The Role of the WISHE Mechanism in Secondary Eyewall Formation

Concentric eyewalls (CEs) and the eyewall replacement cycle (ERC) in tropical cyclones (TCs) have been widely documented by both observational and numerical studies. The accompanied change of TC characteristics associated with CE and ERC is of scientific importance, and is an interesting issue for TC intensity prediction. While previous studies had provided a number of possible mechanisms for the secondary eyewall formation (SEF) from different dynamical aspects, consensus remains to be reached in literature.

A number of studies pointed out that the intensification of TC is associated with the wind-induced surface heat exchange (WISHE) mechanism. In addition, some studies (Nong and Emanuel 2003; Terwey and Montgomery 2008) mentioned the possible role of the WISHE mechanism on the route to SEF. In this study, the high-resolution numerical simulations (based on WRF model) are conducted to examine the role of the WISHE mechanism in SEF. Analysis of the control experiment (CTL) shows an increase of inertial stability and an expansion of tangential winds in the SEF region. Moreover, the supergradient wind near the top and above the boundary layer (BL) increases with time within the SEF region before SEF, accompanied with an increase of convergence in the BL and an increase of divergence near the top and above the boundary layer. These characteristics are consistent with those in previous studies (Huang et al. 2012).

To examine the sensitivity of SEF to surface flux in both core and outer rainband regions, in the sensitivity experiments, the surface wind is capped at several designated values in the calculation of surface latent and sensible heat flux. The values of the capped surface winds are 15 (m/s, W15), 10 (10m/s, W10), 5 (5m/s, W05), and 1 (1m/s, W01), respectively. Preliminary results show that surface heat flux derived from W10 and W15 can support SEF, but the intensity of the TC and the time of SEF are weaker than CTL. Nevertheless, In W05 and W01, the vortices decay with time, and no SEF occurs. In addition, features of progressively strengthening supergradient forces and the accompanied strengthening convection in the SEF region are found in W15 and W10, which supports the role of unbalanced dynamics for SEF in and just above boundary layer. The analysis of both W15 and W10 shows that the inertial stability increases near the SEF region, mainly contributed by the broadening of tangential wind, and the area of increasing inertial stability is reduced as compared to that in CTL. Further work is ongoing to weight the contribution from dynamic/thermodynamic mechanisms (such as tangential wind tendency budget, PV budget, Sawyer-Eliassen diagnosis) among the experiments. Additional sensitivity experiments (varying capped wind speed at different regions) are also carried out to further evaluate the role of WISHE on SEF.