An Analysis on the Sources of Tropical Cyclone Forecast Track Bifurcation in the COAMPS-TC and GEFS Ensembles

Track and intensity forecasts for Tropical Cyclones (TCs) have improved significantly over the past few decades, largely due to improvements in numerical modeling. The Coupled Ocean/Atmosphere Mesoscale Prediction System for Tropical Cyclones Ensemble (COAMPS-TC Ensemble) developed by the Naval Research Laboratory (NRL) is a reliable prediction system, with forecast skill for intensity and track on par with top-performing TC forecast models. However, cases still exist where the skill for both track and intensity in the ensemble drops significantly and spread in the ensemble grows unusually large. This study compares TC forecasts from both the COAMPS-TC Ensemble and the Global Ensemble Forecast System (GEFS) for cases in which track bifurcation occurs. Both ensembles are initialized from the Global Forecast System (GFS), yet have differing parameterizations and initialization techniques. Global TC forecasts are retrieved for three seasons spanning from 2020 to 2022, and then analyzed to find cases where the ensemble forecast tracks bifurcate. It is found that the COAMPS-TC Ensemble bifurcates approximately twice as frequently as the GEFS, with many of the TCs being in a regime with weak steering flow. It is hypothesized that the increase in the frequency of track bifurcation in the COAMPS-TC Ensemble is primarily driven by minor differences in the synoptic-scale, which are the result of varying methodologies in initializing the model (initial and boundary conditions) and the vortex initialization scheme representing the TC. To address this hypothesis, the ensemble sensitivity metric, as defined in previous works, is calculated on pertinent fields to help identify key differences in the model, and the inherent sensitivity of the track to those fields. Preliminary results suggest that the existence of large spread/bifurcation in the ensemble tracks are primarily driven by the representation and evolution of the large-scale synoptic flow immediately in the vicinity of the TC.