Edilbert Kirk

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

On ENSO Physics

Two extended integrations of general circulation models (GCMs) are examined to determine the physical processes operating during an ENSO cycle. The first integration is from the Hamburg version of the ECMWF T21 atmospheric model forced with observed global sea surface temperatures (SST) over the period 1970–85. The second integration is from a Max Planck Institut model of the tropical Pacific forced by observed wind stress for the same period. Both integrations produce key observed features of the tropical ocean-atmosphere system during the 1970–85 period. The atmospheric model results show an eastward propagation of information from the western to eastern Pacific along the equator, although this signal is somewhat weaker than observed. The Laplacian of SST largely drives the surface wind field convergence and hence determines the position of large scale precipitation-condensation heating. This statement is valid only in the near-equatorial zone. Air-sea heat exchange is important in the planetary boundary layer in forcing the wind field convergence but not so important to the main troposphere, which is heated largely by condensation heating. The monopole response seen in the atmosphere above about 500 mb is due to a combination of factors, the most important being adiabatic heating associated with subsidence and tropic-wide variations in precipitation. The models show the role of air-sea heat exchange in the ocean heat balance in the wave guide is one of dissipation/damping. Total air-sea heat exchange is well represented by a simple Newtonian cooling parameterization in the near-equatorial region. In the wave guide, advection dominates the oceanic heat balance with meridional advection being numerically the most important in all regions except right on the equator. The meridional term is largely explained by local Ekman dynamics that generally overwhelm other processes in the regions of significant wind stress. The principal element in this advection term is the anomalous meridional current acting on the climatological mean meridional SST gradient. The eastward motion of the anomalies seen in both models is driven primarily by the ocean. The wind stress associated with the SST anomalies forces an equatorial convergence of heat and mass in the ocean. Outside the region of significant wind forcing, the mass source leads to a convergent geostrophic flow, which drives the meridional heat flux and causes warming to the east of the main wind anomaly. West of the main anomaly the wind and geostrophic divergence cause advective cooling. The result is that the main SST anomaly appears to move eastward. Since the direct SST forcing drives the anomalous wind, surface wind convergence, and associated precipitation, these fields are seen also to move eastward.

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

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.

Simulation of ENSO Related Surface Wind Anomalies with an Atmospheric GCM Forced by Observed SST

The ECMWF-T21 atmospheric GCM is forced by observed near-global SST from January 1970 to December 1985. Its response in low level winds and surface wind stress over the Pacific Ocean is compared with various observations. The time dependent SST clearly induces a Southern Oscillation (SO) in the model run which is apparent in the time series of all variables considered. The phase of the GCM SO is as observed, but its low frequency variance is too weak and is mainly confined to the western Pacific. Because of the GCMs use as the atmospheric component in a coupled ocean-atmosphere model, the response of an equatorial oceanic primitive equation model to both the modeled and observed wind stress is examined. The ocean model responds to the full observed wind stress forcing in a manner almost identical to that when it is forced by the first two low frequency EOFs of the observations only. These first two EOFs describe a regular eastward propagation of the SO signal from the western Pacific to the central Pacific within about a year. The ocean models response to the modeled wind stress is too weak and similar to the response when the observed forcing is truncated to the first EOF only. In other words, the observed SO appears as a sequence of propagating patterns but the simulated SO as a standing oscillation. The nature of the deviation of the simulated wind stress from observations is analyzed by means of Model Output Statistics (MOS). It is shown that a MOS-corrected simulated wind stress, if used to force an ocean GCM, leads to a significant enhancement of low frequency SST variance, which is most pronounced in the western Pacific.

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...

Habitability and multistability in earth-like planets

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 aims at presenting a general methodology for dealing with exoplanets. We provide a thorough analysis of the non-equilibrium thermodynamical properties of the climate system and explore, using a 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. It is possible to reproduce the multi-stability properties reminiscent of the paleoclimatologically relevant snowball (SB) - warm (W) conditions. We then study the thermodynamics of the W and SB states, clarifying the 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 parametrizations where the main thermodynamic properties are expressed as functions of the emission temperature of the planet only. Such functions are rather robust with respect to changing the rotation rate of the planet from the current terrestrial one to half of it. We then explore the dynamical range of slowy rotating and phase locked planets. There is a critical rotation rate below which the multi-stability properties are lost. 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.

Habitability and multistablility in earth-like plantets

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 aims at presenting a general methodology for dealing with exoplanets. We provide a thorough analysis of the non-equilibrium thermodynamical properties of the climate system and explore, using a 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. It is possible to reproduce the multi-stability properties reminiscent of the paleoclimatologically relevant snowball (SB) - warm (W) conditions. We then study the thermodynamics of the W and SB states, clarifying the 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 parametrizations where the main thermodynamic properties are expressed as functions of the emission temperature of the planet only. Such functions are rather robust with respect to changing the rotation rate of the planet from the current terrestrial one to half of it. We then explore the dynamical range of slowy rotating and phase locked planets. There is a critical rotation rate below which the multi-stability properties are lost. 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.

Statistical Single-Station Short-Term Forecasting of Temperature and Probability of Precipitation: Area Interpolation and NWP Combination

Two statistical single-station short-term forecast schemes are introduced and applied to real-time weather prediction. A multiple regression model (R model) predicting the temperature anomaly and a multiple regression Markov model (M model) forecasting the probability of precipitation are shown. The following forecast experiments conducted for central European weather stations are analyzed: (a) The single-station performance of the statistical models, (b) a linear error minimizing combination of independent forecasts of numerical weather prediction and statistical models, and (c) the forecast representation for a region deduced by applying a suitable interpolation technique. This leads to an operational weather forecasting system for the temperature anomaly and the probability of precipitation; the statistical techniques demonstrated provide a potential for future applications in operational weather forecasts.