How accurate is the Taiwan Central Weather Bureau (CWB) regional spectral model (RSM)? An assessment is conducted by numerical simulation of Mei-Yu fronts and their associated precipitation patterns during May to August of 1998 using CWB RSM, driven by analysis from the CWB global forecast system.

For this assessment, three episodes of East Asian Summer Monsoon heavy rainfall shown by Ding and Liu (2001) and an additional transition period from 26 May to 10 June were chosen to test the respective roles of domain size and model resolution sensitivity. Results indicate that the small domain (~3500 by 3000 km²) simulation, but not the larger domain (≥ 7000 by 6000 km²) simulations, simulated the monsoon and Mei-Yu front heavy precipitation events accurately for all of the above periods.

By comparing experimental results from various combinations of domain size and model resolution, we demonstrate that the simulated large-scale circulation and precipitation distribution were more sensitive to model domain size than to model horizontal resolution, whereas finer structures could only be achieved in the higher resolution simulations. Local characteristics of the precipitation over Taiwan Island, especially over the Central Mountains, were reasonably reproduced in the 20 km resolution simulation in terms of averages taken during the transition period and over the seasonal period from May to August. However, the CWB RSM with 20 km resolution might still be insufficient to resolve the severe precipitation events in southern Taiwan.