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How Far is Far Enough?: The Fetch Requirements for Micrometeorological Measurement of Surface Fluxes

T. W. HorstNational Center for Atmospheric Research, Boulder, Colorado

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J. C. WeilNational Center for Atmospheric Research, Boulder, Colorado

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Abstract

Recent model estimates of the flux footprint are used to examine the fetch requirements for accurate micro-meteorological measurement of surface fluxes of passive, conservative scalars within the surface flux layer. 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/zo, where zo 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.

Abstract

Recent model estimates of the flux footprint are used to examine the fetch requirements for accurate micro-meteorological measurement of surface fluxes of passive, conservative scalars within the surface flux layer. 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/zo, where zo 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.

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