The roles of moat width and outer eyewall contraction in affecting the timescale of eyewall replacement cycle

The timescale of the eyewall replacement cycle (ERC) is critical for the
prediction of intensity and structure changes of tropical cyclones (TCs)
with concentric eyewall (CE) structures. Previous studies have indicated
that the moat width can regulate the interaction between the inner and
outer eyewalls and has a salient relationship with the ERC timescale. In
this study, a series of sensitivity experiments are carried out to
investigate the essential mechanisms resulting in the diversity of the
duration of CEs using both simple and full-physics models. Results reveal
that a larger moat can induce stronger inflow under the same inner eyewall
intensity by providing a longer distance for air parcels to accelerate in
the boundary layer, thus there is greater inward absolute vorticity flux
to sustain the inner eyewall. Besides, the equivalent potential
temperature budget indicates that the vertical advection and surface flux
of moist entropy can overbalance the negative contribution from the
horizontal advection and lead to an increasing trend of equivalent
potential temperature in the inner eyewall. This suggests that the
thermodynamic process in the boundary layer is not indispensable to the
inner eyewall weakening. It is also found that the contraction rate of the
secondary eyewall, which directly influences the moat width, is subject to
the activity of outer spiral rainbands. By directly introducing positive
wind tendency outside the eyewall and indirectly promoting a vertically
tilted eyewall structure, active convection in the outer region will
impede or even suspend the contraction of the outer eyewall hence
extending the ERC timescale.