Spectral Signatures during Nocturnal Stable Boundary Conditions over a Mature Vineyard

Vineyards are agricultural surfaces that present a unique structural perturbation to the mean wind flow. As part of the Grape Remote Sensing Atmospheric Profiling and Evapotranspiration Experiment (GRAPEX) a 10-m profile tower of three sonic anemometers (2, 3.75 and 8 m above the vine canopy) was located in a mature vineyard to measure high frequency diurnal variations of 3-dimensional velocity components (u, v, w) and temperature (T) throughout the growing season from March through October for three years. Previous work has been published involving eddy covariance measurements in vineyards but these mostly represented convective daytime unstable conditions. Significantly less has been published about turbulence in vineyards during stable nocturnal periods. Hence, in this study we focused on the nocturnal stable periods under clear skies and relatively light winds typical of the northern portion of California's Central Valley. Our objective was to characterize and evaluate turbulence exchange processes in the layer near the canopy top up to 8 m during calm stable nocturnal periods which are characterized by weak and intermittent turbulence. Spectra, cospectra and coherence plots were evaluated for calm nocturnal periods. The spectra suggest there are periods of intermittent turbulence with features indicative of local and regional scale processes. Additionally the impact of vine structure and spacing on slow meandering flows enhance the decomposition of organized turbulent eddies. These preliminary results provide insight on how vine structure, row spacing, wind speed, direction and stability affect turbulence characteristics during the nocturnal stable periods in the shallow surface layer above a vineyard.