Ensemble Simulations of Hurricane Katrina (2005) from TCLAPS: Intensity and Track Sensitivity and Vortex Structure Evolution

Hurricane Katrina (2005) is the focus of this study. Both intensity and track forecasts from TCLAPS (Davidson and Weber, 2000) show encouraging skill for this storm, and thus, have been analysed in detail to understand the evolution of the vortex structure during the model integration.

Sensitivity studies of the model performance at 5km resolution were carried out to assess the robustness of the forecasts with slightly different model configurations and initial conditions. For this purpose, uncertainties in the internal vortex structure, large-scale environment, and sea surface temperature (SST) were introduced in the form of randomly perturbed conventional and synthetic observations, different boundary conditions, and different sea surface temperatures, respectively. It was found that the intensity and track forecasts of the perturbed observation members are very similar to each other. On the other hand, the use of different sea surface temperatures results in larger changes in intensity and track forecasts. Similarly, different boundary conditions (i.e., essentially different storm environments) have a large effect on track forecasts and intensity. This dependency between track and intensity forecasts in this ensemble experiment can be explained largely by the change in the SST along the forecast track, with the average correlation coefficient between SST and minimum surface pressure of -0.8. Furthermore, additional experiments with different initial vortex sizes also show variations both in track and intensity. Thus, the vortex structures were then analysed in more detail to understand its possible association with TC intensity change

The evolution of the symmetric and asymmetric vortex structures were analysed in order to understand the role of asymmetries, including Vortex Rossby Waves, in the intensification process. During the model integration, it was observed that there exist two distinct regimes: 1) periods of increased maximum asymmetry (measured by the standard deviation of PV along each radius circle) associated with a decrease in the tendency of the maximum tangential wind at the same level; and 2) the reverse process with a decrease in maximum asymmetries associated with a more rapid acceleration of the mean flow. Even though this relationship appears in the majority of ensemble runs, its detailed characteristics tend to vary between different members. In order to understand the mechanisms behind this oscillating pattern, a simulation with the best model configuration and boundary conditions (i.e. based on analyses), was chosen for detailed study. Preliminary results show that the first regime may be associated with the development of mesovortices near the inner side of the eye-wall. This process is consistent with the barotropic unstable configuration and the mixing effects of mesovortices proposed by Schubert et al. (1999) and Kossin and Schubert (2001). On the other hand, the second regime with a decrease in asymmetries and strengthening of the mean flow appears to be accompanied with the emergence of convective bands into the region of high vorticity in the core region. More detailed results from this work will be presented at the Conference.