T. Iversen | University of Oslo (original) (raw)

Papers by T. Iversen

Atmospheric Environment (1967), 1989

A model for the simulation of long range transport (LRT) of SO, and particulate SO:-has been cons... more A model for the simulation of long range transport (LRT) of SO, and particulate SO:-has been constructed. It empioys 10 layers with isentropic coordinate surfaces and is Eulerian. The horizontal and vertical advection is calculated by an anti-diffusively corrected upwind scheme (Smolarkiewica, Mon. We@. Rev, 111, 479-486, 1983). The oxidation of SO, is linearized (e.g. Eliassen and Saltbones, Atmospheric Environment, 17,1457-1473,1983) with a reaction rate depending on latitude and season. Vertical turbulent diffusion and wet deposition are parameterized, and the emissions are mixed vertically to a height dependent on the local static stability in the source areas. The model includes a meteorological 'preprocessor which estimates heating and precipitation from analyses of wind, mass field and humidity, together with knowledge ofgrouud surface properties. It has been applied to estimate the transport to the Arctic during two periods in 1983. Emission surveys were given by Semb (NILU, 1985), and separated into four main source regions in order to estimate the relative contribution from each. The model gives reasonable results as compared with measurements, however, the ground level S concentrations are somewhat exaggerated by the model. The difference in concentration level between the cold and warm seasons is reproduced, and the day to day variations are simulated reasonably well. During the simulation periods, the European and Soviet emissions are estimated to be the major contributors to Arctic air pollution. The North American emissions contribute sigticantly to upper level Arctic haze. Sources far from the Arctic contribute to upper level pollution, while close sources contribute at low levels. This pattern is much more pronounced in the cold period than during the warm, in accordance with the seasonal variation in the tropospheric static stability. The simulations indicate that the annual variation in meridional transport processes is a more important factor than the annual variation in Arctic wet deposition processes in determining the seasonal cycle in Arctic air pollution.

Atmospheric Environment (1967), 1989

Regular ground level measurements of particulate sulphate in air taken at Bjiarnerya (Bear Island... more Regular ground level measurements of particulate sulphate in air taken at Bjiarnerya (Bear Island) and Ny Alesund (Svalbard) are studied. Annual cycles in mean values and variances are observed, both having maxima during winter/spring. The two time-series are transformed into approximately second order stationarity, and the high-pass residuals are used to identify positive and negative anomalies. Days with positive anomalies are frequent during winter/spring and early autumn, however, the autumn anomalies rarely represent pollution episodes. The meteorological analysis selects quasi-stationary, extratropical flow systems on the planetary scale, creating extended meridional exchange of air. Two indices are applied to the 500 hPa flow; a meridional index created by wave numbers 1,2,3 and 4, and a zonal index. The agreement between the values of the indices and Arctic ground level air pollution, on the annual as well as the,episodic time scale, strongly suggests that the occurrence of quasi-stationary, atmospheric flow systems on the planetary scale is the major cause of long range transport of polluted air to the Arctic.

Atmospheric Environment (1967), 1985

Three characteristics of air pollution in the Norwegian Arctic are documented by measurements: th... more Three characteristics of air pollution in the Norwegian Arctic are documented by measurements: the seasonal cycle, the episodic behaviour and the deep tropospheric structure. The quasistationary, large scale, atmospheric phenomenon termed blocking is proposed to be a fundamental mechanism that provides conditions for poleward transport of ~~titudin~ air pollution. The seasonal variation of blocking is an important factor, in addition to scavenging and other removal processes, for the explanation of the annual cycle of Arctic air pollution.

Tellus B, 1992

A 3-dimensional Eulerian model is used to evaluate the influence of North American anthropogenic ... more A 3-dimensional Eulerian model is used to evaluate the influence of North American anthropogenic sulphur emissions over the western coast of Europe. Transport of sulphur across the North Atlantic ocean is simulated on the basis of the actual meteorology for 4 different months during 1982 and 1983. Results show large regional variations and indicate that the relative importance of North American emissions is largest over north western Europe. The North American contribution to pollution levels over western Europe is shown to occur mainly as wet deposition. Maximum wet deposition of North American origin over north western Europe is predicted for January 1983. On average over the four simulated months, the wet deposition of North American origin is estimated to be 2mgSm2month1 which represents 10% of the calculated total deposition over western Europe. The influence of North American sulphur on the air concentrations over the west coast of Europe is relatively larger at higher levels in the atmosphere. At ground level, the North American contribution to sulphate in air represents only 2.5% of the averaged total calculated, but it ascends to 50% at 5000m. Maximum contributions to the sulphate concentrations in air over western Europe are estimated to occur during July 1983.

Geoscientific Model Development, 2013

The objective of this study is to document and evaluate recent changes and updates to the module ... more The objective of this study is to document and evaluate recent changes and updates to the module for aerosols and aerosol-cloud-radiation interactions in the atmospheric module CAM4-Oslo of the core version of the Norwegian Earth System Model (NorESM), NorESM1-M. Particular attention is paid to the role of natural organics, sea salt, and mineral dust in determining the gross aerosol properties as well as the anthropogenic contribution to these properties and the associated direct and indirect radiative forcing. The aerosol module is extended from earlier versions that have been published, and includes life-cycling of sea salt, mineral dust, particulate sulphate, black carbon, and primary and secondary organics. The impacts of most of the numerous changes since previous versions are thoroughly explored by sensitivity experiments. The most important changes are: modified prognostic sea salt emissions; updated treatment of precipitation scavenging and gravitational settling; inclusion of biogenic primary organics and methane sulphonic acid (MSA) from oceans; almost doubled production of landbased biogenic secondary organic aerosols (SOA); and increased ratio of organic matter to organic carbon (OM/OC) for biomass burning aerosols from 1.4 to 2.6. Compared with in situ measurements and remotely sensed data, the new treatments of sea salt and dust aerosols give smaller biases in near-surface mass concentrations and aerosol optical depth than in the earlier model version. The model biases for mass concentrations are approximately unchanged for sulphate and BC. The enhanced levels of mod--1.2 W m -2 , which is closer to the IPCC AR4 estimates than the previous estimate of -1.9 W m -2 , has thus been obtained without imposing unrealistic artificial lower bounds on cloud droplet number concentrations.

Bulletin of the American Meteorological Society, 2011

We describe a field campaign out of Northern Norway in late winter 2008, providing new insight in... more We describe a field campaign out of Northern Norway in late winter 2008, providing new insight into the dynamics and predictability of polar lows and Arctic fronts.

… under global warming …, 2003

... During this visit Øyvind Seland implemented PNNL's aerosol schem... more ... During this visit Øyvind Seland implemented PNNL's aerosol scheme into the new version of NCAR's climate model, the Community Climate System Model (CCSM). ... Wahl, D. DNMI 8 Wettre, C. DNMI 7 Ødegård, V. DNMI 5 Ødegård, M. DNMI 1, 7 Ådlandsvik, B. IMR 2 ...

Atmospheric Chemistry and Physics, 2011

In this paper we describe and summarize the main achievements of the European Aerosol Cloud Clima... more In this paper we describe and summarize the main achievements of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). EUCAARI started on 1 January 2007 and ended on 31 December 2010 leaving a rich legacy including: (a) a comprehensive database with a year of observations of the physical, chemical and optical properties of aerosol particles over Europe, (b) comprehensive aerosol measurements in four developing countries, (c) a database of airborne measurements of aerosols and clouds over Europe during May 2008, (d) comprehensive modeling tools to study aerosol processes fron nano to global scale and their effects on climate and air quality. In addition a new Pan-European aerosol emissions inventory was developed and evaluated, a new cluster spectrometer was built and tested in the field and several new aerosol parameterizations and computations modules for chemical transport and global climate models were developed and evaluated. These achievements and related studies have substantially improved our understanding and reduced the uncertainties of aerosol radiative forcing and air quality-climate interactions. The EUCAARI results can be utilized in European and global environmental policy to assess the aerosol impacts and the corresponding abatement strategies.

Atmospheric Chemistry and Physics, 2006

Nine different global models with detailed aerosol modules have independently produced instantane... more Nine different global models with detailed aerosol modules have independently produced instantaneous direct radiative forcing due to anthropogenic aerosols. The anthropogenic impact is derived from the difference of two model simulations with prescribed aerosol emissions, one for present-day and one for pre-industrial conditions. The difference in the solar energy budget at the top of the atmosphere (ToA) yields a new harmonized estimate for the aerosol direct radiative forcing (RF) under all-sky conditions. On a global annual basis RF is −0.22 Wm −2 , ranging from +0.04 to −0.41 Wm −2 , with a standard deviation of ±0.16 Wm −2. Anthropogenic nitrate and dust are not included in this estimate. No model shows a significant positive all-sky RF. The corresponding clear-sky RF is −0.68 Wm −2. The cloud-sky RF was derived based on all-sky and clear-sky RF and modelled cloud cover. It was significantly different from zero and ranged between −0.16 and +0.34 Wm −2. A sensitivity analysis shows that the total aerosol RF is influenced by considerable diversity in simulated residence times, mass extinction coefficients and most importantly forcing efficiencies (forcing per unit optical depth). The clear-sky forcing efficiency (forcing per unit optical depth) has diversity comparable to that for the all-sky/ clear-sky forcing ratio. While the diversity in clear-sky forcing efficiency is impacted by factors

Atmospheric Chemistry and Physics, 2006

The AeroCom exercise diagnoses multicomponent aerosol modules in global modeling. In an initial a... more The AeroCom exercise diagnoses multicomponent aerosol modules in global modeling. In an initial assessment simulated global distributions for mass and mid-visible aerosol optical thickness (aot) were compared among 20 different modules. Model diversity was also explored in the context of previous comparisons. For the component combined aot general agreement has improved for the annual global mean. At 0.11 to 0.14, simulated aot values are at the lower end of global averages suggested by remote sensing from ground (AERONET ca. 0.135) and space (satellite composite ca. 0.15). More detailed comparisons, however, reveal that larger differences in regional distribution and significant differences in compositional mixture remain. Of

Atmospheric Chemistry and Physics, 2009

We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by... more We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by considering the diversity among year 2000 model simulations and comparing model predictions with available measurements. These model-measurement intercomparisons include BC surface and aircraft concentrations, aerosol absorption optical depth (AAOD) retrievals from AERONET and Ozone Monitoring Instrument (OMI) and BC column estimations based on AERONET. In regions other than Asia, most models are biased high compared to surface concentration measurements. However compared with (column) AAOD or BC burden retreivals, the models are generally biased low. The average ratio of model to retrieved AAOD is less than 0.7 in South American and 0.6 in African biomass burning regions; both of these regions lack surface concentration measurements. In Asia the average model to observed ratio is 0.7 for AAOD and 0.5 for BC surface concentrations. Compared with aircraft measurements over the Americas at latitudes between 0 and 50N, the average model is a factor of 8 larger than observed, and most models exceed the measured BC standard deviation in the mid to upper troposphere. At higher latitudes the average model to aircraft BC ratio is 0.4 and models underestimate the observed BC loading in the lower and middle troposphere associated with springtime Arctic haze. Low model bias for AAOD but overestimation of surface and upper atmospheric BC concentrations at lower latitudes suggests that most models are underestimating BC absorption and should improve estimates for refractive index, particle size, and optical effects of BC coating. Retrieval uncertainties and/or differences with model diagnostic treatment may also contribute to the model-measurement disparity. Largest AeroCom model diversity occurred in northern Eurasia and the remote Arctic, regions influenced by anthropogenic sources. Changing emissions, aging, removal, or optical properties within a single model generated a smaller change in model predictions than the range represented by the full set of AeroCom models. Upper tropospheric concentrations of BC mass from the aircraft measurements are suggested to provide a unique new benchmark to test scavenging and vertical dispersion of BC in global models.

Atmospheric Chemistry and Physics, 2007

The effects of unified aerosol sources on global aerosol fields simulated by different models are... more The effects of unified aerosol sources on global aerosol fields simulated by different models are examined in this paper. We compare results from two AeroCom experiments, one with different (ExpA) and one with unified emissions, injection heights, and particle sizes at the source (ExpB). Surprisingly, harmonization of aerosol sources has only a small impact on the simulated diversity for aerosol burden, and consequently optical properties, as the results are largely controlled by model-specific transport, removal, chemistry (leading to the formation of secondary aerosols) and parameterizations of aerosol microphysics (e.g. the split between deposition pathways) and to a lesser extent on the spatial and temporal distributions of the (precursor) emissions. The burdens of black carbon and especially sea salt become more coherent in ExpB only, because the large ExpA diversity for these two species was caused by few outliers. The experiment also indicated that despite prescribing emission fluxes and size distributions, ambiguities in the implementation in individual models can lead to substantial differences. These results indicate the need for a better understanding of aerosol life cycles at process level (including spatial dispersal and interaction with meteorological parameters) in order to obtain more reliable results from global aerosol simulations. This is particularly important as such model results are used to assess the consequences of specific air pollution abatement strategies.

Air Pollution Modeling and …, 2004

Aerosols influence processes in the climate system in a variety of ways. Particulate matter inter... more Aerosols influence processes in the climate system in a variety of ways. Particulate matter interacts with electromagnetic radiation by scattering and absorption, thus exerting direct radiative forcing. By extraction of moisture from ambient air or by their similarity to ice-crystals, ...

... Ghan, SJ, Leung, LR, Easter, RC, and Abdul-Razzak, H. (1997). Prediction of cloud droplet num... more ... Ghan, SJ, Leung, LR, Easter, RC, and Abdul-Razzak, H. (1997). Prediction of cloud droplet number in a general circulation model. J. Geophys. Res., 102(D18):21777–21794. Lohmann, U., Stier, P., Hoose, C., Ferrachat, S., Kloster, S., Roeckner, E., and Zhang, J. (2007). ...

We use global models to explore the microphysical effects of carbonaceous aerosols on liquid clou... more We use global models to explore the microphysical effects of carbonaceous aerosols on liquid clouds. Although absorption of solar radiation by soot warms the atmosphere, soot may cause climate cooling due to its contribution to cloud condensation nuclei (CCN) and therefore cloud brightness. Six global models conducted three soot experiments; four of the models had detailed aerosol microphysical schemes. The average cloud radiative response to biofuel soot (black and organic carbon), including both indirect and semi-direct effects, is −0.11 Wm −2 , comparable in size but opposite in sign to the respective direct effect. In a more idealized fossil fuel black carbon experiment, some models calculated a positive cloud response because soot provides a deposition sink for sulfuric and nitric acids and secondary organics, decreasing nucleation and evolution of viable CCN. Biofuel soot particles were also typically assumed to be larger and more hygroscopic than for fossil fuel soot and therefore caused more negative forcing, as also found in previous studies. Diesel soot (black and organic carbon) experiments had relatively smaller cloud impacts with five

The core version of the Norwegian Climate Center's Earth System Model, named NorESM1-M, is presen... more The core version of the Norwegian Climate Center's Earth System Model, named NorESM1-M, is presented. The NorESM family of models are based on the Community Climate System Model version 4 (CCSM4) of the University Corporation for Atmospheric Research, but differs from the latter by, in particular, an isopycnic coordinate ocean model and advanced chemistry-aerosol-cloud-radiation interaction schemes. NorESM1-M has a horizontal resolution of approximately 2 • for the atmosphere and land components and 1 • for the ocean and ice components. NorESM is also available in a lower resolution version (NorESM1-L) and a version that includes prognostic biogeochemical cycling (NorESM1-ME). The latter two model configurations are not part of this paper. Here, a first-order assessment of the model stability, the mean model state and the internal variability based on the model experiments made available to CMIP5 are presented. Further analysis of the model performance is provided in an accompanying paper (Iversen et al., 2013), presenting the corresponding climate response and scenario projections made with NorESM1-M. 1 Introduction In the following, the Norwegian Earth System Model (NorESM) is presented. NorESM is a nationally coordinated effort, building on the heritage of the research project Reg

Aerosol indirect effects continue to constitute one of the most important uncertainties for anthr... more Aerosol indirect effects continue to constitute one of the most important uncertainties for anthropogenic climate perturbations. Within the international AEROCOM initiative, the representation of aerosol-cloud-radiation interactions in ten different general circulation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects, and none of the model explicitly parameterises aerosol effects on convective clouds. We compute statistical relationships between aerosol optical depth (τ a) and various cloud and radiation quantities in a manner that is consistent between the models and the satellite data. It is found that the model-simulated influence of aerosols on

Atmospheric Environment. Part A. General Topics, 1993

Atmospheric Environment (1967), 1989

A model for the simulation of long range transport (LRT) of SO, and particulate SO:-has been cons... more A model for the simulation of long range transport (LRT) of SO, and particulate SO:-has been constructed. It empioys 10 layers with isentropic coordinate surfaces and is Eulerian. The horizontal and vertical advection is calculated by an anti-diffusively corrected upwind scheme (Smolarkiewica, Mon. We@. Rev, 111, 479-486, 1983). The oxidation of SO, is linearized (e.g. Eliassen and Saltbones, Atmospheric Environment, 17,1457-1473,1983) with a reaction rate depending on latitude and season. Vertical turbulent diffusion and wet deposition are parameterized, and the emissions are mixed vertically to a height dependent on the local static stability in the source areas. The model includes a meteorological 'preprocessor which estimates heating and precipitation from analyses of wind, mass field and humidity, together with knowledge ofgrouud surface properties. It has been applied to estimate the transport to the Arctic during two periods in 1983. Emission surveys were given by Semb (NILU, 1985), and separated into four main source regions in order to estimate the relative contribution from each. The model gives reasonable results as compared with measurements, however, the ground level S concentrations are somewhat exaggerated by the model. The difference in concentration level between the cold and warm seasons is reproduced, and the day to day variations are simulated reasonably well. During the simulation periods, the European and Soviet emissions are estimated to be the major contributors to Arctic air pollution. The North American emissions contribute sigticantly to upper level Arctic haze. Sources far from the Arctic contribute to upper level pollution, while close sources contribute at low levels. This pattern is much more pronounced in the cold period than during the warm, in accordance with the seasonal variation in the tropospheric static stability. The simulations indicate that the annual variation in meridional transport processes is a more important factor than the annual variation in Arctic wet deposition processes in determining the seasonal cycle in Arctic air pollution.

Atmospheric Environment (1967), 1989

Regular ground level measurements of particulate sulphate in air taken at Bjiarnerya (Bear Island... more Regular ground level measurements of particulate sulphate in air taken at Bjiarnerya (Bear Island) and Ny Alesund (Svalbard) are studied. Annual cycles in mean values and variances are observed, both having maxima during winter/spring. The two time-series are transformed into approximately second order stationarity, and the high-pass residuals are used to identify positive and negative anomalies. Days with positive anomalies are frequent during winter/spring and early autumn, however, the autumn anomalies rarely represent pollution episodes. The meteorological analysis selects quasi-stationary, extratropical flow systems on the planetary scale, creating extended meridional exchange of air. Two indices are applied to the 500 hPa flow; a meridional index created by wave numbers 1,2,3 and 4, and a zonal index. The agreement between the values of the indices and Arctic ground level air pollution, on the annual as well as the,episodic time scale, strongly suggests that the occurrence of quasi-stationary, atmospheric flow systems on the planetary scale is the major cause of long range transport of polluted air to the Arctic.

Atmospheric Environment (1967), 1985

Three characteristics of air pollution in the Norwegian Arctic are documented by measurements: th... more Three characteristics of air pollution in the Norwegian Arctic are documented by measurements: the seasonal cycle, the episodic behaviour and the deep tropospheric structure. The quasistationary, large scale, atmospheric phenomenon termed blocking is proposed to be a fundamental mechanism that provides conditions for poleward transport of ~~titudin~ air pollution. The seasonal variation of blocking is an important factor, in addition to scavenging and other removal processes, for the explanation of the annual cycle of Arctic air pollution.

Tellus B, 1992

A 3-dimensional Eulerian model is used to evaluate the influence of North American anthropogenic ... more A 3-dimensional Eulerian model is used to evaluate the influence of North American anthropogenic sulphur emissions over the western coast of Europe. Transport of sulphur across the North Atlantic ocean is simulated on the basis of the actual meteorology for 4 different months during 1982 and 1983. Results show large regional variations and indicate that the relative importance of North American emissions is largest over north western Europe. The North American contribution to pollution levels over western Europe is shown to occur mainly as wet deposition. Maximum wet deposition of North American origin over north western Europe is predicted for January 1983. On average over the four simulated months, the wet deposition of North American origin is estimated to be 2mgSm2month1 which represents 10% of the calculated total deposition over western Europe. The influence of North American sulphur on the air concentrations over the west coast of Europe is relatively larger at higher levels in the atmosphere. At ground level, the North American contribution to sulphate in air represents only 2.5% of the averaged total calculated, but it ascends to 50% at 5000m. Maximum contributions to the sulphate concentrations in air over western Europe are estimated to occur during July 1983.

Geoscientific Model Development, 2013

The objective of this study is to document and evaluate recent changes and updates to the module ... more The objective of this study is to document and evaluate recent changes and updates to the module for aerosols and aerosol-cloud-radiation interactions in the atmospheric module CAM4-Oslo of the core version of the Norwegian Earth System Model (NorESM), NorESM1-M. Particular attention is paid to the role of natural organics, sea salt, and mineral dust in determining the gross aerosol properties as well as the anthropogenic contribution to these properties and the associated direct and indirect radiative forcing. The aerosol module is extended from earlier versions that have been published, and includes life-cycling of sea salt, mineral dust, particulate sulphate, black carbon, and primary and secondary organics. The impacts of most of the numerous changes since previous versions are thoroughly explored by sensitivity experiments. The most important changes are: modified prognostic sea salt emissions; updated treatment of precipitation scavenging and gravitational settling; inclusion of biogenic primary organics and methane sulphonic acid (MSA) from oceans; almost doubled production of landbased biogenic secondary organic aerosols (SOA); and increased ratio of organic matter to organic carbon (OM/OC) for biomass burning aerosols from 1.4 to 2.6. Compared with in situ measurements and remotely sensed data, the new treatments of sea salt and dust aerosols give smaller biases in near-surface mass concentrations and aerosol optical depth than in the earlier model version. The model biases for mass concentrations are approximately unchanged for sulphate and BC. The enhanced levels of mod--1.2 W m -2 , which is closer to the IPCC AR4 estimates than the previous estimate of -1.9 W m -2 , has thus been obtained without imposing unrealistic artificial lower bounds on cloud droplet number concentrations.

Bulletin of the American Meteorological Society, 2011

We describe a field campaign out of Northern Norway in late winter 2008, providing new insight in... more We describe a field campaign out of Northern Norway in late winter 2008, providing new insight into the dynamics and predictability of polar lows and Arctic fronts.

… under global warming …, 2003

... During this visit Øyvind Seland implemented PNNL's aerosol schem... more ... During this visit Øyvind Seland implemented PNNL's aerosol scheme into the new version of NCAR's climate model, the Community Climate System Model (CCSM). ... Wahl, D. DNMI 8 Wettre, C. DNMI 7 Ødegård, V. DNMI 5 Ødegård, M. DNMI 1, 7 Ådlandsvik, B. IMR 2 ...

Atmospheric Chemistry and Physics, 2011

In this paper we describe and summarize the main achievements of the European Aerosol Cloud Clima... more In this paper we describe and summarize the main achievements of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). EUCAARI started on 1 January 2007 and ended on 31 December 2010 leaving a rich legacy including: (a) a comprehensive database with a year of observations of the physical, chemical and optical properties of aerosol particles over Europe, (b) comprehensive aerosol measurements in four developing countries, (c) a database of airborne measurements of aerosols and clouds over Europe during May 2008, (d) comprehensive modeling tools to study aerosol processes fron nano to global scale and their effects on climate and air quality. In addition a new Pan-European aerosol emissions inventory was developed and evaluated, a new cluster spectrometer was built and tested in the field and several new aerosol parameterizations and computations modules for chemical transport and global climate models were developed and evaluated. These achievements and related studies have substantially improved our understanding and reduced the uncertainties of aerosol radiative forcing and air quality-climate interactions. The EUCAARI results can be utilized in European and global environmental policy to assess the aerosol impacts and the corresponding abatement strategies.

Atmospheric Chemistry and Physics, 2006

Nine different global models with detailed aerosol modules have independently produced instantane... more Nine different global models with detailed aerosol modules have independently produced instantaneous direct radiative forcing due to anthropogenic aerosols. The anthropogenic impact is derived from the difference of two model simulations with prescribed aerosol emissions, one for present-day and one for pre-industrial conditions. The difference in the solar energy budget at the top of the atmosphere (ToA) yields a new harmonized estimate for the aerosol direct radiative forcing (RF) under all-sky conditions. On a global annual basis RF is −0.22 Wm −2 , ranging from +0.04 to −0.41 Wm −2 , with a standard deviation of ±0.16 Wm −2. Anthropogenic nitrate and dust are not included in this estimate. No model shows a significant positive all-sky RF. The corresponding clear-sky RF is −0.68 Wm −2. The cloud-sky RF was derived based on all-sky and clear-sky RF and modelled cloud cover. It was significantly different from zero and ranged between −0.16 and +0.34 Wm −2. A sensitivity analysis shows that the total aerosol RF is influenced by considerable diversity in simulated residence times, mass extinction coefficients and most importantly forcing efficiencies (forcing per unit optical depth). The clear-sky forcing efficiency (forcing per unit optical depth) has diversity comparable to that for the all-sky/ clear-sky forcing ratio. While the diversity in clear-sky forcing efficiency is impacted by factors

Atmospheric Chemistry and Physics, 2006

The AeroCom exercise diagnoses multicomponent aerosol modules in global modeling. In an initial a... more The AeroCom exercise diagnoses multicomponent aerosol modules in global modeling. In an initial assessment simulated global distributions for mass and mid-visible aerosol optical thickness (aot) were compared among 20 different modules. Model diversity was also explored in the context of previous comparisons. For the component combined aot general agreement has improved for the annual global mean. At 0.11 to 0.14, simulated aot values are at the lower end of global averages suggested by remote sensing from ground (AERONET ca. 0.135) and space (satellite composite ca. 0.15). More detailed comparisons, however, reveal that larger differences in regional distribution and significant differences in compositional mixture remain. Of

Atmospheric Chemistry and Physics, 2009

We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by... more We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by considering the diversity among year 2000 model simulations and comparing model predictions with available measurements. These model-measurement intercomparisons include BC surface and aircraft concentrations, aerosol absorption optical depth (AAOD) retrievals from AERONET and Ozone Monitoring Instrument (OMI) and BC column estimations based on AERONET. In regions other than Asia, most models are biased high compared to surface concentration measurements. However compared with (column) AAOD or BC burden retreivals, the models are generally biased low. The average ratio of model to retrieved AAOD is less than 0.7 in South American and 0.6 in African biomass burning regions; both of these regions lack surface concentration measurements. In Asia the average model to observed ratio is 0.7 for AAOD and 0.5 for BC surface concentrations. Compared with aircraft measurements over the Americas at latitudes between 0 and 50N, the average model is a factor of 8 larger than observed, and most models exceed the measured BC standard deviation in the mid to upper troposphere. At higher latitudes the average model to aircraft BC ratio is 0.4 and models underestimate the observed BC loading in the lower and middle troposphere associated with springtime Arctic haze. Low model bias for AAOD but overestimation of surface and upper atmospheric BC concentrations at lower latitudes suggests that most models are underestimating BC absorption and should improve estimates for refractive index, particle size, and optical effects of BC coating. Retrieval uncertainties and/or differences with model diagnostic treatment may also contribute to the model-measurement disparity. Largest AeroCom model diversity occurred in northern Eurasia and the remote Arctic, regions influenced by anthropogenic sources. Changing emissions, aging, removal, or optical properties within a single model generated a smaller change in model predictions than the range represented by the full set of AeroCom models. Upper tropospheric concentrations of BC mass from the aircraft measurements are suggested to provide a unique new benchmark to test scavenging and vertical dispersion of BC in global models.

Atmospheric Chemistry and Physics, 2007

The effects of unified aerosol sources on global aerosol fields simulated by different models are... more The effects of unified aerosol sources on global aerosol fields simulated by different models are examined in this paper. We compare results from two AeroCom experiments, one with different (ExpA) and one with unified emissions, injection heights, and particle sizes at the source (ExpB). Surprisingly, harmonization of aerosol sources has only a small impact on the simulated diversity for aerosol burden, and consequently optical properties, as the results are largely controlled by model-specific transport, removal, chemistry (leading to the formation of secondary aerosols) and parameterizations of aerosol microphysics (e.g. the split between deposition pathways) and to a lesser extent on the spatial and temporal distributions of the (precursor) emissions. The burdens of black carbon and especially sea salt become more coherent in ExpB only, because the large ExpA diversity for these two species was caused by few outliers. The experiment also indicated that despite prescribing emission fluxes and size distributions, ambiguities in the implementation in individual models can lead to substantial differences. These results indicate the need for a better understanding of aerosol life cycles at process level (including spatial dispersal and interaction with meteorological parameters) in order to obtain more reliable results from global aerosol simulations. This is particularly important as such model results are used to assess the consequences of specific air pollution abatement strategies.

Air Pollution Modeling and …, 2004

Aerosols influence processes in the climate system in a variety of ways. Particulate matter inter... more Aerosols influence processes in the climate system in a variety of ways. Particulate matter interacts with electromagnetic radiation by scattering and absorption, thus exerting direct radiative forcing. By extraction of moisture from ambient air or by their similarity to ice-crystals, ...

... Ghan, SJ, Leung, LR, Easter, RC, and Abdul-Razzak, H. (1997). Prediction of cloud droplet num... more ... Ghan, SJ, Leung, LR, Easter, RC, and Abdul-Razzak, H. (1997). Prediction of cloud droplet number in a general circulation model. J. Geophys. Res., 102(D18):21777–21794. Lohmann, U., Stier, P., Hoose, C., Ferrachat, S., Kloster, S., Roeckner, E., and Zhang, J. (2007). ...

We use global models to explore the microphysical effects of carbonaceous aerosols on liquid clou... more We use global models to explore the microphysical effects of carbonaceous aerosols on liquid clouds. Although absorption of solar radiation by soot warms the atmosphere, soot may cause climate cooling due to its contribution to cloud condensation nuclei (CCN) and therefore cloud brightness. Six global models conducted three soot experiments; four of the models had detailed aerosol microphysical schemes. The average cloud radiative response to biofuel soot (black and organic carbon), including both indirect and semi-direct effects, is −0.11 Wm −2 , comparable in size but opposite in sign to the respective direct effect. In a more idealized fossil fuel black carbon experiment, some models calculated a positive cloud response because soot provides a deposition sink for sulfuric and nitric acids and secondary organics, decreasing nucleation and evolution of viable CCN. Biofuel soot particles were also typically assumed to be larger and more hygroscopic than for fossil fuel soot and therefore caused more negative forcing, as also found in previous studies. Diesel soot (black and organic carbon) experiments had relatively smaller cloud impacts with five

The core version of the Norwegian Climate Center's Earth System Model, named NorESM1-M, is presen... more The core version of the Norwegian Climate Center's Earth System Model, named NorESM1-M, is presented. The NorESM family of models are based on the Community Climate System Model version 4 (CCSM4) of the University Corporation for Atmospheric Research, but differs from the latter by, in particular, an isopycnic coordinate ocean model and advanced chemistry-aerosol-cloud-radiation interaction schemes. NorESM1-M has a horizontal resolution of approximately 2 • for the atmosphere and land components and 1 • for the ocean and ice components. NorESM is also available in a lower resolution version (NorESM1-L) and a version that includes prognostic biogeochemical cycling (NorESM1-ME). The latter two model configurations are not part of this paper. Here, a first-order assessment of the model stability, the mean model state and the internal variability based on the model experiments made available to CMIP5 are presented. Further analysis of the model performance is provided in an accompanying paper (Iversen et al., 2013), presenting the corresponding climate response and scenario projections made with NorESM1-M. 1 Introduction In the following, the Norwegian Earth System Model (NorESM) is presented. NorESM is a nationally coordinated effort, building on the heritage of the research project Reg

Aerosol indirect effects continue to constitute one of the most important uncertainties for anthr... more Aerosol indirect effects continue to constitute one of the most important uncertainties for anthropogenic climate perturbations. Within the international AEROCOM initiative, the representation of aerosol-cloud-radiation interactions in ten different general circulation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects, and none of the model explicitly parameterises aerosol effects on convective clouds. We compute statistical relationships between aerosol optical depth (τ a) and various cloud and radiation quantities in a manner that is consistent between the models and the satellite data. It is found that the model-simulated influence of aerosols on

Atmospheric Environment. Part A. General Topics, 1993