We nominally define the hurricane boundary layer as the layer in which the effects of surface friction are associated with significant departures from gradient wind balance. Using this definition, the boundary layer in the intensifying core region of the vortex is found to be strongly nonlinear. At large radii, exterior to the eyewalls, Ekman-like balance as traditionally defined, is found to hold true. Where significant departures from Ekman-like balance are found, the departures are characterized by large departures from gradient wind balance in the boundary layer and large vertical advection of horizontal velocity through the depth of the boundary layer. These effects cannot be described by either of the balance models so defined and the effects are shown to make important contributions to the maximum winds and the primary and secondary circulations. We search also for the existence also of shock-like structures in the azimuthally averaged view of the vortex boundary layer. Implications of our findings will be discussed.
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