Predictability and Dynamics of Hurricane Joaquin (2015) Explored through Convection-Permitting Ensemble Sensitivity Experiments

The official track and intensity forecasts of Joaquin (2015) were uncharacteristically poor. The 72 to 120-h track errors were more than double the mean track errors over the previous 5 years and the Official NHC intensity forecast errors between 72 and 96 h were ~70% larger than intensity errors over the previous 5 years. In this study, we examine real-time ensemble forecasts from the PSU WRF-EnKF system for Hurricane Joaquin. The ensemble forecasts, from early in Joaquin’s lifecycle, displayed large track spread, with nearly half of the ensemble members tracking Joaquin towards the United States East coast and the other half tracking Joaquin out to sea. The ensemble forecasts also displayed large intensity spread with many of the members developing into major hurricanes and other ensemble members not intensifying at all. This study examines the cause of the high uncertainty in the PSU WRF-EnKF ensemble track and intensity forecasts.

Initial condition differences from the regions greater than (less than) 300 km were isolated by effectively removing initial condition differences in desired regions through relaxing each ensemble member to GFS (APSU) initial conditions, Renv (Rcore). The regions of initial condition errors contributing to the track spread were examined, and the dominant source of track errors arose from the region greater than 300 km from the tropical cyclone center. Further examination of the track divergence revealed that the region between 600 and 900 km from the initial position of Joaquin was found to be the largest source of initial condition errors that contributed to this divergence. Small differences in the low-level steering flow (~700 hPa), originating from perturbations between 600 and 900 km from the initial position, appear to have resulted in the bifurcation of the forecast tracks of Joaquin. The initial condition errors north of the initial position of Joaquin were also shown to contribute most significantly to the track divergence. The region inside of 300 km, specifically the initial intensity of Joaquin, was the dominant source of initial condition errors contributing to the intensity spread. Stronger initial members are associated with a deeper initial vortex and higher relative humidity throughout the inner-core. All members with stronger intensity than best track intensify while only some members with initial intensity weaker than best track intensify. The intensification of members that are initially weaker than best track is more sensitive to the initial inner-core moisture.

Figure 1: Ensemble (a) track, (b) minimum central pressure, and (c) maximum wind speed spread comparison of CNTL, Renv, Rcore, and Rloc ensembles