5A.1 Comparing limited-area 3DVAR and hybrid variational-ensemble data assimilation methods for typhoon track forecasts: Sensitivity to outer loops and vortex relocation

Schwartz, Craig, Zhiquan Liu, Xiang-Yu Huang, Ying-Hwa Kuo, National Center for Atmospheric Research, and Chin-Tzu Fong, Central Weather Bureau

The Weather Research and Forecasting (WRF) model’s “hybrid” variational-ensemble data assimilation (DA) algorithm was used to initialize WRF model forecasts of three tropical cyclones (TCs). The hybrid-initialized forecasts were compared to forecasts initialized by WRF’s three-dimensional variational (3DVAR) DA system. An ensemble adjustment Kalman filter (EAKF) updated a 32-member WRF-based ensemble system that provided flow-dependent background error covariances for the hybrid. The 3DVAR, hybrid, and EAKF configurations cycled continuously for ~3.5 weeks and produced new analyses every 6-hrs that initialized 72-hr WRF forecasts. Additionally, the impact of employing a TC relocation technique designed for WRF and using multiple outer loops (OLs) in the 3DVAR and hybrid minimizations were explored.

Model output was compared to TC “best-track” observations. On average, the hybrid produced superior forecasts compared to 3DVAR when only 1 OL was used during minimization. However, when 3 OLs were employed, 3DVAR forecasts were dramatically improved but the mean hybrid performance changed little. Additionally, incorporation of TC relocation within the cycling systems further improved the mean 3DVAR-initialized forecasts but the average hybrid-initialized forecasts were nearly unchanged.

These results will be presented and practical considerations of using multiple OLs in WRFDA will be discussed.