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The ability of general circulation models (GCMs) to correctly simulate precipitation is usually assessed by comparing simulated mean precipitation with observed climatologies. However, to what extent the skill in simulating average precipitation indicates how well the models represent temporal changes is unclear. A direct assessment of the latter is hampered by the fact that freely evolving climate simulations for past periods are not set up to reproduce the specific evolution of internal atmospheric variability. Therefore, model-to-real-world comparisons of time series of daily, monthly, or annual precipitation are not meaningful. Here, for the first time, the authors quantify GCM skill in simulating precipitation variability using simulations in which the temporal evolution of the large-scale atmospheric state closely matches that of the real world. This is achieved by nudging the atmospheric states in the ECHAM5 GCM, but crucially not the precipitation field itself, toward the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). Global correlation maps between observed and simulated seasonal precipitation allow areas in which simulated future precipitation changes are likely to be meaningful to be identified. In many areas, correlations higher than 0.8 are found. This means also that in these regions the simulated precipitation is a very good predictor for the true precipitation, and thus a statistical correction of the simulated precipitation, which can include a downscaling component, can provide useful estimates for local-scale precipitation. The authors show that a simple scaling of the simulated precipitation performs well in a cross validation and thus appears to be a promising alternative to standard statistical downscaling approaches.

A simple plume implementation of the second version (v2) of the Max Planck Institute Aerosol Climatology, MACv2-SP, is described. MACv2-SP provides a prescription of anthropogenic aerosol optical properties and an associated Twomey effect. It was created to provide a harmonized description of post-1850 anthropogenic aerosol radiative forcing for climate modeling studies. MACv2-SP has been designed to be easy to implement, change and use, and thereby enable studies exploring the climatic effects of different patterns of aerosol radiative forcing, including a Twomey effect. MACv2-SP is formulated in terms of nine spatial plumes associated with different major anthropogenic source regions. The shape of the plumes is fit to the Max Planck Institute Aerosol Climatology, version 2, whose present-day (2005) distribution is anchored by surface-based observations. Two types of plumes are considered: one predominantly associated with biomass burning, the other with industrial emissions. These differ in the prescription of their annual cycle and in their optical properties, thereby implicitly accounting for different contributions of absorbing aerosol to the different plumes. A Twomey effect for each plume is prescribed as a change in the host model's background cloud-droplet population density using relationships derived from satellite data. Year-to-year variations in the amplitude of the plumes over the historical period (1850-2016) are derived by scaling the plumes with associated national emission sources of SO2 and NH3. Experiments using MACv2-SP are performed with the Max Planck Institute Earth System Model. The globally and annually averaged instantaneous and effective aerosol radiative forcings are estimated to be -0.6 and -0.5Wm-2, respectively. Forcing from aerosol-cloud interactions (the Twomey effect) offsets the reduction of clear-sky forcing by clouds, so that the net effect of clouds on the aerosol forcing is small; hence, the clear-sky forcing, which is more readily measurable, provides a good estimate of the total aerosol forcing.

ECHAM6, the sixth generation of the atmospheric general circulation model ECHAM, is described. Major changes with respect to its predecessor affect the representation of shortwave radiative transfer, the height of the model top. Minor changes have been made to model tuning and convective triggering. Several model configurations, differing in horizontal and vertical resolution, are compared. As horizontal resolution is increased beyond T63, the simulated climate improves but changes are incremental; major biases appear to be limited by the parameterization of small‐scale physical processes, such as clouds and convection. Higher vertical resolution in the middle atmosphere leads to a systematic reduction in temperature biases in the upper troposphere, and a better representation of the middle atmosphere and its modes of variability. ECHAM6 represents the present climate as well as, or better than, its predecessor. The most marked improvements are evident in the circulation of the extratropics. ECHAM6 continues to have a good representation of tropical variability. A number of biases, however, remain. These include a poor representation of low‐level clouds, systematic shifts in major precipitation features, biases in the partitioning of precipitation between land and sea (particularly in the tropics), and midlatitude jets that appear to be insufficiently poleward. The response of ECHAM6 to increasing concentrations of greenhouse gases is similar to that of ECHAM5. The equilibrium climate sensitivity of the mixed‐resolution (T63L95) configuration is between 2.9 and 3.4 K and is somewhat larger for the 47 level model. Cloud feedbacks and adjustments contribute positively to warming from increasing greenhouse gases. Key Points To describe ECHAM6, as it was configured for participation in CMIP5 To describe the climate of ECHAM6 To describe the climate sensitivity of ECHAM6

We introduce and evaluate aerosol simulations with the global aerosol–climate model ECHAM6.3–HAM2.3, which is the aerosol component of the fully coupled aerosol–chemistry–climate model ECHAM–HAMMOZ. Both the host atmospheric climate model ECHAM6.3 and the aerosol model HAM2.3 were updated from previous versions. The updated version of the HAM aerosol model contains improved parameterizations of aerosol processes such as cloud activation, as well as updated emission fields for anthropogenic aerosol species and modifications in the online computation of sea salt and mineral dust aerosol emissions. Aerosol results from nudged and free-running simulations for the 10-year period 2003 to 2012 are compared to various measurements of aerosol properties. While there are regional deviations between the model and observations, the model performs well overall in terms of aerosol optical thickness, but may underestimate coarse-mode aerosol concentrations to some extent so that the modeled particles are smaller than indicated by the observations. Sulfate aerosol measurements in the US and Europe are reproduced well by the model, while carbonaceous aerosol species are biased low. Both mineral dust and sea salt aerosol concentrations are improved compared to previous versions of ECHAM–HAM. The evaluation of the simulated aerosol distributions serves as a basis for the suitability of the model for simulating aerosol–climate interactions in a changing climate.

We investigate the effect of systematic model biases on teleconnections influencing the Northern Hemisphere wintertime circulation. We perform a two-step nudging and bias-correcting scheme for the dynamic variables of the ECHAM6 atmospheric model to reduce errors in the model climatology relative to ERA-Interim. One result is a significant increase in the strength of the Northern Hemisphere wintertime stratospheric polar vortex, reducing errors in the December–February mean zonal stratospheric winds by up to 75%. The bias corrections are applied to the full atmosphere or the stratosphere only. We compare the response of the bias-corrected and control runs to an increase in Siberian snow cover in October—a surface forcing that, in our experiments, weakens the stratospheric polar vortex from October to December. We find that despite large differences in the vortex strength the magnitude of the stratospheric weakening is similar among the different climatologies, with some differences in the timing and length of the response. Differences are more pronounced in the stratosphere–troposphere coupling, and the subsequent surface response. The snow forcing with the stratosphere-only bias corrections results in a stratospheric response that is comparable to control, yet with an enhanced surface response that extends into early January. The full-atmosphere bias correction’s snow response also has a comparable stratospheric response but a somewhat suppressed surface response. Despite these differences, our results show an overall small sensitivity of the Eurasian snow teleconnection to the background climatology.

The new Max‐Planck‐Institute Earth System Model (MPI‐ESM) is used in the Coupled Model Intercomparison Project phase 5 (CMIP5) in a series of climate change experiments for either idealized CO2‐only forcing or forcings based on observations and the Representative Concentration Pathway (RCP) scenarios. The paper gives an overview of the model configurations, experiments related forcings, and initialization procedures and presents results for the simulated changes in climate and carbon cycle. It is found that the climate feedback depends on the global warming and possibly the forcing history. The global warming from climatological 1850 conditions to 2080–2100 ranges from 1.5°C under the RCP2.6 scenario to 4.4°C under the RCP8.5 scenario. Over this range, the patterns of temperature and precipitation change are nearly independent of the global warming. The model shows a tendency to reduce the ocean heat uptake efficiency toward a warmer climate, and hence acceleration in warming in the later years. The precipitation sensitivity can be as high as 2.5% K−1 if the CO2 concentration is constant, or as small as 1.6% K−1, if the CO2 concentration is increasing. The oceanic uptake of anthropogenic carbon increases over time in all scenarios, being smallest in the experiment forced by RCP2.6 and largest in that for RCP8.5. The land also serves as a net carbon sink in all scenarios, predominantly in boreal regions. The strong tropical carbon sources found in the RCP2.6 and RCP8.5 experiments are almost absent in the RCP4.5 experiment, which can be explained by reforestation in the RCP4.5 scenario. Key Points The climate feedback in MPI‐ESM is non‐linear and depends on the forcing history Ocean heat uptake is reduced in a warmer climate. Patterns of temperature and precipitation changes are robust for RCP26/45/85.

The Max‐Planck‐Institute Aerosol Climatology version 1 (MAC‐v1) is introduced. It describes the optical properties of tropospheric aerosols on monthly timescales and with global coverage at a spatial resolution of 1° in latitude and longitude. By providing aerosol radiative properties for any wavelength of the solar (or shortwave) and of the terrestrial (or longwave) radiation spectrum, as needed in radiative transfer applications, this MAC‐v1 data set lends itself to simplified and computationally efficient representations of tropospheric aerosol in climate studies. Estimates of aerosol radiative properties are provided for both total and anthropogenic aerosol in annual time steps from preindustrial times (i.e., starting with year 1860) well into the future (until the year 2100). Central to the aerosol climatology is the merging of monthly statistics of aerosol optical properties for current (year 2000) conditions. Hereby locally sparse but trusted high‐quality data by ground‐based sun‐photometer networks are merged onto complete background maps defined by central data from global modeling with complex aerosol modules. This merging yields 0.13 for the global annual midvisible aerosol optical depth (AOD), with 0.07 attributed to aerosol sizes larger than 1 µm in diameter and 0.06 of attributed to aerosol sizes smaller than 1 µm in diameter. Hereby larger particles are less absorbing with a single scattering albedo (SSA) of 0.98 compared to 0.93 for smaller sizes. Simulation results of a global model are applied to prescribe the vertical distribution and to estimate anthropogenic contributions to the smaller size AOD as a function of time, with a 0.037 value for current conditions. In a demonstration application, the associated aerosol direct radiative effects are determined. For current conditions, total aerosol is estimated to reduce the combined shortwave and longwave net‐flux balance at the top of the atmosphere by about −1.6 W/m2 from which −0.5 W/m2 (with an uncertainty of ±0.2 W/m2) is attributed to anthropogenic activities. Based on past and projected aerosol emission data, the global anthropogenic direct aerosol impact (i.e., ToA cooling) is currently near the maximum and is projected to drop by 2100 to about −0.3 W/m2. The reported global averages are driven by considerable spatial and temporal variability. To better convey this diversity, regional and seasonal distributions of aerosol optical properties and their radiative effects are presented. On regional scales, the anthropogenic direct aerosol forcing can be an order of magnitude stronger than the global average and it can be of either sign. It is also shown that maximum anthropogenic impacts have shifted during the last 30 years from the U.S. and Europe to eastern and southern Asia. Key Points Aerosol monthly climatology for global modeling Aerosol direct radiative effects Temporal change in aerosol direct radiative forcing

The explicit calculation of stratospheric ozone in global climate models still comes at a high computational cost. Here, the usefulness of a linear ozone parameterization in a global climate model is assessed by comparing it to an explicit chemistry scheme and to observations. It is shown that the annual mean total ozone column and the tropical ozone profile agree well for the linear and the explicit chemistry schemes and the observations. Ozone variability caused by the quasi‐biennial oscillation and by extratropical quasi‐stationary planetary waves is reproduced qualitatively, but its magnitude is underestimated in particular in the simulations using the linear parameterization. The response of ozone to a quadrupling of CO 2 simulated with both schemes is in the range of earlier simulations with explicit schemes. This concerns in particular ozone decreases in the tropical lower stratosphere and increases above predicted as a consequence of a strengthening of tropical upwelling and potentially affecting climate sensitivity. This study demonstrates that despite existing weaknesses a linear ozone parameterization can be a useful tool to represent stratospheric ozone in climate models at negligible computational cost. Plain Language Summary Ozone is the main absorber of solar radiation in the stratosphere. Feedbacks between ozone, temperature, and circulation are therefore crucial for stratospheric variability and responses to external forcings like anthropogenic greenhouse gas emissions. However, the comprehensive representation of stratospheric ozone in global climate models comes at a high computational cost. Here, we analyze to what extent a computationally cheap linear ozone parameterization can overcome this problem. It is shown that a linear ozone scheme reproduces well key features of the mean ozone distribution but underestimates variability induced by circulation patterns like the quasi‐biennial oscillation of winds in the tropical stratosphere. The response of ozone to an idealized global warming scenario (instantaneous quadrupling of the atmospheric CO 2 concentration) simulated with the linear ozone scheme is in the range of earlier model studies with more complex chemistry schemes. We demonstrate that despite existing weaknesses, a linear ozone parameterization can be a useful tool to represent stratospheric ozone in climate models at negligible computational cost. Key Points Linear scheme can be a useful tool to interactively represent ozone at low computational cost Responses of ozone to 4xCO 2 are very similar for an explicit and a linear ozone scheme Linear scheme underestimates amplitudes of circulation‐induced variability patterns

This study analyzes a method of constructing a homogeneous, high-resolution global atmospheric hindcast. The method is the spectral nudging technique, which was applied to a state-of-the-art general circulation model (ECHAM6, T255L95). Large spatial scales of the global climate model prognostic variables were spectrally nudged toward a reanalysis dataset (NCEP-1, T62L28) for the past few decades. The main idea is the addition of dynamically consistent regional weather details to the coarse-grid NCEP-1 reanalysis. A large number of sensitivity experiments was performed, using different nudging e-folding times, vertical profiles, wavenumbers, and variables. Comparisons with observations and several reanalyses showed a high dependency on the variations of the nudging configuration. At the global scale, the accordance is very high for extratropical regions and lower in the tropics. A wavenumber truncation of 30, a relatively short e-folding time of 50 min, and a plateau-shaped nudging profile applied only to divergence and vorticity generally yielded the best results. This is one of the first global spectral nudging hindcast studies and the first applying an altitude-dependent profile to selected prognostic variables. The method can be applied to reconstructing the history of extreme events such as intense storms within the context of ongoing climate change.

State-of-the-art Earth system models typically employ grid spacings of O(100 km), which is too coarse to explicitly resolve main drivers of the flow of energy and matter across the Earth system. In this paper, we present the new ICON-Sapphire model configuration, which targets a representation of the components of the Earth system and their interactions with a grid spacing of 10 km and finer. Through the use of selected simulation examples, we demonstrate that ICON-Sapphire can (i) be run coupled globally on seasonal timescales with a grid spacing of 5 km, on monthly timescales with a grid spacing of 2.5 km, and on daily timescales with a grid spacing of 1.25 km; (ii) resolve large eddies in the atmosphere using hectometer grid spacings on limited-area domains in atmosphere-only simulations; (iii) resolve submesoscale ocean eddies by using a global uniform grid of 1.25 km or a telescoping grid with the finest grid spacing at 530 m, the latter coupled to a uniform atmosphere; and (iv) simulate biogeochemistry in an ocean-only simulation integrated for 4 years at 10 km. Comparison of basic features of the climate system to observations reveals no obvious pitfalls, even though some observed aspects remain difficult to capture. The throughput of the coupled 5 km global simulation is 126 simulated days per day employing 21 % of the latest machine of the German Climate Computing Center. Extrapolating from these results, multi-decadal global simulations including interactive carbon are now possible, and short global simulations resolving large eddies in the atmosphere and submesoscale eddies in the ocean are within reach.