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A. Hannart, C. Vera, B. Cerne, and F. E. L. Otto
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T. R. Marthews, F. E. L. Otto, D. Mitchell, S. J. Dadson, and R. G. Jones
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A. Hannart, J. Pearl, F. E. L. Otto, P. Naveau, and M. Ghil


The emergence of clear semantics for causal claims and of a sound logic for causal reasoning is relatively recent, with the consolidation over the past decades of a coherent theoretical corpus of definitions, concepts, and methods of general applicability that is anchored into counterfactuals. The latter corpus has proved to be of high practical interest in numerous applied fields (e.g., epidemiology, economics, and social science). In spite of their rather consensual nature and proven efficacy, these definitions and methods are to a large extent not used in detection and attribution (D&A). This article gives a brief overview of the main concepts underpinning the causal theory and proposes some methodological extensions for the causal attribution of weather and climate-related events that are rooted into the latter. Implications for the formulation of causal claims and their uncertainty are finally discussed.

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K. Bergaoui, D. Mitchell, F. Otto, M. Allen, R. Zaaboul, and R. McDonnell
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Sarah f. Kew, Sjoukje Y. Philip, Geert Jan van Oldenborgh, Gerard van der Schrier, Friederike E. L. Otto, and Robert Vautard
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Jarkko T. Koskinen, Jani Poutiainen, David M. Schultz, Sylvain Joffre, Jarmo Koistinen, Elena Saltikoff, Erik Gregow, Heikki Turtiainen, Walter F. Dabberdt, Juhani Damski, Noora Eresmaa, Sabine Göke, Otto Hyvärinen, Leena Järvi, Ari Karppinen, Janne Kotro, Timo Kuitunen, Jaakko Kukkonen, Markku Kulmala, Dmitri Moisseev, Pertti Nurmi, Heikki Pohjola, Pirkko Pylkkö, Timo Vesala, and Yrjö Viisanen


The Finnish Meteorological Institute and Vaisala have established a mesoscale weather observational network in southern Finland. The Helsinki Testbed is an open research and quasi-operational program designed to provide new information on observing systems and strategies, mesoscale weather phenomena, urban and regional modeling, and end-user applications in a high-latitude (~60°N) coastal environment. The Helsinki Testbed and related programs feature several components: observing system design and implementation, small-scale data assimilation, nowcasting and short-range numerical weather prediction, public service, and commercial development of applications. Specifically, the observing instrumentation focuses on meteorological observations of meso-gamma-scale phenomena that are often too small to be detected adequately by traditional observing networks. In particular, more than 40 telecommunication masts (40 that are 120 m high and one that is 300 m high) are instrumented at multiple heights. Other instrumentation includes one operational radio sounding (and occasional supplemental ones), ceilometers, aerosol-particle and trace-gas instrumentation on an urban flux-measurement tower, a wind profiler, and four Doppler weather radars, three of which have dual-polarimetric capability. The Helsinki Testbed supports the development and testing of new observational instruments, systems, and methods during coordinated field experiments, such as the NASA Global Precipitation Measurement (GPM). Currently, the Helsinki Testbed Web site typically receives more than 450,000 weekly visits, and more than 600 users have registered to use historical data records. This article discusses the three different phases of development and associated activities of the Helsinki Testbed from network development and observational campaigns, development of the local analysis and prediction system, and testing of applications for commercial services. Finally, the Helsinki Testbed is evaluated based on previously published criteria, indicating both successes and shortcomings of this approach.

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Maurice Blackmon, Byron Boville, Frank Bryan, Robert Dickinson, Peter Gent, Jeffrey Kiehl, Richard Moritz, David Randall, Jagadish Shukla, Susan Solomon, Gordon Bonan, Scott Doney, Inez Fung, James Hack, Elizabeth Hunke, James Hurrell, John Kutzbach, Jerry Meehl, Bette Otto-Bliesner, R. Saravanan, Edwin K. Schneider, Lisa Sloan, Michael Spall, Karl Taylor, Joseph Tribbia, and Warren Washington

The Community Climate System Model (CCSM) has been created to represent the principal components of the climate system and their interactions. Development and applications of the model are carried out by the U.S. climate research community, thus taking advantage of both wide intellectual participation and computing capabilities beyond those available to most individual U.S. institutions. This article outlines the history of the CCSM, its current capabilities, and plans for its future development and applications, with the goal of providing a summary useful to present and future users.

The initial version of the CCSM included atmosphere and ocean general circulation models, a land surface model that was grafted onto the atmosphere model, a sea-ice model, and a “flux coupler” that facilitates information exchanges among the component models with their differing grids. This version of the model produced a successful 300-yr simulation of the current climate without artificial flux adjustments. The model was then used to perform a coupled simulation in which the atmospheric CO2 concentration increased by 1 % per year.

In this version of the coupled model, the ocean salinity and deep-ocean temperature slowly drifted away from observed values. A subsequent correction to the roughness length used for sea ice significantly reduced these errors. An updated version of the CCSM was used to perform three simulations of the twentieth century's climate, and several projections of the climate of the twenty-first century.

The CCSM's simulation of the tropical ocean circulation has been significantly improved by reducing the background vertical diffusivity and incorporating an anisotropic horizontal viscosity tensor. The meridional resolution of the ocean model was also refined near the equator. These changes have resulted in a greatly improved simulation of both the Pacific equatorial undercurrent and the surface countercurrents. The interannual variability of the sea surface temperature in the central and eastern tropical Pacific is also more realistic in simulations with the updated model.

Scientific challenges to be addressed with future versions of the CCSM include realistic simulation of the whole atmosphere, including the middle and upper atmosphere, as well as the troposphere; simulation of changes in the chemical composition of the atmosphere through the incorporation of an integrated chemistry model; inclusion of global, prognostic biogeochemical components for land, ocean, and atmosphere; simulations of past climates, including times of extensive continental glaciation as well as times with little or no ice; studies of natural climate variability on seasonal-to-centennial timescales; and investigations of anthropogenic climate change. In order to make such studies possible, work is under way to improve all components of the model. Plans call for a new version of the CCSM to be released in 2002. Planned studies with the CCSM will require much more computer power than is currently available.

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