The role of advection of fluxes in modelling dispersion in convective boundary layers

Belušić

2008

ABSTRACT:We intend to improve the 'z-less' mixing length-scale for parametrization of turbulence in the stable atmospheric boundary layers (SABL). Since the SABL structures are far from being fully understood today, their parametrizations or explicit treatment are still usually very sketchy in most mesoscale and climate models. Typically an over-diffusion through the SABL occurs in most numerical models. With the 'z-less' mixing length-scale proposed here the over-diffusion is absent. In particular, the mesoscale model used gave results similar to those from an improved (calibrated) Prandtl model, i.e. katabatic flow occupying the lower and more active part of the SABL developed in both models. The corresponding low-level jet that is embedded in the strong near-surface inversion appears similar in both models. Certain details vary, simply because of the very different nature of the models deployed. The results, that should be also valid for other types of SABL flows, could be used in different types of numerical modelling, parametrizations of the SABL, further development of analytical models and data interpretation.

Improving mixing length‐scale for stable boundary layers

Belušić

2008

Generalizing ‘z‐less’ mixing length for stable boundary layers

2010

A couple of important shortcomings of the current turbulence parametrizations for the SABL, as modelled in numerical weather prediction, 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 were derived from simplified main principles, instead of only being guessed from plausible reasoning or dimensional analysis. In particular, it has often been assumed that K and profiles could be chosen for modelling purposes more or less independently from each other. It is shown, based on a simple renormalization for , that this is not the case if one wishes to use a more consistent parametrization scheme.

Boundary‐layer variations due to orographic‐wave breaking in the presence of rotation

Enger

2004

SUMMARY A mesoscale numerical model is used to study the atmospheric boundary-layer (ABL) response to nonlinear orographic forcing with Coriolis effect, f , over a mountain with length (the cross-wind component) comparable to the Rossby radius of deformation, L R . The orographic-wave breaking occurring for Froude number Fr < 1, affected by f > 0, intensifies on the northern flank for westerly flows, as also found in other recent studies. A cumulative effect occurs as the Coriolis force lifts the northern ABL top and generates a stronger low-level jet (LLJ) than on the southern side. A differential layering also appears, since the specific humidity is higher in the lower southern ABL than in the related northern ABL, and vice versa. By contrast, there are higher values of the turbulent kinetic energy and humidity in the upper northern ABL.The breaking of flow symmetry around the orography due to f changes both the vertical vorticity and horizontal divergence field, (ζ, D), it modulates eddies and turbulence leading to the differential layering of the ABL. The stronger northern LLJ and its weaker southern counterpart, both meandering, together with the asymmetric wave breaking, induce strong lee-side fluctuations of the (ζ, D) field in the presence of f . The enhanced (ζ, D) production due to wave breaking over the distance ≈L R , the primary atmosphere-orography resonance occurs mainly in the vertical, while the 'f -enhancement' occurs in the horizontal plane. In this way, the initial mesoscale forcing may extend its effects over the synoptic scale.