“Log-Chipper” Turbulence in the Convective Boundary Layer

Abstract

Turbulent fluctuations of a conservative scalar in the atmospheric boundary layer (ABL) can be generated by a scalar flux at the surface, a scalar flux of entrainment at the ABL top, and the ?chewing up? of scalar variations on the mesoscale. The first two have been previously studied, while the third is examined in this paper through large-eddy simulation (LES). The LES results show that the scalar fluctuations due to the breakdown of mesoscale variations in advected conservative scalar fields, which the authors call the ?log-chipper? component of scalar fluctuations, are uniformly distributed through the depth of the convective ABL, unlike the top?down and bottom?up components. A similarity function, similar to those for the top?down and bottom?up scalars, is derived for the log-chipper scalar variance in the convective ABL and used to compare the relative importance of these three processes for generating scalar fluctuations. Representative mesoscale gradients for water vapor mixing ratio and potential temperature are computed from airplane measurements over both land and water. In situations where the entrainment and surface fluxes are sufficiently small, or the ABL depth, turbulence intensity, or the mesoscale scalar gradient is sufficiently large, the variance of the log-chipper scalar fluctuations in mid-ABL can be of the order of the variance of top?down and bottom?up scalars.