Using scintillometry to understand MOST validity over a trellised canopy

Accurate modeling of flow over canopies is important for understanding the water use and disease propagation in agronomic, urban, and natural ecosystems. Typical flux measurement methods are very localized and are of limited use at grid sizes of meso-scale models and remote sensing platforms. The scintillometer method allows a non-invasive determination of surface heat and moisture fluxes in areas where setting up traditional instruments can be logistically difficult and has potential to help validate models at larger scales. However, scintillometry derived surface fluxes require Monin-Obukhov Similarity Theory (MOST) which limits the method's applicability when used over heterogeneous terrain.

The validity of MOST is questionable within a few canopy heights, the roughness sub-layer, as fluxes may not be homogeneous. However, many models use MOST to determine boundary conditions over rough terrain. We aim to exploit the MOST requirement of scintillometer fluxes to better understand the conditions when MOST is violated at field scales and determine how sensitive are the scintillometer fluxes to errors in the parameters required in MOST. To help answer these questions a field experiment was conducted in a vineyard near Monmouth, Oregon.

A trellised vineyard can be seen as a 'simple' heterogeneous canopy. Previous research in trellised canopies shows a wind direction dependence of displacement height as well as a rotation of the mean wind within the canopy. These dependencies complicate the general application of similarity theory. Additionally, the validity of MOST on the scale of a vineyard, which at best is a single grid cell, has implications on how agricultural fields are represented in numerical models and field scale evapotranspiration estimates.

In August 2016, a 1 km² vineyard was instrumented with an array of four eddy covariance towers as well as a two-wavelength scintillometer system spanning 750 m across the vineyard. This experiment will allow a detailed analysis of above, and below, canopy flow conditions when the spatially averaged fluxes from the scintillometer deviate significantly from the eddy covariance fluxes and if this deviation is caused by a breakdown in the similarity theory.