In the last decades flux footprint modeling has evolved to an indispensable quality assessment
tool in micrometeorology. In studies which include routine footprint estimates for long-term
or continuous flux observations, analytical models are the most commonly used class of
footprint models, due to their mathematical simplicity, and hence their low computational
expense. This practical advantage outweighs the two main drawbacks of such models: their
common assumption of homogeneous turbulence, which is not usually fulfilled in practical
flux measurement conditions, and the non-consideration of flux contributions from sources
downwind of the measurement system.
To demonstrate that downwind flux contributions are present and measurable, we conducted
tracer experiments at a grassland site in Graswang, southern Germany. The site is part of the
TERENO.net preAlpine observatory and is located on a flat alluvial valley bottom (ca. 1 km
wide), flanked by steep sides. An artificial tracer (methane) was released continuously over
one averaging period from a surface source of 1m2 size located downstream of an eddy
covariance measurement system. Our measurements show that, depending on along-wind
turbulence intensity σu/u̅, downwind sources can contribute considerably to a flux
measurement. We introduce a new version of the existing flux footprint model FSAM
(Schmid, 1994) that now takes along-wind diffusion into account and thus is capable of
accounting for downwind flux contributions. A programming error that encumbered later
versions of FSAM has also been corrected in the new version. Further, we are able to evaluate
the overall performance of the new model by means of additional tracer experiments
