Section 6: Tropical Cyclones 6.4 Theories for Genesis CISK WISHE Discussion Resources: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html http://wind.mit.edu/~emanuel/ http://hurricanes.noaa.gov/ http://www.nhc.noaa.gov/pastall.shtml http://www.bom.gov.au/bmrc/pubs/tcguide/globa_guide_intro.htm
Section 6.4: Theories for Genesis : CISK Moisture Convergence Rules! See Notes
Section 6.4: Theories for Genesis : WISHE American Scientist 1988 A nice summary paper of the basics See also: Emanuel, K.A., 1986: An air-sea interaction theory for tropical cyclones. Part I: Steady state maintenance. J. Atmos. Sci., 43, 585-604. Rotunno, R., and K.A. Emanuel, 1987: An air-sea interaction theory for tropical cyclones, Part II: Evolutionary study using axisymmetric nonhydrostatic numerical model. J. Atmos. Sci., 44, 542-561.
Section 6.4: Theories for Genesis : WISHE Large-Scale View of the Tropics
Section 6.4: Theories for Genesis : WISHE What is the energy source for hurricanes?
Section 6.4: Theories for Genesis: Discussion Why don’t we see more tropical cyclones? See: Emanuel, K.A., 1989: The finite-amplitude nature of tropical cyclogenesis. J. Atmos. Sci., 46, 3431-3456.
Section 6.4: Theories for Genesis : Discussion Can WISHE account for observed genesis which is always 3D initially? Hurricane Charley Hurricane Jeanne August 3, 12Z September 6, 06Z
Section 6.4: Theories for Genesis : Discussion • What are the key factors in the “Pre-WISHE” phase? • Elimination of downdrafts through moistening at mid-levels • Competition among multiple pre-hurricane vortices (MCVs)
Section 6.4: Theories for Genesis : Discussion Pre-WISHE Theories or Tropical Depression Formation (i) “Top-down”: Vortex in mid-troposphere somehow affects the low- level circulation Bister and Emanuel (1997) Advection of MCV-vorticity downwards in a moist environment Ritchie and Holland (1999) Merging of several MCVs, inducing winds at low-levels interpreted using PV-thinking and “action-at-a-distance” concepts (importance of small penetration depth: HR=fL/N)
Section 6.4: Theories for Genesis : Discussion • Pre-WISHE Theories or Tropical Depression Formation • “Bottom-up”: Vorticity increases at low-levels via vortex stretching and can be interpreted by considering PV production in a region of heating gradients in the vertical. • Raymond et al (1998)/ Case studies that start to promote this view • Hendricks et al (2004) • Montgomery et al (2006) Vortical Hot Towers; Low-level PV generated via latent heating. Modeling and theory describe how VHTs, interact with the background vorticity field to provide a finite amplitude low-level seedling. • Important to note that VHTs are smaller scale (~10km) than MCVs (~100km)
Section 6.4: Theories for Genesis : Discussion We still do not have a good idea a priori which AEW will likely trigger a TD or TC Some recent work by Dunkerton et al (2007) has proposed a “marsupial paradigm” hypothesis: AEWs posess “sweet spots” within them - regions where the seedlings are somewhat shielded from the detrimental affects of vertical shear and entrainment of dry air. This happens in the critical layer where the wave speed matches the environmental flow speed.
Section 6.4: Theories for Genesis : Discussion Tropical Cyclogensis is viewed as a two-stage process: Stage I: Generation of a seedling (Vortex generation, merger, interactions with the environment) A background state characterised by high vorticity and high humidity is favorable for this but the details are still up for debate Stage II: Consensus is that a WISHE feedback mechanism is operating.