An Ensemble Approach to Investigate Tropical Cyclone Intensification in Sheared Environments. Part II: Ophelia (2011)

Abstract

he mechanisms leading to tropical cyclone (TC) intensification amid moderate vertical wind shear can vary from case to case, depending on the vortex structure and the large-scale conditions. To search for similarities between cases, this second part investigates the rapid intensification of Hurricane Ophelia (2011) in an environment characterized by 200?850-hPa westerly shear exceeding 8 m s?1. Similar to Part I, a 96-member ensemble was employed to compare a subset of members that predicted Ophelia would intensify with another subset that predicted Ophelia would weaken. This comparison revealed that the intensification of Ophelia was aided by enhanced convection and midtropospheric moisture in the downshear and left-of-shear quadrants. Enhanced left-of-shear convection was key to the establishment of an anticyclonic divergent outflow that forced a nearby upper-tropospheric trough to wrap around Ophelia. A vorticity budget showed that deep convection also contributed to the enhancement of vorticity within the inner core of Ophelia via vortex stretching and tilting of horizontal vorticity enhanced by the upper-tropospheric trough. These results suggest that TC intensity changes in sheared environments and in the presence of upper-tropospheric troughs highly depend on the interaction between convective-scale processes and the large-scale flow. Given the similarities between Part I and this part, the results suggest that observations from the three-dimensional moisture and wind fields could improve both forecasting and understanding of TC intensification in moderately sheared environments.