Structure of temperature fluctuations in turbulent convective boundary layers

The Large-Eddy Simulation technique is exploited to investigate statistics of temperature fluctuations, Δ _r θ, in Atmospheric Boundary Layers (ABLs) with different degrees of convection. We found statistical characterizations for both strong and weak fluctuations. In terms of probability density functions (pdfs) of Δ _r θ, weak and strong fluctuations reflect themselves in different rescaling properties of pdf cores and tails, respectively. For the cores, the observed rescaling is P(Δ _r θ) = r ^−α (Δ _r θ/r ^α); while for the tails, data are compatible with P(Δ _r θ)∝ r ^ζ_∞ . Such two rescaling properties are equivalent to saying ⟨|Δ _r θ| ^p ⟩∼ r ^{ζ _p} , with ζ _p = α p for small p's and ζ _p = ζ_∞ = constant for large p's. Both α and ζ_∞ turn out to be z-independent within the mixed layer and, more importantly, they do not appreciably vary by changing the degree of convection in the ABL. We also address the question related to the geometrical structure of temperature jumps contributing to large |Δ _r θ|. Finally, the possible relevance of our results to the long-standing problem of subgrid scale parameterizations is discussed.

Acknowledgments

We are particularly grateful to Chin-Hoh Moeng and Peter Sullivan, for providing us with their LES code as well as many useful comments, discussions and suggestions. Helpful discussions and suggestions by Antonio Celani, who inspired the analysis in terms of mature and nonmature fronts, are also acknowledged.

This work has been partially supported by the projects Cofin 2003, prot. 020302002038 Simulations have been performed at CINECA (INFM parallel computing initiative)