PV diagnostics of the impact of model initialization on the performance of a typhoon prediction system

The modified 1997 version of the Typhoon Track Forecast System (TFS) of the Central Weather Bureau in Taiwan had made significant improvement on typhoon track forecasts (Huang et al., 1999). The mean 48h distance forecast error was decreased from 338km in1996 to 264km in 1997. Some changes, mainly on model initialization processes, have been made from version of 1996 to that of 1997. It is worthwhile of investigating how the different initial conditions affect the different movements of the storms. In this study, both the 1996 version and 1997 version of TFS are examined. The basic initial conditions are from the archived fields of CWB. For selected cases, the ECMWF post-analyed fields also have been used for comparison. Potential vorticity diagnostics has been used to help identifying dynamic features affecting the steering flow, and thus to know how different initialization procedures can result in different track forecasts. Some major characteristics of difference on PV distribution on different track categories has been identified, and will be presented in the conference. Some of these findings may be applicable to develop ensemble forecasts for operational use.