More than ten state-of-the-art regional air quality models have been applied as part of the Air Quality Model Evaluation International Initiative (AQMEII). These models were run by twenty independent groups in Europe and North America. Standardised modelling outputs over a full year (2006) from each group have been shared on the web-distributed ENSEMBLE system, which allows for statistical and ensemble analyses to be performed by each group. The estimated ground-level ozone mixing ratios from the models are collectively examined in an ensemble fashion and evaluated against a large set of observations from both continents. The scale of the exercise is unprecedented and offers a unique opportunity to investigate methodologies for generating skilful ensembles of regional air quality models outputs. Despite the remarkable progress of ensemble air quality modelling over the past decade, there are still outstanding questions regarding this technique. Among them, what is the best and most beneficial way to build an ensemble of members? And how should the optimum size of the ensemble be determined in order to capture data variability as well as keeping the error low? These questions are addressed here by looking at optimal ensemble size and quality of the members. The analysis carried out is based on systematic minimization of the model error and is important for performing diagnostic/probabilistic model evaluation. It is shown that the most commonly used multi-model approach, namely the average over all available members, can be outperformed by subsets of members optimally selected in terms of bias, error, and correlation. More importantly, this result does not strictly depend on the skill of the individual members, but may require the inclusion of low-ranking skill-score members. A clustering methodology is applied to discern among members and to build a skilful ensemble based on model association and data clustering, which makes no use of priori knowledge of model skill. Results show that, while the methodology needs further refinement, by optimally selecting the cluster distance and association criteria, this approach can be useful for model applications beyond those strictly related to model evaluation, such as air quality forecasting

ABSTRACT:In this short note we discuss a long-standing problem in modelling the atmospheric boundary layer (ABL) over complex terrain: namely, an excessive use of the Monin-Obukhov length scale L MO . This issue becomes increasingly relevant with the ever-increasing resolution of numerical weather-prediction and climate models, which typically use L MO in one way or another for parametrizing the surface layer, or at least for formulating the lower boundary conditions. Hence, inevitably, the models under-represent a significant part of the mesoscale flow variability.We focus here on the stable ABL over land: in particular, sloped cooled flows. However, a qualitatively similar reasoning applies to the corresponding unstable ABL. We show that for sufficiently stratified flows over moderately sloped surfaces, Monin-Obukhov scaling is inadequate for describing the basic ABL dynamics, which is often governed by katabatic and drainage flows.

Abstract. This paper introduces two changes of the turbulence parameterization for the EMEP (European Monitoring and Evaluation Programme) Eulerian air pollution model: the replacement of the Blackadar in stable and O'Brien in unstable turbulence formulations with an analytical vertical diffusion profile (K(z)) called Grisogono, and a different mixing height determination, based on a bulk Richardson number formulation (RiB). The operational or standard (STD) and proposed new parameterization for eddy diffusivity have been validated in all stability conditions against the observed daily surface nitrogen dioxide (NO2), sulphur dioxide (SO2) and sulphate (SO42−) concentrations at different EMEP stations during the year 2001. A moderate improvement in the correlation coefficient and bias for NO2 and SO2 and a slight improvement for sulphate is found for the most of the analyzed stations with the Grisogono K(z) scheme, which is recommended for further application due to its scientific and technical advantages. The newly extended approach for the mechanical eddy diffusivity is applied to the Large Eddy Simulation data focusing at the bulk properties of the neutral and stable atmospheric boundary layer. A summary and extension of the previous work on the empirical coefficients in neutral and stable conditions is provided with the recommendations to the further model development. Special emphasis is given to the representation of the ABL in order to capture the vertical transport and dispersion of the atmospheric air pollution. Two different schemes for the ABL height determination are evaluated against the radiosounding data in January and July 2001, and against the data from the Cabauw tower, the Netherlands, for the same year. The validation of the ABL parameterizations has shown that the EMEP model is able to reproduce spatial and temporal mixing height variability. Improvements are identified especially in stable conditions with the new ABL height scheme based on the RiB number.

A COAMPS (TM) nonhydrostatic numerical model with a higher order turbulence closure scheme is used to study the effect of the sea surface temperature (SST) on the idealized nonlinear flow over an idealized mountain in the presence of rotation. The low-level jet (LLJ) that develops at both flanks of the mountain is intensified by the Coriolis effect on the northern flank for a westerly flow. Shooting flow develops down the slope ending over the sea while resembling a hydraulic jump. This is considered as bora (bura) like flow. The front is related to the abrupt slowdown of the shooting flow through the hydraulic jump.Seven different idealized cases are addressed, the control run, linear case with Fr = 1.2, and the cases with the SST 10 K colder, and 2.5 K, 5 K, 7.5 K and 10 K warmer than the control run. The maximum wind speeds in the shooting flow and the LLJs are around two times higher than the background wind speeds. The interplay of SST effects and the effects of the asymmetric lee-side vortices modify the location and the shape of the bora front which is found not to be parallel with the shoreline. The front is not stationary in time but exhibits vibrations which are more pronounced at the southern flank associated with the weaker LLJ.

Abstract. A new vertical diffusion scheme, called Grisogono, has been implemented in the Unified EMEP (European Monitoring and Evaluation Programme) model. It is shown based on Large Eddy Simulation (LES) that the Grisogono method performs better than the operational O'Brien's polynomial, especially in the stable conditions. In this work, the operational and proposed new parameterization for eddy diffusivity K(z) have been validated against observed daily surface nitrogen dioxide (NO2), sulphur dioxide (SO2) and sulphate (SO4−2) concentrations at different EMEP stations during year 2001. Moderate improvement in the correlation coefficient and bias for NO2 and SO2 and slight improvement for sulphate is found for most of the analyzed stations with the Grisogono K(z) scheme, which is recommended for further application due to its scientific and technical advantages. Special emphasis is given to the representation of the atmospheric boundary layer (ABL) in order to capture vertical transport and dispersion of atmospheric air pollution. Two different ABL schemes are evaluated against radiosounding data in January and July 2001, and against data from the Cabauw tower, the Netherlands, in the same year. Based on validation of the ABL parameterizations, it is found that the EMEP model is able to reproduce spatial and temporal mixing height variability. Improvements are identified especially in stable conditions with the new ABL scheme based on the bulk Richardson number (RiB).