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An Air-Sea Interaction Theory for Tropical Cyclones
Part I: Steady-State Maintenance
LIU Yan
- Emanuel, K.A. (1986) An Air-Sea Interaction Theory for Tropical Cyclones. Part I: Steady-State Maintenance. Journal of Atmospheric Sciences, 43, 585-605.
Conditional Instability VS Air-Sea Interaction
Which is the key process to the development and maintenance of TC?
Conditional Instability VS Air-Sea Interaction
“entirely by the air-sea interaction”
• Hydrostatic and gradient wind balance (steady-state, cyclone scale)
• Thermodynamically reversible (constant 𝜽↓𝒆↑∗ , M ǁ 𝜽↓𝒆↑∗ )
• Neutrality to slantwise moist convection (no ambient CAPE)
An idealized axisymmetric steady-state model of a mature TC
Central Pressure
Central RH=100% Ambient RH=80%
Temperature Surpluses
How to explain the observed absence of TCs when SST<26℃
Central Pressure
Minimum attainable surface central pressure in September
Central RH=100%
Ambient RH: observation or 78%
• Region 1:Eye (unsaturated) • Region 2:Eyewall (saturated) • Region 3:Constant RH
The Boundary Layer
outer region inner region
Outer region: enhanced evaporation balanced by turbulent flux through BL Inner region: turbulent flux through BL is negligible
r
Z
Analytical Solutions
outer region
inner region
free atmosphere
• Free atmosphere
• Outer region
• Inner region RMW
• An analytic but highly idealized nonlinear axisymmetric tropical cyclone model has been formulated.
• TC can be maintained in an intense steady state without any contribution from ambient conditional instability.
• The absence of TC when SST<26℃ is due to the shallow depth of the conditional neutral or unstable layer.
• The model of TC can be imagined as a simple Carnot heat engine.
• The boundary later physics and the dynamics of the eye have not been accounted for properly.
Conclusions