WRF simulations of convectively generated gravity waves in opposite QBO phases

In this study, the Weather Research and Forecasting (WRF) model is used to assess gravity wave forcing of the Quasi-Biennial Oscillation (QBO) in January-February 2006 and 2007. The model domain is configured as a tropical channel with a horizontal grid-spacing of 37 km and a top at 0.1 hPa. The European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis Interim data provide initial and boundary conditions. In these simulations a large part of the gravity wave spectrum is explicitly simulated. The model is shown to have a realistic representation of tropical precipitation variability, mean tropical ascent, and evolution of the stratospheric zonal mean wind. The high-resolution model outputs are used to compute gravity wave forcing of the QBO. It is shown that gravity waves account for similar to 60% of the total eastward forcing during the westerly shear phase and for similar to 80% of the total westward forcing during the easterly shear phase. These estimates are in agreement with previous evaluations of gravity wave forcing of the QBO. In addition, wave forcing associated with large-horizontal-scale inertia-gravity waves is computed. In the simulations, sufficient vertical resolution in the stratosphere is required to properly resolve large-horizontal-scale inertia-gravity waves near the peak in QBO forcing. In the WRF experiments this wave type represents similar to 30% of the total gravity wave forcing. This suggests that large-horizontal-scale inertia-gravity waves can play an important role in the forcing of the QBO.