Frank Lunkeit

The Planet Simulator: Towards a user friendly model

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 atmosphericfields 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

A Green Planet versus a Desert World: Estimating the Effect of Vegetation Extremes on the Atmosphere

Abstract The effect of vegetation extremes on the general circulation is estimated by two atmospheric GCM simulations using global desert and forest boundary conditions over land. The difference between the climates of a “green planet” and a “desert world” is dominated by the changes of the hydrological cycle, which is intensified substantially. Enhanced evapotranspiration over land reduces the near-surface temperatures; enhanced precipitation leads to a warmer mid- and upper troposphere extending from the subtropics (induced by ITCZ, monsoon, and Hadley cell dynamics) to the midlatitudes (over the cyclogenesis area of Northern Hemisphere storm tracks). These regional changes of the surface water and energy balances, and of the atmospheric circulation, have potential impact on the ocean and the atmospheric greenhouse.

The planet simulator : Green planet and desert world

An application of the Planet Simulator is presented to estimate the maximum effect of vegetation on the Earth’s climate. Four sets of sensitivity experiments are performed: (1) All vegetation related land surface parameters are changed simultaneously. (2) Only one effect of vegetation on climate is considered: albedo, surface roughness and soil hydrology. To identify the nature of vegetation-climate interaction, linear superposition and non-linear interaction of these three effects are compared. (3) The first experimental set-up is repeated but with mixed-layer ocean and thermodynamic sea-ice. (4) The effect of enhanced greenhouse gas concentrations on extreme vegetation climates is analysed repeating the preceding experimental setup with twice the CO2 concentration (compared to set-3). Zusammenfassung Der Planet Simulator wird hier benutzt, um den maximalen Effekt der Vegetation auf das Erdklima

Thermodynamic analysis of snowball Earth hysteresis experiment: Efficiency, entropy production and irreversibility

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 a substantial bistability: for a large range of values the realization of snowball (SB) or of warm (W) climate conditions depend on the history of the system. Using recent results on the global climate thermodynamics, we show that that the two regimes feature radically different properties. The efficiency of the climate machine increases with decreasing solar constant in W climate conditions, whereas the opposite takes place in the SB regime. Instead, entropy production is increasing with the solar constant in both branches of climate conditions, and its value is about 4 times as large in the W branch than in the corresponding C state. Finally, the degree of irreversibility of the system is much higher in the W conditions, with an explosive growth in the upper range of the considered values of solar constants. Whereas in the SB regime a dominating role is played by changes in the meridional albedo contrast, in the W 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 transitions at the boundary of the bistable region based upon macro-scale thermodynamic properties is also proposed. Our results support the adoption of a new generation of diagnostic tools based on the 2nd law of thermodynamics for auditing climate model and outline a set of parameterizations to be used in conceptual and intermediate complexity models or for the reconstruction of the past climate conditions.

The portable university model of the atmosphere (PUMA): Storm track dynamics and low-frequency variability

The ability of analysing atmospheric dynamics by idealized experiments using a simplified circulation model is illustrated in three related studies. The investigation s focus on the organization of localized strom tracks, their impact on low-frequency variability, and on the response to external thermal forcing. A localized storm track in agreement with observations is forced by a heating dipole embedded in a zonally symmetric field if the dipole orientation corresponds to the Northern Hemisphere winter case. The interaction of two storm tracks leads to low-frequency variability. Spatial resona nce between a low-frequency large scale retrograde travelling wave and the storm track eddies is identified caus ing the fluctuations. Teleconnection pattern remarkably similar to the observed North Atlantic Oscillation (NAO) and Pacific North American (PNA) pattern emerge when the distance of the two storm tracks is set to the observed value of about 150 � . While the spatial resonance mechanism forces the NAO-like pattern, baroclinic processes are related to the PNA-like teleconnection. Anomalies induced by large scale thermal forcing strongly depend on the background flow. A non-linear response is observed in the model depending on the sign of the forcing and its position relative to the storm track. A baroclinic and an equivalent barotropic component defines the response. In addition, the change of the space-time variability is affected by eddy feedbacks. Zusammenfassung Die Leistungsfahigkeit von idealisierten Experimenten mit einem vereinfachten Zirkulationsmodell bei der Analyse der atmospharischen Dynamik wird anhand dreier miteinander verbundener Studien veranschaulicht. Die Untersuchungen konzentrieren sich auf die Organisation lokalisierter Stormtracks und deren Einfluss auf die niederfrequente Variabilitat sowie die Antwort auf einen externen Antrieb. Ein lokalisierter Stormtrack in Ubereinstimung mit Beobachtungen wird durch einen Heizungs-Dipol erzeugt, der in ein zonal symmetrisches Feld eingebettet ist und dessen Orientierung dem nordhemispharischen Winter Fall entspricht. Die Wechselwirkung zweier Stormtracks fuhrt zu langperiodischen Schwankungen. Ein Mechanismus raumlicher Resonanz zwischen langperiodischen, mit einer retrograd wandernden Welle verbundenen Schwankungen, und den synoptischen Storungen des Stormtracks wird als Ursache der Variabilitat identifiziert. Telekonnektionsmuster, die bemerkenswert mit der Nord-Atlantischen Oszillation (NAO) und dem Pazifik NordAmerikanischen (PNA) Muster ubereinstimmen, treten bei einem Stormtrack Abstand von 150 � auf. Wahrend die raumliche Resonanz das simulierte NAO-Muster antreibt, sind im Modell barokline Prozesse mit dem PNA-Muster verbunden. Durch einen grosskaligen thermischen Antrieb verursachte Anomalien hangen vom Grundzustand ab. Eine nichtlineare Antwort abhangig vom Vorzeichen und der Position des Antriebs wird beobachtet, die sich aus einem baroklinen und einem aquivalent barotropen Anteil zusammensetzt. Auch die Variabilitat in Raum und Zeit wird durch Ruckkopplungsmechanismen mit den Stormtrack Storungen beeinflusst.

Large-Scale Flow and the Long-Lasting Blocking High over Russia: Summer 2010

AbstractSeveral 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 (~10–60 days). One pathway commences over the eastern Nort...

Regime Transitions in a Stochastically Forced Double-Gyre Model

A reduced-gravity double-gyre ocean model is used to study the influence of an additive stochastic wind stress component on the regime behavior of the wind-driven circulation. The variance of the stochastic component (spatially coherent white noise) representing the effect of atmospheric transient eddies is chosen to be spatially inhomogeneous. This is done to account for the observed concentration of eddy activity along the North Atlantic and North Pacific storm tracks. As a result the double-gyre model with a spatially inhomogeneous stochastic forcing shows a bimodal behavior. One regime shows a quasi-antisymmetric; the second regime a nonsymmetric flow pattern. It is suggested that the nonsymmetric regime corresponds to one member of a known nonsymmetric pair of stationary solutions. Actually no stationary solutions are explicitly calculated in this study. The bimodality does not appear without a spatially inhomogeneous stochastic forcing nor with spatially homogeneous stochastic forcing. Therefore, the regime transitions are induced by the inhomogeneity of the white noise variance. The study suggests that the stochastic forcing enables the system to reach the neighborhood of an unstable fixed point that is not reached without the spatially inhomogeneous stochastic wind field. The unstable fixed point then acts to steer the model in a temporarily persistent regime.

Climate simulations with the global coupled atmosphere-ocean model ECHAM2/OPYC Part I: present-day climate and ENSO events

In this study the global coupled atmosphere-ocean general circulation model ECHAM2/OPYC and its performance in simulating the present-day climate is presented. The model consists of the T21-spectral atmosphere general circulation model ECHAM2 and the ocean general circulation model OPYC with a resolution corresponding to a T42 Gaussian grid, with increasing resolution towards the equator. The sea-ice is represented by a dynamic thermodynamic sea-ice model with rheology. Both models are coupled using the flux correction technique. With the coupled model ECHAM2/OPYC a 210-year integration under present-day greenhouse gas conditions has been performed. The coupled model simulates a realistic mean climate state, which is close to the observations. The model generates several ENSO events without external forcing. Using traditional and advanced (POP-technique) methods these ENSO events have been analyzed. The results are consistent with the “delayed action oscillator” theory. The model simulates both a tropical and an extra-tropical response to ENSO, which are in good agreement with observations.

Effects of Mountains and Ice Sheets on Global Ocean Circulation

AbstractThe 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 th...

Diagnosing the entropy budget of a climate model

A general circulation model (GCM) of intermediate complexity (Planet Simulator) is subjected to an analysis of the entropy budget and its sensitivity. The entropy production is computed directly based on temperature and temperature tendencies and estimated indirectly based on boundary fluxes. For indirect estimates, the model shows reasonably good agreement with observations. The direct computation indicates deficits of the indirect measures, as they, for example, overestimate the material entropy production (that is, the production by turbulent fluxes). Sensitivity analyses of entropy production are provided, which, depending on changing parameters, hint to a possible applicability of maximum entropy production (MEP) under the constrained dynamics of a complex GCM.