|
E. Linacre and B. Geerts |
4/'99 |
The roughness length is used in numerical models to express the roughness of the surface. It affects the intensity of mechanical turbulence and the fluxes of varies quantitities above the surface. The 'roughness length' zo, depends on the frontal area of the average element (facing the wind) divided by the ground width it occupies. Vertical sub-gridscale heat exchange (by turbulent eddies) can be expressed as the vertical gradient of potential temperature times the roughness length. A lower roughness length implies less exchange between the surface and the atmosphere, but also stronger wind near the ground (e.g. at the standard height of 10 m). A terrain classification based on roughness length is given in Table 1.
Table 1. Terrain classification due to Davenport and quoted by Wieringa (1)
|
class |
roughness length: m |
landscape features |
|
|
no. |
name |
||
|
1 |
sea |
0.0002 |
open water, tidal flat, snow with fetch above 3 km |
|
2 |
smooth |
0.005 |
featureless land, ice |
|
3 |
open |
0.03 |
flat terrain with grass or very low vegetation, airport runway |
|
4 |
roughly open |
0.10 |
cultivated area, low crops, obstacles of height H separated by at least 20 H |
|
5 |
rough |
0.25 |
open landscape, scattered shelter belts, obstacles separated by 15 H or so |
|
6 |
very rough |
0.5 |
landscape with bushes, young dense forest etc separated by 10 H or so |
|
7 |
closed |
1.0 |
open spaces comparable with H, eg mature forest, low-rise built-up area |
|
8 |
chaotic |
over 2.0 |
irregular distribution of large elements, eg city centre, large forest with clearings |
Trees have a high roughness length compared to tree-free suburban areas. Tree planting therefore reduces the wind speed. Another example is the clearing of the Amazon rainforest to make way for pastures and crops. This would reduce the roughness length from class 7 to class 4. This has two complementary effects on the change of evapotranspiration. Even though the strengthened surface wind speed can mitigate the effects of the reduction in surface roughness, the net effect of these two processes is that the surface evapotranspiration is decreased (2).
References