Revisiting Tropical Cyclone Potential Intensity Theory from an Unbalanced Flow Perspective.

The importance of the ratio of the surface exchange coefficients of enthalpy (C_K) to drag (C_D) to hurricane potential intensity (PI) is examined using numerical simulations. Unlike previous studies, this study compares model results to PI theory for simulated intensity and consistency of the theory with internal storm structure. Hurricanes are simulated for a range of C_K and C_D values using the axisymmetric cloud model 1 (CM1). A contraction of the radius of maximum wind occurs in all sensitivity experiments where C_K and C_D are increased yet the C_K/C_D ratio maintained, and it is suggested that the redistribution of angular momentum in the boundary layer contributes to this structural change. Another primary finding is the gradient wind balance approximation of the Emanuel PI theory is not valid in certain conditions. Following on from previous work in this field, we show as C_K/C_D increases then the ability for supergradient overshoot increases, thus the inclusion of inertial (unbalanced) terms in PI theory is necessary. We establish that the azimuthal vorticity has greater influence on the unbalanced terms than the contribution from the vertical velocity. The initial vortex is unlikely to be as important in controlling the unbalanced terms as the C_K specification (the terms increase markedly for increasing C_K), whereas an increase in C_D leads to a decrease in the unbalanced terms. This results in substantial structure and intensity changes. We believe further assessment of the role of structure in PI is merited.