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

The mechanisms responsible for tropical cyclone (TC) intensification in the presence of moderate vertical shear magnitudes are not well understood. To investigate how TCs intensify in spite of moderate shear, this study employed a 96-member ensemble generated with the Advanced Hurricane Weather Research and Forecasting (AHW) Model. In this first part, AHW ensemble forecasts for TC Katia (2011) were evaluated when Katia was a weak tropical storm in an environment of 12 m s(-1) easterly shear. The 5-day AHW forecasts for Katia were characterized by large variability in the intensity, presenting an opportunity to compare the underlying mechanisms between two subsets of members that predicted different intensity scenarios: intensification and weakening. The key difference between these two subsets was found in the lower-tropospheric moisture north of Katia (i.e., right-of-shear quadrant). With more water vapor in the lower troposphere, buoyant updrafts helped to moisten the midtroposphere and enhanced the likelihood of deep and organized convection in the subset that predicted intensification. This finding was validated with a vorticity budget, which showed that deep cyclonic vortex stretching and tilting contributed to spinning up the circulation after the midtroposphere had moistened. Sensitivity experiments, in which the initial conditions were perturbed, also demonstrated the importance of lower-tropospheric moisture, which suggests that moisture observations may help reduce uncertainty in forecasts of weak, sheared tropical storms.