Orographic Gravity Wave Drag and the Large Scale Atmospheric Circulation

The need for accurate climate modelling demands an understanding of the physical processes in the atmosphere in order to improve model capability. Much of this modelling is concerned with the general circulation, which is affected by orographic gravity waves and the drag force that they exert on the mean flow. The effects of Orographic Gravity Wave Drag (OGWD) are what is explored in this thesis. By experimenting on the OGWD forcing in an idealized General Circulation Model. Firstly, the impact of OGWD in the model is quantified for the wintertime hemispheres, particularly the change to the zonal wind profile, where the polar vortex and eddy driven jet are shifted equatorward in the Northern Hemisphere. The sensitivity of the circulation to the strength of the OGWD is then explored. In the Northern Hemisphere, a reduction of the resolved wave drag compensates an increase of the parameterized OGWD. Weak OGWD yields a similar response to strong OGWD, suggesting that the atmosphere can become ‘saturated’ with OGWD. A brief comparison of OGWD from CMIP6 models shows the importance of constraining uncertainty. The saturation effect is explored further by examining OGWD in a regional case by case basis, allowed for in an idealised modelling framework. It is found that different mountainous regions (the Andes, the Rockies, Asia and Antarctica) have differing impacts on the general circulation, due to their size, roughness and location, and that there is mutual interference between them. Generally there is a split between the Southern Hemisphere, where the OGWD adds together approximately linearly, and the Northern Hemisphere, where it does not. It can be shown, through a similar mechanism, that the response of the general circulation depends on the background atmospheric state, and whether it is already saturated with OGWD or not. The importance of the non-mountainous terrain to the global OGWD budget is shown to be similar to that in mountainous regions, due to the large spatial distribution of the base momentum flux and resilience of the saturation flux. In addition to different horizontal regions of drag deposition, a similar method is used to show that OGWD in the troposphere/lower stratosphere impacts the surface winds alone and reduces the stratospheric drag response. The compensation due to resolved wave driving is due to the OGWD in the upper stratosphere alone. This then leads to an examination of how circulation changes due to global warming are altered by OGWD and the feedback between the changing circulation and OGWD. Although there is some reliance on the base state, there is also direct forcing from the modification of OGWD in the Northern Hemisphere.<p></p>

Author dc:creator

• Ross Castle (24321425)

• Climate modelling
• Orographic Gravity Wave Drag

Statement dc:rights

• All rights reserved
• Open Access after 2028-01-06

Identifier

10779/exe.32861501.v1

OAI identifier oai:identifier

oai:figshare.com:article/32861501