Prof. Zhemin Tan's group has made progress in the research of the role of vertical wind shear on tropical cyclone intensity and structure. The research paper, titled The Evolution of Vortex Tilt and Vertical Motion of Tropical Cyclones in Directional Shear Flows, was published on Sep. 14, in Journal of the Atmospheric Sciences (JAS, subordinated to American Meteorological Society).
Large-scale vertical wind shear (VWS) is commonly regarded as an important environmental factor, which affects the intensity and structure of tropical cyclone (TC). It is also a difficult part of studying tropical cyclone dynamics. Large-scale VWS, also referred to as deep-layer shear, is commonly defined as the difference between horizontal wind vectors in the 200- and 850-hPa layers, averaged over an area in an annular region or within a given radius from the TC center. Most studies simplify the VWS as a unidirectional shear with the same shear direction throughout the deep layer. Observational and theoretical studies suggested that with an identical deep-layer shear, TC intensification may reduce. Although deep-layer shear is responsible for TC genesis, structure, and intensity change, it alone is not enough to represent the overall vertical structure of a large-scale environmental flow. The profile of large-scale environmental wind can be considerably more complex in the real atmosphere. Different vertical structures of an environment flow will lead to distinctions between the real VWS and the overall deep-layer shear, due to which different VWS will cause different results of TC intensification. For example, with an identical deep-layer shear, the TC embedded in a clockwise (CW) hodograph significantly intensifies, whereas that in a counterclockwise (CC) hodograph does not. However, the inner dynamical mechanism is not clear. Some studies demonstrate it is the vortex tilt that essentially leads to this. Since the change of vortex tilt depends on its kinematic and thermodynamic structures, the TC intensification theoretically depends on its kinematic and thermodynamic structures. Therefore, the authors present an alternative mechanism that is associated with balanced dynamics in response to vortex tilt to explain the coincidence and the distribution variability of vertical motions, as well as local helicity in directional shear flows.
In this research, the Weather Research and Forecasting (WRF) Model, version 3.4, is used with the point-downscaling (PDS) method to investigate vortex evolution in a directional shear flow. The results show that the vortex tilt has significant difference in the rotated shear flows: in a clockwise rotated environmental flow, the low-level vortex tilt is closer to the left-of-shear (with respect to the deep-layer shear) region; in a anticlockwise rotated flow, the low-level vortex tilt is closer to the right-of-shear region; in a uniform sheared flow, the upper and lower vortex tilt has consistent direction. In the clockwise rotated sheared flow, the whole layevortex tilt moves the fastest and the tilt direction is on the left-of-shear when balanced. In the anticlockwise rotated flow, the vortex tilt moves the slowest, and the whole layer vortex tilt direction stays in the right-of-shear. These indicate that in clockwise rotated veritical sheared flow TC tends to move into anticlockwise direction, which reduces the vortex tilt and further intensifies the TC strength. These could be understood through the interaction between cyclonic circulations at different levels, and through vortex Rossby wave (VRW) dynamics.
In addition, such a configuration of vortex tilt in CW hodographs is potentially favorable for the continuous precession of convection into the upshear region but in CC hodographs, it is unfavorable. Most of the upward motions within a TC undergoing CW shear are concentrated in the downshear-left region, whereas those in the CC shear are located in the downshear-right region. Moreover, the upward (downward) motions are in phase with positive (negative) local helicity in both CW and CC hodographs.
FIG. 3. Schematic diagram showing the configuration of lowlevel vortex tilt, overall vortex tilt, and low-level upward motions in directional and unidirectional shear flows. The largest thin black circle represents the TC's inner-core region. The smallest black circle at the center of the inner-core region represents the vortex center at the surface.
In all, this research demonstrates an alternative TC intensification mechanism in complex environmental vertical wind shear: the balanced dynamics that is propelled by vortex tilt essentially accounts for firstly, the structural evolution of TC, and secondly, TC intensification.
The first author of this research is Dr. Jianfeng Gu; the corresponding author is Prof. Zhemin Tan. The National Key R&D Program of China and other program grants jointly supported this work.
Gu, J.-F., Z.-M. Tan, and X. Qiu, 2018: The evolution of vortex tilt and vertical motion of tropical cyclones in directional shear flows. J. Atmos. Sci., 75(10), 3565-3578.