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1.1. Surface Winds

1.1.1. Mean Winds

Typically the winds in the boundary layer are backed and decreased from the geostrophic values through the retarding action of surface friction.  Frictional retardation varies according to the differing surface features – quite high over rugged terrain or forests, moderate over open grassland or farmland, small over featureless tundra or deserts.  Over open water the friction varies according to the size of waves which in turn varies in part according to the wind itself – generally minimal over smooth water but significant where swell or wind waves are high (e.g. near major storms).  

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A verifying anemometer at A could reasonably be associated with a roughness length appropriate to short grass, but an unconventionally placed anemometer at B might be associated with a higher roughness length due to proximity of larger vegetation.  Both locations would be affected by valley winds, blocked flow and possibly gravity waves.  Therefore local effects should be considered when interpreting grid point values and especially meteograms.

1.1.2. Forecast 10m wind output

ECMWF's standard "10m wind components" - surface parameters - differ fundamentally from wind components on the lowest model levels, even if those are nominally at 10m.  These "10m wind components" are diagnostic quantities generally computed not by using the roughness length of the tile itself, but instead assuming a roughness length for short grass (=0.03m), the surface over which (by WMO convention) winds should be measured.  ECMWF creates these fields using a 'blending height' (40m since 2013), by extracting the model wind speed there, and by then effectively extrapolating that down to the surface on the assumption of a 0.03m roughness length. The new speed is converted to velocity (and thereby u and v components) by replicating the wind direction on the lowest model level.  It is these wind values which are shown as surface winds on ecCharts, Meteograms etc.  If the tile roughness length itself is ≤0.03m (e.g. over desert or ocean) then the above adjustments are not made.

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Fig9.3.3:  Mean sea level pressure (MSLP) isobars and 10m wind from data time 00UTC 2 March 2011 T+12hr forecast verifying 12UTC 2 March 2011.  The 10m winds are unrealistically weak over the rugged Norwegian mountains.  Values of 10m/s might be realistic in sheltered valleys, but not on exposed mountain ranges.

1.1.3. Sub-grid drag processes

1.1.3.1. Parameterisation of Surface Roughness - Horizontal scales <5km.  

A roughness length parameterisation (z) is used to obtain a realistic area-averaged turbulent drag according to the underlying surface.  This is used during the calculation of winds at the lowest levels.  Surface roughness:

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Fig9.3.4: Sub-grid drag mechanisms. Scales smaller than 5km. Roughness length parameters (z) indicate the relative effects of different surfaces.  It is important to note that roughness length (z) is only a parameter for use within the calculations; it does not represent typical heights of the obstruction.

1.1.3.2. Parameterisation of Surface Roughness - Horizontal scales between 5km and the model grid resolution

To represent orographic features that are too small to be resolved by the ensemble model grid (9km).  Orographic parameters are derived from the height of valleys, hills and mountains, taken from a land elevation data set with ~1km resolution which is first averaged to ~5km resolution.

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The orographic drag parameterisations are fairly simplistic and don’t necessarily behave well with differing model resolutions.

1.1.4. Verification of forecast winds

Users should be cautious when verifying forecast winds against observed values, and in particular should be aware that:

• Local surface winds depend strongly on local exposure. 
• Surface winds vary on small space and time scales.
• Sometimes verifying sites are not well exposed, particularly at unofficial locations.  Winds may not be measured as well in some directions as in others.
  
• Near-surface wind forecasts have weaknesses in many mountain areas, due to the difficulty of parameterising interaction between the airflow and the highly varying sub-grid orography (see Fig9.3.3).
• When interpolating (as for meteograms) the positions and altitudes of IFS grid points surrounding a selected location of interest have a significant impact upon the forecast values.
  

1.1.5. Wind Gusts

10m wind gusts (U_gust) are computed in the IFS by adding a turbulence component, and (at times) a convective component to the 10m mean wind:

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• In spite of there being a convective contribution in the above equation, which does help, extreme gusts associated with vigorous convection (e.g. MCSs or squall lines) are generally under-estimated, sometimes by a factor of 2 (e.g. 60+kt gusts when 30-40kt gusts predicted).
• Spuriously large gusts can occur associated with dynamically active, strongly sheared warm fronts, particularly in cases where convection (even quite weak convection) has been identified in the lowest layers (say below about 850hPa).
• Current resolution cannot represent the internal circulations of convective systems and empirical rules relating instability to gusts do not work in all cases.
• Gusts can be as much as 2 to 3 times the mean wind speed, but this depends on stability of the atmosphere and roughness of the surface.  Users should consider local effects (e.g. funnelling, particularly in stable conditions, katabatic flow, CAPE, CAPE-shear and convective activity).
• ECMWF's CAPE and CAPE-shear EFI fields have been designed to help the forecaster to predict extreme weather related to deep moist convection; this includes forewarning of the potential for extreme convective gusts.
• There can be an unrealistic localised reduction in gustiness over a relatively short distance over water (~2 grid lengths).  Strong gusts may be underestimated just offshore (lake or sea) when the wind is blowing off the land.
  

1.1.6. Winds related to Intense Extra-tropical Cyclones

More extreme extra-tropical cyclones can give rise to swathes of damaging winds (Fig 9.3.6). Verification using many case studies has shown that in most cases both the mean winds and the gusts are reasonably well predicted, on average.  That is to say if the cyclone shape, central pressure and pressure gradients are well-forecast, then in general so are the winds.  There is also no clear evidence to suggest that extreme cyclones are under- or over-deepened on average, though some case studies show that the onset of the phase of most rapid deepening can sometimes come a little too late (meaning, for a standard eastward-moving low, a little too far east).  In turn this tends to mean that the onset of very strong winds may actually be further west than predicted, but conversely those very strong winds may actually not extend as far east as in model output.  A particularly difficult strong wind swathe to predict is the one associated with the rare phenomena known as the "sting jet".  Evidence from a few cases suggests that IFS gusts are not biased but are representative of the real gusts experienced during a sting jet event.  However because this phenomena is intrinsically difficult to predict, and because the damage swathe associated with it is narrow, and typically short-lived, one has to expect large errors in forecasts for specific locations in potential sting jet scenarios.   Because of these factors and the often finely balanced nature of extreme cyclogenesis events (that relate to sting jets) one must expect some jumpiness in wind forecasts in these situations, even at relatively short ranges.  Nevertheless, the ensemble can help a great deal with highlighting the intrinsic uncertainty in gust forecasts in very cyclogenetic situations.  Users are also encouraged to make use of the cyclone database products also extratropical cyclone diagrams when dealing with strong winds related to cyclogenesis events.  These products were designed, in part, with that particular use in mind.

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See cyclone database products for a key to colours used (for spots) in the cyclone life-cycle phases.

1.1.6.1. Very small cyclones

The guidelines in Section 1.1.6 hold for most cyclones, but very small cyclones pose extra difficulties. They may be beyond the capabilities of the IFS horizontal resolution.  And even very small cyclones can have a sting jet associated.  Accordingly forecasters need to treat ensemble output with particular care when there is potential for very small cyclones to develop - say with lateral dimensions of order 200km or less.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

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