 3). When background rotation is present, the aggregating convection can transform into a tropical cyclone at a lower critical temperature. Khairoutdinov and Emanuel (2010) and Wing and Emanuel (2014) report that the critical temperature at which self-aggregation occurs is close to the peak sea surface temperatures found in the tropics. As noted by Muller and Held (2012) and Emanuel et al. (2014), the closeness of the critical temperature to the peak tropical temperature in the current climate could be the reason why self-aggregation of convection, and potentially tropical cyclone formation, are so sensitive to physical parameters in observations and numerical models. The aggregation of moist convection is found to be also associated with drying of the troposphere outside the region of convection. This, in effect, may act as a regulator of tropical climate via radiative feedbacks Mechanism of Tropical Cyclone Formation Tropical cyclogenesis is generally understood to be a multi-scale process. Earlier viewpoints (e.g., Zehr 1993; McBride 1995; Karyampudi and Pierce 2002) emphasized a two-stage process with environmental preconditioning followed by mesoscale development. Recent studies (e.g., Dunkerton et al. 2009) have begun to further refine this viewpoint with the recognition that tropical storm formation is a continuous process. It involves scales that are both larger and smaller than the tropical cyclone. The environmental factors highlighted in the previous section

5 represent the large- and synoptic-scale end of the spectrum. Individual thunderstorms, gust fronts and downdraughts, along with turbulent eddies, comprise the other end of the spectrum. The study of Dunkerton et al. (2009) envisions tropical cyclogenesis as a meeting point of downscale enstrophy cascade and upscale transfer of energy (Fig. 4). This approach offers a continuum view of tropical cyclogenesis than what may be implied by the two-stage process. Fig. 4. Dunkerton et al. (2009) view of tropical cyclogenesis as a meeting point between downscale and upscale cascades (from Montgomery and Smith (2012) Downscale cascade Fig. 5: Modelling ITCZ breakdown in a shallow water model. Potential vorticity at initial time (top panel) and after 10 days (bottom panel) showing a zonally uniform strip breaking up into a series of cyclonic eddies. Figure adapted from Ferreira and Schubert (1997)

6 A precursor to tropical cyclone such as an easterly wave is much larger in scale than the pre-tropical cyclone proto-vortex. The transformation of the quasi-linear wave to a nonlinear vortex remains to be fully understood and is an area of active research. The reduction in scale of the parent wave can be achieved through wave energy accumulation (e.g. Shaprio 1977; Sobel and Bretherton 1999) or eddy shedding. The latter pathway includes barotropic instability of the intertropical convergence zone (e.g., Ferreira and Schubert 1997; Wang and Magnusdottir 2005) and wave-breaking of easterly waves (e.g., Lindzen 1974; Dunkerton et al. 2009; Tyner and Aiyyer 2013) Upscale Cascade Although tropical cyclones can develop in different synoptic-scale environments in different ocean basins, it is generally believed that their formation proceeds through essentially the same mesoscale evolution. That is, the mesoscale aspects of the upscale energy cascade leading to a tropical cyclone-scale vortex are likely to be universally valid. There are, however, several conceptual gaps that remain with regard to the precise pathway through which this vortex is established. There are two groups of theories regarding the development of the tropical cyclone proto-vortex near the surface. In recent literature, these two are often referred to as the top-down development and the bottom-up development. (a) The top-down mechanism The top-down view of tropical cyclogenesis includes both dynamic and thermodyamic aspects. In one proposed pathway, the merger of multiple mid-level vortices in the stratiform precipitation region can create a vortex of a stronger intensity and a larger spatial scale (Simpson et al. 1997; Ritchie and Holland 1997). The downward extension of the vortex engenders a cyclonic circulation near the surface, which is assumed to further intensify through the wind-induced surface heat exchange (WISHE) mechanism. Bister and Emanuel (1997) examined the formation of a tropical cyclone from a mesoscale convective system (MCS) and proposed a mechanism for generation of a low-level vortex (Fig. 6). Herein, stratiform rain from the MCS spins up a midlevel vortex. The mid-level vortex forms in response to the vertical gradient of heating in the MCS associated with latent heating in the anvil and evaporative cooling below. The cold anomaly associated with the MCS descends towards the boundary layer. Saturation of the low level ensues and this prevents further evaporatively driven downdraughts. Deep convection develops and a surface based warm core vortex forms. Evidence for downward building of the protovortex was also provided by Raymond et al. (1998). They showed that the level of maximum vertical mass flux progressively descended down and inflow became concentrated at low-levels. Recent studies have further emphasized that a midlevel vortex, with the associated cold core in the lower troposphere and warm core in the upper troposphere, modifies the thermodynamic environment and makes the vertical mass flux profile more bottom-heavy, which is favourable for the intensification of the low-level circulation (e.g., Raymond and Sessions 2007; Davis and Ahijevych 2013)

7 Fig. 6. Tropical cyclogenesis from an MCS as proposed by Bister and Emanuel. Figure from Bister and Emanuel (1987) (b) The bottom-up mechanism The bottom-up theory proposed by Montgomery and colleagues (Hendricks et al. 2004; Montgomery et al. 2006) emphasizes the role of vortical hot towers (VHTs), or rotating cumulonimbus clouds in tropical cyclone formation. Although the lifetime of convective updrafts is of the order of one hour, convective vorticity anomalies last much longer. It is suggested that VHTs and their vortical remnants are the essential building blocks for the tropical cyclone proto-vortex and that VHTs collectively act as a quasi-steady heat source for driving the transverse circulation. The associated low-level convergence concentrates vorticity and intensifies the system-scale circulation. The top-down development emphasizes the importance of stratiform processes and a mid-level vortex in initiating the surface cyclonic circulation, while the bottom-up development emphasizes the critical role of deep convection and the associated low-level convergence. While low-level convergence is the most effective way to intensify the surface circulation, it is worth noting that the bottomup and top-down development routes may not be mutually exclusive (Tory and Frank 2010). Different theories may be more relevant in different regions, and

8 vorticity evolution at different spatial scales may take different vertical development routes (Wang 2012). Besides, recent studies also suggest that other modes of moisture convection, besides VHTs, produce a wide range of vorticity anomalies and contribute to genesis (Fang and Zhang 2011; Kilroy and Smith 2013; Wang 2014). In particular, cumulus congestus is proposed to play a dominant role in moistening the lower and middle troposphere and preconditioning the atmosphere for the transition to sustained deep convection and genesis (Wang 2014) The meeting point: Meso-alpha scale In the continuum view of tropical cyclogenesis, an important milestone for the upscale and downscale cascades is the meso-alpha scale which bridges the large scale and tropical cyclone scale processes. The establishment of this intermediate scale of closed circulation has been proposed as a key step for tropical cyclogenesis (Dunkerton et al. 2009). Over the Atlantic, the majority of the tropical cyclones originate from tropical easterly waves (Landsea 1993). Recent studies (e.g., Dunkerton et al. 2009; Wang et al. 2009; Montgomery et al. 2010) suggested that a region of approximately closed Lagrangian circulation within the wave critical layer (i.e., the Kelvin cat s eye) plays an important role in tropical cyclone formation. The quasi-closed circulation occurs about the trough axis inside the wave s critical layer, which forms from the nonlinear interaction between the wave and the flow around the critical latitude (Fig. 7). As demonstrated by both observational diagnoses (e.g., Wang et al. 2009; Wang et al. 2012a; Raymond and Carrillo 2011; Montgomery et al. 2012) and numerical model simulations (Wang et al. 2010a, b; Montgomery et al. 2010b; Fang and Zhang 2010; Fritz and Wang 2013), the quasi-closed circulation provides a favourable environment for vorticity aggregation and convective organization leading to tropical cyclone formation. In addition, it protects moist convection inside from dry air intrusion to some extent. Fig 7. A Conceptual representation of the wave pouch (Wang et al. 2010). This conceptual model is labelled the marsupial paradigm as the cyclogenesis sequence is likened to the development of a marsupial infant in its mother s

9 pouch. The cat s eye within the wave critical layer is dubbed the wave s pouch or simply pouch. It was also shown that the pouch center is the preferred location for genesis due to its low strain rate and high moisture content. In particular, Wang (2012) found that the mid-level equivalent potential temperature increases several degrees one to two days prior to genesis in the inner pouch region but changes little away from the pouch center. The role of mid-level moistening in tropical cyclogenesis was also emphasized by Nolan (2007). Although the marsupial paradigm was originally proposed for tropical easterly waves, subsequent studies (Wang et al. 2012b; Montgomery et al. 2010a) show that it is also valid for tropical cyclogenesis associated with other types of waves. The formation of the wave pouch, perhaps a necessary condition for wave to tropical cyclone transition, however may not be sufficient to ensure tropical cyclone formation. For instance, in a case study of the case examined by Fritz and Wang (2013) upper level dry air intrusion prevented a tropical cyclone from forming despite the existence of a low-level wave pouch. The search for robust distinguishing characteristics that separate developing and non-developing systems will likely involve careful investigation of the structural characteristics of the wave pouch and attendant large-scale factors Environmental baroclinicity The impact of environmental baroclinicity on tropical cyclogenesis is not thoroughly understood. The tropical transition (TT) formation pathway proposed by Davis and Bosart (2003, 2004) is a very direct example of baroclinic involvement, wherein a midlatitude precursor disturbance transforms into a TC. This and other more subtle influences were found by McTaggart-Cowan et al. (2008) to be implicated in more than half of development events in the North Atlantic basin. Using satellite imagery, Payne and Methven (2012) confirmed the presence of upper-level baroclinic troughs during the majority of tropical cyclone formations in the southern Indian Ocean. In a global climatology of baroclinically influenced tropical cyclogenesis, McTaggart-Cowan et al. (2013) found that approximately 30% of developments occur in the presence of environmental baroclinic structures, although this value is strongly region-dependent. The basin with the broadest spectrum of baroclinic pathways is the North Atlantic, although over 40% of storms forming in the northern Indian Ocean are also influenced by baroclinic features. They demonstrated the importance of the tropical upper tropospheric troughs (TUTTs) in creating environments favourable to the various baroclinically influenced tropical cyclone development pathways. Non-baroclinic environments are most frequently encountered in the equatorial region, and baroclinic environments are found near coastal regions, weak TT environments occur preferentially along the TUTT axes. To the west of the TUTTs, peaks in the frequency of the trough-induced environment are associated with the migration of isolated TUTT cells (upper cold lows). Combining the environment climatology with observed tropical cyclone developments, McTaggart-Cowan et al. (2013) recast the results of their study in terms of upper- and lower-level baroclinicity to

10 demonstrate that the optimal formation environment is characterized by the presence of a baroclinic feature aloft (Fig. 8 below). This finding supports and extends previous investigations (e.g. Montgomery and Shaprio, 1993; Bracken and Bosart, 2000) wherein the reduced stability and quasigeostrophic forcing for ascent associated with a cold upper-level feature are found to favour tropical cyclone development. Fig. 8. Normalized yield (efficiency) of TC formation plotted as a function of upper-level quasigeostrophic forcing for ascent (abscissa) and lower-level baroclinicity (ordinate). The yield is computed as the ratio of development events to climatological prevalence of each point in this plane, normalized by the mean yield of the climatology. The background shading represents the development pathway (classified in the space shown here), and the squares represent the class centroids. Reproduced from McTaggart-Cowan et al. (2013) Bibliography Bister, M., and K. A. Emanuel, 1997: The genesis of Hurricane Guillermo: TEXMEX analyses and a modeling Bracken, W. E., and L. F. Bosart, 2000: The role of synoptic-scale flow during tropical cyclogenesis over the North Atlantic Ocean. Mon. Wea. Rev., 128, Bretherton, C., P. Blossey, and M. Khairoutdinov (2005), An energy-balance analysis of deep convective self-aggregation above uniformsst, J. Atmos. Sci., 62, Dare, R. A. and J. L. McBride, 2011: The Threshold Sea Surface Temperature Condition for Tropical Cyclogenesis. J. Climate, 24, Davis, C. A., and D. A. Ahijevych, 2013: Thermodynamic Environments of Deep Convection in Atlantic Tropical Disturbances. J. Atmos. Sci., 70, Davis, C., and L. F. Bosart, 2003: Baroclinically induced tropical cyclogenesis. Mon. Wea. Rev., 131, Davis, C., and L. F. Bosart, 2004: The TT problem: Forecasting the tropical transition of cyclones. Bull. Amer. Meteor. Soc., 85, DeMaria, M. and John Kaplan, 1994: A Statistical Hurricane Intensity Prediction Scheme (SHIPS) for the Atlantic Basin. Wea. Forecasting, 9,

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13 Tory, K. J., and W. M. Frank, 2010: Tropical cyclone formation. Global Perspectives on Tropical Cyclones, 2nd ed. J. Chan and J. D. Kepert, Eds., World Scientific, Tuleya, R. E. and Y. Kurihara, 1981: A Numerical Study on the Effects of Environmental Flow on Tropical Storm Genesis. Mon. Wea. Rev., 109, Tyner, B. P., and A. Aiyyer, 2012: Evolution of African easterly waves in isentropic potential vorticity fields. Mon. Wea. Rev., 140, Vincent E.M., K. A. Emanuel, M. Lengaigne, J. Vialard, G. Madec, 2014: Influence of upper-ocean stratification interannual variability on Tropical Cyclones, J. Adv. Model. Earth Syst. Wang Z., T. J. Dunkerton, and M. T. Montgomery, 2012b: Application of the marsupial paradigm to tropical cyclogenesis from northwestward propagating disturbances. Mon. Wea. Rev., 140, Wang, Z., 2012: Thermodynamic aspects of tropical cyclone formation. J. Atmos. Sci., 69, Wang, Z., 2014: Role of Cumulus Congestus in Tropical cyclogenesisin a High- Resolution Numerical Model Simulation. J. Atmos. Sci., 71, Wang, Z., M. T. Montgomery, and T. J. Dunkerton, 2009: A dynamically-based method for forecasting tropical cyclogenesis location in the Atlantic sector using global model products. Geophys. Res. Lett., 36, L03801, doi: /2008gl Wang, Z., M. T. Montgomery, and T. J. Dunkerton, 2010a: Genesis of Pre- Hurricane Felix (2007). Part I: The role of the easterly wave critical layer. J. Atmos. Sci., 67, Wang, Z., M. T. Montgomery, and T. J. Dunkerton, 2010b: Genesis of Pre- Hurricane Felix (2007). Part II: Warm core formation, precipitation evolution, and predictability. J. Atmos. Sci., 67, Wang,Z., M. T. Montgomery, and C. Fritz, 2012a: A first look at the structure of the wave pouch during the 2009 PREDICT- GRIP dry runs over the Atlantic. Mon. Wea. Rev., 4,