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Editorial The main objective of atmospheric dynamics is to study of those processes of the atmosphere that are associated with weather and climate, in order to understand and simulate with numerical models the different motion systems ranging from the micro-scale to the global circulation. In atmospheric dynamics the fluid is regarded as a continuous medium, and the fundamental laws of fluid mechanics and thermodynamics are expressed in terms of partial differential equations involving the fluid velocity, pressure, density, and temperature. The integration of the governing hydro-thermodynamic equations by numerical methods offers an opportunity to investigate basic theoretical problems (such as the interactions and energy transport processes among the motion systems of different spatial and temporal scales), it paves the way for objective and reliable weather and air-pollution dispersion forecasts, and it constitutes the single possibility to describe the forced and free changes of the climate system. Successful modeling work requires a close co-operation between the meteorologist and physicist, who are experienced in fluid dynamics and nonlinear processes, mathematics and computer sciences, data assimilation, model initialization, and numerical methods. Development of different scale dispersion models demands the collaboration with air chemistry researchers. In addition, parameterization techniques of the soil–vegetation– atmosphere exchange processes call for solving problems of soil science, biology, and ecology. For a productive scientific co-operation, it is inevitable to clearly define the objective of the common research, assure continuous communication among the different teams, and demand regular publications of high quality. The principal mission of the Working Group on Atmospheric Dynamics, belonging to the Scientific Committee for Meteorology of the Hungarian Academy of Sciences, is to create the conditions mentioned above. The tasks of the members of the Working Group include provision of a scientific forum for the different research teams and for the Hungarian investigators working in foreign institutes in order to exchange results related to geophysical fluid dynamics, assisting scientific co-operations and team work. Programs of the Working Group contribute to these goals by organizing joint lectures, conferences, and presentations of the following scientific groups: Numerical Weather Prediction and Climate Dynamics Division of the Hungarian Meteorological Service (HMS), Department of Meteorology, Department of Applied Analysis and Computational Mathematics, and von Kármán Laboratory of Environmental Flows of the Eötvös Loránd University (ELTE, Budapest), and Department of Fluid Mechanics, Budapest University of Technology and Economics. One of the most important events of the Hungarian meteorological community is the Scientific Days of Meteorology, organized every year at the headquarters of the Hungarian Academy of Sciences. Recent topics of these conferences, arranged by our Working Group, included numerical modeling (in 2003), climate dynamics research (in 2004), and cloud physics and micrometeorology (in 2006). The present thematic issue of IDŐJÁRÁS, with 14 scientific papers, is also the results of our activity. The leading paper is dedicated to recall the first steps of numerical weather prediction in Hungary. The following papers describe the present status of numerical models, as ALADIN or MM5, for the Carpathian Basin. Meso-scale processes, ensemble forecasts and dynamic downscaling methods represent the main fields of this research work at the HMS. I Fluid mechanics laboratory experiments offer a powerful tool for the analysis of atmospheric dynamics. This type of experimental work can successfully illustrate the structure of mountain waves. Investigations of model initialization and data assimilation methods are examples of the most fruitful research activities at the Numerical Weather Prediction and Climate Dynamics Division of the HMS. Development of a variational data assimilation system for a limited area model and the application of high-resolution satellite observations in the ALADIN/HU model are presented. Elaborating a new meso-scale transport model for the Carpathian Basin at ELTE and HMS (methodology and applications) illustrates the research activity in the field of air pollution. Up-to-date mathematical background in the theory of partial differential equation systems and numerical methods is an indispensable knowledge. Four papers are dedicated to these questions from the Mathematical Institute of ELTE. Theoretical and applied results of splitting methods, main attributes and applications of semi-groups, and possible application of discontinuous Galerkin methods are demonstrated. The 30 authors of the 14 papers represent different generations, from the pioneers of numerical modeling activity in Hungary, “the elderly generation”, through present-day researchers successfully continuing the numerical modeling work, up to the new generation consisting of PhD students and young scientists. The editors and invited authors dedicate this thematic issue of IDŐJÁRÁS to the illustration of the status and main results of atmospheric dynamics and numerical weather prediction research in Hungary at the beginning of the 21st century for the enrichment of the knowledge of the readers. Gusztáv Götz and Tamás Weidinger Guest editors Working Group on Atmospheric Dynamics Scientific Committee for Meteorology Hungarian Academy of Sciences II