The effect of internally generated inner-core asymmetries on tropical cyclone potential intensity

In a quiescent environment on an f plane, the internal dynamic processes of a tropical cyclone (TC) can generate axially asymmetric circulations (asymmetries) in its inner-core region. The present study investigates how these inner-core asymmetries affect TC intensity. For this purpose, a three-dimensional (3D) TC model and its axisymmetric (2D) version were used. Both have identical model vertical structure and use the same set of parameters and the same initial conditions. The differences between the two model runs are considered to be due to mainly the effects of the TC asymmetries. The results show that the presence of asymmetries in the 3D run reduces the TC final intensity by about 15% compared with the 2D run, suggesting that the TC asymmetry is a limiting factor to the potential intensity ( PI). In the 2D run without asymmetries, the convective heating in the eyewall generates an annular tower of high potential vorticity (PV) with relatively low PV in the eye. The eyewall tilts outward with height significantly. Underneath the tilted eyewall the downdrafts induced by evaporation of rain and melting of snow and graupel make the subcloud-layer inflow dry and cool, which lowers the boundary layer equivalent potential temperature (theta(e)), thus increasing the entropy difference between the air and sea in the vicinity of the radius of maximum wind (RMW). The increased air-sea entropy deficit leads to more energy input into TC from the underlying ocean and thus a greater final intensity. On the other hand, in the 3D run, the model-resolved asymmetric eddies, which are characterized by the vortex Rossby waves in the mid-lower troposphere, play important roles in modifying the symmetric structure of the TC. Potential vorticity and theta(e) budgets indicate that significant inward PV mixing from the eyewall into the eye results in a less-tilted eyewall, which in turn limits the drying and cooling effects of downdrafts in the subcloud layer and reduces the air-sea entropy deficit under the eyewall, thereby reducing the TC intensity. The angular momentum budget analysis shows that the asymmetric eddies tend to reduce the strength of the primary circulation in the vicinity of the RMW. This eddy contribution to the azimuthal mean angular momentum budget is larger than the parameterized horizontal diffusion contribution in the 3D run, suggesting an overall diffusive effect of the asymmetric eddies on the symmetric circulation.