A comparison of local and non-local turbulence closure methods for the case of a cold air outbreak

Abstract

Numerical experiments have shown that large-eddy-simulation models (LES) are able to reproduce the common features of convective boundary layers (CBL) quite well. Models which cannot resolve the convective motions due to their grid structure (lD-models or models with coarse horizontal and/or vertical resolution) have to take into account the effects of large eddies within their subgrid diffusion terms. Turbulent fluxes are frequently parameterized through first-order-closure methods (Ktheory). Recently, non-local closure schemes have also been developed. In this paper we compare IDand 2D-models using different local and non-local first-order closure methods. The analysis is carried out for the case of an idealized cold air outbreak (CAO). One of the non-local closures is based on the so-called transilient turbulence theory. The reference states are given by a bulk-model and a 2D-model which resolves the large eddies explicitly. A comparison of the results is presented for characteristic quantities such as evolution of boundary-layer height and surface heat flux as well as mean wind and temperature profiles. It is found that simple local first-order closure does not give good agreement with the reference models. The results of the transilient turbulence model shows that a non-local closure is able to parameterize the effects of the large eddies. Comparable results are produced by a local closure where eddy diffusivities are parameterized by dimensionless gradient-functions,

Modelling the arctic convective boundary-layer with different turbulence parameterizations

“…The neglect of countergradient transports gives similar results for the nonlocal closure scheme as for local and profile closure schemes. Our findings support results of Chrobok et al (1992) who showed that a nonlocal closure (top-down and bottom-up type) is superior to a purely local closure of Prandtl-Kolmogorov type.…”

“…The comparison of these model results with those achieved with a twodimensional large-eddy simulation model showed that both closures (a) and (b), 108 CHRISTOFLijPKESANDK.HEINKESCHLijNZEN give substantially better results than a simple local first-order closure scheme, in which the eddy diffusivity is a function of local gradients only (Chrobok et al, 1992). However, a slightly positive vertical gradient of the temperature within the CBL, obtained with a large-eddy simulation model as a result of countergradient transport was not reproduced when applying the closures (a) and (b).…”

“…Since we apply a model with high vertical resolution, it is possible to determine z; diagnosticly as the level at which the heat flux attains a minimum. This has also been done by Chrobok et al (1992) for their top-down and bottom-up type closure.…”

“…Closures of type (a) and (b) were used by Chrobok et al (1992) to simulate an Arctic CBL with a mesoscale model, in which thermal convection was not resolved explicitly. The comparison of these model results with those achieved with a twodimensional large-eddy simulation model showed that both closures (a) and (b), 108 CHRISTOFLijPKESANDK.HEINKESCHLijNZEN give substantially better results than a simple local first-order closure scheme, in which the eddy diffusivity is a function of local gradients only (Chrobok et al, 1992).…”

Mesoscale Modelling of the Arctic Atmospheric Boundary Layer and Its Interaction with Sea Ice

“…For a case of extreme convection observed during a cold-air outbreak by Kottmeier et al ( 1994 ) , it was shown that only schemes allowing very effi cient turbulent mixing were able to reproduce the typical temperature structure of the well-mixed convective ABL. According to their study and a similar one by Chrobok et al ( 1992 ) , a strong mixing can be realised by closures allowing large eddy diffusivities with maxima in the order of several 100 m 2 s -1 . Furthermore, in a simple nonlocal closure as that of Troen and Mahrt ( 1986 ) or of Lüpkes and Schlünzen ( 1996 ) , countergradient transport of heat and humidity contributes to the mixing.…”