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J. R. Garratt


Observations of wind stress and wind profiles over the ocean reported in the literature over the past 10 years are consistent with Charnock's (1955) relation between aerodynamic roughness length (z 0) and friction velocity (u *), viz, z 0= αu * 2/g, with α= 0.0144 and g= 9.81 m s−2. They also imply a von Kármán constant = 0.41±0.025. For practical purposes Charnock's relation may he closely approximated in the range 4&<V<21 m s−1 by a neutral drag coefficient (referred to 10 m) varying with the 10 m wind speed V (m s−1), either by a power law relation
CDN 3V0.46
or a linear form

Results of recent turbulence sensor comparison experiments suggest that much of the source of data scatter in CDN (V) plots and of the systematic differences between data sets is due to calibration uncertainties associated with sensor performance in the field. The effects (if any) of fetch, wind duration and unsteadiness remain obscured in this experimental data scatter.

Vertical transfer of momentum over land may be described in terms of an effective roughness length or geostrophic drag coefficient which incorporates the effects of both friction and form drag introduced by flow perturbation around uneven topographical features.

Typically low relief topography and low mountains (peaks <0.5–1 km) require a geostrophic drag coefficient CDN ≈ 3×10−3, while land surfaces in general require CDN ≈ 2×10−3 for which CDN (10)≈ 10×10−3 and the effective aerodynamic roughness length 0(eff)≈ 0.2 m. The latter values satisfy, very approximately, the requirement of global angular momentum balance.

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