Typhoon Lekima (2019) possessed a double‐eyewall structure before making landfall in eastern China, with its outer eyewall showing quasi‐periodic convective intensification. Several physical mechanisms that may cause eyewall convection asymmetries were examined. The upshear occurrence of the strongest convection could not be explained by either the typhoon's motion or the effect of descending inflow from outer rainbands. Radar reflectivity analysis showed that phase locking occurred between the wavenumber‐2 vortex Rossby waves (VRWs) propagating radially outward from the inner eyewall and the azimuthally propagating wavenumber‐1 VRWs on the inner edge of the outer eyewall. Additional phase locking further arose between the aforementioned wavenumber‐1 VRWs and the azimuthally propagating wavenumber‐2 VRWs on the outer edge of the outer eyewall. These two phase‐locking processes led to the pronounced quasi‐periodic intensification of the convective asymmetry in the western semicircle of Lekima's outer eyewall.

In this study, a quadruply nested, nonhydrostatic tropical cyclone (TC) model is used to investigate how the structure and intensity of a mature TC respond differently to imposed lower‐layer and upper‐layer unidirectional environmental vertical wind shears (VWSs). Results show that TC intensity in both cases decrease shortly after the VWS is imposed but with quite different subsequent evolutions. The TC weakens much more rapidly for a relatively long period in the upper‐layer shear than in the lower‐layer shear, which is found to be related to the stronger storm‐relative asymmetric flow in the middle‐upper troposphere and the larger vertical vortex tilt in the former than in the latter. The stronger storm‐relative flow in the former imposes a greater ventilation of the warm core in the middle‐upper troposphere, leading to a more significant weakening of the storm. The storm in the lower‐layer shear only weakens initially after the VWS is imposed but then experiences a quasi periodic intensity oscillation with a period of about 24 hr. This quasi periodic behavior is found to be closely related to the boundary layer thermodynamic “discharge/recharge” mechanism associated with the activity of shear‐induced outer spiral rainbands. There is no significant intensity oscillation for the storm embedded in the upper‐layer shear, even though outer spiral rainbands develop quasi periodically also. The boundary layer inflow is very weak in that case and the low equivalent potential temperature air induced by downdrafts in outer spiral rainbands therefore cannot penetrate into the inner core but remains in the outer region.

The effects of diabatic heating and cooling in the rapid filamentation zone (RFZ), within which inner rainbands are often active, on tropical cyclone (TC) structure and intensity are investigated based on idealized numerical experiments using a cloud-resolving TC model (TCM4). The results show that removal of heating (cooling) in the RFZ would reduce (increase) the TC intensity. Diabatic heating in the RFZ plays an important role in increasing the inner-core size whereas diabatic cooling tends to limit the inner-core size increase or even reduce the inner-core size of a TC. Removal of both diabatic heating and cooling in the RFZ greatly suppresses the activity of inner rainbands but leads to the quasi-periodic development of a convective ring immediately outside of the inner core. A similar convective ring also develops in an experiment with the removal of diabatic heating only in the RFZ. With diabatic cooling removed only in the RFZ, an annular-hurricane-like structure arises with the outer rainbands largely suppressed.