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Stations sited according to WMO guidelines (which ECMWF uses for verification) are located in open terrain, over short grass and with no obstacles in the vicinity; hence a low roughness length parameter is appropriate. However the roughness length parameter assigned to the grid square will usually be greater.  Consequently observed winds at WMO station sites will often be stronger than those forecast at the lowest model level (level 137).  This is because of the influence of areas of greater roughness within the grid square. ECMWF addresses this discrepancy as described immediately below.

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Fig9.3.2: The red lines show the extent of a very approximate 9km x 9km schematic HRES grid square surrounding a grid point (flag).  This is an example of the variability of land surface within an IFS grid box illustrating the difficulty in assigning a representative roughness length for the whole surrounding grid square.  HTESSEL uses up to six "tiles" to describe the different surfaces in the square to assess fluxes of momentum (Fig 9.3.4), and also fluxes of heat and moisture.  These values are used to evaluate the forecasted parameters (temperature, wind etc) at the grid point (flag).  An HRES meteogram for a given location is interpolated from adjacent three grid points (flags) each derived from HTESSEL within its own surrounding grid square.  In the figure these are from adjoining grid squares that are outside of the picture.  An ENS meteogram is derived from the four grid points surrounding the station.  In this example the grid points might not include any wooded low-level grid points at all.  See Section on Selection of gridpoints for Meteograms for details.    

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

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