A Triggering Mechanism for Rapid Intensification of Tropical Cyclones

Triggering processes for the rapidly intensifying phase of a tropical cyclone (TC) were investigated on the basis of numerical experiments using a three-dimensional nonhydrostatic model. The results revealed that the rapid intensification of the simulated TC commenced following the formation of a ci...

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Bibliographic Details
Published in:Journal of the atmospheric sciences 2015-07, Vol.72 (7), p.2666-2681
Main Authors: Miyamoto, Yoshiaki, Takemi, Tetsuya
Format: Article
Language:English
Subjects:
Amplification
Axisymmetric flow
Boundary layers
Circularity
Cloud formation
Clouds
Computer simulation
Convergence
Coriolis parameters
Cyclones
Experiments
Fluid flow
Hurricanes
Mathematical models
Maximum winds
Meteorology
Numerical experiments
Parameters
Rossby number
Rotating fluids
Simulation
Studies
Temperature
Three dimensional models
Tropical cyclones
Velocity
Vortices
Weather
Wind speed
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Summary:Triggering processes for the rapidly intensifying phase of a tropical cyclone (TC) were investigated on the basis of numerical experiments using a three-dimensional nonhydrostatic model. The results revealed that the rapid intensification of the simulated TC commenced following the formation of a circular cloud, which occurred about 12 h after the TC became essentially axisymmetric. The circular cloud (eyewall) evolved from a cloudy convective cell that was originally generated near the radius of maximum wind speed (RMW). The development of the convective cell in the eyewall was closely related to the radial location of the strong boundary layer convergence of axisymmetric flow. The radius of maximum convergence (RMC) was small relative to the RMW when the TC vortex was weak, which is consistent with the boundary layer theory for a rotating fluid system on a frictional surface. As the TC intensified, the RMC approached the RMW. An eyewall was very likely to form in the simulated TC when the RMC approached the RMW. Because the RMC is theoretically determined by a Rossby number defined by the maximum tangential velocity, RMW, and Coriolis parameter, a series of numerical experiments was conducted by changing the three parameters. The results were consistent with the hypothesis that intensification occurs earlier for larger Rossby numbers. This finding indicates that initial TC vortices with larger Rossby numbers are more likely to experience rapid intensification and, hence, to evolve into strong hurricanes.
ISSN:0022-4928
1520-0469
DOI:10.1175/JAS-D-14-0193.1