Unbalanced Dynamics of Secondary Eyewall Formation in Tropical Cyclones - Part II: Analyses from Higher-Resolution Simulations

n part I, new evidence for the dynamically unbalanced pathway to secondary eyewall formation (SEF) is provided based on a dataset with a relatively coarse horizontal resolution (5 km) (hereafter, CTRL), which may not adequately represent dynamics in the inner core of a tropical cyclone. Therefore, in part II, the pathway is examined based on numerical simulations of Typhoon Sinlaku (2008) with finer horizontal model resolution (1.67 km) (hereafter, HIGH).

The result with this finer grid spacing yields a stronger typhoon and shows distinct asymmetric features throughout the integration. Compared to CTRL, the timing for SEF delays by about one day. Before SEF, supergradient forces/winds progressively strengthen in the SEF region within and just above the boundary layer. The supergradient force is approximately four times of that in CTRL. The evident feature of developing unbalanced flow prior to SEF shows supporting evidence for the dynamical pathway to SEF articulated in our previous two studies in 2012. The budget analysis of axisymmetric tangential wind is generally consistent with the findings in the part I presentation. Though the asymmetric structure is more prominent in terms of each individual quantity (e.g., tangential winds, vertical velocity, vorticity, etc.), the contribution from the eddy terms in the momentum budget remains similar to results assessed from CTRL.

The gradient/agradient wind analysis and momentum budget in HIGH supports the recently-proposed unbalanced pathway to SEF. Analyses of the rest 28 ensemble members are also carried out to further validate the robustness of the presented pathway to SEF and to assess the predictability and uncertainty of SEF under such numerical design. In addition, given adequately fine resolution in HIGH, data denial experiments are performed to investigate whether the model dynamics can capture SEF and the corresponding mechanism in an SEF or a non-SEF case.