Initializing the Taiwan WRF-based Regional Ensemble Prediction System with an Ensemble Partial Cycling Strategy

A 20-member WRF-based regional Ensemble Prediction System (WEPS) is operationally run at the Central Weather Administration (CWA) to provide up to 5-day ensemble forecasts over the East Asian region with a 15-km grid spacing and a 3-km nest centered over Taiwan. Since becoming operational in 2011, WEPS has been under continuous development that aims to improve the construction of its perturbations in initial conditions (IC), boundary conditions (BC), as well as model uncertainties due to numerical approximations and physical parameterizations. Among these uncertainties, construction of IC perturbations for WEPS is the focus of this study.

 

An initialization method named ensemble partial cycling (EnPC) is proposed for the WEPS. The EnPC method combines partial cycling data assimilation (DA) and the ensemble of DA approach with an additional blending procedure that merges large-scale global features with small-scale regional information, leveraging the DA efforts from the deterministic system of CWA. EnPC is compared with three other initialization methods that are popularly used for regional ensemble forecasting, including dynamic downscaling from a global EPS, Ensemble Adjustment Kalman Filter (EAKF) based regional ensemble DA, and a blended version of the two, the last of which is equivalent to the current operational configuration of WEPS. Among all 4 methods, EnPC is the only method that allows separate initializations for the parent and the nested domains while the initialization for the nested domain in the other three methods is simply a downscale-interpolation from the corresponding parent grid.

 

Several sets of WEPS experiments are conducted over a 5-week period, including five typhoons. EnPC-initialized WEPS forecasts are found to be comparable to the dynamically downscaled forecasts in many evaluation metrics and have more accurate near-surface forecasts over the first 12 h and better precipitation forecast discrimination ability for typhoon events. Compared to the EAKF and the blended methods, forecasts initialized from EnPC have overall smaller errors in most of the evaluation metrics by both deterministic and probabilistic measures and better spread-to-error ratios. As an alternative initialization method, EnPC not only adds some regional benefits on top of downscaling, but also shows some advantages over the operational method. With the planned retirement of EAKF and the anticipation of a more unified production suite at CWA, EnPC will replace the current operational method.