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On wind and roughness over land

JW Verkaik

The relation between wind, momentum flux, roughness and land-use in disturbed, non-homogeneous boundary layers is studied. Key questions are: `how is the roughness related to land-use?', `how are wind and friction related to the upstream land-use and roughness?', and `is Monin-Obukhov theory still useful over non-homogeneous terrain?'.

To address these questions wind profiles and momentum fluxes up to 180 m height were measured at the Cabauw tower in 1996. The K-Gill propeller vane (k-vane) was used as flux measuring device. Before installing it at Cabauw this instrument was intensively tested and analyzed. This was done using perturbation theory, wind tunnel tests, and a field comparison experiment. The k-vane's response to a turbulent wind field is equal to that of a regular first-order sensor with a response length of 2.9 m. During the measurement campaign at Cabauw the k-vane and its electronics proved to be very vulnerable to atmospheric electricity and contamination.

The roughness in the Cabauw environment is determined from the wind speed profiles, the gustiness and the drag coefficient. The upper and lower part of the profile yield different roughness lengths, and so do the gustiness and the drag coefficients at different heights. The differences found are explained qualitatively by the difference in source area of the measurements and the inhomogeneities in the Cabauw environment. Within the range of the tower, the momentum and heat flux decrease significantly with height, so regular surface layer similarity relations may not apply. The dimensionless wind speed gradient is also influenced by distant roughness transitions. Nevertheless, when local scaling is applied, the wind speed gradient can still be described by the functions that apply in homogeneous surface layers, both in stable and unstable conditions.

To determine the roughness length at synoptic or climatological stations gustiness models can be used. The roughness information helps to interpret the wind speed data and to monitor the exposure of the station. The gustiness model that has been used for long at the Royal Netherlands Meteorological Institute (KNMI) was not suited for the modern digital recording techniques. A new gustiness model has been proposed. The differences between the old and the new model are assessed and evaluated using field cases.

The gustiness derived roughness lengths of the synoptic and climatological stations are used to validate a roughness map that is derived from a land-use map. The roughness map covers The Netherlands with a resolution of 0.5 km. This map is used as input to a downscaling method which corrects weather model wind forecasts for the influence of local roughness variations. The downscaling method is successfully validated using in situ wind measurements from an airport and the coastal zone.

Bibliografische gegevens

JW Verkaik. On wind and roughness over land
published, Wageningen Universiteit, 2006

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