Relationships Between Second and Third Moments in the Surface Layer Under Different Stratification over Grassland and Urban Landscapes

We present results from the multi-level mast eddy-covariance measurements conducted over flat grassland and over complex urban terrain composed of trees and small buildings. Relationships between the stability parameter and turbulent statistical properties were analysed. Over flat grassland, non-dimensional turbulent moments up to the third-order obey Monin-Obukhov similarity theory. Third-order moments increase for increasing instability, indicating nonlocal turbulent transport of heat flux due to convective motions, and in this case, the relationship between the third- and second-order moments obtained using the framework of the mass-flux approach might be applicable. In stable conditions, the turbulent transport of heat flux is positive and does not demonstrate an explicit dependence on stability but might be associated with large eddies and sweep motions transporting downwards the heat flux originating near the top of a stable boundary layer. Such a downward transport of negative heat flux by large eddies makes applicable the mass-flux approach even in stable boundary layers at higher levels. Over the urban landscape, for all wind directions the third moment corresponds well with the theoretical curve in cases of stable and unstable stratification, but there is a wide spread of data in near-neutral conditions. Skewness of the vertical velocity component is negative over the urban landscape and its value depends on wind direction. At lower levels, the standard deviation of the vertical velocity component is increasing with stability faster than expected from the theoretical curve. Over the urban landscape, large roughness elements do not affect the performance of the mass-flux approach in the convective boundary layer. Furthermore, there is a link between the second and third moments even in stable boundary layers over heterogeneous landscape.