Mesoscale Fluctuations in Scalars Generated by Boundary Layer Convection

Harm J. J. Jonker Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, the Netherlands, and Royal Netherlands Meteorological Institute, De Bilt, the Netherlands

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Peter G. Duynkerke Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, the Netherlands

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Joannes W. M. Cuijpers Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, the Netherlands, and Royal Netherlands Meteorological Institute, De Bilt, the Netherlands

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Abstract

This study has determined energy spectra of turbulent variables in large eddy 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 both 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 the convective boundary layer can behave completely differently from the velocity and temperature spectra. Depending on the boundary conditions of the scalar, that is, the surface flux and the entrainment flux, the spectrum is either classical in the aforementioned sense or it is dominated by the smallest wavenumbers, implying that the fluctuations are dominated by the largest scales. Loosely speaking the results can be summarized as follows: if the scalar entrainment flux is a negative fraction (about −½) of the surface flux, the scalar fluctuations are dominated by relatively small scales (∼ boundary layer depth), whereas in most other cases the scalar fluctuations tend to be dominated by the largest scales resolved (∼ tenths of kilometers, i.e., mesoscales). The latter result is rather peculiar since neither the velocity components nor the temperature field contains these large-scale fluctuations.

Corresponding author address: Dr. Harm J. J. Jonker, Royal Netherlands Meteorological Institute, P.O. Box 201, 3730 AE De Bilt, the Netherlands.

Email: jonker@knmi.nl

Abstract

This study has determined energy spectra of turbulent variables in large eddy 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 both 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 the convective boundary layer can behave completely differently from the velocity and temperature spectra. Depending on the boundary conditions of the scalar, that is, the surface flux and the entrainment flux, the spectrum is either classical in the aforementioned sense or it is dominated by the smallest wavenumbers, implying that the fluctuations are dominated by the largest scales. Loosely speaking the results can be summarized as follows: if the scalar entrainment flux is a negative fraction (about −½) of the surface flux, the scalar fluctuations are dominated by relatively small scales (∼ boundary layer depth), whereas in most other cases the scalar fluctuations tend to be dominated by the largest scales resolved (∼ tenths of kilometers, i.e., mesoscales). The latter result is rather peculiar since neither the velocity components nor the temperature field contains these large-scale fluctuations.

Corresponding author address: Dr. Harm J. J. Jonker, Royal Netherlands Meteorological Institute, P.O. Box 201, 3730 AE De Bilt, the Netherlands.

Email: jonker@knmi.nl

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