ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-5587-2009 Tropical cyclogenesis in a tropical wave critical layer: easterly waves Dunkerton T. J. 1 2 Montgomery M. T. 2 Wang Z. 2 NorthWest Research Associates, Bellevue WA, USA Naval Postgraduate School, Monterey CA, USA 06 08 2009 9 15 5587 5646 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/9/5587/2009/acp-9-5587-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/5587/2009/acp-9-5587-2009.pdf

The development of tropical depressions within tropical waves over the Atlantic and eastern Pacific is usually preceded by a "surface low along the wave" as if to suggest a hybrid wave-vortex structure in which flow streamlines not only undulate with the waves, but form a closed circulation in the lower troposphere surrounding the low. This structure, equatorward of the easterly jet axis, is identified herein as the familiar critical layer of waves in shear flow, a flow configuration which arguably provides the simplest conceptual framework for tropical cyclogenesis resulting from tropical waves, their interaction with the mean flow, and with diabatic processes associated with deep moist convection. The recirculating Kelvin cat's eye within the critical layer represents a sweet spot for tropical cyclogenesis in which a proto-vortex may form and grow within its parent wave. A common location for storm development is given by the intersection of the wave's critical latitude and trough axis at the center of the cat's eye, with analyzed vorticity centroid nearby. The wave and vortex live together for a time, and initially propagate at approximately the same speed. In most cases this coupled propagation continues for a few days after a tropical depression is identified. For easterly waves, as the name suggests, the propagation is westward. It is shown that in order to visualize optimally the associated Lagrangian motions, one should view the flow streamlines, or stream function, in a frame of reference translating horizontally with the phase propagation of the parent wave. In this co-moving frame, streamlines are approximately equivalent to particle trajectories. The closed circulation is quasi-stationary, and a dividing streamline separates air within the cat's eye from air outside. The critical layer equatorward of the easterly jet axis is important to tropical cyclogenesis because its cat's eye provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the developing gyre and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. The entire sequence is likened to the development of a marsupial infant in its mother's pouch. These ideas are formulated in three new hypotheses describing the flow kinematics and dynamics, moist thermodynamics and wave/vortex interactions comprising the "marsupial paradigm". A survey of 55 named tropical storms in 1998–2001 reveals that actual critical layers sometimes resemble the ideal east-west train of cat's eyes, but are usually less regular, with one or more recirculation regions in the co-moving frame. It is shown that the kinematics of isolated proto-vortices carried by the wave also can be visualized in a frame of reference translating at or near the phase speed of the parent wave. The proper translation speeds for wave and vortex may vary with height owing to vertical shear and wave-vortex interaction. Some implications for entrainment/containment of vorticity and moisture in the cat's eye are discussed from this perspective, based on the observational survey.

 References Aiyyer, A. R. and Molinari, J.: Evolution of mixed Rossby-gravity waves in idealized MJO environments, J. Atmos. Sci., 60, 2837–2855, 2003. Aiyyer, A. and Molinari, J.: MJO and tropical cyclogenesis in the Gulf of Mexico and Eastern Pacific: case study and idealized numerical modeling, J. Atmos. Sci., 65, 2691–2704, 2008. Alexander, M. J. and Dunkerton, T. J.: A spectral parameterization of mean-flow forcing due to breaking gravity waves, J. Atmos. Sci., 56, 4167–4182, 1999. Andrews, D. G. and McIntyre, M. E.: An exact theory of nonlinear waves on a Lagrangian mean flow, J. Fluid Mech., 89, 609–646, 1978. Andrews, D. G., Holton, J. R., and Leovy, C. B.: Middle Atmosphere Dynamics, Academic Press, 504 pp, 1987. Bassom, A. P. and Gilbert, A. D.: The spiral wind-up and dissipation of vorticity and a passive scalar in a strained planar vortex, J. Fluid Mech., 398, 245–270, 1999. Bender, M.: Simulation of intense Atlantic hurricane activity in a twenty-first century warmed climate, using the GFDL high-resolution, coupled hurricane model, 3rd Workshop on High-resolution and Cloud Modeling Tropical Cyclones and Climate, University of Hawaii at Manoa, 2008. Bengtsson, L., Hodges, K. I., Esch, M., Keenlyside, N., Kornblueh, L., Luo, J.-J., and Yamagata, T.: How may tropical cyclones change in a warmer climate?, Tellus, 59A, 539–561, 2007. Berry, G. J. and Thorncroft, C.: Case study of an intense African easterly wave, Mon. Weather Rev., 133, 752–766, 2005. Bessafi, M. and Wheeler, M. C.: Modulation of South Indian Ocean tropical cyclones by the Madden-Julian oscillation and convectively coupled equatorial waves, Mon. Weather Rev., 134, 638–656, 2006. Bister, M. and Emanuel, K. A.: The genesis of Hurricane Guillermo: TEXMEX analyses and a modeling study, Mon. Weather Rev., 125, 2662–2682, 1997. Black, P. G., Katsaros, K. B., Drennan, W. M., Lehner, S., and Vachon, P. W.: Interpretation of SAR-observed boundary layer flow structures in hurricanes. NASA Final Report, Work Order No. W-19-835, 58 pp, 2005. Borth, H.: Fundamental circulation modes in moist rotating atmospheres. Talk presented at a workshop on tropical dynamics, University of Mainz, Mainz DE, 2007. Bosart, L.: The tropical transition of hurricane Alex (2004): An observational perspective, 27th Conference on Hurricanes and Tropical Meteorology, American Meteorological Society, Monterey CA, 2006. Bracken, W. E. and Bosart, L. F.: The role of synoptic-scale flow during tropical cyclogenesis over the North Atlantic Ocean, Mon. Weather Rev., 128, 353–376, 2000. Bretherton, C. S., Peters, M. E., and Back, L. E.: Relationships between water vapor path and precipitation over the tropical oceans, J. Climate, 17, 1517–1528, 2004. Brill, O. L. and Goodman, B.: Causality in the Coulomb gauge, Am. J. Phys., 35, 832–837, 1967. Brunet, G. and Warn, T.: Rossby wave critical layers on a jet, J. Atmos. Sci., 47, 1173–1178, 1990. Brunet, G. and Haynes, P.: The nonlinear evolution of disturbances to a parabolic jet, J. Atmos. Sci., 52, 464–477, 1995. Burpee, B.: Characteristics of North African easterly waves during the summers of 1968 and 1969, J. Atmos. Sci., 31, 1556–1570, 1974. Carlson, T. N.: Synoptic histories of three African disturbances that developed into Atlantic hurricanes, Mon. Weather Rev., 97, 256–276, 1969. Chang, C. P., Morris, V. F., and Wallace, J. M.: A statistical study of easterly waves in the western Pacific: July–December 1964, J. Atmos. Sci., 27, 195–201, 1970. Chang C.-P., Liu, C.-H., and Kuo, H.-C.: Typhoon Vamei: an equatorial tropical cyclone formation, Geophys. Res. Lett., 30(3), 50–53, 2003. Crum, F. X. and Dunkerton, T. J.: Analytic and numerical models of wave-CISK with conditional heating, J. Atmos. Sci., 49, 1693–1708, 1992. Crum, F. X. and Dunkerton, T. J.: CISK and evaporation-wind feedback with conditional heating on an equatorial beta-plane, J. Meteorol. Soc. Japan, 72(1), 11–18, 1994. Davis, C. A. and Bosart, L. F.: Numerical simulations of the genesis of Hurricane Diana (1984). Part I: control simulation, Mon. Weather Rev., 129, 1859–1881, 2001. Davis, C. A. and Bosart, L. F.: The TT problem: forecasting the tropical transition of cyclones, B. Am. Meteorol. Soc., 85, 1657–1662, 2004. DeMaria, M., Mainelli, M., Shay, L. K., Knaff, J. A., and Kaplan, J.: Further improvements to the Statistical Hurricane Intensity Prediction Scheme (SHIPS), Wea. Forecasting, 20, 531–543, 2005. Dickinson, M. and Molinari, J.: Mixed Rossby–gravity waves and western Pacific tropical cyclogenesis. Part I: synoptic evolution, J. Atmos. Sci., 59, 2183–2196, 2002. Doblas-Reyes, F. J. and Déqué, M.: A flexible bandpass filter design procedure applied to midlatitude intraseasonal variability, Mon. Weather Rev., 126, 3326–3335, 1998. Dunion, J. P. and Velden, C. S.: The impact of the Saharan Air Layer on Atlantic tropical cyclone activity, B. Am. Meteorol. Soc., 85, 353–365, 2004. Dunkerton, T. J.: Intensity variation and coherence of 3-6 day equatorial waves, Geophys. Res. Lett., 18(8), 1469–1472, 1991. Dunkerton, T. J.: Observation of 3–6-day meridional wind oscillations over the tropical Pacific, 1973–1992: vertical structure and interannual variability, J. Atmos. Sci., 50, 3292–3307, 1993. Dunkerton, T. J.: Evidence of meridional motion in the summer lower stratosphere adjacent to monsoon regions, J. Geophys. Res., 100(D8), 16675–16688, 1995. Dunkerton, T. J.: A tale of two ITCZs – the Jim Holton perspective, B. Am. Meteorol. Soc., 87, 1492–1495, 2006. Dunkerton, T. J. and Baldwin, M. P.: Observation of 3–6-day meridional wind oscillations over the tropical Pacific, 1973–1992: horizontal structure and propagation, J. Atmos. Sci., 52, 1585–1601, 1995. Dunkerton, T. J. and Crum, F. X.: Eastward propagating $\sim $2- to 15-day equatorial convection and its relation to the tropical intraseasonal oscillation, J. Geophys. Res., 100(D12), 25781–25790, 1995. Dunkerton, T. J. and Scott, R. K.: A barotropic model of the angular-momentum conserving potential vorticity staircase in spherical geometry, J. Atmos. Sci., 65, 1105–1136, 2008. Dunkerton, T. J., Lussier III, L. L., Montgomery, M. T., Wang, Z., and Tory, K. J.: Spatial and statistical distribution of convective and stratiform clouds in the gyre-pouch of incipient tropical cyclones, J. Geophys Res., in preparation, 2009a. Dunkerton, T. J., Walter, B. A., Perrie, W., Long, D. G., Nie, C., Zhang, J., Rogers, R., Uhlhorn, E., and Black, P.: Images of Hurricane Katrina (2005) below the cloud, Science, in preparation, 2009b. Edson, R. T. and Lander, M.: Characteristics of the early stages of tropical cyclones as viewed with microwave data, AMS 27th Conference on Hurricanes and Tropical Meteorology, Monterey CA, 2006. Emanuel, K. A.: The finite-amplitude nature of tropical cyclogenesis, J. Atmos. Sci., 46, 3431–3456, 1989. Emanuel, K. A.: The physics of tropical cyclogenesis over the eastern Pacific, in: Tropical Cyclone Disasters: Proceedings of ICSU/WMO International Symposium, edited by: Lighthill, J. Zhemin, Z., Holland, G. J., and Emanuel, K., Peking University Press, 136–142, 1993. Emanuel, K. A.: Some aspects of hurricane inner-core dynamics and energetics, J. Atmos. Sci., 54, 1014–1026, 1997. Emanuel, K. A.: Increasing destructiveness of tropical cyclones over the past 30 years, Nature, 436, 686–688, 2005. Emanuel, K. A. and Nolan, D. S.: Tropical cyclone activity and the global climate system. Talk presented at the AMS 26th Conference on Hurricanes and Tropical Meteorology, Miami, FL, 2004. Emanuel, K. A.: Divine Wind: The History and Science of Hurricanes, Oxford University Press, New York, 285 pp, 2005. Emanuel, K. A.: The hurricane embryo. Talk presented at short program workshop entitled \textitSmall scales and extreme events: The Hurricane, NSF Institute for Pure and Applied Mathematics (IPAM), UCLA, 2007. Eliassen, A.: Slow thermally or frictionally controlled meridional circulation in a circular vortex, Astrophys. Norv., 5, 19–60, 1951. Eliassen, A.: On the Ekman layer in a circular vortex, J. Meteorol. Soc. Japan, 49, 784–789, 1971. Eliassen, A. and Lystad, M.: The Ekman layer of a circular vortex: A numerical and theoretical study, Geophys. Norv., 31, 1–16, 1977. Ferreira, R. N. and Schubert, W. H.: Barotropic aspects of ITCZ breakdown, J. Atmos. Sci., 54, 261–285, 1997. Frank, N. L.: Atlantic tropical systems of 1969, Mon. Weather Rev., 98, 307–314, 1970. Frank, N. L. and Johnson, H. M.: Vortical cloud systems over the tropical Atlantic during the 1967 hurricane season, Mon. Weather Rev., 97, 124–129, 1969. Frank, W. M. and Roundy, P. E.: The role of tropical waves in tropical cyclogenesis, Mon. Weather Rev., 134, 2397–2417, 2006. Frank, W. M. and Young, G. S.: The interannual variability of tropical cyclones, Mon. Weather Rev., 135, 3587–3598, 2007. Gentry, R. C., Fujita, T. T., and Sheets, R. C.: Aircraft, spacecraft, satellite and radar observations of Hurricane Gladys, 1968, J. Appl. Meteor., 9, 837–850, 1970. Goldenberg, S. B. and Shapiro, L. J.: Physical mechanisms for the association of El Niño and West African rainfall with Atlantic major hurricane activity, J. Climate, 9, 1169–1187, 1996. Gore, A.: An Inconvenient Truth, Paramount Classics, US gross revenue to date: $49,047,567, 2006. Gray, W. M.: Atlantic seasonal hurricane frequency. Part I: El Niño and 30 mb Quasi-Biennial Oscillation influences, Mon. Weather Rev., 112, 1649–1668, 1984a. Gray, W. M.: Atlantic seasonal hurricane frequency. Part II: forecasting its variability, Mon. Weather Rev., 112, 1669–1683, 1984b. Gray, W. M.: The formation of tropical cyclones, Meteorol. Atmos. Phys., 67, 37–69, 1998. Green, M. A., Rowley, C. W., and Haller, G.: Detection of Lagrangian coherent structures in 3D turbulence, J. Fluid Mech., 572, 111–120, 2007. Grotjahn, R.: Different data, different general circulations? A comparison of selected fields in NCEP/DOE AMIP-II and ECMWF ERA-40 Reanalyses, Dyn. Atmos. Oceans, 44, 108–142, 2008. Hack, J. J. and Schubert, W. H.: Nonlinear response of atmospheric vortices to heating by organized cumulus convection, J. Atmos. Sci., 43, 1559–1573, 1986. Hack, J. J., Schubert, W. H., Stevens, D. E., and Kuo, H.-C.: Response of the Hadley circulation to convective forcing in the ITCZ, J. Atmos. Sci., 46, 2957–2973, 1989. Hall, N. M. J., Kiladis, G. N., and Thorncroft, C. D.: Three-dimensional structure and dynamics of African easterly waves. Part II: dynamical modes, J. Atmos. Sci., 63, 2231–2245, 2006. Haller, G. and Yuan, G.: Lagrangian coherent structures and mixing in two-dimensional turbulence, Physica D, 147, 352–370, 2000. Halverson, J., Black, M., Braun, S., et al.: NASA's Tropical Cloud Systems and Processes Experiment, B. Am. Meteorol. Soc., 88, 867–882, 2007. Harr, P. A. and Elsberry, R.: Structure of a mesoscale convective system embedded in typhoon Robyn during TCM-93, Mon. Weather Rev., 124, 634–652, 1996. Hartmann, D. L., Hendon, H. H., and Houze, R. A.: Some implications of the mesoscale circulations in tropical cloud clusters for large-scale dynamics and climate, J. Atmos. Sci., 41, 113–121, 1984. Hayashi, Y.: A theory of large-scale equatorial waves generated by condensation heat and accelerating the zonal wind, J. Meteorol. Soc. Japan, 48(2) 140–160, 1970. Heifetz, E., Bishop, C. H., and Alpert, P.: Counter-propagating Rossby waves in the barotropic Rayleigh model of shear instability, Q. J. Roy. Meteorol. Soc., 125, 2835–2853, 1999. Held, I. M.: Rotating radiative-convective equilibria at low resolution in large domains, Talk presented at short program workshop entitled \textitSmall scales and extreme events: The Hurricane, NSF Institute for Pure and Applied Mathematics (IPAM), UCLA, 2007. Held, I. M. and Ting, M.: Orographic versus thermal forcing of stationary waves: the importance of the mean low-level wind, J. Atmos. Sci., 47, 495–500, 1990. Hendon, H. H. and Liebmann, B.: The structure and annual variation of antisymmetric fluctuations of tropical convection and their association with Rossby-gravity waves, J. Atmos. Sci., 48, 2127–2140, 1991. Hendricks, E. A., Montgomery, M. T., and Davis, C. A.: On the role of "vortical" hot towers in the formation of tropical cyclone Diana, J. Atmos. Sci., 61, 1209–1232, 2004. Hendricks, E. A. and Montgomery, M. T.: Rapid scan views of convectively generated mesovortices in sheared tropical cyclone Gustav (2002), Wea. Forecasting, 21, 1041–1050, 2006. Herring, J. R. and McWilliams, J. C.: Lecture notes on turbulence from the NCAR-GTP summer school, June 1987. Lecture notes by Douglas Lilly, p. 171–218, World Scientific, ISBN 9971-50-805-2, 1989. Hidalgo, J. M.: Vortical hot towers, their aggregate effects and their resolution dependence in the formation of Hurricane Diana (1984), PhD dissertation, Colorado State University, 184 pp., 2007. Holton, J. R.: An Introduction to Dynamic Meteorology, Elsevier Academic Press, 4th Ed., 535 pp., 2004. Hoskins, B. J., McIntyre, M. E., and Robertson, A. W.: On the use and significance of isentropic potential vorticity maps, Q. J. Roy. Meteorol. Soc., 111, 877–946, 1985. Houze Jr., R. A.: Observed structure of mesoscale convective systems and implications for large-scale heating, Q. J. Roy. Meteorol. Soc., 115, 425–461, 1989. Ide, K., Kuznetsov, L., and Jones, C. K. R. T.: Lagrangian data assimilation for point-vortex systems, J. Turbulence, 3, 053, 1–7, 2002. IPCC: Climate Change 2007, Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change, 2007. Julian, P. R.: Objective analysis in the tropics: a proposed scheme, Mon. Weather Rev., 112, 1752–1767, 1984. Katsaros, K. B., Vachon, P. W., Black, P. G., Dodge, P. P., and Uhlhorn, E. W.: Wind fields from SAR: could they improve our understanding of storm dynamics?, Johns Hopkins APL Tech. Digest, 21(1), 86–93, 2000. Kiladis, G. N., Thorncroft, C. D., and Hall, N. M. J.: Three-dimensional structure and dynamics of African easterly waves. Part I: observations, J. Atmos. Sci., 63, 2212–2230, 2006. Killworth, P. D. and McIntyre, M. E.: Do Rossby-wave critical layers absorb, reflect or overreflect?, J. Fluid Mech., 161, 449–492, 1985. Kimball, S. K. and Mulekar, M. S.: A 15-year climatology of North Atlantic tropical cyclones. Part I: size parameters, J. Climate, 17, 3555–3575, 2004. Knutson, T. R.: High resolution modeling of hurricanes in a climate context, presentation at AGU Fall Meeting, San Francisco, 2007. Knutson, T. R., Sirutis, J. J., Garner, S. T., Held, I. M., and Tuleya, R. E.: Simulation of the recent multidecadal increase of Atlantic hurricane activity using an 18-km-grid regional model, B. Am. Meteorol. Soc., 88, 1549–1565, 2007. Knutson, T. R., Sirutis, J. J., Garner, S. T., Vecchi, G. A., and Held, I. M.: Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions, Nature Geoscience, 1, 359–364, doi:10.1038/ngeo202, 2008. Krishnamurti, T. N., Bedi, H. S., Oosterhof, D., and Hardiker, V.: The formation of Hurricane Frederic (1979), Mon. Weather Rev., 122, 1050–1074, 1994. Kurgansky, M. V.: Helicity production and maintenance in a baroclinic atmosphere, Meteorologische Zeitschrift, 15, 409–416, 2006. Lapeyre, G.: Topologie du mélange dans un fluide turbulent géophysique, Doctoral Thesis, Univ. of Paris, 204 pp., 2000. Lau, K.-H. and Lau, N.-C.: Observed structure and propagation characteristics of tropical summertime synoptic scale disturbances, Mon. Weather Rev., 118, 1888–1913, 1990. LeMone, M. A., Zipser, E. J., and Trier, S. B.: The role of environmental shear and thermodynamic conditions in determining the structure and evolution of mesoscale convective systems during TOGA COARE, J. Atmos. Sci., 55, 3493–3518, 1998. Liebmann, B. and Hendon, H. H.: Synoptic-scale disturbances near the equator, J. Atmos. Sci., 47, 1463–1479, 1990. Lighthill, M. J.: Waves in Fluids, Cambridge University Press, 504 pp., 1978. Lilly, D. K.: Stratified turbulence and the mesoscale variability of the atmosphere, J. Atmos. Sci., 40, 749–761, 1983. Lilly, D. K.: The structure and propagation of rotating convective storms. Part I: energy exchange with the mean flow, J. Atmos. Sci., 43, 113–125, 1986a. Lilly, D. K.: The structure and propagation of rotating convective storms. Part II: helicity and storm stabilization, J. Atmos. Sci., 43, 126–140, 1986b. Lin, J., Mapes, B., Zhang, M., and Newman, M.: Stratiform precipitation, vertical heating profiles, and the Madden-Julian Oscillation, J. Atmos. Sci., 61, 296–309, 2004. Lindzen, R. S.: Wave-CISK in the tropics, J. Atmos. Sci., 31, 156–179, 1974. Lindzen, R. S. and Tung, K. K.: Wave overreflection and shear instability, J. Atmos. Sci., 35, 1626–1632, 1978. Lindzen, R. S., Farrell, B. F., and Rosenthal, A.: Absolute barotropic instability and monsoon depressions, J. Atmos. Sci., 40, 1178–1184, 1983. Lukovich, J. V. and Shepherd, T. G.: Stirring and mixing in two-dimensional divergent flow, J. Atmos. Sci., 62, 3933–3954, 2005. Magnusdottir, G.: The modeled response of the mean winter circulation to zonally averaged SST trends, J. Climate, 14, 4166–4190, 2001. Maloney, E. D. and Dickinson, M. J.: The intraseasonal oscillation and the energetics of summertime tropical western North Pacific synoptic-scale disturbances, J. Atmos. Sci., 60, 2153–2168, 2003. Maloney, E. D. and Hartmann, D. L.: Modulation of eastern North Pacific hurricanes by the Madden-Julian oscillation, J. Climate, 13, 1451–1460, 2000. Mandelbrot, B. B.: Intermittent turbulence in self-similar cascades: divergence of high moments and dimension of the carrier, J. Fluid Mech., 62, 331–358, 1974. Mapes, B. E. and Houze, R. A.: Diabatic divergence profiles in western Pacific mesoscale convective systems, J. Atmos. Sci., 52, 1807–1828, 1995. Matsuno, T.: Kelvin waves, internal gravity waves and their behaviors in numerical models, Talk presented at the 86th AMS Annual Meeting, Atlanta GA, 2006. Matsuno, T.: Modeling tropical convective cloud systems by a very high resolution atmosphere model NICAM. Talk presented at ESSL, National Center for Atmospheric Research, Boulder CO, 2007. McBride, J. L. and Zehr, R.: Observational analysis of tropical cyclone formation. Part II: comparison of non-developing versus developing systems, J. Atmos. Sci., 38, 1132–1151, 1981. McIntyre, M. E.: On the Antarctic ozone hole, J. Atmos. Terr. Phys., 51, 2973–2994, 1989. McIntyre, M. E.: Isentropic distributions of potential vorticity and their relevance to tropical cyclone dynamics, Tropical Cyclone Disasters: Proceedings of ISCU/WMO International Symposium, edited by: Lighthill, J. Zhemin, Z., Holland, G. J., and Emanuel, K., Peking University Press, 143–156, 1993. McIntyre, M. E.: On global-scale atmospheric circulations, in: Perspectives in Fluid Dynamics: A Collective Introduction to Current Research, edited by: Batchelor, G. K., Moffatt, H. K., and Worster, M. G., Cambridge, University Press, paperback edition, 557–624, 2003. McWilliams, J. C.: The emergence of isolated coherent vortices in turbulent flow, J. Fluid Mech., 140, 21–43, 1984. McWilliams, J. C. and Flierl, G. R.: On the evolution of isolated, nonlinear vortices, J. Phys. Oceanogr., 9, 1155–1182, 1979. Moffat, H. K.: The degree of knottendness of tangled vortex lines, J. Fluid Mech., 35, 117–129, 1969. Molinari, J.: Paradigms of tropical cyclogenesis, B. Am. Meteorol. Soc., 85, 662–663, 2004. Molinari, J., Skubis, S., and Vollaro, D.:~External influences on hurricane intensity. Part III: potential vorticity structure, J. Atmos. Sci., 52, 3593–3606, 1995. Molinari, J., Knight, D., Dickinson, M., Vollaro, D., and Skubis, S.: Potential vorticity, easterly waves, and eastern Pacific tropical cyclogenesis, Mon. Weather Rev., 125, 2699–2708, 1997. Molinari, J., Vollaro, D., Skubis, S., and Dickinson, M.: Origins and mechanisms of eastern Pacific tropical cyclogenesis: a case study, Mon. Weather Rev., 128, 125–139, 2000. Molinari, J. and Vollaro, D.: Planetary- and synoptic-scale influences on eastern Pacific tropical cyclogenesis, Mon. Weather Rev., 128, 3296–3307, 2000. Molinari, J., Vollaro, D., and Corbosiero, K. L.: Tropical cyclone formation in a sheared environment: a case study, J. Atmos. Sci., 61, 2493–2509, 2004. Molinari, J., Dodge, P., Vollaro, D., Corbosiero, K. L., and Marks, F.: Mesoscale aspects of the downshear reformation of a tropical cyclone, J. Atmos. Sci., 63, 341–354, 2006. Molinari, J., Lombardo, K., and Vollaro, D.: Tropical cyclogenesis within an equatorial Rossby wave packet, J. Atmos. Sci., 64, 1301–1317, 2007. Montgomery, M. T. and Farrell, B. F.: Tropical cyclone formation, J. Atmos. Sci., 50, 285–310, 1993. Montgomery, M. T. and Kallenbach, R. J.: A theory for vortex Rossby waves and its application to spiral bands and intensity changes in hurricanes, Q. J. Roy. Meteorol. Soc., 123, 435–465, 1997. Montgomery, M. T., Moller, J. D., and Nicklas, C. T.: Linear and nonlinear vortex motion in an asymmetric balance shallow water model, J. Atmos. Sci., 56, 749–768, 1999. Montgomery, M. T., Bell, M. M., Aberson, S. D., and Black, M. L.: Hurricane Isabel (2003): new insights into the physics of intense storms. Part I: mean vortex structure and maximum intensity estimates, B. Am. Meteorol. Soc., 87, 1335–1347, 2006a. Montgomery, M. T., Nicholls, M. E., Cram, T. A., and Saunders, A. B.: A vortical hot tower route to tropical cyclogenesis, J. Atmos. Sci., 63, 355–386, 2006b. Montgomery, M. T., Nguyen, V. S., and Smith, R. K.: Do tropical cyclones intensify by WISHE?, Q. J. Roy. Meteorol. Soc., in press, 2009. Montgomery, M. T., Wang, Z., and Dunkerton, T. J.: Intermediate and high-resolution simulations of the transition of a tropical wave critical layer to a tropical depression, Atmos. Chem. Phys. Discuss., in review, 2009. Montgomery, M. T.: Interactions of tropical waves and diabatic vortices in tropical cyclone formation, NASA-TCSP/NAMMA workshop, Invited plenary presentation, Baltimore, Maryland, May 2007. Moore, R. W. and Montgomery, M. T.: Re-examining the dynamics of short-scale, diabatic Rossby waves and their role in midlatitude moist cyclogenesis, J. Atmos. Sci., 61, 754–768, 2004. Moore, R. W. and Montgomery, M. T.: An idealized three-dimensional analysis of the diabatic Rossby vortex: A coherent structure of the moist baroclinic atmosphere, J. Atmos. Sci., 62, 2703–2725, 2005. Moore, R. W. and Montgomery, M. T.: Dynamics of the 24–25 February 2005 cyclone, 13th Cyclone Workshop, Pacific Grove CA, 2006. Moulin, F. Y. and Flor, J. B.: Experimental study on wave breaking and mixing properties in the periphery of an intense vortex, Dyn. Atmos. Oceans, 40, 115–130, 2005. Nolan, D. S., Rappin, E. D., and Emanuel, K. A.: Could hurricanes form from random convection in a warmer world?, AMS 27th Conference on Hurricanes and Tropical Meteorology, 2006. Ooyama, K.: Numerical simulation of the life cycle of tropical cyclones, J. Atmos. Sci., 26, 3–40, 1969. Panetta, R. L.: Zonal jets in wide baroclinically unstable regions: persistence and scale selection, J. Atmos. Sci., 50, 2073–2106, 1993. Pierrehumbert, R.: Local and global baroclinic instability of zonally varying flow, J. Atmos. Sci., 41, 2141–2162, 1984. Pozrikidis, C.: Introduction to Theoretical and Computational Fluid Dynamics, Oxford University Press, 675 pp, 1997. Randel, W. J.: Upper tropospheric equatorial waves in ECMWF analyses, Q. J. Roy. Meteorol. Soc., 118, 365–394, 1992. Raymond, D. J., López-Carrillo, C., and López Cavazos, L.: Case-studies of developing east Pacific easterly waves, Q. J. Roy. Meteorol. Soc., 124, 2005–2034, 1998. Raymond, D. J., Bretherton, C. S., and Molinari, J.: Dynamics of the Intertropical Convergence Zone of the East Pacific, J. Atmos. Sci., 63, 582–597, 2006. Raymond, D. J. and Sessions, S. L.: Evolution of convection during tropical cyclogenesis, Geophys. Res. Lett., 34, L06811, doi:10.1029/2006GL028607, 2007. Reasor, P. D. and Montgomery, M. T.: Three dimensional alignment and corotation of weak, TC-like vortices via linear vortex Rossby waves, J. Atmos. Sci., 58, 2306–2330, 2001. Reasor, P. D., Montgomery, M. T., and Grasso, L. D.: A new look at the problem of tropical cyclones in vertical shear flow: vortex resiliency, J. Atmos. Sci., 61, 3–22, 2004. Reed, R. J., Norquist, D. C., and Recker, E. E.: The structure and properties of African wave disturbances as observed during phase III of GATE, Mon. Weather Rev., 105, 317–333, 1977. Ritchie, E. A. and Holland, G. J.: Scale interactions during the formation of Typhoon Irving, Mon. Weather Rev., 125, 1377–1396, 1997. Ritchie, E. A. and Holland, G. J.: Large-scale patterns associated with tropical cyclogenesis in the western Pacific, Mon. Weather Rev., 127, 2027–2043, 1999. Ritchie, E. A., Simpson, J., Liu, W. T., Halverson, J., Velden, C., Brueske, K. F., and Pierce, H.: Present day satellite technology for hurricane research: a closer look at formation and intensification, Hurricane! Coping with disaster: progress and challenges since Galveston, 1900, AGU monograph, 359 p., 2003. Rotunno, R. and Emanuel, K. A.: An air-sea interaction theory for tropical cyclones. Part II: Evolutionary study using a nonhydrostatic axisymmetric numerical model, J. Atmos. Sci., 44, 542–561, 1987. Rozoff, C. M., Schubert, W. H., McNoldy, B. D., and Kossin, J. P.: Rapid filamentation zones in intense tropical cyclones, J. Atmos. Sci., 63, 325–340, 2006. Schecter, D. A., Montgomery, M. T., and Reasor, P. D.: A theory for the vertical alignment of a quasi-geostrophic vortex, J. Atmos. Sci., 59, 150–168, 2002. Schecter, D. A. and Dunkerton, T. J.: Hurricane formation in diabatic Ekman turbulence, Q. J. Roy. Meteorol. Soc., 135(641), 823–838, 2009. Schubert, W. H. and Hack, J. J.: Transformed Eliassen balanced vortex model, J. Atmos. Sci., 40, 1571–1583, 1983. Schubert, W. H., Ciesielski, P. E., Stevens, D. E., and Kuo, H.-C.: Potential vorticity modeling of the ITCZ and the Hadley circulation, J. Atmos. Sci., 48, 1493–1509, 1991. Schumacher, A. B., DeMaria, M., and Knaff, J. A.: Objective estimation of the 24-h probability of tropical cyclone formation, Wea. Forecasting, 24, 456–471, 2009. Shapiro, L. J.: Tropical storm formation from easterly waves: a criterion for development, J. Atmos. Sci., 34, 1007–1022, 1977. Shapiro, L. J.: The effect of nonlinearities on the evolution of barotropic easterly waves in a nonuniform environment, J. Atmos. Sci., 37, 2631–2643, 1980. Shapiro, L. J.: The asymmetric boundary layer flow under a translating hurricane, J. Atmos. Sci., 40, 1984–1998, 1983. Shapiro, L. J. and Willoughby, H. E.: The response of balanced hurricanes to local sources of heat and momentum, J. Atmos. Sci., 39, 378–394, 1982. Sharp, R. J., Bourassa, M. A., and O'Brien, J. J.: Early detection of tropical cyclones using Seawinds-derived vorticity, B. Am. Meteorol. Soc., 83, 879–889, 2002. Simpson, R. H., Frank, N., Shideler, D., and Johnson, H. M.: Atlantic tropical disturbances, 1967, Mon. Weather Rev., 96, 251–259, 1968. Simpson, J., Ritchie, E., Holland, G. J., Halverson, J., and Stewart, S.: Mesoscale interactions in tropical cyclone genesis, Mon. Weather Rev., 125, 2643–2661, 1997. Smith, R. K.: The surface boundary layer of a hurricane, Tellus, 20, 473–484, 1968. Smith, R. K., Montgomery, M. T., and Vogl, S.: A critique of Emanuel's hurricane model and potential intensity theory, Q. J. Roy. Meteorol. Soc., 134, 551–561, 2008. Sobel, A. H. and Bretherton, C. S.: Development of synoptic-scale disturbances over the summertime tropical northwest Pacific, J. Atmos. Sci., 56, 3106–3127, 1999. Sobel, A. H., Nilsson, J., and Polvani, L. M.: The weak temperature gradient approximation and balanced tropical moisture waves, J. Atmos. Sci., 58, 3650–3665, 2001. Takayabu, Y. N. and Nitta, T.: 3–5 day-period disturbances coupled with convection over the tropical Pacific Ocean, J. Meteorol. Soc. Japan, 71(2), 221–246, 1993. Thorncroft, C. D. and Hoskins, B. J.: An idealized study of African easterly waves. I: a linear view, Q. J. Roy. Meteorol. Soc., 120, 953–982, 1994a. Thorncroft, C. D. and Hodges, K.: African easterly wave variability and its relationship to Atlantic tropical cyclone activity, J. Climate, 14, 1166–1179, 2001. Thorncroft, C. D., Parker, D. J., Burton, R. R., et al.: The JET2000 project: aircraft observations of the African easterly jet and African easterly waves, B. Am. Meteorol. Soc., 84, 337–351, 2003. Tory, K. J. and Montgomery, M. T.: Internal influences on tropical cyclone~formation. Topic 2.2 in: Sixth International Workshop on Tropical Cyclones, San Jose, Costa Rica, World Meteorological Organization, 22 pp, 2006. Tory, K. J., Montgomery, M. T., and Davidson, N. E.: Prediction and diagnosis of tropical cyclone formation in an NWP system. Part I: the critical role of vortex enhancement in deep convection, J. Atmos. Sci., 63, 3077–3090, 2006a. Tory, K. J., Montgomery, M. T., Davidson, N. E., and Kepert, J. E.: Prediction and diagnosis of tropical cyclone formation in a NWP system. Part II: a diagnosis of tropical cyclone Chris formation, J. Atmos. Sci., 63, 3091–3113, 2006b. Tory, K. J., Davidson, N. E., and Montgomery, M. T.: Prediction and diagnosis of tropical cyclone formation in an NWP system. Part III: diagnosis of developing and non-developing storms, J. Atmos. Sci., 64, 3195–3213, 2007. Tuleya, R. E. and Kurihara, Y.: A numerical study on the effects of environmental flow on tropical storm genesis, Mon. Weather Rev., 109, 2487–2506, 1981. Van Sang, N., Smith, R., and Montgomery, M. T.: Tropical cyclone intensification and predictability in three dimensions, Q. J. Roy. Meteorol. Soc., 134, 563–582, 2008. Vecchi, G. A. and Soden, B. J.: Increased tropical Atlantic wind shear in model projections of global warming, Geophys. Res. Lett., 34, L08702, doi:10.1029/2006GL028905, 2007. Velden, C. S., Hayden, C. M., Nieman, S. J., Menzel, W. P., Wanzong, S., and Goerss, J. S.: Upper-tropospheric winds derived from geostationary satellite water vapor observations, B. Am. Meteorol. Soc., 78, 173–195, 1997. Velden, C., Daniels, J., Stettner, D., et al.: Recent innovations in deriving tropospheric winds from meteorological satellites, B. Am. Meteorol. Soc., 86, 205–223, 2005. Ventham, J. D. and Wang, B.: Large-scale flow patterns and their influence on the intensification rates of Western North Pacific tropical storms, Mon. Weather Rev., 135, 1110–1127, 2007. Vimont, D. J. and Kossin, J. P.: The Atlantic Meridional Mode and hurricane activity Geophys. Res. Lett., 34, L07709, doi:10.1029/2007GL029683, 2007. Wang, C.-C. and Magnusdottir, G.: ITCZ breakdown in three-dimensional flows, J. Atmos. Sci., 62, 1497–1512, 2005. Wang, C.-C. and Magnusdottir, G.: The ITCZ in the central and eastern Pacific on synoptic time scales, Mon. Weather Rev., 134, 1405–1421, 2006. Webster, P. J. and Holton, J. R.: Cross-equatorial response to middle-latitude forcing in a zonally varying basic state, J. Atmos. Sci., 39, 722–733, 1982. Weisman, M. L. and Rotunno, R.: The use of vertical wind shear versus helicity in interpreting supercell dynamics, J. Atmos. Sci., 57, 1452–1472, 2000. Wernli, H. and Kenzelmann, P.: Diabatic Rossby waves: Aspects of their dynamics, climatology and predictability, 13th Cyclone Workshop, Pacific Grove CA, 2006. Wheeler, M. and Kiladis, G. N.: Convectively coupled equatorial waves: analysis of clouds and temperature in the wavenumber-frequency domain, J. Atmos. Sci., 56, 374–399, 1999. Wheeler, M., Kiladis, G. N., and Webster, P. J.: Large-scale dynamical fields associated with convectively coupled equatorial waves, J. Atmos. Sci., 57, 613–640, 2000. Wilson, J. D. and Mak, M.: Tropical response to lateral forcing with a latitudinally and zonally nonuniform basic state, J. Atmos. Sci., 41, 1187–1201, 1984. Wing, A. A., Sobel, A. H., and Camargo, S. J.: Relationship between the potential and actual intensities of tropical cyclones on interannual time scales, Geophys. Res. Lett., 34, L08810, doi:10.1029/2006GL028581, 2007. Wirth, V. and Dunkerton, T. J.: A unified perspective on the dynamics of axisymmetric hurricanes and monsoons, J. Atmos. Sci., 63, 2529–2547, 2006. Wong, S. and Dessler, A. E.: Suppression of deep convection over the tropical North Atlantic by the Saharan Air Layer, Geophys. Res. Lett., 32, L09808, doi:10.1029/2004GL022295, 2005. Zehnder, J. A., Powell, D. M., and Ropp, D. L.: The interaction of easterly waves, orography, and the Intertropical Convergence Zone in the genesis of eastern Pacific tropical cyclones, Mon. Weather Rev., 127, 1566–1585, 1999. Zhang, C. and Webster, P. J.: Laterally forced equatorial perturbations in a linear model. Part I: stationary transient forcing, J. Atmos. Sci., 49, 585–607, 1992. Zhu, Y. and Newell, R. E.: A proposed algorithm for moisture fluxes from atmospheric rivers, Mon. Weather Rev., 126, 725–735, 1998.