The impact of orographic gravity wave drag on the GCM prediction of atmospheric angular momentum

The impact of the parameterized gravity wave drag (GWD) on the prediction of atmospheric angular momentum and zonal wind is investigated by using extensive sets of medium range forecasts with identical initial conditions but varied strength of GWD. It is found that, although GWD causes a partial reduction of the error of local angular momentum in the Northern Hemisphere, it degrades the forecast of global angular momentum by creating a systematic negative bias in it. The unphysical sink of angular momentum due to GWD indicates that the parameterization scheme is not consistent with the balance of global angular momentum. The negative error of global angular momentum, initially caused by an excessive damping in GWD, saturates in week 2 in the medium range forecasts. This saturation process involves a compensating mechanism in the surface torques, namely, the friction and mountain torques weaken following the initial reduction of low-level zonal wind due to GWD. The weakenning of mountain and friction torques slows down the GWD-induced downward trend of angular momentum, eventually causing the error of global angular momentum to saturate.