Seminar: Diverging Behaviors of Tropical Cyclones in Moderate Vertical Wind Shear

Abstract:

Tropical cyclones in moderate wind shear, around 5–10 m s-1 of bulk shear between 850 and 200 hPa, have a large variability of structure and intensity change outcomes. One factor that appears to be important for governing this variability is the tropical cyclone vortex structure itself, namely the amplitude of the inner-core vorticity aloft. Through a set of idealized CM1 simulations, we explore tropical cyclone evolution in 7.5 m s-1 of shear by systematically amplifying the inner-core, mid-to-upper-level vorticity at a restart point. Without any amplification, the tropical cyclone tilt increases and the storm stays weak. With amplification, the tropical cyclone tilt precesses cyclonically, aligns, and the storm reintensifies.

There are both dynamical and thermodynamic processes important to controlling this diverging behavior. First, mid-level vorticity generation in the stratiform portion of the shear-organized principal rainband contributes to increasing the vortex tilt and slowing the precession rate when the tilt is downshear left, resulting in a precession “hiatus”. Overcoming this hiatus is necessary for a tropical cyclone to be resilient against shear. Second, strong inner-core downdrafts flush low-θe air into the boundary layer, disrupting the inner-core vorticity and preventing alignment and intensification. Strong diabatic cooling adjacent to intense downshear convection, along with an inertial instability-like mechanism, are responsible for the strong downdraft and low-θe intrusion. The downdraft is strongest for the simulation without vorticity amplification. These results demonstrate that moist, convective processes, along with the internal tropical cyclone structure, are important for tropical cyclone evolution in moderate shear.