Scalar Dissimilarity and Coherent Structures in the Atmospheric Surface Layer over Farmland

The oasis-desert system is a typical heterogeneous surface, over which the atmospheric surface layer is usually influenced by the advection flow. This situation violates the application of Monin-Obukhov similarity and induces the uncertainty in quantifying turbulent exchange of momentum and scalars between lands and the atmosphere. To better understand this issue requires more studies about turbulence structures and flux exchange. In this study, using the eddy covariance time series data at 3.24 m, 8.25 m, and 16.32 m on a tower over an oasis farmland, the near-surface scalar dissimilarity is analyzed. The correlation coefficient and coherent spectra between scalars are examined. The correlation between temperature and humidity increases with the observed heights. High correlation coefficients occur over the wind speed range of 2 to 4 m s-1. The results from coherent spectra indicate that it has strong range-dependency, the high values occur at the energy-containing range, and it has definitive effect on the correlation coefficients. Spectra gaps are present for spectra at the low frequency range for the data with low correlation coefficients, which indicates that the site is disturbed by meso-scale motions. The temporal scales of coherent structures are further investigated, showing similar scales between vertical and horizontal velocity components and between temperature and humidity, but substantially different scales between velocity components and scalars. The physical processes for these features are also analyzed.