Climate and Tropical Cyclones

Part I of the work presented here investigates the impact of relative humidity on TC size in idealized WRF simulations. It is hypothesized that outer-core precipitation influences TC size, and therefore environmental factors that influence the amount of outer-core precipitation in turn influence TC size. Outer-core precipitation affects TC size through the diabatic generation of lower-tropospheric PV, which can amalgamate with the TC central PV tower or remain at radius. Four idealized high-resolution numerical simulations with the WRF model were performed in order to test the hypothesized sensitivity of TC size to environmental humidity. Differences in TC size between the runs were substantial, consistent with the hypothesis; several TC size metrics demonstrate this difference, including differences of a factor of 3 in the size of the RMW in the simulations.;During the simulation period, moisture fluxes led to similar moisture content in the boundary layer in all simulations, although large differences remained above the boundary layer and outside of the initial moist envelope. A close correspondence between the inward protrusion of the dry air and the formation of persistent precipitation was found, indicating that the presence of dry air was responsible for differences in simulated precipitation outside of the eyewall. PV budget analysis reveals that size differences can be linked to differences in precipitation in outer rainbands and associated diabatic production of lower tropospheric PV, which both were larger in the more moist simulations. Several feedback mechanisms serve to reinforce TC growth.;Part II of this research involved an investigation of climate change and its impact on TC intensity and structure. Previous work was extended in this study by utilizing a larger number of GCMs forced with 3 different greenhouse gas emissions scenarios to estimate climate change, which allowed for a detailed analysis of uncertainty. TC simulations featured higher resolution than in previous idealized downscaling studies, and the explicit convection simulations allowed for a more realistic representation of TC structure and analysis of TC structure changes in a future climate. The high-resolution model output was used to investigate structural changes, and to explore the mechanism of future intensity changes.;A large sample of model simulations with 6-km grid spacing indicate an average increase in future TC maximum intensity of ~9%. A smaller sample of simulations with 2- km grid spacing indicate a slightly larger increase of 13%. Tropospheric stabilization, which varies largely in GCM projections, plays a key role in influencing future outflow temperature and structure. Simulations with no tropospheric stabilization had an average increase in central pressure deficit of 28%, slightly more than double that found with an identical SST increase along with projected tropospheric stabilization. Stratospheric cooling was found to not impact TC intensity in the model simulations. In addition to intensity increases being tied to warmer SSTs, it was also discovered that increases in rainfall lead to a stronger central PV tower, and that the increase in PV is highly correlated with intensity. Therefore, it is proposed that changes in both thermodynamic efficiency and in rainfall are responsible for future intensity increase.