Quantifying the role of orographic gravity waves on polar stratospheric cloud occurrence in the Antarctic and the Arctic

[1] The proportion of polar stratospheric clouds due to orographic gravity wave (OGW) forcing is quantified during four Antarctic (2007-2010) and four Arctic (2006/2007 to 2009/2010) winter seasons. OGW-active days are defined as those days above major polar mountain ranges which have wave-ice polar stratospheric clouds (PSCs), tropospheric wind conditions appropriate for orographic wave generation and propagation, and stratospheric temperatures below the frost point: 37% of Antarctic days and 12% of Arctic days are OGW-active. Regions downstream of these mountain ranges are defined using a forward-trajectory model which follows particle movement from ridge lines for 24 h periods. In both hemispheres in these mountain regions, more than 75% of H2O ice PSCs and around 50% of a high number density liquid-nitric acid trihydrate mixture class (Mix 2-enh) are attributed to OGW activity, with the balances due to non-orographic formation. For the whole Arctic (equatorward of 82 degrees), 25% of Mix 2-enh and 54% of H2O ice PSCs are attributed to OGWs, while for the whole Antarctic, 7% of Mix 2-enh and 13% of H2O ice PSCs are attributed to OGWs. For all types of PSC, 5% in the whole Antarctic and 12% in the whole Arctic are attributed to OGW forcing. While gravity waves play a role in PSC formation in the Antarctic, overall it is minor compared with other forcing sources. However, in the synoptically warmer Arctic, much larger proportions of PSCs are due to OGW activity.