Comparison of TC Track Forecast Based on 3D EnVar and 4D EnVar Data Assimilation

The hybrid 4DEnVar was successfully implemented operationally at NCEP on May 11, 2016. Results from an observing system simulation experiment show that, compared with its hybrid 3DEnVar predecessor, the hybrid 4DEnVar reduces the analysis errors for most variables and levels. The hybrid 4DEnVar has several attractive aspects compared with traditional 4DVar. One notable advantage is the replacement of the tangent linear and adjoint models with an ensemble of full nonlinear forecasts, thus significantly reducing the computational cost and need for the generation and maintenance of the tangent linear and adjoint models.

This study focuses on the comparison of GEFS TC track forecasts initialized from the hybrid 3DEnVar and 4DEnVar data assimilation methods. We selected a few major hurricanes/typhoons from various basins to evaluate and compare the extended-range (beyond day-5) ensemble TC track forecast error and spread between the two methods. Both experiments with and without TC relocation are examined. The results from this study can help us to understand how the initial conditions impact the track forecast and ensemble spread.