An Evaluation of the Performance of the CWB NFS Model for Warm-Season Rainfall Distribution and Propagation over the East Asian Continent

Abstract

This study evaluates the performance of the regional operational model at Central Weather Bureau (CWB), the Non-hydrostatic Forecast System (NFS), in capturing the general distribution and eastward propagation of warm-season rainfall within the diurnal cycle in Hovmoler (longitude-time) space over the East Asian continent. The Tropical Rainfall Measuring Mission (TRMM) 0.25° rain-rates are used to evaluate the NFS coarse domain (45 km) 12 - 36 h QPFs during May - August, 2002 - 2005 both qualitatively and quantitatively.

Our results show that the propagating rainfall signals to the lee of the Tibetan Plateau (TP) in the diurnal cycle, evident in TRMM data, are poorly captured in the NFS QPFs throughout the warm season, similar to earlier results in the United States. The nocturnal rainfall peak near the Sichuan Basin in the NFS is unclear, and the propagation is confined to a smaller region in May - June and almost missing entirely in mid-summer. Overall, the model QPFs exhibit largest disagreement with observations in June, and smallest in May.

There is a tendency for the NFS to over-predict rainfall in eastern TP. However, both the total amount and the diurnalwave amplitude are under-predicted to the lee, where a lack of propagation signals also leads to increased phase error farther downstream. A persistent phase error (at least 7 h) is also found over 110 - 120°E, with early morning maxima in the model but afternoon in TRMM data. Overall, the 1200 UTC model runs predict less rainfall compared to 0000 UTC runs, while the NFS also showed some improvements from 2002 to 2005 but the leeside propagation is still under-represented.

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