Flux Footprints Over a Forested Hill Derived From a Lagrangian Particle Model Coupled Into a Large-Eddy Simulation Model

Abstract

Flux footprints are widely used in the study of turbulent flux measurements. Most of the existing footprint models assume horizontal homogeneity. However, as more and more flux towers are established over complex terrain, it is necessary to advance our understanding of footprints over complex terrain. Here we use a Lagrangian particle model coupled into a large-eddy simulation model to investigate footprints over an idealized 2-dimensional forested hill. Coordinate rotation, which is similar to that performed in real eddy-covariance measurement, is considered in the calculation of footprints. For detectors over the upwind slope, their footprints are generally larger than the footprints of the detectors over the upwind flat ground. For detectors over the separation point, which is slightly downwind of the hill crest, their footprints extend both in the upwind and downwind directions. For detectors over the downwind slope and away from the separation point, their footprints also extend to the downwind direction, provided that the sources are released at the lower half of the canopy. This substantial downwind extension is in contrast to the conventional viewpoint. It is found that the footprints for the whole soil-canopy system can be calculated by assuming that the canopy source/sink occurs at the single layer with the strongest source/sink. Compared to the footprints calculated with coordinate rotation, footprints calculated without coordinate rotation extend much farther upwind for detectors over the upwind slope, and have opposite signs for detectors over the downwind slope.