The major mountain ranges of the world play an important role in determining the strength and location of the atmospheric jet streams, mainly by generating planetary-scale Rossby waves and through surface drag. Orography acts both through large-scale resolved lifting and diversion of the flow over and around major mountain ranges, and through sub-grid scale momentum transport due to vertically propagating gravity waves at horizontal scales between 10 and 100 km. The limited horizontal resolution of climate GCMs implies a smoothing of the underlying topography which has sometimes been counteracted by enhancing the terrain by using an envelope orography, but this has adverse effects by displacing other surface physical processes. The sub-grid scale momentum transport acts to decelerate the upper-level flow and is included by gravity-wave drag parametrization schemes (e.g., Palmer et al., 1986; Kim, 1996; Lott and Miller, 1997).

No systematic studies are available to assess the impacts of these procedures and schemes on climate sensitivity and variability. The orographic impact is most pronounced in the Northern Hemisphere winter. Gravity wave drag implies a reduction in the strength of the mid-latitude jet stream by almost 20 ms^-1 (Kim, 1996), and alters the amplitude and location of the planetary-scale wave structure (e.g., Zhou et al., 1996). Convective precipitation in models is often spuriously locked onto high topography. Thus the numerical simulation of many key climatic elements, such as rainfall and cloud cover, strongly depends upon orography and is strongly sensitive to the horizontal resolution employed. As such, it is possible that phenomena of climate variability are sensitive to orographic effects and their parametrization (Palmer and Mansfield, 1986). These issues may have potentially important consequences for the planetary-scale distribution of climate change.

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