The improvements in tropical cyclone track prediction by T639L60 of the National Meteorological Center, CMA

The global spectral model T639L60 started to provide tropical cyclone (TC) track forecast in 2009 at the National Meteorological Center, CMA, China. The initial vortex in T639L60 is constructed by inserting a BOGUS vortex for the first forecast once a TC is initially named. In the following forecast, the forecast TC vortex from previous forecast is relocated to match the observed TC location and the vortex intensity is modified to match the observed TC intensity. The forecast TC tracks from 2009-2012 by this system show obvious northeastward bias during the early several forecasts for TCs with westward or northwestward motions.

A new initial vortex scheme based on the bogus data assimilation (BDA) technique was developed for the first forecast as the vortex initialization and tested for 16 TCs in 2012. The BOGUS data in the BDA are constructed based on the analyzed TC data, including the minimum sea level pressure, maximum surface wind and the radius of 15m/s. The data include u, v and sea level pressure from 1000 hPa to 300 hPa with 3D asymmetric structure. The BOGUS data are analyzed through GSI data assimilation system as conventional observations. The initialized TC vortex from the BDA scheme has larger size and weaker intensity in the TC core compared to the BOGUS vortex. The experimental prediction results for TCs in 2012 indicate that the northeastward bias was greatly reduced by using the BDA vortex initialization at the first forecast once a TC was first named.