Section 6: Tropical Cyclones

1
Section 6 Tropical Cyclones
6.4 Theories for Genesis CISK WISHE Discussion
Resources http//www.aoml.noaa.gov/hrd/tcfaq/tcf
aqHED.html http//wind.mit.edu/emanuel/ http//hu
rricanes.noaa.gov/ http//www.nhc.noaa.gov/pastall
.shtml http//www.bom.gov.au/bmrc/pubs/tcguide/glo
ba_guide_intro.htm
2
Section 6 Tropical Cyclones
3
Section 6.4 Theories for Genesis CISK
Moisture Convergence Rules! See Notes
4
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.
5
Section 6.4 Theories for Genesis WISHE
Large-Scale View of the Tropics
6
Section 6.4 Theories for Genesis WISHE
What is the energy source for hurricanes?
7
Section 6.4 Theories for Genesis WISHE
8
Section 6.4 Theories for Genesis Discussion
Why dont we see more tropical cyclones?
See Emanuel, K.A., 1989  The finite-amplitude
nature of tropical cyclogenesis.  J. Atmos. Sci.,
46, 3431-3456.
9
Section 6.4 Theories for Genesis Discussion
Can WISHE account for observed genesis which is
always 3D initially?
Hurricane Jeanne
Hurricane Charley
September 6, 06Z
August 3, 12Z
10
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)

11
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
HRfL/N)
12
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)

13
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.
14
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.