Sources of Tropical Cyclone Track Bifurcation in the COAMPS-TC and GFS Ensembles

• DOI: 10.1175/waf-d-24-0012.1
• Publication Date: 2024-08-22T00:31:55Z
• Authors: Kevin C. Prince; Jonathan R. Moska...; James D. Doyle; William A. Komaromi
• Keywords: 0207 environmental engineering; 02 engineering and technology

Abstract As tropical cyclone (TC) official and model track forecasts improve, cases still exist where model position errors and uncertainty are large. The goal of this study is to identify forecast track bifurcations in both the Coupled Ocean–Atmosphere Mesoscale Prediction System–tropical cyclone (COAMPS-TC) ensemble and the Global Ensemble Forecast System (GEFS), with subsequent analyses focusing on the comparison of the synoptic environments of the bifurcating cases between the ensembles. Thirty-three bifurcating cases are identified in the COAMPS-TC ensemble, while 38 are identified in the GEFS over a time period spanning the 2020–22 TC seasons in the Northern Hemisphere. For these bifurcating cases, early lead time position of an ensemble member relative to the ensemble mean shows little correlation to the 72-h member’s position relative to the ensemble mean. Rather, the TC’s 72-h forecast position relative to the ensemble mean shows high sensitivity to the tropospheric deep-layer steering flow at early lead times, particularly in the COAMPS-TC ensemble. For both ensembles, a region of weak flow is located near the TCs at the forecast initial time. Minor differences in the steering flow at early lead times induced by variations in the synoptic-scale environment cause the TC’s position relative to the col to vary across the ensemble members, resulting in a bifurcation. Differences in the results for the COAMPS-TC ensemble and GEFS are thought to be driven, in part, by the fact that the COAMPS-TC ensemble utilizes flow-independent initial-state perturbations, while the perturbations in the GEFS are flow dependent.