This paper is a summary of the "Best Practice Guideline" (BPG) document (Franke et al., 2007) produced in the framework of the European COST Action 732 "Quality assurance and improvement of micro-scale meteorological models", available from the site given in the reference section. The full document provides guidelines for undertaking simulations that are used to evaluate micro-scale obstacle-accommodating meteorological models. This paper provides an overview of the topics covered in the full document without reproducing the specific recommendations

Published by Copernicus Publications on behalf of the European Geosciences Union. A. Baklanov et al.: Online coupled regional meteorology chemistry models in EuropeAbstract. Online coupled mesoscale meteorology atmospheric chemistry models have undergone a rapid evolution in recent years. Although mainly developed by the air quality modelling community, these models are also of interest for numerical weather prediction and regional climate modelling as they can consider not only the effects of meteorology on air quality, but also the potentially important effects of atmospheric composition on weather. Two ways of online coupling can be distinguished: online integrated and online access coupling. Online integrated models simulate meteorology and chemistry over the same grid in one model using one main time step for integration. Online access models use independent meteorology and chemistry modules that might even have different grids, but exchange meteorology and chemistry data on a regular and frequent basis. This article offers a comprehensive review of the current research status of online coupled meteorology and atmospheric chemistry modelling within Europe. Eighteen regional online coupled models developed or being used in Europe are described and compared. Topics discussed include a survey of processes relevant to the interactions between atmospheric physics, dynamics and composition; a brief overview of existing online mesoscale models and European model developments; an analysis on how feedback processes are treated in these models; numerical issues associated with coupled models; and several case studies and model performance evaluation methods. Finally, this article highlights selected scientific issues and emerging challenges that require proper consideration to improve the reliability and usability of these models for the three scientific communities: air quality, numerical meteorology modelling (including weather prediction) and climate modelling. This review will be of particular interest to model developers and users in all three fields as it presents a synthesis of scientific progress and provides recommendations for future research directions and priorities in the development, application and evaluation of online coupled models.

[1] A model is presented that describes the simultaneous growth and ablation of a layer of ice between an under-ice melt pond and the underlying ocean. Such ''false bottoms'' are the only significant source of ice formation in the Arctic during summer. Analytical solutions for diffusional transport of heat and salt are calculated that illustrate the importance of salt transport in effecting phase change. The model is extended to account for turbulent transports and applied to make predictions of bottom ablation rates of sea ice given the far-field properties of the ocean from the AIDJEX and SHEBA field experiments. The model predictions show that false bottoms may play a significant role in the summer heat budget of the ice-ocean system, causing localized heat fluxes of more than 10 W m À2 into the mixed layer. The thickening of thin ice by false-bottom formation leads to longer-lasting sea ice and thus smaller ice-free areas, which might be an important mechanism affecting the surface albedo.

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Different parameterizations of subgrid-scale fluxes are utilized in a nonhydrostatic and anelastic mesoscale model to study their influence on simulated Arctic cold air outbreaks. A local closure, a profile closure and two nonlocal closure schemes are applied, including an improved scheme, which is based on other nonlocal closures. It accounts for continuous subgrid-scale fluxes at the top of the surface layer and a continuous Prandtl number with respect to stratification. In the limit of neutral stratification the improved scheme gives eddy diffusivities similar to other parameterizations, whereas for strong unstable stratifications they become much larger and thus turbulent transports are more efficient. It is shown by comparison of model results with observations that the application of simple nonlocal closure schemes results in a more realistic simulation of a convective boundary layer than that of a local or a profile closure scheme. Improvements are due to the nonlocal formulation of the eddy diffusivities and to the inclusion of heat transport, which is independent of local gradients (countergradient transport).