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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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large scale flow response antarctica
Large scale flow responseAntarctica

Experiment simulating westerly flow

Baines and Fraedrich, 1989

Turner et al., 2009

Mean JJA 700hPa height

large scale flow response greenland
Large scale flow responseGreenland

Surface wind speed

Mean sea level pressure

Petersen et al., 2003

Orr et al., 2005

complex orography
Complex orography

High horizontal resolution required to represent complex orography and associated processes

Powers et al., 2003

Spiers et al., 2010

slide5

Mean orography

UK Met Office Unified Model (UM)

Smith et al., 2006

slide6
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

basic flow response to isolated mountain
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

horizontal and vertical wind unified model 12km res

Antarctic Peninsula

Horizontal and vertical windUnified Model, 12km res

Blocking conditions, 29 Jan 2002

Flow-over conditions, 21 Feb 2002

Orr et al., 2007

slide10

Potential temperatureUnified Model, 12km res

Blocking conditions, 29 Jan 2002

Flow-over conditions, 21 Feb 2002

slide11

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

slide12

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

slide13

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

slide14

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

slide15

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

slide16

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

slide17

Down-slope wind storms

Observations over Rocky mountains

Wave reflection, hydraulic jump, trapped lee waves

Lilly and Kennedy, 1973

slide18

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

slide19

Rotors

Wind component simulated by Met Office model BLASIUS at 200 m resolution

Sheridan and Vosper, 2006

Mobbs et al., 2005

slide20

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

slide23

Tip-jets

Comparison of observations and COAMPS simulated surface wind greater than 30 m/s on 18 Feb 1997

Doyle and Shapiro, 1999

slide24

Katabatic winds

Mean wintertime streamlines over the surface of the Antarctic

Parish and Bromwich, 2007

slide26

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

slide27

Strong wind events

Turner et al, 2009

slide28

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

slide29

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

slide30

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

slide31

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

slide32

Coastal jets (4 km res)

Very sharp gradients in

velocity across coastline

~30 m/s

~20 m/s

slide33

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

slide37

Case study over New Zealand

Qualitative agreement at 12km resolution and quantitative agreement at <4 km resolution

Webster et al., 2008

slide38

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