Impact of vertical turbulent mixing on inner-core asymmetric structure and vortex Rossby wave propagation of tropical cyclones

In this study, the inner-core asymmetric structure and vortex Rossby wave (VRW) propagation in a tropical cyclone (TC)-like vortex are investigated using the Weather Research and Forecasting Model (WRF). In the first experiment, the asymmetries and VRWs obtained in the classic two-dimensional unforced nondivergent framework are revisited by a carefully designed three-dimensional unforced WRF simulation with no physics. It is found that the redistribution of potential vorticity (PV) and the inward/outward propagation of VRW in the 3D simulation are similar to those in the 2D framework. However, the mixing process of PV in the 3D simulation is almost as twice slower as that in the 2D framework. Moreover, unlike the vortex in a 2D framework, it appears that there exists an upper limit that the core PV cannot exceed. In order to investigate the effect of vertical turbulent mixing on inner-core asymmetric structure and VRW propagation, model physics are activated in the sensitivity experiments with the same initialization and model configuration as the first experiment. In the first stage, with little moisture in the atmosphere, the eyewall PV is quickly dissipated due to the friction induced by the vertical turbulent mixing in the lower layer and the initial vortex is spanned down to a weak midlevel vortex. The amplitude and propagation of VRW is largely affected by the turbulent mixing during this stage. In the second stage, the vortex intensifies due to the moist convection resultant from the increased atmospheric moisture from surface evaporation. However, unlike the unforced vortex, the inner-core asymmetries of the intensified vortex are dominated by the high frequency PV perturbations, which show a profound impact on the propagation of low frequency VRWs. The sensitivity of inner-core asymmetries and VRW propagation to vertical turbulent mixing is investigated by three experiments with different vertical turbulent mixing schemes.