Turbulent Flow over a California Vineyard Under Non-Advective and Advective Conditions

To effectively manage limited water resources and ensure the long-term sustainability of the California’s wine industry, robust methods for monitoring evapotranspiration (ET) and vine water needs across the continuum from sub-field to regional scales are needed. Remote sensing-based models represent the most practical approach fulfilling this need, particularly over the large spatial domains that typify wine grape growing regions. However, the relationships used by these models to describe turbulent exchange and transport processes, which were developed and validated primarily over more homogeneous landscapes, may not be fully applicable to vineyards due to their unique canopy structure. As a result, the models may not accurately estimate ET and the other surface fluxes from vineyards. Thus, it is essential that the turbulent structure within and above vineyards be fully characterized as a key first step toward refining these models and enhancing their utility over vineyards and other highly structured crops. Using data collected during the 2017 growing season as a part of the Grape Remote sensing Atmospheric Profiling and Evapotranspiration eXperiment (GRAPEX), this study investigated the structure of turbulence over vineyards under both non-advective and advective conditions bringing hot dry air from the surroundings and altering the ET. Specifically, wind velocity data collected at four heights (2.50, 3.75, 5.00, and 8.00 m agl) via sonic anemometer were evaluated using a combination of Fourier and Wavelet-based approaches. While the power spectra for both non-advective and advective periods share some characteristics, such as the decrease of the total power with height following a well-defined exponential decay relationship, the spectra for the advective periods have several unique features. Under advective conditions, the power spectra for the horizontal wind components have a clearly defined spectral gap between the low frequency peak near 0.002 Hz and the primary peak near 0.1 Hz that is not evident during non-advective periods. At the same time, the power spectra of the vertical wind component are shifted to higher frequencies under advective conditions compared to non-advective conditions suggesting greater interactions with the vines. Moreover, the wavelet analysis indicates the low-frequency contribution to the turbulence under advective conditions is due to distinct entrainment events when large low-frequency eddies intrude into the surface layer. In turn, this suggests that the advective enhancement of evapotranspiration may also be episodic in nature, occurring intermittently following the transport of warm dry air toward the surface. Since flux transport parameterizations used in remote sensing-based energy balance models do not currently consider the effects of intermittency, this may be an important source of model uncertainty, particularly under advective conditions.