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Mellado J.P.,Max Planck Institute for Meteorology
Journal of Fluid Mechanics | Year: 2012

Direct numerical simulations of free convection over a smooth, heated plate are used to investigate unbounded, unsteady turbulent convection. Four different boundary conditions are considered: free-slip or no-slip walls, and constant buoyancy or constant buoyancy flux. It is first shown that, after the initial transient, the vertical structure agrees with observations in the atmospheric boundary layer and predictions from classical similarity theory. A quasi-steady inner layer and a self-preserving outer layer are clearly distinguished, with an overlap region between them of constant turbulent buoyancy flux. The extension of the overlap region reached in our simulations is more than 100 wall units κ3/Bs)1/4, where Bs is the surface buoyancy flux and κ the corresponding molecular diffusivity (the Prandtl number is one). The buoyancy fluctuation inside the overlap region already exhibits the-1/3 power-law scaling with height for the four types of boundary conditions, as expected in the local, free-convection regime. However, the mean buoyancy gradient and the vertical velocity fluctuation are still evolving toward the corresponding power laws predicted by the similarity theory. The second major result is that the relation between the Nusselt and Rayleigh numbers agrees with that reported in Rayleigh-Bénard convection when the heated plate is interpreted as half a convection cell. The range of Rayleigh numbers covered in the simulations is then 5×107-107-109. Further analogies between the two problems indicate that knowledge can be transferred between steady Rayleigh-Bénard and unsteady convection. Last, we find that the inner scaling based on {Bsκ reduces the effect of the boundary conditions to, mainly, the diffusive wall layer, the first 10 wall units. There, near the plate, free-slip conditions allow stronger mixing than no-slip ones, which results in 30 % less buoyancy difference between the surface and the overlap region and 30-40 % thinner diffusive sublayers. However, this local effect also entails one global, substantial effect: with an imposed buoyancy, free-slip systems develop a surface flux 60 % higher than that obtained with no-slip walls, which implies more intense turbulent fluctuations across the whole boundary layer and a faster growth. © 2012 Cambridge University Press.

Stevens B.,Max Planck Institute for Meteorology
Journal of Climate | Year: 2015

Based on research showing that in the case of a strong aerosol forcing, this forcing establishes itself early in the historical record, a simple model is constructed to explore the implications of a strongly negative aerosol forcing on the early (pre-1950) part of the instrumental record. This model, which contains terms representing both aerosol-radiation and aerosol-cloud interactions, well represents the known time history of aerosol radiative forcing as well as the effect of the natural state on the strength of aerosol forcing. Model parameters, randomly drawn to represent uncertainty in understanding, demonstrate that a forcing more negative than -1.0Wm-2 is implausible, as it implies that none of the approximately 0.3-K temperature rise between 1850 and 1950 can be attributed to Northern Hemisphere forcing. The individual terms of the model are interpreted in light of comprehensive modeling, constraints from observations, and physical understanding to provide further support for the less negative (-1.0Wm-2) lower bound. These findings suggest that aerosol radiative forcing is less negative and more certain than is commonly believed. © 2015 American Meteorological Society.

Marotzke J.,Max Planck Institute for Meteorology | Forster P.M.,University of Leeds
Nature | Year: 2015

Most present-generation climate models simulate an increase in global-mean surface temperature (GMST) since 1998, whereas observations suggest a warming hiatus. It is unclear to what extent this mismatch is caused by incorrect model forcing, by incorrect model response to forcing or by random factors. Here we analyse simulations and observations of GMST from 1900 to 2012, and show that the distribution of simulated 15-year trends shows no systematic bias against the observations. Using a multiple regression approach that is physically motivated by surface energy balance, we isolate the impact of radiative forcing, climate feedback and ocean heat uptake on GMST - with the regression residual interpreted as internal variability - and assess all possible 15- and 62-year trends. The differences between simulated and observed trends are dominated by random internal variability over the shorter timescale and by variations in the radiative forcings used to drive models over the longer timescale. For either trend length, spread in simulated climate feedback leaves no traceable imprint on GMST trends or, consequently, on the difference between simulations and observations. The claim that climate models systematically overestimate the response to radiative forcing from increasing greenhouse gas concentrations therefore seems to be unfounded. © 2015 Macmillan Publishers Limited. All rights reserved.

Pithan F.,Max Planck Institute for Meteorology | Mauritsen T.,Max Planck Institute for Meteorology
Nature Geoscience | Year: 2014

Climate change is amplified in the Arctic region. Arctic amplification has been found in past warm and glacial periods, as well as in historical observations and climate model experiments. Feedback effects associated with temperature, water vapour and clouds have been suggested to contribute to amplified warming in the Arctic, but the surface albedo feedback - the increase in surface absorption of solar radiation when snow and ice retreat - is often cited as the main contributor. However, Arctic amplification is also found in models without changes in snow and ice cover. Here we analyse climate model simulations from the Coupled Model Intercomparison Project Phase 5 archive to quantify the contributions of the various feedbacks. We find that in the simulations, the largest contribution to Arctic amplification comes from a temperature feedbacks: as the surface warms, more energy is radiated back to space in low latitudes, compared with the Arctic. This effect can be attributed to both the different vertical structure of the warming in high and low latitudes, and a smaller increase in emitted blackbody radiation per unit warming at colder temperatures. We find that the surface albedo feedback is the second main contributor to Arctic amplification and that other contributions are substantially smaller or even opposeArctic amplification. © 2014 Macmillan Publishers Limited.

Notz D.,Max Planck Institute for Meteorology
Cryosphere | Year: 2014

We examine how the evaluation of modelled sea-ice coverage against reality is affected by uncertainties in the retrieval of sea-ice coverage from satellite, by the usage of sea-ice extent to overcome these uncertainties, and by internal variability. We find that for Arctic summer sea ice, model biases in sea-ice extent can be qualitatively different from biases in sea-ice area. This is because about half of the CMIP5 models and satellite retrievals based on the Bootstrap and the ASI algorithm show a compact ice cover in summer with large areas of high-concentration sea ice, while the other half of the CMIP5 models and satellite retrievals based on the NASA Team algorithm show a loose ice cover. For the Arctic winter sea-ice cover, differences in grid geometry can cause synthetic biases in sea-ice extent that are larger than the observational uncertainty. Comparing the uncertainty arising directly from the satellite retrievals with those that arise from internal variability, we find that the latter by far dominates the uncertainty estimate for trends in sea-ice extent and area: most of the differences between modelled and observed trends can simply be explained by internal variability. For absolute sea-ice area and sea-ice extent, however, internal variability cannot explain the difference between model and observations for about half the CMIP5 models that we analyse here. All models that we examined have regional biases, as expressed by the root-mean-square error in concentration, that are larger than the differences between individual satellite algorithms.

Seifert A.,Hans Ertel Center for Weather Research | Heus T.,Max Planck Institute for Meteorology
Atmospheric Chemistry and Physics | Year: 2013

Trade wind cumulus clouds often organize in along-wind cloud streets and across-wind mesoscale arcs. We present a benchmark large-eddy simulation which resolves the individual clouds as well as the mesoscale organization on scales of O(10 km). Different methods to quantify organization of cloud fields are applied and discussed. Using perturbed physics large-eddy simulation experiments, the processes leading to the formation of cloud clusters and the mesoscale arcs are revealed. We find that both cold pools as well as the sub-cloud layer moisture field are crucial to understand the organization of precipitating shallow convection. Further sensitivity studies show that microphysical assumptions can have a pronounced impact on the onset of cloud organization. © 2013 Author(s).

Notz D.,Max Planck Institute for Meteorology | Marotzke J.,Max Planck Institute for Meteorology
Geophysical Research Letters | Year: 2012

The very low summer extent of Arctic sea ice that has been observed in recent years is often casually interpreted as an early-warning sign of anthropogenic global warming. For examining the validity of this claim, previously IPCC model simulations have been used. Here, we focus on the available observational record to examine if this record allows us to identify either internal variability, self-acceleration, or a specific external forcing as the main driver for the observed sea-ice retreat. We find that the available observations are sufficient to virtually exclude internal variability and self-acceleration as an explanation for the observed long-term trend, clustering, and magnitude of recent sea-ice minima. Instead, the recent retreat is well described by the superposition of an externally forced linear trend and internal variability. For the externally forced trend, we find a physically plausible strong correlation only with increasing atmospheric CO 2 concentration. Our results hence show that the observed evolution of Arctic sea-ice extent is consistent with the claim that virtually certainly the impact of an anthropogenic climate change is observable in Arctic sea ice already today. Copyright 2012 by the American Geophysical Union.

Grassl H.,Max Planck Institute for Meteorology
Surveys in Geophysics | Year: 2011

Anthropogenic climate change has emerged as one of the major challenges for mankind in the centuries to come. The strongly modified composition of the atmosphere, due to emissions of greenhouse gases and aerosol particles, leads to an enhanced greenhouse effect and also intensified backscattering of solar radiation by aerosol particles. The resulting global mean warming will have a major impact on the entire cryosphere, with global consequences via mean sea level rise and redistributed precipitation. This introductory presentation will summarize the emergence of the topic, its already observed consequences for the cryosphere, and it will also discuss issues in climate policy making when dealing with the climate change challenge. © 2011 Springer Science+Business Media B.V.

Graf H.-F.,University of Cambridge | Zanchettin D.,Max Planck Institute for Meteorology
Journal of Geophysical Research: Atmospheres | Year: 2012

This study contributes to the discussion on possible effects of El Nio on North Atlantic/European regional climates. We use NCEP/NCAR reanalysis data to show how the two different types of El Nios (the central Pacific, or CP, and the east Pacific, or EP) result in remarkably different European winter temperature anomalies, specifically weak warming during EP and significant cooling during CP El Nios, the latter being associated with a negative phase of the winter North Atlantic Oscillation (NAO). Our results diverge from former suggestions addressing the weakened stratospheric polar vortex as the dominant factor contributing to the El Nio/NAO teleconnection. We propose a tropospheric bridge as the mechanism primarily responsible for the establishment of a negative NAO phase and of associated cold European winters. This mechanism includes the subtropical jet (STJ) waveguide being activated only during CP El Nios, when anomalous convective heating occurs near the edge of the Pacific warm pool. Under these conditions the STJ is enhanced by planetary wave flux divergence in the subtropical upper troposphere, providing favorable conditions for the propagation of a wave number 5 disturbance around the subtropical Northern Hemisphere. This wave contributes to weakening of the Azores High and, hence, to the negative NAO phase. As global warming scenarios project an increase in the frequency of CP El Nio events, the distinctive nature of this mechanism implies that the probability of cold European winters may increase as well in future decades. Copyright 2012 by the American Geophysical Union.

Notz D.,Max Planck Institute for Meteorology
Wiley Interdisciplinary Reviews: Climate Change | Year: 2012

Sea ice is a key element of the Earth's climate system, and also of significant ecological, geo-political, and economic importance. Understanding the ongoing changes of the Earth's sea-ice cover is therefore not only scientifically interesting in itself, but also crucial for a large number of different stakeholders. Without such understanding, a reliable projection of possible future changes will be impossible. A main focus of ongoing sea-ice research is therefore aimed at identifying the factors that modulate the ice's variability on seasonal and longer time scales. For such efforts, coupled Climate Models or Earth System Models are used. To give trustworthy results, these models must be able to realistically simulate the mechanical and thermodynamic interaction of sea ice with the atmosphere and the ocean, which determine the resulting sea-ice thickness distribution. While the representation of such air-ice-sea interaction has seen some major advances in the most complex sea-ice models during the past decade, a number of fundamental processes of air-ice-sea interaction are still only crudely understood and currently not realistically represented in models. This article provides a succinct description of these processes and discusses necessary research directions for their improved representation in models. © 2012 John Wiley & Sons, Ltd.

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