Orographic Processes
Andrew Orr
Large scale flow response
Antarctica
Experiment simulating westerly flow
Turner et al., 2009
Baines and Fraedrich, 1989
Mean JJA 700hPa height
Large scale flow response
Greenland
Surface wind speed
Orr et al., 2005
Mean sea level pressure
Petersen et al., 2003
Complex orography
High horizontal resolution required to
represent complex orography and associated
processes
Powers et al., 2003
Spiers et al., 2010
Mean orography
UK Met Office Unified Model (UM)
Smith et al., 2006
Sensitivity to
resolution
Streamlines 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
Nh/U=0.5
Nh/U=1
Olafsson and Bougeault, 1996
Nh/U=1.4
Froude number, F 
Nh/U=2.2
Nh
1
U
Sensitivity to the Coriolis force
F  1.5; Ro  
Rossby deformatio n radius, LR 
Petersen et al, 2003
F  1.5; Ro  0.42
Nh
~ 100km
f
U
Rossby number , Ro 
fLx
f : Coriolis parameter
Antarctic Peninsula
Horizontal and vertical wind
Unified Model, 12km res
Blocking conditions, 29 Jan 2002
Orr et al., 2007
Flow-over conditions, 21 Feb 2002
Potential temperature
Unified Model, 12km res
Blocking conditions, 29 Jan 2002
Flow-over conditions, 21 Feb 2002
Aircraft observations
Flight 19: Jan 2006, ascent from Rothera
and descent over the Larsen Ice Shelf
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
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
Polar MM5
1) Modified parametrization for the prediction of ice cloud fraction
2) Improved cloud-radiation interactions
3) An optimal stable boundary layer treatment
4) Improved calculation of heat transfer through snow and ice
surfaces
5) The addition of fractional sea ice surface type
Is further optimization required?
Is higher horizontal or vertical resolution required?
Bromwich et al., 2001
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
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
Down-slope wind storms
Observations over Rocky mountains
Wave reflection, hydraulic jump, trapped lee waves
Lilly and Kennedy, 1973
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 largeamplitude 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
Rotors
Wind component simulated by
Met Office model BLASIUS at
200 m resolution
Sheridan and Vosper, 2006
Mobbs et al., 2005
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
Cross section: winds, terrainparallel wind (solid line), potential
temperature (dashed line)
Hybrid gap-barrier jet
13 Oct 2003
Tip-jets
Comparison of observations and COAMPS simulated surface
wind greater than 30 m/s on 18 Feb 1997
Doyle and Shapiro, 1999
Katabatic winds
Mean wintertime streamlines over the surface of the Antarctic
Parish and Bromwich, 2007
Mean AMPS surface wind speed from June 2003 – May 2004
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
Strong wind events
Turner et al, 2009
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
Comparison of ERA40 and UM 12 km simulation
10 m winds and MSLP
ERA40
Wind speed at Mawson
Observed: 37 m/s
ERA40: 20 m/s
UM 12 km: 22 m/s
UM 12 km
Minimum MSLP
ERA40: 944 hPa
UM 12km: 936 hPa
UM captures synoptic
forcing and simulates
stronger katabatic winds
Comparison of UM 12 and 4 km simulations
Observed: 37 m/s
UM 12 km: 22 m/s
UM 4 km: 24 m/s
Is higher resolution required to capture local
topographical conditions?
Is optimization of model required ?
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
Coastal jets (4 km res)
Very sharp gradients in
velocity across coastline
~30 m/s
~20 m/s
Mechanism
1) Offshore winds cross coastline
2) Accelerate due to reduced drag
Land
3) Turn to the left in the Southern
Hemisphere
4) If coast is on the left of the wind results in
horizontal convergence
5) Associated with this is the inversion height
rising offshore, due to conservation of
mass
6) Coriolis force induces a wind jet parallel to
coastline (see Hunt et al., 2004)
7) Temperature falls offshore encouraging
condensation and more cloud
Orr et al., 2005
convergence
Sea
h
~  fus
n
Laboratory investigation
UM simulation at 12 and 2 km
Orographic rain
Smith
Case study over New Zealand
Qualitative agreement at 12km
resolution and quantitative
agreement at <4 km resolution
Webster et al., 2008
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