Impact of Assimilation of GPS Radio Occultation Refractivity on the Forecast of Typhoon Usagi in 2007

Abstract

The purpose of this study is to improve the initial field accuracy for the simulation of tropical cyclones (TCs) in their initial and developing stages through mesoscale data assimilation with Global Positioning System (GPS) radio occultation (RO) soundings. The mesoscale 4-dimension variational data assimilation (Meso 4D-Var) system of the Japan Meteorological Agency (JMA), modified for low latitudes, is used to investigate Typhoon Usagi in July 2007. The performance of the Meso 4D-Var system and the impact of the GPS RO refractivity are compared against the JMA’s global analysis by conducting forecasts using the JMA non-hydrostatic model.When the JMA global analysis is used for the initial field, no typhoon is formed in the forecast model. In contrast, when the global analysis is replaced by the Meso 4D-Var analysis, the generation of the TC is successfully simulated. With GPS refractivity assimilated, the intensity forecast is significantly improved. The results indicate that the mesoscale data assimilation system performs well in low latitudes and that GPS RO data, which have high accuracy and considerably robust characteristics, are beneficial for the TC forecast. It is expected that the operational use of the assimilation system with GPS RO will contribute to the mitigation of meteorological disasters in low-latitude regions.