A Large-Eddy Simulation Study of Scalar Dissimilarity in the Convective Atmospheric Boundary Layer

A numerical study of the effect of entrainment fluxes at the top of the atmospheric boundary layer (ABL) on dissimilarity between scalars within the mixed and surface layers is conducted. Simulation results clearly show that entrainment fluxes of opposite sign cause decorrelation between the scalars throughout the entire ABL. In the upper part of the mixed layer, this decorrelation is caused by changes in the covariance between the scalars and the scalar variance as well, and is distributed over the entire range of scales resolved in the simulation. Near the surface, the reduction in correlation coefficient originates from an increasing scalar variance, which is present exclusively in the large scales. These effects are noticeable on time scales of about 24 min or longer, and could be interpreted as nonstationarity for the typical 30-min periods used in surface-layer data processing. In addition, it is shown that, for the conditions studied here, the scalar correlation coefficient within the surface layer scales with the measurement height normalized by the ABL depth and not by the Obukhov length.