The spectral dynamics of TKE and scalar variance in clear convective and cloud topped boundary layers

Recent Large-Eddy Simulation (LES) of clear and cloud topped convective boundary layers show the formation and slow but steady growth of spatial fluctuations in the (thermo)dynamical variables, with a (lateral) size far exceeding the depth of the boundary layer. If the simulation period is long enough and the lateral domain size large enough, these large scale fluctuations will start to dominate the resolved variance of the simulated variables; see for example the recent papers by de Roode et al. (2004) and Schroter et al. (2005). There appears to be also ample observational evidence for this phenomenon (see e.g. Nucciarone and Young, 1991), but in observations it is always hard to exclude possible direct forcings in the mesoscale range. For this reason LES may be considered as an ideal research tool to study the issue. So far, however, these LES studies (including those of the present author) have had a rather phenomenological character, based on analysing the resulting energy-spectra and variance-spectra for different cases and studying the effects of parameter changes and modified atmospheric conditions (boundary conditions, presence of shear, presence of clouds, etc). Conclusions are therefore inevitably based on circumstantial evidence.

The present paper attempts a more direct approach to finding the responsible mechanism. We determine in the Large-Eddy Simulations the spectral decomposition of all terms contributing to the budgets of turbulence kinetic energy and variances of the involved (thermo)dynamical variables. These terms include the spectral transfer (cascade) and spectrally decomposed production/dissipation and pressure-correlation terms, etc. This analysis yields a direct insight into those processes that favor and those processes that inhibit the formation of large scale fluctuations. We conduct the analysis on clear convective boundary layers and on cloud topped boundary layers, and discuss the generic underlying mechanism.

S de Roode, P Duynkerke and H Jonker, 2004 "Large Eddy Simulation of atmospheric boundary layers: How large is large enough?" Journal of the Atmospheric Sciences, 61, 403-421. M Schroter, S Raasch, H Jansen 2005, "Cell broadening revisited: Results from high-resolution large-eddy simulations of cold air outbreaks" Journal of the Atmospheric Sciences, 62, 2023-2032. J Nucciarone and G Young, 1991 "Aircraft measurements of turbulence spectra in the marine stratocumulus-topped boundary layer", Journal of the Atmospheric Sciences, 48, 2382-2392