Experimental Evaluation of Flux Footprint Models

Abstract:

The widespread use of footprint estimates in conjunction with eddy covariance measurements illustrates that flux footprint modelling is an important and therefore commonly used data quality assessment tool in micrometeorology. Mostly, analytical and computationally inexpensive models are applied, although their assumption of homogeneous turbulence is usually not fulfilled in practical flux measurement conditions. This difficulty raises the question of how reliable footprint model results are at real-world flux sites. In order to evaluate the model’s performance, we conducted tracer experiments at a grassland site in Graswang, southern Germany. The site is part of the TERENO.net pre-Alps 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. It was measured simultaneously with two eddy covariance systems which were arranged in different configurations downwind of the source. In this way we are able to evaluate the alongwind as well as the crosswind component of footprint models at real observation conditions. By comparing measured flux contributions from the tracer source to those predicted by footprint models the accuracy of the modelled 2-dimensional flux footprint can be assessed. We compare our measurements to estimates of analytical models as well as models based on Lagrangian stochastic particle dispersion. First results indicate that even analytical models perform adequately, except that they miss downwind source contributions which can be important.