The Footprint for Estimation of Atmosphere-Surface Exchange Fluxes by Profile Techniques

Abstract

The flux footprint, that is the contribution per unit emission from each element of the upwind surface area to measurement of the vertical flux of a passive scalar, is calculated for fluxes estimated by micrometeorological profile techniques. It is found that the upwind extent of the footprint for concentration-profile flux estimates is similar to that of the footprint for eddy-covariance flux measurements, when the eddy-covariance measurement is made at a height equal to the arithmetic mean of the highest and lowest profile measurement heights for stable stratification or the geometric mean for unstable stratification. The concentration-profile flux footprint depends on the ratio of the highest to the lowest measurement height, but is insensitive to the number of measurement levels. The concentration-profile flux footprint extends closer to the measurement location than does the 'equivalent’ eddy-covariance flux footprint, and the difference becomes more pronounced as the ratio of the profile measurement heights increases. The flux footprint for the Bowen-ratio technique is identical to that for a two-level profile measurement only for very limited circumstances. In the more general case, a flux footprint cannot be defined for the Bowen-ratio technique and the uniform upwind fetch required for representative flux measurements depends on the specific spatial distribution of surface fluxes.