Determination of Optimal Gravity Wave Source Parameters for Improved Seasonal Forecasting of the Quasi-Biennial Oscillation

In this study, we employ a prototype high-altitude version of the Navy Global Environmental Model (NAVGEM-HA) to ascertain the forcing needed to minimize hindcast errors compared to analysis in the equatorial lower stratosphere in order to improve forecasts of the quasi-biennial oscillation (QBO) over subseasonal-to-seasonal (S2S) time scales. NAVGEM-HA is a numerical weather prediction (NWP) system developed for middle atmosphere research that combines a hybrid 4DVar data assimilation system with a global semi-implicit semi-Lagrangian forecast model. As the QBO is primarily a wave-driven phenomenon, its structure and evolution in NWP models depends strongly on both resolved tropical wave activity and parameterized sub-grid scale gravity wave drag (GWD). To better constrain the equatorial GWD parameters that contribute to the forcing of the QBO, we use ensembles of 30-day NAVGEM-HA hindcasts over the 2014-2016 period with perturbed gravity wave source parameters. By developing a cost function to minimize errors in the equatorial lower stratosphere compared to analysis, we have determined the set of GWD source parameters that produces a forecast state that most closely agrees with observed QBO winds over each optimization time interval. Applying this methodology to multiple seasons reveals a time evolution in optimal gravity wave source parameters that may be implemented in NWP models to improve the QBO over both S2S time scales and longer interannual simulations. Specifically, we report findings on the optimal structure of the parameterized gravity wave spectrum and its seasonal variability.