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.

[1] Boundary layer turbulence plays a central role in determining the strength of the overall atmospheric circulation by affecting the intensity of exchange of heat, mass, and momentum at the Earth's surface. It is often parameterized using the bulk formula, in which the vonKarman parameter plays a critical role. Here we conducted a range of sensitivity simulations with an atmospheric general circulation model in which we modified the strength of boundary layer turbulence by varying the von-Karman parameter. These simulations show that the maximum of entropy production associated with boundary layer dissipation is consistent with the observed value of the von-Karman parameter of 0.4 and maximizes the planetary rate of entropy production with the global radiative temperature being close to its minimum value. Additional sensitivity simulations were conducted with an increased concentration of atmospheric carbon dioxide, which affects the relative radiative forcing of tropical vs. polar regions. We find that the global climate sensitivity is more-or-less independent of the assumed strength of boundary layer turbulence in our idealized setup. The difference in climate sensitivities of tropical and polar regions is at a minimum at a climatic state of MEP. Citation: Kleidon, A., K. Fraedrich, E. Kirk, and F. Lunkeit (2006), Maximum entropy production and the strength of boundary layer exchange in an atmospheric general circulation model, Geophys. Res. Lett., 33, L06706,

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The impact of mountains and ice sheets on the large-scale circulation of the world’s oceans is investigated in a series of simulations with a new coupled ocean–atmosphere model [Oregon State University–University of Victoria model (OSUVic)], in which the height of orography is scaled from 1.5 times the actual height (at T42 resolution) to 0 (no mountains). The results suggest that the effects of mountains and ice sheets on the buoyancy and momentum transfer from the atmosphere to the surface ocean determine the present pattern of deep ocean circulation. Higher mountains reduce water vapor transport from the Pacific and Indian Oceans into the Atlantic Ocean and contribute to increased (decreased) salinities and enhanced (reduced) deep-water formation and meridional overturning circulation in the Atlantic (Pacific). Orographic effects also lead to the observed interhemispheric asymmetry of midlatitude zonal wind stress. The presence of the Antarctic ice sheet cools winter air temperatures by more than 20°C directly above the ice sheet and sets up a polar meridional overturning cell in the atmosphere. The resulting increased meridional temperature gradient strengthens midlatitude westerlies by ~25% and shifts them poleward by ~10°. This leads to enhanced and poleward-shifted upwelling of deep waters in the Southern Ocean, a stronger Antarctic Circumpolar Current, increased poleward atmospheric moisture transport, and more advection of high-salinity Indian Ocean water into the South Atlantic. Thus, it is the current configuration of mountains and ice sheets on earth that determines the difference in deep-water formation between the Atlantic and the Pacific.

In this paper we explore the potential multistability of the climate for a planet around the habitable zone. We focus on conditions reminiscent to those of the Earth system, but our investigation has more general relevance and aims at presenting a general methodology for dealing with exoplanets. We describe a formalism able to provide a thorough analysis of the non-equilibrium thermodynamical properties of the climate system and explore, using a flexible climate model, how such properties depend on the energy input of the parent star, on the infrared atmospheric opacity, and on the rotation rate of the planet. We first show that it is possible to reproduce the multi-stability properties reminiscent of the paleoclimatologically relevant snowball (SB)-warm (W) conditions. We then characterise the thermodynamics of the simulated W and SB states, clarifying the central role of the hydrological cycle in shaping the irreversibility and the efficiency of the W states, and emphasizing the extreme diversity of the SB states, where dry conditions are realized. Thermodynamics provides the clue for studying the tipping points of the system and leads us to constructing empirical parametrizations allowing for expressing the main thermodynamic properties as functions of the emission temperature of the planet only. Such empirical functions are shown to be rather robust with respect to changing the rotation rate of the planet from the current terrestrial one to half of it. Furthermore, we explore the dynamical range where the length of the day and the length of the year are comparable. We clearly find that there is a critical rotation rate below which the multi-stability properties are lost, and the ice-albedo feedback responsible for the presence of SB and W conditions is damped. The bifurcation graph of the system suggests the presence of a phase transition in the planetary system. Such critical rotation rate corresponds roughly to the phase-lock 2:1 condition. Therefore, if an Earth-like planet is 1:1 phase-locked with respect to the parent star, only one climatic state would be compatible with a given set of astronomical and astrophysical parameters. These results have relevance for the general theory of planetary circulation and for the definition of necessary and sufficient conditions for habitability.