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Orographic Processes Andrew Orr

Orographic Processes Andrew Orr. Large scale flow response Antarctica. Experiment simulating westerly flow. Baines and Fraedrich, 1989. Turner et al., 2009. Mean JJA 700hPa height. Large scale flow response Greenland. Surface wind speed. Mean sea level pressure. Petersen et al., 2003.

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Orographic Processes Andrew Orr

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  1. Orographic Processes Andrew Orr

  2. Large scale flow responseAntarctica Experiment simulating westerly flow Baines and Fraedrich, 1989 Turner et al., 2009 Mean JJA 700hPa height

  3. Large scale flow responseGreenland Surface wind speed Mean sea level pressure Petersen et al., 2003 Orr et al., 2005

  4. Complex orography High horizontal resolution required to represent complex orography and associated processes Powers et al., 2003 Spiers et al., 2010

  5. Mean orography UK Met Office Unified Model (UM) Smith et al., 2006

  6. Sensitivity to resolutionStreamlines over the Carpathian profile with different resolutions: orography smoothed to 32, 10, and 3.3 km Topographic map of Carpathian mountains Rontu, 2007

  7. Basic flow response to isolated mountain Nh/U=1.4 Nh/U=2.2 Nh/U=0.5 Nh/U=1 Olafsson and Bougeault, 1996

  8. Sensitivity to the Coriolis force Petersen et al, 2003

  9. Antarctic Peninsula Horizontal and vertical windUnified Model, 12km res Blocking conditions, 29 Jan 2002 Flow-over conditions, 21 Feb 2002 Orr et al., 2007

  10. Potential temperatureUnified Model, 12km res Blocking conditions, 29 Jan 2002 Flow-over conditions, 21 Feb 2002

  11. Aircraft observationsFlight 19: Jan 2006, ascent from Rothera and descent over the Larsen Ice Shelf

  12. Eastern Peninsula summer warming of 2oC over 40 years Difference in ERA40 10 m winds and surface temperature between years with strongly positive and strongly negative summer Southern Annular Model (SAM) Marshall et al., 2006

  13. Comparison of observations and AMPS Flight 19: Jan 2006, perturbations in vertical velocity and temperature, ascent from Rothera and descent over the Larsen Ice Shelf against Polar MM5, 10 km res

  14. Polar MM5 • Modified parametrization for the prediction of ice cloud fraction • Improved cloud-radiation interactions • An optimal stable boundary layer treatment • Improved calculation of heat transfer through snow and ice surfaces • The addition of fractional sea ice surface type Is further optimization required? Is higher horizontal or vertical resolution required? Bromwich et al., 2001

  15. Is a higher resolution required? Comparison of observations and COAMPS 1.7 km resolution simulation on 29 Jan 1997 over Greenland Doyle et al., 2005

  16. Comparison of observations and AMPS: impact of cold pool Polar MM5 at 2.2km resolution on a grid encompassing the Ross Island Area Spiers et al., 2010

  17. Down-slope wind storms Observations over Rocky mountains Wave reflection, hydraulic jump, trapped lee waves Lilly and Kennedy, 1973

  18. Evaluation of AMPS 15-16 May Ross Island severe wind storm case study simulated by AMPS (Polar MM5) at 3.3 km, res + Formation of barrier jet + Interacts with pre-existing near-surface radiation inversion over Ross Ice Shelf +Resulting conditions favourable for development of large-amplitude mountain waves + Leads to down slope windstorm in Ross Island Area + Underestimation of wind speed due to misplacement of hydraulic jump + Originates from inaccuracies in storm track + Migration to WRF and 3dVar assimilation might lead to improvement Steinhoff et al., 2008

  19. Rotors Wind component simulated by Met Office model BLASIUS at 200 m resolution Sheridan and Vosper, 2006 Mobbs et al., 2005

  20. Barrier jet Comparison of winds and temperatures (dashed) at 150 m from observations and 4km MM5 simulation on 26 Sep 2004 Olson et al., 2007

  21. Cross section: winds, terrain-parallel wind (solid line), potential temperature (dashed line)

  22. Hybrid gap-barrier jet 13 Oct 2003

  23. Tip-jets Comparison of observations and COAMPS simulated surface wind greater than 30 m/s on 18 Feb 1997 Doyle and Shapiro, 1999

  24. Katabatic winds Mean wintertime streamlines over the surface of the Antarctic Parish and Bromwich, 2007

  25. Mean AMPS surface wind speed from June 2003 – May 2004

  26. Comparison of observations and forecast over Greenland Polar MM5, 40 km res ‘reproduce the observed atmospheric state with a high degree of realism’ Brmowich et al., 2001

  27. Strong wind events Turner et al, 2009

  28. Case study: ERA40 MSLP at 0600 GMT 25 July 2004 when Mawson experienced a hurricane force wind of 37.5 m/s Interaction between katabtic pressure gradient force and synoptic pressure gradient force

  29. Comparison of ERA40 and UM 12 km simulation 10 m winds and MSLP ERA40 UM 12 km Wind speed at Mawson Observed: 37 m/s ERA40: 20 m/s UM 12 km: 22 m/s Minimum MSLP ERA40: 944 hPa UM 12km: 936 hPa UM captures synoptic forcing and simulates stronger katabatic winds

  30. Comparison of UM 12 and 4 km simulations Is higher resolution required to capture local topographical conditions? Is optimization of model required ? Observed: 37 m/s UM 12 km: 22 m/s UM 4 km: 24 m/s

  31. Evaluation of AMPS Polar MM5 at 30 km resolution from Sep 2001 to Aug 2003, 12-36 h Reduced surface wind speed correlation at coast line reflecting complex topography Bromwich et al., 2005

  32. Coastal jets (4 km res) Very sharp gradients in velocity across coastline ~30 m/s ~20 m/s

  33. Mechanism • Offshore winds cross coastline • Accelerate due to reduced drag • Turn to the left in the Southern Hemisphere • If coast is on the left of the wind results in horizontal convergence • Associated with this is the inversion height rising offshore, due to conservation of mass • Coriolis force induces a wind jet parallel to coastline (see Hunt et al., 2004) • Temperature falls offshore encouraging condensation and more cloud convergence Sea Land Orr et al., 2005

  34. Laboratory investigation

  35. UM simulation at 12 and 2 km

  36. Orographic rain Smith

  37. Case study over New Zealand Qualitative agreement at 12km resolution and quantitative agreement at <4 km resolution Webster et al., 2008

  38. Summary + Polar regions marked by complex orography. Requires a high resolution to resolve. + Some processes forecast well at medium resolution, such as barrier jets, katabatic winds + Some processes dependent on resolution (for example, gravity waves, rotors, precipitation, coastal jets) + Some processes dependent on boundary layer, etc, and complex interactions (for example, fohn winds) + Initial conditions important, both upstream and downstream

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