Climate model biases introduced by parameterized orographic gravity wave drag

The impact of the parameterized gravity wave drag (GWD) on a GCM's prediction of atmospheric zonal wind and angular momentum is systematically investigated in the model parameter space. Our experiments comprise extensive sets of medium range forecasts with identical initial conditions but varied strength of the parameterized GWD. Analyzing the systematic errors in these forecasts, we discover two previously-unknown characteristics of the parameterized GWD: (1) Although an inclusion of GWD modestly improves the forecast of local angular momentum, it degrades that of the globally-integrated angular momentum. (2) The benefit in the improved forecast of local angular momentum quickly saturates as the strength of GWD increases. At this saturation level a large systematic error of local and global angular momentum remains, indicating the need of other treatments beyond the mere installation of a momentum sink such as the parameterized GWD. Since we have shown previously that the forecast error of atmospheric angular momentum saturates after about 10 days, the systematic errors in the medium range forecasts discussed here are closely related to the climate drift of the general circulation model. Thus, our results have implications not only for short-term weather forecasts but also long-term climate predictions.