The Impacts of COSMIC Radio Occultation Observations on 2008 Typhoon Forecasts

Improvement of typhoon forecast has been challenging because only limited observations are available over the ocean. The six-satellite COSMIC constellation, launched in April 2006, is providing approximately 2,300 radio occultation (RO) soundings per day, uniformly distributed around the globe. With high accuracy, high vertical resolution, and not affected by clouds and precipitation, COSMIC GPS RO soundings are becoming a key type of observations over the oceans, which would be extremely valuable for typhoon prediction.

COSMIC data is available whenever radio occultation event happens, and the total number of daily observations exceeds that of operational radiosonde observations. In this study, we perform data impact studies to assess the impact of COSMIC GPS RO soundings on the prediction of three typhoons over the Western North Pacific during the 2008 season that affect Taiwan. Because the short time window of data assimilation (DA) could not assimilate too many COSMIC GPS RO soundings, DA with a time window for 24 hours (hourly cycling) and longer is performed with the WRF 3DVAR system. In addition, conventional observations which are commonly assimilated in operational forecast centers are also assimilated to evaluate additional impact with COSMIC data.

The impact of the COSMIC data has been studied for three 2008 typhoon landfall cases in Taiwan. All experiments on these three cases with different initialization show positive impact on typhoon track forecasts, where the improvement is more significant with DA for a 24-hour time window than for cold (one-time DA with a 3-hour time window) 3DVAR. However, it takes about one day's free forecast to see improvement of typhoon intensity forecast because of periodic initialization in cycling 3DVAR DA. The results suggest that assimilation of radio occultation data with longer DA time window is important to improve typhoon forecast.