Abstract/Summary

Recent studies have demonstrated that the representation of orography in models is highly uncertain. Motivated by the large spread in the climatological circulation and the circulation response to climate change seen among models, the primary aim of this work is to quantify the uncertainty introduced by their representation of orography. This is done through a number of experiments using different comprehensive atmospheric models across horizontal resolutions and timescales. First, it is shown that two of the models considered are unable to maintain an equivalent total (resolved plus parameterized) orographic drag across resolutions over the Northern Hemisphere (NH) mid-latitudes, leading to systematic biases at lower climate model resolutions. The suitability of substituting one drag parameterization scheme for another is also investigated. It emerges that there is a strong regional dependence of the model error on the drag parameterization scheme employed. High-resolution global and limited area models analysed over the Himalayan Plateau are used as a proxy for the truth. The non-robustness to resolution over this region is attributed to particular components of the orographic drag parameterization scheme and its formulation. It is shown that most of the reduction in short-range forecast error that occurs with increasing resolution is due to a reduction in the parameterized orographic drag, as opposed to the addition of resolved orographic drag. Finally, the impact of the uncertainty in the parameterized orographic drag scheme on the circulation and its response to climate change is investigated. The low-level parameterized orographic drag is found to be beneficial for the modelled stationary wave field over the NH and for the jet latitude in both hemispheres. Over the NH, the amplitude of the stationary wave response to climate change across the experiments is shown to scale with the magnitude of low-level parameterized orographic drag through its influence on the present-day climatological stationary wave amplitudes.