Impact of GPS precipitable water on WRF 3DVAR rainfall forecast

We performed single point data assimilation test, observation system experiment (OSE), and observation system simulation experiment (OSSE), of GPS PWV in the 3 dimensional data assimilation system of the weather research and forecasting (WRF) model with a 4-km high-resolution grid domain. The OSE results showed improvement of moisture field by assimilating GPS PWV both in the U.S. and Japanese domains although the improvement was not always seen over the ocean in the Japanese domain. The results of the OSSE show that denser spacing of GPS sites improves moisture profiles. Positive impact of GPS PWV on rainfall forecast without cumulus parameterizations was observed for several active weather conditions. GPS PWV did not only help to increase the amount of rainfall but also helped to remove false rainfall forecast in the cycling (multiple) assimilation. The best impact on rainfall forecast was observed in the experiment of combined use of GPS PWV and wind profiler data. The positive impact of GPS PWV started from a few hours after free forecasts to the end of the forecast (18 hours) in the U.S. domain, but was limited to several hours in the Japanese domain due to lack of moisture information over the ocean.