Orographic Processes Andrew Orr
Jan 14, 2016
Orographic Processes
Andrew Orr
Turner et al., 2009 Baines and Fraedrich, 1989 Mean JJA 700hPa height
Experiment simulating westerly flow
Large scale flow responseAntarctica
Orr et al., 2005 Petersen et al., 2003
Large scale flow responseGreenland
Surface wind speed Mean sea level pressure
Powers et al., 2003
High horizontal resolution required to represent complex orography and associated processes
Complex orography
Spiers et al., 2010
Smith et al., 2006
Mean orography UK Met Office Unified Model (UM)
Rontu, 2007
Topographic map of Carpathian mountains
Sensitivity to resolution
Streamlines over the Carpathian profile with different resolutions:
orography smoothed to 32, 10, and 3.3 km
Nh/U=0.5 Nh/U=1
Olafsson and Bougeault, 1996
Nh/U=1.4 Nh/U=2.2
1 number, Froude U
NhF
Basic flow response to isolated mountain
Petersen et al, 2003 parameter Coriolis:
,numberRossby o
f
fL
UR
x
RoF ;5.1 42.0;5.1 RoF
Sensitivity to the Coriolis force
km100~ radius,n deformatioRossby Rf
NhL
Horizontal and vertical windUnified Model, 12km res
Blocking conditions, 29 Jan 2002 Flow-over conditions, 21 Feb 2002
Orr et al., 2007
Antarctic Peninsula
Potential temperatureUnified Model, 12km res
Blocking conditions, 29 Jan 2002 Flow-over conditions, 21 Feb 2002
Aircraft observationsFlight 19: Jan 2006, ascent from Rothera
and descent over the Larsen Ice Shelf
Marshall et al., 2006
Eastern Peninsula summer warming of 2oC over 40 yearsDifference in ERA40 10 m winds and surface temperature between years with strongly positive and strongly negative summer Southern Annular Model (SAM)
Comparison of observations and AMPSFlight 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
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
Polar MM5
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
Spiers et al., 2010
Polar MM5 at 2.2km resolution on a grid encompassing the Ross Island Area
Lilly and Kennedy, 1973
Down-slope wind stormsObservations over Rocky mountainsWave reflection, hydraulic jump, trapped lee waves
Evaluation of AMPS15-16 May Ross Island severe wind storm case study simulated by AMPS (Polar MM5) at 3.3 km, res
Steinhoff et al., 2008
+ 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
Mobbs et al., 2005
Rotors
Sheridan and Vosper, 2006
Wind component simulated by Met Office model BLASIUS at 200 m resolution
Barrier jet
Olson et al., 2007
Comparison of winds and temperatures (dashed) at 150 m from observations and 4km MM5 simulation on 26 Sep 2004
Cross section: winds, terrain-parallel wind (solid line), potential temperature (dashed line)
Hybrid gap-barrier jet13 Oct 2003
Doyle and Shapiro, 1999
Tip-jetsComparison of observations and COAMPS simulated surface wind greater than 30 m/s on 18 Feb 1997
Parish and Bromwich, 2007
Katabatic windsMean wintertime streamlines over the surface of the Antarctic
Mean AMPS surface wind speed from June 2003 – May 2004
Comparison of observations and forecast over Greenland
Brmowich et al., 2001
Polar MM5, 40 km res
‘reproduce the observed atmospheric state with a high degree of realism’
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
ERA40 UM 12 km
Wind speed at MawsonObserved: 37 m/sERA40: 20 m/sUM 12 km: 22 m/s
Comparison of ERA40 and UM 12 km simulation10 m winds and MSLP
Minimum MSLPERA40: 944 hPaUM 12km: 936 hPa
UM captures synoptic forcing and simulates stronger katabatic winds
Observed: 37 m/sUM 12 km: 22 m/sUM 4 km: 24 m/s
Comparison of UM 12 and 4 km simulations
Is higher resolution required to capture local topographical conditions?
Is optimization of model required ?
Bromwich et al., 2005
Evaluation of AMPSPolar 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
Coastal jets (4 km res)Very sharp gradients in velocity across coastline
~30 m/s
~20 m/s
Mechanism
1) Offshore winds cross coastline2) Accelerate due to reduced drag3) Turn to the left in the Southern
Hemisphere4) If coast is on the left of the wind results in
horizontal convergence5) 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
Land
Sea
convergence
Orr et al., 2005
sfun
h
~
Laboratory investigation
UM simulation at 12 and 2 km
Orographic rain
Smith
Webster et al., 2008
Case study over New ZealandQualitative agreement at 12km resolution and quantitative agreement at <4 km resolution
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