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Federal Department of Home Affairs FDHA Federal Office of Meteorology and Climatology MeteoSwiss Parametrization of Subgrid- Scale Orographic Drag in the COSMO Model: An Alpine Perspective Oliver Fuhrer, Andries de Vries, MO MeteoSwiss

Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

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Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective. Oliver Fuhrer, Andries de Vries, MO MeteoSwiss. Overview. Introduction Motivation SSO Scheme Improvements Case Study Verification Conclusions & Outlook. Motivation. Motivation. - PowerPoint PPT Presentation

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Page 1: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Federal Department of Home Affairs FDHAFederal Office of Meteorology and Climatology MeteoSwiss

Parametrization of Subgrid-ScaleOrographic Drag in the COSMO Model: An Alpine Perspective

Oliver Fuhrer, Andries de Vries, MOMeteoSwiss

Page 2: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Overview

• Introduction• Motivation• SSO Scheme

• Improvements• Case Study• Verification• Conclusions & Outlook

Page 3: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Motivation

Page 4: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Motivation

• Hope for similar improvements in COSMO-7• Performance over the Alps unclear• Usefulness in COSMO-2 unclear

Page 5: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Subgrid-Scale Orography

Page 6: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

no

n-d

imen

sio

na

l mo

un

tain

he

igh

t G

h

Processes

non-dimensional mountain width Ga

flow blockingbreaking waves

vertically propagatingbuoyancy waves

SSO parametrizationeffective roughness length z0

turbulence

Page 7: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Scale-separation

• Need to separate turbulent and mesoscale unresolved drag

~ 31 km ~ 10 km ~ 4 km

subgrid-scaleresolved SSO turbulentresolved SSO turbulent

Page 8: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

OLD NEW

GLOBE topographic dataset(1 km resolution)

SRTM topographic dataset(92 m resolution)

Unsmoothed model topography Smoothed model topography

Use full sub-grid scale orography for SSO scheme

Use scales > 4 km for SSO scheme

Use full sub-grid scale orography for Z0

Use scales < 4 km for Z0

New External Parameters

Page 9: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

New External Parameters

• SSO parametrization external parameters change only slightly

• Substantial decrease in roughness length over topography

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OLD z0 NEW z0

Page 10: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Case Study

• Winter storm Quinten (10.2.2009)• Cold start• ECMWF

→ COSMO-7 +72h→ COSMO-2 +24h

Page 11: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Impact of SSO parametrization (1 of 3)

• Enhanced flow blocking• fast process (already after +1h)• local + large scale effect over orography• changes in pressure of 0.15 hPa and wind up to 10 m/s

Surface Pressure +6h SSO - REF Wind @ 2km +6h SSO - REF

Page 12: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Impact of SSO parametrization (2 of 3)

• Enhanced cross-isobar flow• slow process (significant after +24h)• limited in importance by domain size• difference in core pressure of ~ 0.4 hPa

Surface Pressure +48h SSO - REF

Page 13: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

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Impact of SSO parametrization (3 of 3)

• Reduction of resolved gravity wave activity• fast process• local and large scale effect over orography

Potential temperature +6h SSO - REF

Page 14: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Relevance of SSO for COSMO-2

• Amplitude of sub-grid scale orography reduced by ~ 35%• Impact of enhanced cross-isobar flow negligible due to

small domain and short integration time (+24h)• Impact of sub-grid scale GWD negligible• Nevertheless, non-negligible impact!

Surface pressure +12h SSO-REF Surface pressure +12h SSO-REF

only GWD only FD

Page 15: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Verification

• Testchain with full assimilation cycle of COSMO-7 and COSMO-2 running with SSO parametrization

• Winter period: 29.2.2008 to 19.3.2008 (20 days)• Summer period: 27.7.2008 to 19.8.2008 (24 days)• Evaluation over EU, ALPS, and CH domain• SYNOP and Upper-air verification

Page 16: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Verification (COSMO-7, EU domain)

• Positive impact in almost all parameters, especially 10m wind speed and direction as well as total precipiation

• In contrast to DWD, no significant reduction in pressure bias

--- a lot worse -- significantly worse - worse ~ same+++ a lot better ++ significantly better + better

Page 17: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

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Verification (COSMO-7, EU domain)

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PSbias

PSSTDE

FF10Mbias

FF10MSTDE

Page 18: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Verification (COSMO-7/2, CH domain)

• Increase of negative bias of wind speed and direction over the Alps

• Other parameters generally positive results

--- a lot worse -- significantly worse - worse ~ same+++ a lot better ++ significantly better + better * contrasing bias/STDE

Page 19: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Verification (COSMO-7/2, CH domain)

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PSbias

PSSTDE

FF10Mbias

FF10MSTDE

Page 20: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Verification (Summary)

• In general positive impact for both COSMO-7 and COSMO-2 for stations outside of significant topography

• Within the Alps, wind speed and direction bias is further increased, but verification is questionable

• Impact in COSMO-2 is roughly 1/3 over boundary conditions and 2/3 due to SSO parametrization

Page 21: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Conclusion

• Improvement of external parameters...• Higher resolution topographic dataset (SRTM)

• More “physical” treatment of processes via scale-separation

• Significant reduction in roughness length (z0)

• Significant impact via several processes...• Enhanced local + large-scale flow blocking

• Enhanced cross-isobar flow

• Reducation in resolved gravity wave activity

• Relevance for COSMO configurations...• COSMO-7: positive impact, GWD and FD relevant

• COSMO-2: positive impact, only FD relevant, cross-isobar flow irrelevant

• Will go operational this Thursday...

Page 22: Parametrization of Subgrid-Scale Orographic Drag in the COSMO Model: An Alpine Perspective

Outlook

• Verification over Topography questionable investigate a MAP case with flight measurements

• Tuning parameters according to GME and considerable differences to Lott and Miller 1997 tuning for COSMO-7/2 resolutions

• Sub-grid scale orographic turbulence still parametrized using effective roughness length approach test a more sophisticated parametrization