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Which value does the roughness length take over open water?

  • A.A value that does not depend on the wind speed, since the surface is smooth regardless of the conditions
  • B.Well under a millimetre in light winds
  • C.About one metre, which is comparable with the value appropriate to a mature forest canopy over land
  • D.About ten centimetres

Answer: B

Calm water is aerodynamically very smooth, but the roughness length increases with wind speed as waves develop, which is the Charnock relation. This dependence on the wind itself makes the ocean surface unusual, since over land the roughness is fixed by the terrain and vegetation. It matters greatly for the drag coefficient in strong winds.

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