CORRESPONDENCE Comments on ''How Does the Boundary Layer Contribute to Eyewall Replacement Cycles in Axisymmetric Tropical Cyclones?''

In a recent paper, Kepert (2013, hereafter K13) investigated the theoretical role of the boundary layer in eyewall replacement cycles. Specifically, he used a family of steady-state, axisymmetric hurricane boundary layer models to examine the boundary layer response to an imposed radial profile of tangential winds with two wind maxima. Based on these solutions, he proposed a new feedback mechanism for secondary eyewall formation (SEF) and pointed to the role of the underlying boundary layer dynamics in this process. Specifically, he proposed (abstract) that ‘‘the boundary layer contributes to the formation of outer eyewalls through a positive feedback among the local enhancement of the radial vorticity gradient, the frictional updraft, and convection,’’ and concluded (section 6) that ‘‘supergradient flow . . . is not essential to SEF.’’ Quoting from K13 (section 6), the proposed contribution of the boundary layer to SEF is as follows: