Overview of Tropical Cyclones

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Overview of Tropical Cyclones AOS 453 April
2004 J. P. Kossin CIMSS/UW-Madison
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HOT
?
COOLER
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• Genesis
• Easterly waves ? African easterly jet ? hot
  Sahara vs cool temps along coast of Gulf of
  Guinea coast ? reversal of meridional PV gradient
  ? combined barotropic-baroclinic instability.
• April-October. Period 3-4 days. ? 2000-2500
  km. N 60/year.

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Genesis may also be instigated by local
baroclinic or upper-level trough forcing along
southermost remnants of fronts. Or perhaps
through barotropic instability of ITCZ (Ferreira
and Schubert 1997).
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Persistent convection diabatically produces PV
and forms Mesoscale Convective Vortices (MCV) in
the mid-levels. Multiple vortices are formed
within tropical cloud clusters. Mid-level vortex
? cold-core system (tangential wind increases
with height). Tropical cyclone ? warm-core. How
does the conversion occur? Modified
Rossby-Burger-Prandtl relationship Vertical
influence ? D (flocal ? a)1/2 L / N Merger
(self organization)
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• Environmental requirements (necessary conditions)
  for genesis
• Warm water SST gt 26.5C (80F)
• Low vertical wind shear (10m/s bottom to top)
• Ambient rotation ( f ) - off equator.
• Moist mid-levels.

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Surface swirling flow ? How does disturbance
amplify? Intensification Conditional Instability
of the Second Kind (CISK) has fallen out of
favor. Convergence related mechanism. Downdrafts
kill moist energy of boundary layer. Convergence
is inefficient at raising air to LFC. Wind
Induced Surface Heat Exchange (WISHE).
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Real CISK

• Scale-dependent feedback from cumulus to system
  by
• momentum forcing
• thermal forcing
• Response of system to cumulus
• Thermal field (mass) adjusts to momentum forcing
  (LltLR , disturbance is smaller than local Rossby
  radius)
• Wind field (momentum) adjusts to thermal (mass)
  forcing (LgtLR)

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Effects of Heating (Global)
When Q represents the diabatic latent heat
release of convection, this is sometimes called
"up moist down dry"
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Where does the warming occur? Not so easy.....
Axisymmetric Dynamics
From Hack and Schubert 1986
Local Response to Local Heating Linear vs.
Nonlinear
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Efficiency There is a nonlinear feedback
mechanism at work. The more intense the local
swirling flow is, the more efficiently the
heating can warm locally. More local warming
increases pressure gradients which further
intensifies the local flow. This can be studied
in the context of an axisymmetric balance model
(Schubert and Hack 1982).

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Heating efficiency as function of inertial
stability
Transverse Circulation
Warming
Less inertial stability
More inertial stability
Schubert and Hack 1982
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Real CISK(continued)

• Since the heating of cumulus projects on to
  multiple scales on either side of LR, a multiple
  of responses to cumulus occur some gravity and
  some rotational.
• Because the properties of the rotational response
  are so different from the gravity wave response,
  the evolving system can be complex.
• Normally, the system is defined by a slow
  mesoscale response that defines the system
  organization over time.

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Slant-Wise Convection

• Two competing stabilities present in the
  atmosphere
• 1. Static Stability (vertical planes)
• 2. Inertial Stability (horizontal planes)
• Stability in one plane limits instability in the
  other
• Both stabilities are represented by gradients of
  a conservative potential

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Slant-Wise Convection(continued)

• There is free movement relative to a particular
  stability along iso-lines of constant potential.
• There is stability induced oscillation for
  movement perpendicular to iso-lines of constant
  potential.

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Slant-Wise Convection(continued)

• The potential for dry static stability is
  potential temperature (q)
• The potential for moist static stability
  (saturated air) is equivalent potential
  temperature(qe)
• The potential for inertial stability is angular
  momentum given by where
  y is the radius from the center of rotation.

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Slant-Wise Convection(continued)

• Lines of constant (q) are usually horizontal but
  dip downward (due to thermal wind balance) into
  the center of a cyclonic vortex whose strength
  decreases with height (warm core) and rise upward
  into the center of vortex whose strength
  increases with height (cold core).
• Lines of constant inertial stability (m) are
  usually vertical, but tilt away from the center
  of a cyclonic warm core vortex because of the
  thermal wind effect and vice versa in a cold core
  vortex.

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Slant-Wise Convection(continue)

• Hence if we have a saturated warm core vortex,
  neutral inertial upward movement (movement along
  an m surface) experiences less static stability
  than pure vertical upward movement .
• Likewise, neutral horizontal movement along a q
  surface, experiences less inertial stability than
  pure horizontal movement
• If vortex is strong enough momentum lines and q
  lines can cross, creating static instability
  along m surfaces or inertial instability along q
  surfaces (isentropes).

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Slant-Wise Convection(continued)

• Hence convection erupting up the tilted momentum
  surface is called slant-wise convection
• Slant-wise convection is due to symmetric
  instability or inertial instability relative to
  the symmetric vortex that defines the radius of
  curvature for the momentum lines.

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Conditional Symmetric Instability

• Conditional Instability along a momentum m
  surface, ie condition for slantwise moist
  convection
• Alternative way of looking at the same thing
  Inertial Instability along a theta_e surface

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Tropical cyclone structure
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