Intensification Variability of Tropical Cyclones in Directional Shear Flows: Vortex Tilt–Convection Coupling

Abstract

The coupling of vortex tilt and convection, and their effects on the intensification variability of tropical cyclones (TCs) in directional shear flows, is investigated in this study. The height-dependent vortex tilt controls TC structural differences in clockwise (CW) and counterclockwise (CC) hodographs during their initial stage of development. Moist convection may enhance the coupling between displaced vortices at different levels and thus reduce the vortex tilt amplitude and enhance precession of the overall vortex tilt during the early stage of development. However, differences in the overall vortex tilt between CW and CC hodographs are further amplified by a feedback from convective heating and therefore result in much higher intensification rates for TCs in CW hodographs than those in CC hodographs. In CW hodographs, convection organization in the left-of-shear region is favored because the low-level vortex tilt is ahead of the overall vortex tilt and the TC moves to the left side of the deep-layer shear. This results in a more humid midtroposphere and stronger surface heat flux on the left side (azimuthally downwind) of the overall vortex tilt, thus providing a positive feedback and supporting continuous precession of the vortex tilt into the upshear-left region. In CC hodographs, convection tends to organize on the right side (azimuthally upwind) of the overall vortex tilt because the low-level vortex tilt is behind the overall vortex tilt and the TC moves to the right side of the deep-layer shear. In addition, convection organizes radially outward near the downshear-right region, which weakens convection within the inner region. These configurations lead to a drier midtroposphere and weaker surface heat flux in the downwind region of the overall vortex tilt and also a broader potential vorticity skirt. As a result, a negative feedback is established that prevents continuous precession of the overall vortex tilt.

Boundary Layer Recovery and Precipitation Symmetrization Preceding Rapid Intensification of Tropical Cyclones under Shear

“…The upward dynamic forcing is likely attributed to the low-level convergence in the downshear convergence zone, and it is difficult to further decompose the dynamical forcing to quantify the relative contribution of the low-level convergence and other mechanisms. The role of dynamic forcing in the convective initiation in the incipient eyewall is consistent with the findings in previous modeling studies of TCs(Zhang et al 2000;Braun 2002;Gu et al 2019). One new finding in this study is that the buoyancy forcing also plays a role in the convective initiation at the inward flank of the CPS (Figs.15c and 15f).Figures 15 and 11 also indicate a striking difference between the two experiments: in CTL a large portion of these parcels have already developed or are going to develop into deep convection in the downwind part of the azimuthally-propagating CPS, as seen in Figs.…”

“…In the presence of VWS, dry TC-like vortices are usually vertically tilted (Jones 1995(Jones , 2004. When coupled with moist processes, the balanced mesoscale lifting associated with the tilted vortex organizes a convective precipitation shield (CPS) in the downtilt side (e.g., Wang and Holland 1996;Corbosiero and Molinari 2002;Reasor et al 2013;Gu et al 2019).…”

“…The boundary layer recovery of these downdraft-cooled parcels by the surface enthalpy fluxes is argued as the key to compensate for the low-level ventilation and impacts the subsequent TC intensity change (Powell 1990;Tang and Emanuel 2012;Molinari et al 2013;Zhang et al 2017b;Zhang and Rogers 2019;Nguyen et al 2019). However, the linkage between the boundary layer recovery and convective development in sheared TCs remains elusive: recent idealized simulations with the same SST attribute convective initiation in the azimuthallypropagating CPS before RI onset to dynamical forcing, rather than buoyancy forcing (Gu et al 2019).…”

The Unexpected Relationship Between Extreme Shear Events and AMO

“…Environmental shear ows can heavily in uence the variability of the intensi cation rate of tropical cyclones 33 . It is important to understand the mechanisms of and relationships between tropical cyclone intensi cation and shear ows.…”

The Role of Low-Level Flow Direction on Tropical Cyclone Intensity Changes in a Moderate-Sheared Environment

“…In addition to the DLS, several studies have examined the impact of assorted characteristics of the sheared background flow profile on TC intensity. Idealized simulations, with 10 m s 21 DLS, have shown that shear height and depth (Finocchio et al 2016) and tropical cyclone-relative environmental helicity (hereafter referred to as TCREH; Nolan 2011; Onderlinde andNolan 2014, 2016;Gu et al 2018Gu et al , 2019 can modulate the intensity evolution and inner-core structure. The deep-sheared layer, upper-level shear, and positive TCREH are more favorable to the upshear precession process and intensification.…”