Page History
| Children Display |
|---|
Surface energy fluxes
Assessment of the thermal, moisture and momentum fluxes between the model atmosphere and the underlying surface is vital. It is necessary to have:
...
Energy exchanges are sub-grid scale so statistical methods and simplified mathematical-physical models have to be used. But of necessity there will be some consequent impact on forecast accuracy. Future higher and more appropriate resolution will allow air-surface interaction to be described more explicitly.
Evaluation of grid point data
For forecast ensemble temperature data, all locations within each grid box surrounding a grid point are considered to have the same values as that forecast at the central grid point. 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 within the surrounding area (where HTESSEL will be used) and lake/coastal seas (where FLake will be used). For a sea grid point well offshore NEMO is be used to determine the surface fluxes of heat, moisture and momentum.
...
See also selection of grid points for meteograms and selection of grid points for vertical profiles. See the selection methods directly compared.
Examples of grid point location and energy flux computation
Grid boxes are coloured by the fraction of land cover - scales are on the right and apply to all figures in this sub-section.
Coastal Area - Isle of Wight, Southern England.
Fig2A.1.4-1: 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. Sea grid points open circles, land grid points (>50% land in the surrounding box) solid circles. Within each rectangle all locations are considered to have the same values as forecast at the central grid point. 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 or sea in the surrounding box.
...
- Locations on the green boxes the land fraction is ~95% and the water fraction ~5%. Therefore HTESSEL will supply ~95% and FLake (rather than NEMO, because the land fraction >50%) will supply ~5%.
- Locations on the island in the blue boxes the land fraction is about 10%, and the water fraction about 90%. Therefore HTESSEL supply ~10% of the flux information and NEMO (rather than FLake, because the land fraction is <50% and the location is not a lake) will supply ~90%.
- Locations on the island in the turquoise boxes where the land fraction is about 60% and the water fraction about 40%. Therefore HTESSEL will supply ~60% of the flux information and FLake (rather than NEMO, because the land fraction >50%) will supply about 40%.
Fig2A.1.4-2: ENS grid points around Lake Geneva. 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 as forecast at the central grid point. 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 or sea in the surrounding box.
- If land fraction is <50% then FLake provides lake fluxes.
- If land fraction is >50% then HTESSEL is used for the proportion of land and FLake is used for the proportion of lake water. The proportions are given by the colouring.
Fig2A.1.4-3: ENS grid points around the Canary Islands. 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 as forecast at the central grid point. 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 or sea in the surrounding box.
...
Note several island points have a fairly high proportion of sea in their grid box, and some points on islands have no land within their grid box and are considered as sea points because their grid box has <50% land cover. All blue boxes are open ocean areas and NEMO is used to provide oceanic fluxes; all turquoise and dark green boxes use HTESSEL and FLake according to the proportion of land and sea.
Small Islands - Isole Eolie
Fig2A.1.4-4: Isole Eolie are the group of islands to the north of Sicily. 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 as forecast at the central grid point. 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 or sea in the surrounding box.
...
Overview
Community Forums
Content Tools