The influence of canopy architecture on the effective surface roughness of vineyards

The highly-structured architecture of vineyards can strongly influence air flow within and above the plant canopy. As a result, the parameterization schemes used by many land surface models may not adequately describe either the aerodynamic characteristics of a vineyard or the efficacy of turbulent exchange within and above the canopy. Using data collected at a two different vineyard sites during 2013 and 2014 as a part of the Grape Remote Sensing Atmospheric Profiling and Evapotranspiration Experiment (GRAPEX), an ongoing multi-agency field campaign conducted in the Central Valley of California, this study sought to determine the effects of vineyard canopy structure on two key aerodynamic variables used in land surface models, namely the displacement height (do) and roughness length (zo). Specifically, wind velocity data collected during neutral conditions at three heights (2.5 m, 3.75 m, and 8 m, agl) were used to estimate do and zo and identify how the varied over time in response changing environmental conditions. The analysis indicates do exhibits a step-like response to changing vegetation density as measured via leaf area index (LAI). In all cases, do increased from ~ 0.75 m to ~1.5 m within 10 days of bud burst and then remained constant for the remainder of the growing season. At the same time, zo was found to vary in response to wind direction. It increased sigmoidally from a minimum averaging 0.15 m when the wind direction was parallel to the vine rows to a maximum near 0.3 m when the winds were perpendicularly to the rows. These results not only demonstrate the unique effects of highly-structured canopies on aerodynamic variables used in describing momentum exchange, it also provides well-behaved relationships that can be used to improve the accuracy of modeled turbulent fluxes over vineyards.