Selection of ensemble grid point relevant for a chosen location

For both land and sea locations:

• Vertical profiles use the nearest grid point to the location of interest, regardless of being land or sea grid point.  Grid resolution ~18km resolution (T_Co640) and 137 levels.  Data is supplied by the ensemble forecast at 9km but reduced to 18km resolution to cope with the large amounts of data forecast at all levels.   

Modelling the surface energy flux is crucial to the effectiveness of the grid point data as a basis for production of a vertical profile at a given location.  A colour code shows the proportion of surface energy fluxes from land or water sources surrounding each grid point.  This is described in the section: Modelling surface energy fluxes. 

Where the selected ensemble point is:

• inland, data is calculated using HTESSEL within the grid point box surrounding the location of interest.
• in coastal areas, data is calculated using HTESSEL and FLake according to the proportions of land and coastal sea cover within the grid point box surrounding the location of interest. 
• over the ocean, data is calculated using NEMO.

More details are given in the sections: Land-sea mask,  Modelling land surfaces, Modelling coastal waters, Modelling lake and coastal waters, Modelling ocean surfaces. 

Some influences of the adjacent sea areas or mountains may be over- or under-represented by the ensemble vertical profiles.  This can significantly affect the forecast parameter on the profile (temperature, wind, etc).    Users should assess differences in vertical profiles for coastal, island or mountainous regions.   In particular consider:

• the impact of the grid point(s) relative to the land-sea mask, especially where surface winds might blow onshore or there are differences in instability over sea or land.
• the variation of the altitude of the land, especially when compared with the model representation of orography.   Forecast values at the grid point nearest to the location are adjusted for altitude using a standard lapse rate assumption.  The difference in surface temperature can be considerable.

Selection of ensemble grid points for vertical profiles using the rule given above is illustrated below, using relatively complex but informative examples.

There can be inconsistencies in surface values because of different rules for selection of grid points for meteograms and vertical profiles. 

Fig9.9.2: Selection of grid points for vertical profiles.  Consider the four ensemble grid points that surround the location of interest.  The nearest grid point will be chosen regardless of being land or sea grid point. This means that vertical profiles at some coastal locations may not reflect the effects of the nearby sea or may not reflect the effects of the land.  Crosses represent grid spacing 0f 18km.  The colour scale represents the proportion of heat, moisture and momentum for each grid point derived by HTESSEL and Flake at 9km resolution. 

(FUG Associated with Cy49r1)