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: Tropical Cyclones

Section 6.4: Theories for Genesis : CISK

Moisture Convergence Rules!

See Notes

Section 6.4: Theories for Genesis : WISHE

A nice summary paper of the basics

See also:

American Scientist 1988

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 : WISHE

Section 6.4: Theories for Genesis: Discussion

See: Emanuel, K.A., 1989:  The finite-amplitude nature of tropical cyclogenesis.  J. Atmos. Sci., 46, 3431-3456.

Why don’t we see more tropical cyclones?

Section 6.4: Theories for Genesis : Discussion

Can WISHE account for observed genesis which is always 3D initially?

September 6, 06Z

Hurricane Jeanne

August 3, 12Z

Hurricane Charley

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 smallpenetration depth: HR=fL/N)

Section 6.4: Theories for Genesis : Discussion

Pre-WISHE Theories or Tropical Depression Formation

(ii) “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 viewHendricks 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.