The common micrometeorological techniques for determining surface fluxes yield flux magnitude and sense but give little information about the location of the area source or sink of the scalar. Knowledge of the upwind fetch is necessary to estimate the location of the actual surface which is involved in the emission or deposition of the scalar investigated. It is this proportionally-contributing area of surface which is described as the flux footprint. The footprint expand or contracts, elongates or widens with different conditions of stability and wind speed. The footprint is smaller when the measurements are made closer to the surface and larger when the measurement height is increased.
Micrometeorological rules-of-thumb probably suffice when the surface is uniform in its deposition/emission characteristics. Such rules generally specify minimum uniform surface fetches. Several models exist which can be used to predict the footprint. However, there is a need for readily applicable models which can be used to design experiments and interpret results for varying conditions. Such models require experimental data sets to validate them under a range of atmospheric conditions. The FOOTPRINT92 field program was undertaken to provide such an experimental data set.
The basic design of the ASTER-supported field study involved the controlled release of sulfur hexafluoride from a crosswind line source and the measurement of the vertical flux at different heights and distances downwind. Fast response SF6 sensors were used with sonic anemometers to measure the SF6 fluxes by eddy correlation. The ASTER facility also served to determine the micrometeorological environment.
This work was sponsored by the National Science Foundation
The ASTER facility is maintained by the Surface and Sounding Systems Facility of the Atmospheric Technology Division of the National Center for Atmospheric Research .