How far is far enough? The fetch requirements for micrometeorological measurement of surface fluxes

Abstract

The required fetch is quantified by specifying an acceptable ratio of the measured flux to the local surface flux. When normalized by the measurement height zm, the fetch is found to be a strong function of atmospheric stability as quantified by zm/L, where L is the Obukhov length, and a weaker function of the normalized measurement height zm/z0, where z0 is the roughness length. Stable conditions are found to require a much greater fetch than do unstable conditions, and the fetch required for even moderately stable conditions is for many situations considerably greater than 100 times the measurement height. -from Authors