Impact of formulation and resolution on ensemble forecast performance in the tropics

A number of configurations of the Navy Operational Global Atmospheric Prediction System (NOGAPS) ensemble forecasting system were run for summer 2007 and 2008 seasons. Experiments were run testing ensembles with different initial perturbation formulations and different representations of model uncertainty. Ensembles were run at 110, 83, and 55km horizontal resolutions. Initial perturbations were formulated based on different implementations of the ensemble transform method. Model perturbations include parameter variations as well as stochastic forcing. The focus will be on the impact on lower and upper tropospheric wind forecasts, but the impact on Madden Julian Oscillation forecasts will also be reported on. It is found that both stochastic perturbations as well as parameter variations can result in improved spread-skill relationships in the tropics, and can also produce more ensemble spread in MJO forecasts, although the ensembles are still generally underdispersive. The impact of ensemble formulation on tropical cyclone track will be reported on in detail a companion abstract.