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- At coastal locations where there is less than 50% land cover in a grid box the water proportion is treated as a lake (using Flake) rather than as an ocean (which would use NEMO).
Some water surfaces (e.g. The Great Lakes) are classed as lakes rather than sea and FLake is used exclusively.
Fig8.1.5.6A6: An example over southern England of "fraction of land cover" values showing the proportion of land and water within each 9km x 9km square centred on each grid point. At grid point X the fluxes of heat, moisture and momentum will be determined by 70%-80% by HTESSEL and 20%30% by FLake. At grid point Y the fluxes of heat, moisture and momentum will be determined by 100% by FLake, even though the grid point lies over land.
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The Isle of Wight in southern England. The island is approximately 40km long by 25km wide. Coastal areas are strongly influenced by the sea while central parts are not.
Fig8.1.5.6A7: ENS grid points over part of southern England. Rectangles surrounding each grid point are coloured according to the "fraction of land cover" assigned to each grid point and shown by the scale on the right. Within each rectangle all locations are considered to have the same values. The fluxes of heat, moisture and momentum which in turn determine the surface values of temperature, dewpoint and wind at the grid point are calculated using the proportion of land (where HTESSEL will be used) and lake/coastal seas (where FLake will be used for lakes or shallow coastal water), or NEMO alone for sea grid points. Towns mentioned below are Ventnor (V), Bembridge (B), Freshwater (F) and the city of Portsmouth (P) and locations are marked by a cross.
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It is for the user to assess critically the representativeness of the Meteogram displayed and to make adjustments in the light of local knowledge and experience. Disentangling coastal effects from altitude effects can be difficult.
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Fig8.1.5.78:10-day medium-range meteogram for Ventnor (a coastal location in southern England) from HRES and ENS data time 00UTC 09 May 2017. During the first three days of the forecast the HRES temperature (blue line) is consistently cooler than the ENS members which are showing very little spread. The ENS grid point is inland but the HRES temperature is interpolated from the three HRES grid points nearest to the location of the selected ENS grid point and adjusted for altitude from the HRES orography to the ENS orography. The diagnosis of discrepancies between HRES and ENS meteograms is complex and it can be difficult to disentangle causes, but users need to be aware of possible reasons in each case. Discrepancy may possibly be due to altitude-related temperature adjustments, and/or to differences in HTESSEL and FLake tiling at the ENS and HRES grid pointsOslo from HRES/Ensemble Control (blue line) and ENS members (box and whiskers) data time 00UTC 26 June 2023. The nearest land grid point to Oslo is at 59.93N 10.83E which lies some 5km away from and some 141m higher than Oslo city centre. This grid point may well be representative of Haugerud on the fringes of Oslo, but temperatures are reduced to near sea level using 6.5K/km lapse rate.
Example2: A lake surrounded by rugged orography.
Eastern Lake Geneva.
Lake Geneva.
Fig8.1.5.8A: An example over Lake Geneva of "fraction of land cover" values showing the proportion of land and water within each 9km x 9km square centred on each grid point. At grid point X the fluxes of heat, moisture and momentum will be determined by 70%-80% by HTESSEL and 20%30% by FLake.
Fig8.1.5.9: ENS grid points over Lake Geneva. Rectangles surrounding each grid point Fig8.1.5.8A: ENS grid points around Lake Geneva. Only one grid box has less than 50% land and any land locations within that box will be considered as if over water. The other turquoise shades show the proportion of water cover within the each box and defines the proportional influence of FLake and HTESSEL for any point within the grid box. For example, a point (T) towards the northeast of the right hand box surrounding grid point Q (coloured turquoise as between 50% and 70% land) even though remote from Lake Geneva will nevertheless experience 50% to 30% influence of the lake. Vevey is marked V on the chart. Grid boxes are coloured according to the "fraction of land cover" assigned to each grid point and shown by the scale on the right.
Fig8.1.5.8B: HRES grid points around Lake Geneva. Much more surface detail is captured by the grid, resulting in a more realistic representation of the influences of land and water within each grid box. Grid boxes are coloured according to the "fraction of land cover" scale on the right.
In the case shown in the diagram:
For ENS Meteograms:
Within each rectangle all locations are considered to have the same values. The fluxes of heat, moisture and momentum which in turn determine the surface values of temperature, dewpoint and wind at the grid point are calculated using the proportion of land (where HTESSEL will be used) and lake (where FLake will be used). Towns mentioned below are Montreux (M) and Vevey (V); locations are marked by a cross.
Example sites are shown on the diagram:
- A lake side location - Montreux (location M). The ENS grid is scanned for the grid points surrounding the location (ENS grid points DEFG) and the nearest land point is chosen (Point D). This is a land point where the "fraction of land cover" is 50%-60%. Surface energy fluxes are determined 50%-60% by HTESSEL and 40%-50% by FLake. The grid point D actually lies at Montreux itself.
- A lake side location - Vevey (location V). The ENS grid is scanned for the grid points surrounding the location (ENS grid points ABCD) and the nearest land point is chosen (Point A). This is a land point where the "fraction of land cover" is 60%-70%. Surface energy fluxes are determined 60%-70% by HTESSEL and 30%-40% by FLake.
In the case shown in the diagram:
For ENS Meteograms:Vevey (marked V in Fig8.1.5.8) is a town on the eastern shores of Lake Geneva and is surrounded by three ENS land grid points (PQR) and one ENS lake grid point (S). The nearest ENS land grid point to location V is grid point Q and this is selected as representative of Vevey, or any other location within the ENS box centred on the ENS grid point (Q). The area around grid point (Q) (coloured turquoise) indicates 50-70% land cover with HTESSEL providing energy fluxes and 30-50% water cover with FLake providing energy fluxes.
The temperature at a given location is adjusted using ENS orography from the ENS temperature (at the ENS altitude of the ENS grid point) to the ENS altitude at the desired location using a standard lapse rate (6.5K/km). Temperatures can be adjusted higher or lower according to the difference in altitude between grid point and the location in question. There are wide variations in orography within the area and the representativeness of a grid point can be uncertain.
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