dc.description.abstract | In order to understand the impact of FORMOSAT-3 GPS RO measurements on the diagnosis and model simulation of Mei-Yu system in the East Asia region, this study investigates a heavy rain case occurring during 6-9 June 2007 by blending FORMOSAT-3 GPS RO data into CWB GFS analyses via objective analysis and WRF 3DVAR methods. The objective analysis method utilized is the optimum interpolation method. For the WRF 3DVAR method, FORMOSAT-3 GPS RO data are assimilated into the CWB GFS fields to get the best initial fields as the first step, then a 72hr simulation is conducted using the WRF model. Differences between analyses with and without FORMOSAT-3 GPS RO data are examined and compared.
Comparison results of objective analyses show no obviously difference over the land. This suggests that the FORMOSAT-3 GPS RO data are highly comparable with the radiosonde observations. However, obvious differences can be found over the ocean with particularly noticeable enhancement of Pacific subtropical high ridge line. In a global model, climatic analysis is usually used as the first guess field during the initial objective analysis procedure. Thus, the differences could be due to the discrepancy of the intensity of Pacific subtropical high ridge between the first guess field and the real atmospheric state of 2007.
After assimilating the FORMOSAT-3 GPS RO data into the model initial fields by the WRF 3DVAR method, results show that the differences of initial fields are highly correlated with the positions of the GPS RO data and the corrections exhibit like concentric circles. The differences will gradually expand during the model integration with the maximum corrections often found in areas of active convection along the Mei-Yu front and the front peripheries of Pacific subtropical high ridge. The results also show that model simulations with FORMOSAT-3 GPS RO data will enhance the characteristics of convective activities such as cloud top cooling and intrusion of high potential-vorticity air in the middle and upper troposphere.
By blending of FORMOSAT-3 GPS RO data into the analysis fields in these two ways, results show different degrees of impact on different meteorological parameters. Some of the impacts are positive and can be verified from other measurements such as satellite images. However, some of them are hard to give precise conclusions. Therefore, with the current data amount and distribution of FORMOSAT-3 GPS RO data, a systematic improvement in the diagnosis and forecast of the Mei-Yu system seems unfeasible at present. Now, many countries are planning on launching more and advanced GPS RO satellites, and in Taiwan we also have a FORMOSAT-3 follow-up mission too. We hope that, in the future, even better quality and denser data will contribute to advanced evolution of meteorology and improvement of weather analysis and forecast. | en_US |