Presently the majority of the inter-comparison projects under GASS are somewhat focused on thermodynamic and/or microphysics processes. At the first Pan-GASS conference, Anton Beljaars presented some results comparing surface stresses from the ECMWF and UKMet models and suggested that more work on momentum exchanges could be of interest to all. During the 2012 WGNE meeting, the Canadian center report showed an example on recent adjustments to the orographic blocking and boundary layer schemes, and the large-scale forecast improvements that followed. WGNE therefore agreed to get started a momentum/drag inter-model comparison study.

Requested Output

At the WGNE meeting, the Canadian center (represented by Dr. A. Zadra) agreed to co-ordinate and produce the first inter-comparison of surface drag for NWP global models.

Please note that -- at least for now -- we'd like to keep this exercise simple, so that we can get some results quickly. Therefore the minimal output requested (please see item 1 in the list below) is supposed to be easily produced.

If possible, we'd like to try and put together first results this summer (say, in August).

Please consider also the optional output (items 2 and 3). If you have any of them already archived -- or if you're planning to do re-runs and are able to output those as well -- that would be great.

1) Basic output: Using your center's global model, in its operational resolution/configuration, the basic required fields are surface stresses averaged over a winter and a summer month. More specifically:

- total surface stress (boundary layer + orographic) averaged over the first day (24h) of a month of forecasts

- the months proposed are Jan & Jul 2012

- output (2D fields) in a netcdf format (details to follow)

2) Optional output A: If possible, the participants are also invited to:

- break down the surface stress into its various components (this partition may vary from one model to another)

- separate averages at 00Z, 06Z, 12Z and 18Z

- produce averages of wind components U and V at 850 hPa

3) Optional output B: Please let us know if you / your center might also be willing and capable of providing:

- time averages of momentum tendency profiles -- from any physics scheme that generates tendencies for momentum, and from the dynamics

- time series of spatially averaged fields (e.g. over land, ocean or prescribed regions; zonal averages; etc.)

- data at distinct resolutions/configurations


(a) Preliminary results for:

- 2-d maps of surface stress for each model

- 850-hPa winds for each model

- zonal average torque components for each model

- inter-model comparison of:

> zonal averages of torques

> zonal averages of the abs. value of surface stresses

- inter-model spread

(b) Reports:

- Report no. 1 (Oct 2013)

Related articles

(a) On orographic torques

Brown, A.R., 2004: Resolution dependence of orographic torques. Q. J. R. Meteorol. Soc., 130, pp. 3029-3046.

Egger, J. and K.-P. Hoinka, 2000: Mountain Torques and the Equatorial Components of Global Angular Momentum. J. Atmos. Sci., 57, 2319-2331.

Lott, F., A.W. Robertson and M. Ghil, 2001: Mountain torques and atmospheric oscillations. Geophysical Research Letters, 28, 1207-1210.

(b) On orography-related processes

Olafsson, H. and P. Bougeault, 1996: Nonlinear Flow Past and Elliptic Mountain Ridge. J. Atmos. Sci., 53, 2465-2489.

Phillips, D.S., 1984: Analytical Surface Pressure and Drag for Linear Hydrostatic Flow over Three-Dimensional Elliptical Mountains. J. Atmos. Sci., 41, 1073-1084.

Vosper, S.B. and A.R. Brown, 2007: The effect of small-scale hills on orographic drag. Q. J. R. Meteorol. Soc. ,133, 1345-1352.

Wells, H., S. Webster and A. Brown, 2005: The effect of rotation on the pressure drag force produced by flow around long mountain ridges. Q. J. R. Meteorol. Soc., 131, 1321-1338.

Wells, H., S.B. Vosper, W. Webster , A.N. Ross and A.R. Brown, 2008: The impact of mountain wakes on the drag exerted on downstream mountains. Q. J. R. Meteorol. Soc., 134, 677-687

Wells, H., S.B. Vosper, A.N. Ross, A.R. Brown and W. Webster, 2008: Wind direction effects on orographic drag. Q. J. R. Meteorol. Soc.,134, 689-701.

Wells, H. and S.B. Vosper, 2010: The accuracy of linear theory for predicting mountain-wave drag: Implications for parametrization schemes. Q. J. R. Meteorol. Soc.,136, 429-441.

(c) On orography-related parametrizations

Beljaars, A.C., A.R. Brown and N. Wood, 2004: A new parametrization of turbulent orographic form drag. Q. J. R. Meteorol. Soc., 130, 1327-1347

Kim, Y.-J., S.D. Eckermann and H.-Y. Chun, 2002: An Overview of the Past, Present and Future of Gravity-Wave Drag Parametrization for Numerical Climate and Weather

Prediction Models. Atmosphere-Ocean, 41, 65-98.

Kim, Y.-J. and J.D. Doyle, 2005: Extension of an orographic-drag parametrization scheme to incorporate orographic anisotropy and flow blocking. Q. J. R. Meteorol. Soc., 131, 1893-1921.

Lott, F. and M.J. Miller, 1997: A new subgrid-scle orographic drag parametrization: Its formulation and testing. Q. J. R. Meteorol. Soc., 123, 2353-2393.

Sandu, I. and Coauthors, 2013: Why is it so difficult to represent stably stratified conditions in numerical weather prediction (NWP) models?, J. Adv. Model. Earth Syst., 5, 1-17.

Scinocca, J.F. and N.A. McFarlane, 2000: The parametrization of drag induced by stratified flow over anisotropic orography. Q. J. R. Meteorol. Soc., 126, 101-127.

Vosper, S.B., H. Wells and A.R. Brown, 2009: Notes and Correspondence - Accounting for non-uniform static stability in orographic drag parametrization. Q. J. R. Meteorol. Soc., 135, 815-822.

Webster, S., A. R. Brown, D. R. Cameron and C. P. Jones, 2002: Improvements to the representation of orography in the MetO Unifed Model. Q. J. R. Meteorol. Soc., 129, 1989-2010.

Zadra, A., M. Roch, S. Laroche and M. Charron, 2003: The subgrid-scale orographic blocking parametrization of the GEM model. Atmosphere-Ocean, 41, 155-170.

(d) On turbulent wind stress over land and over water

Blackadar, A.K., 1962: The Vertical Distribution of Wind and Turbulent Exchange in a Neutral Atmosphere. J.G.R., 67, 3095-3102.

Bougeault, P., P. Lacarrere, 1989: Parameterization of Orography-Induced Turbulence in a Mesobeta--Scale Model. Mon. Wea. Rev., 117, 1872-1890.

Charnock, H., 1955: Wind stress on a water surface. Q. J. R. Meteorol. Soc., 81, 639-640.

Goode, K. and S.E. Belcher, 1999: On the parameterisation of the effective roughness length for momentum transfer over heterogeneous terrain. Boundary-Layer Meteorology, 93, 133-154.

Pena, A., S.-E. gryning and C.B. Hasager, 2010: Comparing mixing-length models of the diabatic wind profile over homogeneous terrain. Theor. Appl. Climato.,100, 325-335.

Wood, N., A.R. Brown and F.E. Hewer, 2001: Parametrizing the effects of orography on the boundary layer: An alternative to effective roughness lengths. Q. J. R. Meteorol. Soc., 127, 759-777.

Wood, N. and P. Mason, 1991: The influence of static stability on the effective roughness lengths for momentum and heat transfer. Q. J. R. Meteorol. Soc., 117, 1025-1056.

(e) Analyses and observations

Risien, C.M. and D.B. Chelton, 2008: A Global Climatology of Surface Wind and Wind Stress Fields from Eight Years of QuikSCAT Scatterometer Data. J. Phys. Oceanogr., 38, 2379-2413.


Ayrton Zadra
Numerical Prediction Research Division
Environment Canada