Derivation of a footprint function designed for scintillometers over complex terrain.

Abstract:

Since surface fluxes are estimated in the atmospheric surface boundary layer, few meters above the ground, micrometeorologists have been looking for the flux source distribution at the ground level. Nowadays, eddy-covariance techniques are deployed in operational mode, sometime in non ideal areas, and footprint analysis is becoming a necessary step to control data quality and to analyse measurements. Following the same way, scintillometers are more and more used by different scientific communities as they offer integrated estimations of surface fluxes with reasonable data processing. This promotes for scintilloters to be good candidates for long term observation of surface fluxes. However, because scintillometers aggregate fluxes over wide areas, it is useful to quantify land cover variability within the scintillometer footprint. More over, it is often an easy way to install transmitter and receiver on high points to avoid high tower design, which leads to varying height of the scintillometer beam above the ground and affect footprint areas. Although large uncertainties may affect data interpretation, very few studies present scintillometer footprints.

This study presents the derivation of a footprint function to be applied for large aperture scintillometers. It is based on the Hsieh analytical footprint model, a Gaussian distribution function for transverse wind diffusivity and the path average scintillometer weighting function. It accounts for varying beam height, varying wind direction, and distributed aerodynamical properties along the scintillometer path. Illustrations for both ideal and real setup are presented, together with the impact of stability regime assumptions.