The Planet Simulator is a Model of Intermediate Complexity (MIC) which can be used to run climate and paleo-climate simulations for time scales up to 10 thousand years or more in an acceptable real time. The priorities in development are set to speed, easy handling and portability. Its modular structure allows a problem dependent configuration. Adaptions exist for the atmospheres of Mars and of Saturn's moon Titan. Common coupling interfaces enable the addition of ocean models, ice models, vegetation and more. An interactive mode with a Model Starter (MoSt) and a Graphical User Interface (GUI) can be used to select a model configuration from the available hierarchy, set its parameters and inspect atmospheric fields while changing model parameters on the fly. This is especially useful for teaching, debugging and tuning of parameterizations. This paper gives an overview of the model's features. The complete model including sources and documentation is available at (www.mi.uni-hamburg.de/plasim). Zusammenfassung Der Planet Simulator ist als "Model of Intermediate Complexity" (MIC) in der Lage, Paläoklima-und andere Simulationen für 10.000 oder mehr Jahre in kurzer Realzeit durchzuführen. Die Prioritäten der Entwicklung liegen in der Geschwindigkeit, der einfachen Handhabung und der Portabilität. Sein modularer Aufbau erlaubt die Konfiguration problemangepasst zu modifizieren. Neben der Erdsystem-Modellierung wurden auch Adaptionen für die Atmosphären des Mars und des Saturnmondes Titan durchgeführt. Kopplungsschnittstellen ermöglichen die Einbindung anderer Komponenten, wie Ozeanmodelle, Eismodelle und andere. Ein interaktiver Modus, Modell-Starter und grafische Benutzeroberfäche, erlaubt eine Auswahl des Modells aus einer Hierarchie, die Voreinstellung der Parameter, die Ansicht von Feldern sowie dieÄnderung von Modellparametern während der Simulation. Dies ist besonders nützlich in der Lehre, beim Austesten vonÄnderungen und der Optimierung von Parameterisierungen. Diese Veröffentlichung gibt einen kurzen Uberblick des Modellaufbaus. Das komplette Modellpaket inklusive Quellcode und Dokumentation kann vom Internet unter (www.mi.uni-hamburg.de/plasim) heruntergeladen werden.

The sensitivity of the climate system to increasing CO2 concentration and the response at decadal time scales are still major factors of uncertainty for the assessment of the long and short term effects of anthropogenic climate change. Here we demonstrate that it is possible to use Ruelle's response theory to predict the impact of an arbitrary CO2 forcing scenario on the global surface temperature of a general circulation model. Response theory puts the concept of climate sensitivity on firm theoretical grounds, and addresses rigorously the problem of predictability at different time scales. Conceptually, our results show that climate change assessment is a well defined problem from a physical and mathematical point of view. Practically, our results show that considering one single CO2 forcing scenario is enough to construct operators able to predict the response of climatic observables to any other CO2 forcing scenario, without the need to perform additional numerical simulations, thus paving the way for redesigning climate change experiments from a radically new perspective

We present an extensive thermodynamic analysis of a hysteresis experiment performed on a simplified yet Earth-like climate model. We slowly vary the solar constant by 20% around the present value and detect that for a large range of values of the solar constant the realization of snowball or of regular climate conditions depends on the history of the system. Using recent results on the global climate thermodynamics, we show that the two regimes feature radically different properties. The efficiency of the climate machine monotonically increases with decreasing solar constant in present climate conditions, whereas the opposite takes place in snowball conditions. Instead, entropy production is monotonically increasing with the solar constant in both branches of climate conditions, and its value is about four times larger in the warm branch than in the corresponding cold state. Finally, the degree of irreversibility of the system, measured as the fraction of excess entropy production due to irreversible heat transport processes, is much higher in the warm climate conditions, with an explosive growth in the upper range of the considered values of solar constants. Whereas in the cold climate regime a dominating role is played by changes in the meridional albedo contrast, in the warm climate regime changes in the intensity of latent heat fluxes are crucial for determining the observed properties. This substantiates the importance of addressing correctly the variations of the hydrological cycle in a changing climate. An interpretation of the climate transitions at the tipping points based upon macro-scale thermodynamic properties is also proposed. Our results support the adoption of a new generation of diagnostic tools based on the second law of thermodynamics for auditing climate models and outline a set of parametrizations to be used in conceptual and intermediate-complexity models or for the reconstruction of the past climate conditions.

The provision of accurate methods for predicting the climate response to anthropogenic and natural forcings is a key contemporary scientific challenge. Using a simplified and efficient open-source general circulation model of the atmosphere featuring O(10 5 ) degrees of freedom, we show how it is possible to approach such a problem using nonequilibrium statistical mechanics. Response theory allows one to practically compute the time-dependent measure supported on the pullback attractor of the climate system, whose dynamics is nonautonomous as a result of time-dependent forcings. We propose a simple yet efficient method for predicting-at any lead time and in an ensemble sense-the change in climate properties resulting from increase in the concentration of CO 2 using test perturbation model runs. We assess strengths and limitations of the response theory in predicting the changes in the globally averaged values of surface temperature and of the yearly total precipitation, as well as in their spatial patterns. The quality of the predictions obtained for the surface temperature fields is rather good, while in the case of precipitation a good skill is observed only for the global average. We also show how it is possible to define accurately concepts like the inertia of the climate system or to predict when climate change is detectable given a scenario of forcing. Our analysis can be extended for dealing with more complex portfolios of forcings and can be adapted to treat, in principle, any climate observable. Our conclusion is that climate change is indeed a problem that can be effectively seen through a statistical mechanical lens, and that there is great potential for optimizing the current coordinated modelling exercises run for the preparation of the subsequent reports of the Intergovernmental Panel for Climate Change.Paper prepared for the special issue of the Journal of Statistical Physics dedicated to the 80th birthday of Y.

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Several studies show that the anomalous long-lasting Russian heat wave during the summer of 2010, linked to a long-persistent blocking high, appears mainly as a result of natural atmospheric variability. This study analyzes the large-scale flow structure based on the ECMWF Re-Analysis Interim (ERA-Interim) data (1989-2010). The anomalous long-lasting blocking high over western Russia including the heat wave occurs as an overlay of a set of anticyclonic contributions on different time scales. (i) A regime change in ENSO toward La Nina modulates the quasi-stationary wave structure in the boreal summer hemisphere supporting the eastern European blocking. The polar Arctic dipole mode is enhanced and shows a projection on the mean blocking high. (ii) Together with the quasi-stationary wave anomaly, the transient eddies maintain the long-lasting blocking. (iii) Three different pathways of wave action are identified on the intermediate time scale (similar to 10-60 days). One pathway commences over the eastern North Pacific and includes the polar Arctic region; another one runs more southward and crossing the North Atlantic, continues to eastern Europe; a third pathway southeast of the blocking high describes the downstream development over South Asia

[1] Energy balance models suggest that the atmospheric circulation operates close to a state of maximum entropy production. Here we support this hypothesis with sensitivity simulations of an atmospheric general circulation model. A state of maximum entropy production is obtained by (i) adjusting boundary layer turbulence and (ii) using a sufficiently high model resolution which allows sufficient degrees of freedom for the atmospheric flow. The state of maximum entropy production is associated with the largest conversion of available potential energy into kinetic energy which is subsequently dissipated by boundary layer turbulence. It exhibits the largest eddy activity in the mid latitudes, resulting in the most effective transport of heat towards the poles and the least equator-pole temperature difference. These results suggest that GCMs have a fundamental tendency to underestimate the magnitude of atmospheric heat transport and, therefore, overestimate the equator-pole temperature gradient for the present-day climate, for the response to global climatic change, and for atmospheres of other planetary bodies.

A Langrangian-type climatology of North Atlantic cyclones is established based on the high-resolution European Centre for Medium-Range Weather Forecasts data-set of the loo0 hPa height-field. First, an algorithm is introduced to identify mid-latitude cyclones and cyclone paths with as few constraints as possible. Cluster analysis of relative cyclone displacements yields three types of cyclone tracks characterizing stationary-, north-eastwardand zonally-travelling storms. The internal Lagrangian statistics of these cyclone-track types reveal representative life-cycles for central pressure and geopotential-height gradients and a power-law scaling behaviour of cyclone displacements. Finally, a basic climatology of North Atlantic cyclone-track regimes is deduced in terms of a time-series and circulation statistics.