Analysis of second-moment budget equations in a slope-oriented coordinate frame exhibits the pathways of exchange between the potential energy of mean flow and the total turbulent mechanical energy. It is shown that this process is controlled by the inclination of the potential temperature gradient. Hence, this parameter should be considered in studies of turbulence in slope flows as well as the slope inclination. The concept of turbulent potential energy is generalized to include baroclinicity, and is used to explain the role of along-slope turbulent heat flux in energy conversions. A generalization of static stability criteria for baroclinic conditions is also proposed. In addition, the presence of feedback between the turbulent heat flux and the temperature variance in stably-stratified flows is identified, which implies the existence of oscillatory modes characterized by the Brunt-Väisäla frequency.
Derivation of surface-layer flux-gradient relationships from a local-equilibrium, turbulence-closure model for a forced flow over inclined terrain is presented. Results are shown as a generalization of Monin-Obukhov universal functions respesenting nondimensional wind and temperature gradients.
Assuming that the vertical turbulent heat flux vanishes at extremely stable conditions, one should expect its maximal absolute value to occur somewhere at moderate stability, between a neutral and extremely stable equilibrium. Consequently, in some situations duality of solutions may be encountered (e.g. two different values of temperature difference associated with the same values of heat flux and wind speed). A quantitative analysis of this feature with a local equilibrium Reynolds-stress model is presented. The fixed-wind / fixed-shear maximum has been identified both in the bulk and in single-point flux-gradient relationships (that is, in the vertical temperature gradient and wind-shear parameter domain). The value of the Richardson number corresponding to this maximum is derived from the model equations. To study the possible feedback in strongly stable conditions, weak and intense cooling scenarios have been simulated with a one-dimensional numerical, high-resolution atmospheric boundary-layer model. Despite the rapid cooling, flow decoupling at the surface has not been observed; instead, a stability-limited heat flux is maintained, with a gradual increase of the Richardson number towards the top of the turbulent layer, with some signs of oscillatory behaviour at intermediate heights. Vertical changes of wind shear and the Brunt-Väisälä frequency display a remarkably non-monotonic character, with some signs of a gradually developing instability.
Abstract. In the scope of the AQMEII Phase 1 project the GEM-AQ model was run over Europe for the year 2006. The modelling domain was defined using a global variable resolution grid with a rotated equator and uniform resolution of 0.2• over the European continent. Spatial distribution and temporal variability of the GEM-AQ model results were analysed for surface ozone and PM 10 concentrations. Model results were compared with measurements available in the ENSEMBLE database. Statistical measures were used to evaluate performance of the GEM-AQ model. The mean bias error, the mean absolute gross error and the Pearson correlation coefficient were calculated for the maximum 8 h running average ozone concentrations and daily mean PM 10 concentrations. The GEM-AQ model performance was characterized for station types, European climatic regions and seasons. The best performance for ozone was obtained at suburban stations, and the worst performance was obtained for rural stations where the model tends to underestimate. The best results for PM 10 were calculated for urban stations, while over most of Europe concentrations at rural sites were too high. Discrepancies between modelled and observed concentrations were discussed in the context of emission data uncertainty as well as the impact of large-scale dynamics and circulation of air masses. Presented analyses suggest that interpretation of modelling results is enhanced when regional climate characteristics are taken into consideration.
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