Typhoon Bilis which struck Taiwan in July 2006 was chosen to assess the potential impact of GPS radio occultation (RO) refractivity soundings on numerical simulation using the WRF model. We found that this case elucidates the impact of the limited GPS RO soundings on typhoon prediction due to their favorable locations. In addition, on top of available precipitable water (PW) and near-surface wind speed from SSM/I data, we have also explored their combined impacts on model prediction.
The two GPS RO soundings available from FORMOSAT-3 near the model initial time are assimilated using the nonlocal operator in WRF 3DVAR. More contributions after the assimilation are found in the moisture increments (up to -1.2 g kg-1) surrounding the corresponding occultation positions to the east of the typhoon vortex and southwest of the subtropical high, with dryness occurring in the lower troposphere. The assimilation ofPWinduces much larger and wider moisture increments to the west of the typhoon center. The ingestion of near-surface wind speeds produces more pronounced temperature and wind increments at lower levels.
The SSM/I data improve the predictions of typhoon intensity and intense rainfall over Taiwan, especially for the later simulation time, but generally have a less impact on track prediction as compared to the GPS RO data. Assimilation of the two GPS RO soundings leads to a marked improvement on the track that otherwise will be southward biased prior to landfall and also enhances the positive impact when the SSM/I data have been assimilated as well. An improvement using the GPS RO data is also found for daily rainfall but only for larger thresholds at later times when the bias in track in the control experiment is remedied. In general, the combined assimilation of both GPS RO and SSM/I data has shown the most promising performances. Finally, the sensitivity tests indicate that the closer GPS RO sounding to the east of the typhoon plays a more crucial role in the positive impact on track prediction.