Applications of the mean recentering scheme to improve the typhoon track forecast. A case study of Typhoon Namadol (2011)

Under the strong nonlinear evolution, the assumption of the Gaussian distribution for the ensemble may be strongly violated and thus the mean of the ensemble cannot be the best estimate for the atmosphere. In this study, a mean-recentering scheme (MRC) is proposed to deal with this issue when the typhoon track prediction is conducted under a highly uncertain condition. The capability of the mean-recentering scheme is investigated by a case of the typhoon Nanmadol in 2011, which moves northward initially, but turns westward sharply at 0000UTC 24 August. Factors contribute to Nanmadol‘s movement prediction include the saddle field between Typhoons Nanmadol and Talas, the development of the subtropical height at the north side of both typhoons and Nanmadol's own circulation.

The MRC method successfully improves the typhoon track prediction with a regional ensemble prediction system based on the Weather and Research Forecasting (WRF) model. The correction from the MRC method allows the ensemble to capture the nature behavior when the original ensemble track prediction is poor. Such ensemble adjustment can have positive feedback to the background error covariance used in the ensemble-based data assimilation system. The MRC method is implemented in the WRF-Local Ensemble Transform Kalman Filter (WRF-LETKF) system. With the MRC method, results show that the ensemble track prediction can be significantly improved during WRF-LETKF's spin-up period when Nanmadol development is highly uncertain. With the dynamical adjustment from the MRC method, the ensemble avoids suffering from the non-Gaussian and underdispersive issues shown in the original ensemble prediction.