Role of Precipitation in Tropical Cyclone Lifecycle and Structure

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Role of Precipitation in Tropical Cyclone
Lifecycle and Structure

• Christopher Hennon
• March 4, 2005

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Outline

• Tropical cyclone structure
• Primary and secondary circulation
• Carnot cycle
• Role of precipitation in tropical cyclone
  lifecycle
• Recent observations
• Hot Towers
• Convective Bursts

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Tropical Cyclone Structure
Outflow Anti-cyclonic, Exhaust
Eyewall Strongest winds, heaviest rainfall
Eye Generally calm, little if any precip
Rainbands Heavy precip, Quasi-stationary
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Hurricane Isabel (2003) 12 September
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Precipitation Structure from TRMM Satellite
(Lili, 2002)
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(No Transcript)
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Hurricane as a Carnot Heat Engine

• 1) Isothermal Expansion
• Large heat flux from ocean into inflowing air
  offsets adiabatic expansion
• 2) Adiabatic Expansion (moist)
• Rising air in eyewall
• 3) Isothermal Compression
• Sinking outflow radiates energy to space
• 4) Adiabatic Compression

Efficiency of Cycle Given By (Tocean Ttrop) /
Tocean
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Adiabatic Expansion
Isothermal Compression
Adiabatic Compression
Isothermal Expansion
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Thermal Structure

• Tropical Cyclones are warm
• core systems
• Core warming due to eye
• subsidence and latent heat
• release from eyewall
• precipitation
• Warm core drives the
• synoptic circulation

Bonnie (1998) AMSU derived cross section (Zhu et
al. 2002)
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Role Of Latent Heat

• There is a net transfer of energy from the ocean
  into the atmosphere
• Latent heat release in the eyewall provides
  positive feedback
• Eyewall becomes more buoyant, creates lower
  pressure, enhances inflow
• Localized strong precipitation may lead to rapid
  changes in intensity

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Cold wake from Bonnie (1998)
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Role of Precipitation in Tropical Cyclone
Lifecycle

• Tropical Cyclogenesis
• Theory still undeveloped
• Important developments
• Favorable large-scale environment must be in
  place
• Smaller-scale convection moistens and heats the
  larger-scale developing circulation
• Bursts of intense convection lead to mesoscale
  convective vortex formation (precursor for
  genesis)

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Tropical Disturbance
- Weak background vorticity - Convection
may be strong but no coherent
structure - Latent heating may be carried
away by gravity waves - ?E values
increasing - Precipitation highly
asymmetric
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Tropical Depression

• Closed surface circulation
• Convection may be more
• symmetrical
• Convection deeper
• Banding in precipitation
• becomes evident
• Latent heat trapped by
• increase in strength and
• contraction of vortex

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Tropical Storm
Damry (2000) 7-May 0835 UTC
Damry (2000) 7-May 1819 UTC

• Convection intensifies
• Convective bursts (above) lead to
  intensification/possible
• eyewall formation
• - Rain rates may approach 100 mm/hr

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Typhoon (Hurricane)

• Development of eye and
• mature eyewall
• Precipitation usually
• highly symmetric in inner
• core region (esp. in low
• wind shear)
• Will maintain or increase
• in intensity as long as
• energy source is available
• and dynamical factors
• support it

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Dissipation

• May be due to one or more
• factors
• 1) Cut off from energy source
• 2) High wind shear
• Highly asymmetric
• precipitation
• Weaker convection
• May make transition to
• extratropical

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New Developments in Tropical Cyclone
Precipitation Structure and Intensity
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Hot Tower Theory

• Hot Towers Undiluted, concentrated updrafts
  that redistribute latent heat high into the
  troposphere
• Lowers surface pressure, increases inflow and
  frictional convergence
• Theory developed in late 1950s (Malkus and
  Riehl), confirmed by recent observations

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Lili (2002)
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Convective Bursts

• A mesoscale (i.e. 100 km x hours) system
  consisting of a cluster of high cumulonimbus
  towers within the inner core region that
  approaches or reaches the tropopause, with nearly
  undiluted towers
• Rodgers et al. 2000
• Large mass flux into upper troposphere
• Storm intensity usually increases
• Connection to latent heat?

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Hurricane Gladys (1968) First picture of a
convective burst
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Convective Bursts and TC Intensity
May 7 0027 Z

• Damry (WPAC,
• 2000).
• Winds increased
• 50 kt. during this
• time

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Summary

• Vigorous rainfall a necessary but not sufficient
  condition for tropical cyclogenesis
• Transfer of latent energy from ocean to tropical
  cyclone drives the circulation and is one factor
  that affects intensity change
• New observation platforms (e.g. TRMM) are
  providing new insights into precipitation
  structure and its relationship to TC lifecycle
• Hot Towers, Convective Bursts