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What causes foehn warming? The Antarctic Peninsula as a natural laboratory Andy Elvidge and Ian Renfrew (School of Environmental Sciences, University of East Anglia) What is foehn warming? Foehn warming is the warming effect experienced in the lee of mountains Generally accompanied by foehn winds and a decrease in humidity Foehn winds Prevailing flow Humidity Temperature mountain Why is leeside air warm and dry? Four mechanisms identified in the literature Only two of which have been widely considered Why is leeside air warm and dry? Why is leeside air warm and dry? Why is leeside air warm and dry? Why is leeside air warm and dry? Foehn paradigms Hann (1901) – first thorough examination of foehn theory “Precipitation on the windward side is not a necessary condition for foehn.” Ficker (1920): Latent heating and Isentropic drawdown Foehn paradigms Hann (1901) – first thorough examination of foehn theory “Precipitation on the windward side is not a necessary condition for foehn.” Ficker (1920): Latent heating and Isentropic drawdown Ficker and de Rudder (1943) – the paradigm shifts: Latent heating In later edited editions of Hann’s famous textbook, the text was changed to mention only the latent heat effect. Foehn paradigms Hann (1901) – first thorough examination of foehn theory “Precipitation on the windward side is not a necessary condition for foehn.” Ficker (1920): Latent heating and Isentropic drawdown Ficker and de Rudder (1943) – the paradigm shifts: Latent heating In later edited editions of Hann’s famous textbook, the text was changed to mention only the latent heat effect. And then… Foehn paradigms Hann (1901) – first thorough examination of foehn theory “Precipitation on the windward side is not a necessary condition for foehn.” Ficker (1920): Latent heating and Isentropic drawdown Ficker and de Rudder (1943) – the paradigm shifts: Latent heating In later edited editions of Hann’s famous textbook, the text was changed to mention only the latent heat effect. ALPEX, Seibert (1990), Ólafsson (2005): Effects of latent heating concluded to be insignificant relative to the effect of isentropic drawdown. Isentropic drawdown Foehn paradigms Hann (1901) – first thorough examination of foehn theory “Precipitation on the windward side is not a necessary condition for foehn.” Ficker (1920): Latent heating and Isentropic drawdown Ficker and de Rudder (1943) – the paradigm shifts: Latent heating In later edited editions of Hann’s famous textbook, the text was changed to mention only the latent heat effect. ALPEX, Seibert (1990), Ólafsson (2005): Effects of latent heating concluded to be insignificant relative to the effect of isentropic drawdown. Isentropic drawdown Seibert et al. (2000), Richner and Hächler (2013): Report foehn cases characterised by upwind ascent from near‐surface level and severe precipitation over the windward slopes Latent heating and Isentropic drawdown Foehn paradigms Hann (1901) – first thorough examination of foehn theory “Precipitation on the windward side is not a necessary condition for foehn.” Ficker (1920): Foehn warming contributions have never before been In later edited editions of Hann’squantified famous textbook, the text was changed to mention only the latent heat effect. Latent heating and Isentropic drawdown Ficker and de Rudder (1943) – the paradigm shifts: Latent heating ALPEX, Seibert (1990), Ólafsson (2005): Effects of latent heating concluded to be insignificant relative to the effect of isentropic drawdown. Isentropic drawdown Seibert et al. (2000), Richner and Hächler (2013): Report foehn cases characterised by upwind ascent from near‐surface level and severe precipitation over the windward slopes Latent heating and Isentropic drawdown The Antarctic Peninsula A Natural laboratory for the study of foehn: ‐ Continuously high altitude (up to ~2000 m) s ‐ Elongated, ridge‐like form (~1500 km long) ~1500km s ‐ Both the westward ocean and the eastward Larsen Ice Shelves are relatively smooth, simple and homogeneous surfaces Van Lipzig et al. (2008) Data • • Aircraft observations as part of the OFCAP (Orographic Flows and Climate of the Antarctic Peninsula) field campaign (Jan‐Feb 2011) UK Met Office Unified Model (MetUM) 1.5km British Antarctic Survey’s MASIN (instrumented Twin Otter aircraft) during the OFCAP field campaign MetUM 4 km MetUM 1.5 km 3 foehn events – foehn winds and warming Plot height = 300 m Case A Case B Case C From MetUM 1.5 km simulation 3 foehn events – foehn winds and warming Plot height = 300 m Case A: model validation Case A From MetUM 1.5 km simulation Observations: solid lines Model: dashed lines Upwind Downwind Further downwind 3 foehn events – back trajectories Plot height = 300 m Case A Case B Case C 3 foehn events – back trajectories Plot height = 300 m Case A Case B Case C 1) WI = Whirlwind Inlet (JET) 2) N of WI = North of Whirlwind Inlet (WAKE) 3) MOI = Mobil Oil Inlet (JET) 4) N of MOI = North of Mobil Oil Inlet (WAKE) 5) CI = Whirlwind Inlet (JET) 6) S of CI = South of Cabinet Inlet (WAKE) • Heini Wernli group back trajectory model (Lagranto). • UM 1.5km as input. • Trajectory initiation at 950 hPa. Plot height = 300 m Case A Case B Case C Blue dots = ~150 km upwind of Peninsula’s crest (150 km ≈ Rossby radius of deformation for the Antarctic Peninsula) Foehn heat budget model Foehn heat budget model C C C Foehn heat budget model B B B Foehn heat budget model A A A Foehn heat budget model Foehn heat budget model Foehn heat budget model Due to mechanical mixing Foehn heat budget model results + = Advective drawdown + = zupwind – zdownwind zupwind zdownwind Foehn heat budget model results + = Advective drawdown + = zupwind – zdownwind zupwind zdownwind Foehn heat budget model CASE A ‐ Isentropic drawdown dominates results Potentially warm, dry air + = Advective drawdown + = zupwind – zdownwind Cool, Moist air Flow blocking zupwind zdownwind Mountain Warm, dry air Foehn heat budget model results + = Advective drawdown + = zupwind – zdownwind zupwind zdownwind Foehn heat budget model results + = Advective drawdown + = zupwind – zdownwind zupwind zdownwind Foehn heat budget model CASE C – Latent heating and precipitation dominates results Latent heat release + = Advective drawdown + = zupwind – zdownwind zupwind Cool, moist air zdownwind Mountain Warm, dry air Foehn heat budget model results + = Advective drawdown + = zupwind – zdownwind zupwind zdownwind Foehn heat budget model CASE B – no dominant mechanism results + = Advective drawdown + = zupwind – zdownwind zupwind zdownwind Foehn heat budget model CASE B – Mechanical mixing is significant results Potentially warm, dry air Sensible heat flux + = Advective drawdown + = zupwind – zdownwind zupwind z downwind Cool, moist air Mountain Warm, dry air Conclusions Conclusions • First quantitative assessment of the causes of foehn warming, using a novel heat budget model. Conclusions • First quantitative assessment of the causes of foehn warming, using a novel heat budget model. • Results show that: – both established foehn‐warming mechanisms are important. – a third, hitherto neglected, foehn‐warming mechanism can also be significant; mechanical mixing. Conclusions • First quantitative assessment of the causes of foehn warming, using a novel heat budget model. • Results show that: – both established foehn‐warming mechanisms are important. – a third, hitherto neglected, foehn‐warming mechanism can also be significant; mechanical mixing. • Any of these three mechanisms can dominate, depending on the larger‐scale meteorological conditions and the orographically‐forced flow dynamics. Conclusions • First quantitative assessment of the causes of foehn warming, using a novel heat budget model. • Results show that: – both established foehn‐warming mechanisms are important. – a third, hitherto neglected, foehn‐warming mechanism can also be significant; mechanical mixing. • Any of these three mechanisms can dominate, depending on the larger‐scale meteorological conditions and the orographically‐forced flow dynamics. • All 3 mechanisms must be well represented in numerical weather prediction models. Notably a need for accurate representation of turbulent‐mixing in foehn flows (known to be underestimated in current models). Conclusions • First quantitative assessment of the causes of foehn warming, using a novel heat budget model. • Results show that: – both established foehn‐warming mechanisms are important. – a third, hitherto neglected, foehn‐warming mechanism can also be significant; mechanical mixing. • Any of these three mechanisms can dominate, depending on the larger‐scale meteorological conditions and the orographically‐forced flow dynamics. • All 3 mechanisms must be well represented in numerical weather prediction models. Notably a need for accurate representation of turbulent‐mixing in foehn flows (known to be underestimated in current models). • Improved forecasting and representation of foehn events would have ramifications for associated hazards such as ice shelf melt, wild fires and avalanches. Californian wildfire, driven by foehn (Santa Ana) winds Thanks for your attention Photo from NASA For more foehn winds over the Antarctic Peninsula, see my second talk at WWOSC at 1pm on Thursday (ISS session): Foehn jets and their implications for the Larsen C Ice Shelf, Antarctica