The impact of gradient wind imbalance on potential intensity of tropical cyclones

The response of tropical cyclones' potential intensity (PI) to climate change represents an important and still unresolved issue. The classical PI theory by Emanuel (E-PI) makes use of the assumption that the flow is quasi-balanced. This does not agree with observations and model simulations. Furthermore, E-PI theory does not provide an explanation for the observed range of the radius of maximum winds (RMW) in tropical cyclones. The present study relaxes the constraint of quasi-balance by coupling the E-PI model with a slab-boundary layer model. The unbalanced boundary layer winds depend upon four parameters: i) the modified Rankine decay exponent, ii) the Rossby number, iii) the RMW multiplied with the drag coefficient divided by boundary layer height and iv) the ratio of surface exchange coefficients. The extension of the E-PI model can explain hurricane superintensity, i.e. a larger maximum wind speed than predicted by E-PI, because supergradient winds arise in the boundary layer. The boundary layer flow exhibits a high sensitivity to parameters i), ii) and iii). Significant supergradient winds only arise in the boundary layer. when the modified Rankine decay exponent is larger than assumed in E-PI theory, and when the RMW lies in a range between 10 and 60km. The latter result hints at the existence of an optimal RMW to which the actual RMW of tropical cyclones trends. The theoretical model is evaluated by comparison to numerical simulations with the axisymmetric cloud model HURMOD.