The roles of an expanding wind field and inertial stability in tropical cyclone secondary eyewall formation

The Weather and Research and Forecasting Model (WRF) is used to simulate secondary yewall formation (SEF) in a tropical cyclone (TC) on the β plane. The simulated SEF proc ss is accompanied by an outward expansion of kinetic energy and theTC warm core. Anabsol te angular momentum budget demonstrates that this outward expansion is predominantly a s mmetric response to the azimuthal-mean and wavenumber-1 components of the transverse cir ulation. As the kinetic energy expands outward, the kinetic energy efficiency in which latent heating can be retained as local kinetic energy increases near the developing outer eyewall. The kinetic energy efficiency associated with SEF is examined further using a s mmetric linearized, nonhydrostatic vortex model that is configured as a balanced vortex model. Given the symmetric tangential wind and temperature structure from WRF, which is close to a state of thermal wind balance above the boundary layer, the idealized model provides the transverse circulation associated with the symmetric latent heating and frictio prescribed fromWRF. In a number ofways, this vortex responsematches the azimuthal-mean s condary circulation in WRF. These calculations suggest that sustained azimuthal-mean lat nt heating outside of the primary eyewall will eventually lead to SEF. Sensitivity experiments with the balanced vortex model show that, for a fixed amount of heating, SEF is facilitated by a broadening TC wind field.