Deep convection in elliptical and polygonal eyewalls of tropical cyclones

Abstract

In observations, tropical cyclones with cyclonically rotating elliptical eyewalls are often characterized by wave number 2 (WN2) deep convection located at the edge of the major axis. A simple modeling framework is used to understand this phenomenon, where a nondivergent barotropic model (NBM) is employed to represent the elliptical vortex in the free atmosphere, and an asymmetric slab boundary layer (SBL) model is used to simulate the frictional boundary layer (BL) underneath the free atmosphere. The interaction is one way in that the overlying cyclonic flow drives the BL, but the BL pumping does not feed back to the overlying flow. The nonlinear‐balanced pressure field from the NBM drives the winds in the SBL model, which then causes BL convergence and pumping near the eyewall. The strong updrafts at the edge of the major axis for the elliptic vortex in the BL are induced by the larger convergent radial wind from the asymmetric distribution of the pressure fields of the free atmosphere with noncircular vortex. The large radial inflow maintains the supergradient wind at the edge of the elliptical vortex. The results emphasize the cyclonic rotation of the WN2 feature of strong updrafts at the top of the BL from the local shock‐like BL radial wind structure. Similar radial profiles and strong BL top updrafts occur at the edges of higher‐order polygonal eyewalls with the magnitude of the peak updraft decreasing as the wave number structure of the vortex increases.

Periodic Cycles of Eyewall Convection Limit the Rapid Intensification of Typhoon Hato (2017)

“…e periodic cycles in the eyewall convection were also reproduced satisfactorily in the simulation during model hours 31-38. We analyzed the stage with a relatively steady IR (model hours [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] to determine the effects of the short-term periodic activities of eyewall convection on TC intensification. e simulation showed that convection of the eyewall of Hato exhibited periodic cycles with a period of 3-5 h, which limited the RI of Hato.…”

“…A similar analysis of Hurricane Olivia (1994) showed an elliptical TC vortex rotating with wavenumber-1 deep convection in the presence of strong vertical wind shear [33]. Kuo et al [34] demonstrated that the boundary layer (BL) radial inflow is also in control of the deep convection period. From radar observations of Typhoon Guillermo (1997), Reasor et al [27] identified convective burst cycles with a period of 0.6-2 h. Cycles began with the convective burst appearing in the downshear-left quadrant and ended with the cyclonic wrapping of upper-level updrafts into the downshear side of the eyewall.…”

Polygonal Eyewall Asymmetries During the Rapid Intensification of Hurricane Michael (2018)

“…Due to sampling limitations of the radar beam, it is unclear whether this coupling originates in the boundary layer or is only in the free troposphere. Kuo et al (2016) used a nondivergent barotropic model for the free atmosphere and an asymmetric slab boundary layer underneath and found a tangential wind maximum at the minor axis of the elliptic vortex in the free atmosphere, whereas the wind maximum was closer to the major axis within the boundary layer. In a real, baroclinic vortex that decays with height, the theoretical wave speed for VRWs is height dependent such that a vertically coherent asymmetry that propagates at the same speed over some depth will deviate from the theory at different levels.…”

“…Polygonal eyewalls are hypothesized to be the result of asymmetric vorticity dynamics that can modulate TC structure and intensity through counter‐propagating vortex Rossby waves (VRWs) (Hendricks et al., 2012; Kuo et al., 1999, 2016; Muramatsu, 1986; Schubert et al., 1999). A sign reversal of the mean radial vorticity gradient that acts as a waveguide for VRWs satisfies the Rayleigh condition for barotropic‐baroclinic instability (Montgomery & Shapiro, 1995), which can lead to a breakdown of the potential vorticity (PV) ring and redistribution of eyewall PV and angular momentum (Bell & Montgomery, 2008; Schubert et al., 1999) that can accelerate the mean wind inside the radius of maximum wind (RMW), and promote the contraction of the RMW.…”

“…Previous theoretical studies have often used a simplified barotropic nondivergent model framework to investigate the dynamical processes of polygonal eyewalls (Kuo et al, 1999(Kuo et al, , 2016Rozoff et al, 2009;Schubert et al, 1999). While this framework can provide substantial insight, it has limitations in investigating eyewall dynamics as moist convective processes are very important to PV generation and the intensification process.…”