Abstract
Large-eddy simulations are performed using the U.K. Met Office Large Eddy Model to study the effects of resolution on turbulent structures in a convective boundary layer. A standard Smagorinsky subgrid scheme is used. As the grid length is increased, the diagnosed height of the boundary layer increases, and the horizontally- and temporally-averaged temperature near the surface and in the inversion layer increase. At the highest resolution, quadrant analysis shows that the majority of events in the lower boundary layer are associated with cold descending air, followed by warm ascending air. The largest contribution to the total heat flux is made by warm ascending air, with associated strong thermals. At lower resolutions, the contribution to the heat flux from cold descending air is increased, and that from cold ascending air is reduced in the lower boundary layer; around the inversion layer, however, the contribution from cold ascending air is increased. Calculations of the heating rate show that the differences in cold ascending air are responsible for the warm bias below the boundary layer top in the low resolution simulations. Correlation length and time scales for coherent resolved structures increase with increasing grid coarseness. The results overall suggest that differences in the simulations are due to weaker mixing between thermals and their environment at lower resolutions. Some simple numerical experiments are performed to increase the mixing in the lower resolution simulations and to investigate backscatter. Such simulations are successful at reducing the contribution of cold ascending air to the heat flux just below the inversion, although the effects in the lower boundary layer are weaker.
Highlights
The development of high resolution numerical weather prediction (NWP) [1] models has brought into sharp focus the appropriateness of existing boundary layer subgrid schemes
Motivated by the outstanding issues outlined above, this paper focuses on the following questions: (i) How does the nature of coherent resolved structures in large eddy simulations (LES) of the convective boundary layer (CBL) change as we enter the grey zone? (ii) How do the scales of the structures change as grid spacing is increased? (iii) What are the impacts of these changes on the representation of the CBL? To answer these questions, we report on simulations at a range of grid spacings, analysing a combination of diagnosed statistics, flow visualisations and quantitative coherent structure analysis
Data were generated from two sets of simulations of a convective boundary layer over a flat homogeneous surface, with different initial conditions, mean wind speed and surface heat flux chosen as contrasting quasi-equilibrium cases from previously-published studies
Summary
The development of high resolution numerical weather prediction (NWP) [1] models has brought into sharp focus the appropriateness of existing boundary layer subgrid schemes. We study in particular the effect of grid spacing on each quadrant’s contribution to the heat flux, which offers a useful diagnosis to inform the development of sub-filter schemes for coarser resolutions This may be valuable for further developing those parameterization approaches that distinguish explicitly between local and non-local contributions to turbulent fluxes (e.g., [8,28]).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
