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A parameter used in representing the variation of horizontal wind speed with height immediately above the ground (or water surface), and typically corresponding to the unevenness of the surface; also "surface roughness length".
1930s; earliest use found in Quarterly Journal of the Royal Meteorological Society. Translating German Linearabmessung der Rauhigkeiten (L. Prandtl 1932, in Beiträge zur Physik der Atmosphäre 19 190.