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Fig9.3.2: The red lines show the extent of a very approximate 9km x 9km schematic 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 ENS meteogram for a given location is interpolated from adjacent four 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.  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.

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• over land, is dependent upon the surface vegetation (See Fig9.3.4).
• over sea, the drag on the low-level air flow is modelled using the Charnock parameter.  This includes an aerodynamical roughness length that is a function of the 10m wind speed, but limited to a maximum value of that associated with 10m winds of 40m/s.  Read more information about coupling between the atmosphere and ocean waves in waves near tropical storms.

Also considered is a representation of drag from small scale orography.  Flow around steep orographic features gives temporary, variable and quickly fading atmospheric waves that lead to drag - Turbulent Orographic Form Drag (TOFD).

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