HOW FAR IS FAR ENOUGH - THE FETCH REQUIREMENTS FOR MICROMETEOROLOGICAL MEASUREMENT OF SURFACE FLUXES

Recent model estimates of the flux footprint are used to examine the f etch requirements for accurate micro-meteorological measurement of sur face fluxes of passive, conservative scalars within the surface flux l ayer. The required fetch is quantified by specifying an acceptable rat io of the measured flux to the local surface flux. When normalized by the measurement height z(m), the fetch is found to be a strong functio n of atmospheric stability as quantified by z(m)/L, where L is the Obu khov length, and a weaker function of the normalized measurement heigh t z(m)/z0, where z0 is the roughness length. Stable conditions are fou nd to require a much greater fetch than do unstable conditions, and th e fetch required for even moderately stable conditions is for many sit uations considerably greater than 100 times the measurement height.