Retrospective and real-time tests of GSI-based EnKF-Var hybrid data assimilation system for HWRF using high resolution ensemble

A hybrid ensemble Kalman filter-variational data assimilation system has been developed based on the US NCEP operational data assimilation system, GSI. The hybrid data assimilation (DA) system became operational for the US Global Forecast System (GFS) since May 22, 2012. The new hybrid DA system has significantly improved many aspects of the operational global forecasts. Since then, efforts have been made to further develop and test the same hybrid system with the operational Hurricane Weather Research and Forecast (HWRF) modeling system to improve high resolution tropical cyclone prediction.

The HWRF hybrid DA system was tested with retrospective hurricane cases from past hurricane seasons and was tested in near-real time during the 2013 hurricane season. The airborne Doppler radar data from NOAA P3 aircraft were assimilated. Verification against independent in situ flight level data and remotely sensed observations such as SFMR wind speed and HRD radar wind composite shows that the analyses provided by the hybrid DA system capture the hurricane structure much better than the GSI 3DVar. Forecast initialized from the analysis of the hybrid system produces smaller track errors and better MSLP and max wind relationship than that initialized from the GSI 3DVar, operational HWRF forecast and forecasts without assimilating the airborne Doppler radar data. The system is further enhanced with the four-dimensional ensemble-variational (4DEnsVar) hybrid data assimilation capability and with dual-resolution assimilation capability where the control forecast is run at even higher resolution than the ensemble. Experiments with the new features will also be presented in the conference.