Insights from the Prototype Hurricane Problem: Asymmetries and WISHE

CIRA Director’s Conference Room
Tuesday, June 17, 2014 - 10:30
Naval Postgraduate School
John Persing
Hosted By: 
John Knaff
The prototype problem for hurricane evolution (environmentally “benign”, favorable for convective development without vertical shear, with an axisymmetric initial vortex and idealized microphysics), while much more simple than reality, plays a dual role in further understanding hurricanes.  First, many important theories for understanding hurricanes can be posed in this simple framework.  Second, when tested, many interesting complications arise, which present opportunities for greater understanding.  Presented first is a detailed study of the role of asymmetries in hurricane intensity.  Common thought suggests a spin down role for asymmetries, but the vertical component of asymmetries in the eyewall (i.e. convection) are found to be commonly of a spin-up effect and during key moments of intensification even the horizontal components of eddies lead to storm spin-up.  Context for this result is presented using new analyses with derived from the eyewall heating rates.  Certain key features of 3D spin-up via asymmetries could not easily be incorporated in analog axisymmetric models, as present parameterizations via K-theory diffusion could not possible represent these processes.
Wind-Induced Surface Heat Exchange (WISHE) is often invoked in various manners to explain hurricane intensification, especially in non-specialist media such as the AMS glossary and Wikipedia.  WISHE describes how surface heat exchange with the ocean increases with surface wind speed.  We find that WISHE is not an essential pathway for tropical cyclone intensification.  The essential requirement is that thermodynamics permit further convection at the eyewall, but so long as that is permitted, then convection will act to converge momentum to permit spin-up.