| Environmental vertical wind shear has been shown to have a generally detrimental impact on tropical cyclone (TC) intensity change. However, many cases of rapidly intensifying (RI) sheared TCs have been observed, and TCs in moderate (5-10 m s-1) shear often have the largest intensity forecast errors. Thus, advancing the understanding of TC-shear interactions is vital to improving TC intensity forecasts, which have not seen much improvement over the past few decades. This dissertation employs both observational and high-resolution numerical modeling approaches to investigate how some TCs are able to resist shear and intensify.;The rapid intensification of Hurricane Irene (1999) was studied using observations, while the short-term RI of Tropical Storm Gabrielle (2001) was simulated using the Weather Research and Forecast (WRF) model run at 1-km horizontal resolution. Both storms exhibited a downshear-left vortex tilt and a marked azimuthal wavenumber-1 convective asymmetry. However, the azimuthally averaged diabatic heating also increased, suggesting that TC intensity may be more sensitive to the azimuthally averaged component of diabatic heating rather than the asymmetric component. Furthermore, this increase occurred within the radius of maximum winds (RMW), a region theorized to favor rapid spinup of the vortex.;A key difference between the Irene and Gabrielle cases was that the latter underwent a downshear reformation. The circulation associated with an intense mesovortex and other localized cyclonic vorticity anomalies comprised a developing "inner vortex" on the downshear-left (downtilt) periphery of the broader parent vortex. This inner vortex was nearly upright within a parent vortex that was tilted significantly with height. The inner vortex became the dominant vortex of the system, advecting and absorbing the broad, tilted parent vortex. A method was developed for diagnosing vortex tilt in the simulation. The reduction of TC vortex tilt from 65 km to 20 km within three hours reflected the emerging dominance of this upright inner vortex. We hypothesize that downshear reformation, resulting from diabatic heating associated with asymmetric convection, can aid the TC's resistance to shear by reducing vortex tilt and by enabling more diabatic heating to occur near the center and inside the RMW. |
