engleski

On parameterization of stratified turbulence in numerical meteorological models.

Current research shows, recent and former suggests, that the nature and evolution of the stable atmospheric boundary layer(s) (SABL) is still understood and modeled inadequately. The “classical” SABL, almost always stratified weakly (i.e. gradient Richardson number, Ri << ∞, typically, 0 < Ri ≤ 1), is modeled reasonably well during the last few decades or so, but very stable cases, i.e. the VSABL (where Ri >> 1) is generally not understood well. Excessively diffusive and much too deep VSABL flows, as often appearing in numerical models, were recently addressed ; the over-diffusion was alleviated by improving the local turbulent mixing length. This demands an explicit inclusion of the vertical shear of horizontal wind, S, in the mixing length, besides the previously known role of buoyancy frequency, N. A generalization of this recent work is given in the view of a simplified turbulent kinetic energy (TKE) equation and a set of subsequent parameterizations for the eddy diffusivity and conductivity, i.e. K-parameterizations, in terms of a generalized "z-less" mixing length, Λ. It is shown that Λ~[(TKE)^1/2]/|S|∙f(Ri, Pr), uniformly valid for 0 ≤ Ri < ∞, regardless of the other parameterization details (the details appear as corrections) ; Pr is turbulent Prandtl number and f(Ri, Pr) is a simple set of derived functions depending on the parameterization properties. A couple of important shortcomings of the current turbulence parameterizations for the SABL, as modeled in NWP, air-chemistry and climate models, will be remedied by using this new generalized “z-less” mixing length. This approach also recommends that it should be better if various mixing length scales are derived from simplified main principles, instead of only being guessed from some plausible reasoning or dimensional analysis. In particular, it has often been assumed that K and Λ functions or profiles, could be chosen for modeling purposes more or less independently from each other. It is shown here, based on a simple renormalization for Λ, that this is not the case if one wishes to use a more consistent parameterization scheme.

diffusion; parametrization; Prandtl number; Richardson gradient number; stably-stratified turbulence; very stable boundary layers

Predavanje je znanstveno-stručna ekspertiza o stabilnim graničnim slojevima u atmosferi. Predavanje je poslužilo kao osnova za diskusiju o primjeni parametrizacija stratificirane turbulencije u numerickim meteoroloskim modelima u zračnom onečišćenju, transportu i disperziji, u budućim klimatskim scenarijima te u proračunima energije vjetra. Znanstveni rezultati prikazani u predavanju objavljeni su 2010. godine u Quarterly Journal of the Royal Meteorological Society, časopisu citiranom u bazi CC (CROSBI 428502). Uz BORA-u, predavanje je realizirano i kroz međunarodni projekt EMEP4HR.