Tropical Cyclone Secondary Eyewall Formation in Environmental Helicity

Abstract Two idealized numerical experiments are conducted to address secondary eyewall formation (SEF) of tropical cyclones (TCs) in positive and negative TC-relative environmental helicity and the underlying dynamics. No SEF is observed in the positive environmental helicity experiment, whereas SEF arises in the negative environmental helicity experiment. The results indicate that an asymmetric SEF process is observed in the negative environmental helicity, with the upshear-left development of convection initially arising near the edge of the inner core and subsequently becoming azimuthally symmetric to form an outer eyewall. However, in contrast with previous findings, a confluent region in the lower layers gives rise to the upshear-left convection initiation. The midlevel dry intrusion, linked to the shear, triggers an intensified descending inflow below the stratiform sector of the principal rainband. Conversely, no such inflow occurs in the positive environmental helicity experiment due to the misalignment between the midlevel dry intrusion and the stratiform portion of the principal rainband. The descending inflow confluences with the supergradient wind-forced outflow from the inner core. A new finding of the study is that tropospheric humidification in the upshear-right quadrant serves to promote the convective axisymmetrization process prior to the onset of SEF in the negative environmental helicity. In particular, the “showerhead effect” markedly contributes to midlevel humidification in the upshear-right quadrant. As the upshear-right troposphere near the edge of the inner core becomes moist, the axisymmetrization process is rapidly accomplished, ultimately leading to the formation of the secondary eyewall in the negative environmental helicity. Significance Statement Our study investigated how environmental helicity (a measure of wind rotation with altitude) influences tropical cyclone secondary eyewall formation. Using numerical simulations, we compared two scenarios: one with positive helicity (winds veering with height) and another with negative helicity (winds shifting counterclockwise with height). The secondary eyewall developed exclusively under negative helicity conditions through an asymmetric formation process. We observed that midlevel moisture buildup—caused by the “showerhead effect”—helped the process of axisymmetrization during secondary eyewall formation. These findings clarify the physical mechanisms behind secondary eyewall formation in complex sheared environments. Future research should validate these model results through observational studies.