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• • the impact of the grid point(s) relative to the land-sea mask, especially where surface winds might blow onshore.
  • 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 temperature can be considerable.

The differences in forecast temperature can be magnified at some coastal locations where sea and land energy fluxes are very different (e.g. Gulf of Bothnia, Arabian Gulf) or where there is complex coastal or mountainous geography.  Even a marked contrast in land characteristics can have an effect.

Also the derivation of forecast 2m temperatures at each grid point depends in part on the modelling of the structure of the lowest layers below 10m (L137). This can be influenced by the stability or instability over the differing surfaces.  In Fig9.9.4: 

• At point (E) there is likely to be near surface overturning due to surface heating and heat flux.
• At point (D) there is likely to be near surface stability over cool waters

Other influences regarding forecast 2m temperatures also have an impact.

Fig9.9.1: Selection of grid points for meteograms.  Consider the four ensemble grid points that surround a location of interest.  If there is at least one land grid point within these four, then the nearest land point will be chosen regardless of distance. This means that meteograms at some coastal locations may not reflect the effects of the nearby sea.  If only sea points are available then the nearest sea grid point will be chosen, even if the location lies over land, and may not reflect the effects of the land.  Dots represent grid spacing of 9km. The colour scale represents the proportion of heat, moisture and momentum for each grid point derived by HTESSEL and Flake at 9km resolution. 

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The chart is a section of the charts above and the data is for T+36 VT 12UTC 25 Aug 2025 DT 00UTC 24 Aug 2025.   The wind arrows are only shown to indicate the location of grid points at 9km resolution although some advection of boundary layer air is forecast moving from sea to land.  The land/sea distribution governs the proportion of energy flux from land or sea at each grid point (coloured as above).  The differences in forecast temperature can be magnified at some coastal locations where the sea and land energy fluxes are very different (e.g. Gulf of Bothnia or  Arabian Gulf) or where there is complex coastal or mountainous geography (e.g. Norwegian coast).  Even a marked contrast in land surfaces can have an effect.

(FUG Associated with Cy49r1)  

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