||Improvement of the initial conditions of typhoon plays the crucial role in the prediction of typhoon. Therefore, observations helpful for analyzing a typhoon should be used to adjust initial fields. This study employs the MM5 4DVAR model not only with CHAMP and FORMOSAT-3 GPS radio occultation data but also with bogus data assimilation (BDA) to provide the optimal initialization that gives a positive impact on typhoon prediction.|
In this paper, two typhoon cases, Dujuan (2003) and Shanshan (2006), were chosen for such an impact study. In the simulation experiments, the NCEP/AVN global analysis is used as the initial fields. Then, we conduct 4-dimensional variational (4DVAR) data assimilation which incorporates GPS refractivity and bogus observations into the model. In such a way, with dynamical and physical constraints of the model, the deviation of the model state from the observations will be minimized and optimized. The assimilation results show that the GPS refractivity data improve the initial field only in the surroundings but with smaller magnitudes. On the other hand, the bogus vortex assimilation method adjusts the weak initial typhoon vortex significantly. It clearly strengthens the structure of the vortex reaching a more realistic and stronger typhoon. Finally, in the assimilation with both GPS refractivity data and a bogus vortex, the BDA provides much larger modifications on the typhoon and thus tends to dominate the impact of the observations on the typhoon prediction.
Due to use of BDA improving both intensity and structure of the typhoon vortex, the track prediction has been improved as well, reducing the 72-h forecast error significantly. The predicted rainfall distribution and intensity for BDA are also greatly improved as compared to no-BDA run. In this study, we also revise the BDA method by including three dimensional gradient wind, which was found to well maintain the typhoon intensity in the whole layer. The results show that this revised BDA gives the best track prediction (with 60-km error on 72-h forecast) as compared to the no-BDA run and other BDA runs (assimilating the surface pressure and wind only).
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