Tropical Cyclone Intensity in Vertical Wind Shear

Abstract

The structure and intensity changes of tropical cyclones (TCs) in environmental vertical wind shear (VWS) are investigated in this study using the fifth-generation Pennsylvania State University?National Center for Atmospheric Research (PSU?NCAR) Mesoscale Model (MM5). Triply nested domains of 36-, 12-, and 4-km resolution are used with fully explicit moisture physics in the 4-km domain. Idealized environments with easterly shears of 2, 4, 6, 8, and 10 m s?1 between 800 and 200 hPa are applied on an f plane. Under small values of VWS (2 and 4 m s?1), the TC intensities are similar to that of the control (CTRL; i.e., no VWS) after initial adjustments. The TCs under 6 and 8 m s?1 of VWS are not as intense, although they do not weaken during the simulation. On the other hand, the TC in 10 m s?1 of VWS weakened significantly. Given the same VWS, the TC intensity is also found to be sensitive to TC size. Experiments with TCs with a smaller radius of 15 m s?1 wind reveal that while the TC in 2 m s?1 of VWS remains as intense as the CTRL, the TC in the 4 m s?1 VWS case weakened significantly to a minimal hurricane by the end of the simulation. A VWS of 6 m s?1 is strong enough to cause dissipation of the TC in 72 h. These results indicate that the size of a TC has to be taken into account in determining the intensity change of a TC in VWS. In the 10 m s?1 VWS case, the average temperature over the lower half of the troposphere within 50 km from the TC surface center is higher than that of the CTRL throughout the simulation. Such a warming, though of a small magnitude, is also observed for a brief period in the upper half of the troposphere before the rapid weakening of the TC and is related to the asymmetry of temperature required for a tilt of the vortex axis. The evolution of the vortex tilt is found to be similar to the dry simulations in previous studies, with the midlevel center (σ = 0.525) located mainly in the southeast quadrant of the surface center. A tendency for the midlevel center to rotate about the surface center is also observed. These results support the idea that the resistance to vertical tilt by the mutual rotation between the low-level and midlevel centers is also valid in the moist simulations. It is hypothesized that the secondary circulation and the associated diabatic heating reduce the vertical tilt and the weakening. Condensation heating offsets the anomalous cooling effect due to the anomalous rising motion ahead of the vortex tilt. For small VWS, the vertical motion asymmetry is not strong enough to destroy the complete secondary circulation and the eyewall. As a result, a large temperature asymmetry and the associated vortex tilt cannot develop. Furthermore, there is no entrainment of cool/dry air in the upper troposphere. Therefore, TCs under small shears can be as intense as the CTRL. Large-scale asymmetries in the form of anticyclones found in previous studies are also observed. These asymmetries are apparently related to the change of shears near the TCs. While the shears at outer radii stay roughly constant with time, the shears near the TC centers can have large temporal fluctuations both in magnitude and orientation. This result suggests that the location at which the VWS is estimated in observational studies could be important in determining the relationship between VWS and TC intensity change.