Mesoscale fluctuations in scalars generated by boundary layer convection

This study has determined energy spectra of turbulent variables in large ed dy simulations of the penetrating dry convective boundary layer (microscale convection). The simulated domain has a large aspect ratio, the horizontal size being roughly 16 times the boundary layer depth. It turns out that bo th the turbulent velocities and the potential temperature exhibit "classic" energy spectra, which means that the dominant contribution to the variance originates from a scale of the order of the boundary layer height. Surprisingly, the authors find that energy spectra of passive scalars in th e convective boundary layer can behave completely differently from the velo city and temperature spectra. Depending on the boundary conditions of the s calar, that is, the surface flux and the entrainment flux, the spectrum is either classical in the aforementioned sense or it is dominated by the smal lest wavenumbers, implying that the fluctuations are dominated by he larges t scales. Loosely speaking the results can be summarized as follows: if the scalar entrainment flux is a negative fraction (about -1/2) of the surface flux, the scalar fluctuations are dominated by relatively small scales (bo undary layer depth), whereas in most other cases the scalar fluctuations te nd to be dominated by the largest scales resolved (similar to tenths of kil ometers, i.e., mesoscales). The latter result is rather peculiar since neit her the velocity components nor the temperature field contains these large- scale fluctuations.