Improving global and regional high resolution hurricane forecasts using the hybrid variational-ensemble data assimilation

Hybrid variational-ensemble data assimilation (DA) system have been developed for both the NCEP operational Global Forecast System (GFS) data assimilation and modeling system (GDAS), and the regional Weather Research and Forecast (WRF) modeling system, and applied for tropical cyclone forecasts.

For the global hybrid DA system, in addition to the ensemble-3DVAR hybrid system, an ensemble-4DVAR system was further developed. Experiments were conducted during 2010 and 2011 hurricane seasons assimilating all operational observations. It was found that hurricane track forecasts initialized by both the ensemble-3DVAR hybrid and ensemble-4DVAR system were significantly improved upon the operational 3DVAR system. Compared to the ensemble-3DVAR hybrid, the ensemble-4DVAR hybrid further improved the hurricane track forecasts.

The high resolution WRF hybrid DA system is applied to the assimilation of radial velocity data from two coastal WSR-88D radars for the prediction of Hurricane Ike (2008) before and during landfall. The hybrid method produces temperature increments showing rainband structures and positive increments in the vortex core region, and a warm core throughout the hurricane depth in the final analysis. In contract, the 3DVAR produces much weaker and smoother increments with negative values at the vortex center at lower levels, and a cold core at lower levels in the final analysis. Wind, precipitation, track and intensity forecasts from the hybrid analyses were all better than those from the 3DVAR. The forecast results indicate that the hybrid method produces more accurate and dynamically more consistent state estimation for the hurricane.