Nonlinear Response of a Tropical Cyclone Vortex to Prescribed Eyewall Heating with and without surface friction in TCM4: Implications for Tropical Cyclone Intensification

The recent debate on whether surface friction contributes positively or negatively to tropical cyclone (TC) intensification has been clarified based on two idealized numerical experiments, one without and the other with surface friction, using the fully-compressible, nonhydrostatic TC model–TCM4, with prescribed eyewall heating. The results show that with surface friction included, the intensification rate of the TC vortex is largely reduced, indicating that surface friction contributes negatively to TC intensification. Results from tangential wind budgets demonstrate that although surface friction largely enhances the boundary layer inflow and the contraction of the radius of maximum wind (RMW), the positive tangential wind tendency resulting from the frictionally-induced inward absolute angular momentum (AAM) transport in the boundary layer is not large enough to offset the negative tendency due to the direct frictional loss of AAM to the surface. Results from the Sawyer-Eliassen equation demonstrate that the balanced response to eyewall heating is the major mechanism for TC intensification. The unbalanced response due to the presence of surface friction is found to spin up tangential wind near the RMW in the surface layer where the flow is subgradient and spin down tangential wind immediately above where the flow is supergradient. Surface friction is shown to have a net negative effect on TC intensification, but it plays a critical role in producing the realistic boundary layer structure with enhanced inflow, a low-level jet in tangential wind with supergradient nature, and a shallow outflow layer at the top of the inflow boundary layer.