Publications (66)82.94 Total impact
-
Article: The present-day decadal solar cycle modulation of Earth's radiative forcing via charged H2SO4/H2O aerosol nucleation
[show abstract] [hide abstract]
ABSTRACT: The decadal solar cycle modulation of Earth's radiative forcing via ionization of the atmosphere by galactic cosmic rays, aerosol formation from the gas phase, and the response of clouds to aerosol is quantified for the first time with a climate model that represents and couples the relevant processes. Simulations are conducted for solar maximum and minimum conditions, with present-day anthropogenic aerosol and aerosol precursor gas emissions, and contemporary large-scale meteorology. The solar cycle signal appears in atmospheric ionization, aerosol formation from the gas phase, aerosol concentrations, aerosol optical depth, and in cloud properties, and is most pronounced at mid- and high latitudes. The resulting solar cycle modulation of Earth's radiative forcing exhibits a distinct hemispheric asymmetry, with peak values of −0.14 W m−2 in the southern and −0.06 W m−2 in the northern mid-latitudes. Globally and annually averaged, the solar cycle modulation of Earth's radiative forcing, arising from the increase in atmospheric ionization by galactic cosmic rays from solar maximum to minimum, via charged nucleation of aerosol, the direct aerosol effect, and the cloud albedo effect, amounts to −0.05 W m−2. A limited relevance of this variation for the Earth's atmosphere and climate can be inferred, given that Earth's radiative forcing changes by −0.24 W m−2 from solar maximum to minimum because of a decrease in total solar irradiance.Geophysical Research Letters 01/2012; 39:L02805. · 3.79 Impact Factor -
Article: Bacteria in the ECHAM5-HAM global climate model
[show abstract] [hide abstract]
ABSTRACT: Some bacteria are among the most active ice nu-clei found in nature due to the ice nucleation active proteins on their surface, which serve as active sites for ice nucle-ation. Their potential impact on clouds and precipitation is not well known and needs to be investigated. Bacteria as a new aerosol species were introduced into the global climate model (GCM) ECHAM5-HAM. The inclusion of bacteria acting as ice nuclei in a GCM leads to only minor changes in cloud formation and precipitation on a global level, how-ever, changes in the liquid water path and ice water path are simulated, specifically in the boreal regions where tun-dra and forests act as sources of bacteria. Although bacteria contribute to heterogeneous freezing, their impact is reduced by their low numbers compared to other heterogeneous IN. This result confirms the outcome of several previous studies.Atmospheric Chemistry and Physics 01/2012; 12:8645-8661. · 4.88 Impact Factor -
Article: Soot microphysical effects on liquid clouds, a multi-model investigation
[show abstract] [hide abstract]
ABSTRACT: We use global models to explore the microphys-ical effects of carbonaceous aerosols on liquid clouds. Al-though absorption of solar radiation by soot warms the atmo-sphere, soot may cause climate cooling due to its contribu-tion to cloud condensation nuclei (CCN) and therefore cloud brightness. Six global models conducted three soot experi-ments; four of the models had detailed aerosol microphysi-cal 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 mod-els calculated a positive cloud response because soot pro-vides a deposition sink for sulfuric and nitric acids and sec-ondary organics, decreasing nucleation and evolution of vi-able 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 Correspondence to: D. Koch (dorothy.koch@science.doe.gov) of the models <±0.06 Wm −2 from clouds. The results are subject to the caveats that variability among models, and re-gional and interrannual variability for each model, are large. This comparison together with previously published results stresses the need to further constrain aerosol microphysical schemes. The non-linearities resulting from the competition of opposing effects on the CCN population make it difficult to extrapolate from idealized experiments to likely impacts of realistic potential emission changes.ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; 11:1051-1064. · 5.52 Impact Factor -
Article: Assessment of aerosol-cloud-precipitation interactions employing parameterizations of various complexities
[show abstract] [hide abstract]
ABSTRACT: Atmospheric aerosols play an important role in the global climate system through modifications of the global radiation budget: directly, by scattering and absorption of radiation and indirectly, by the modification of cloud properties and abundance with impacts on the hydrological cycle. In particular the indirect aerosol effects on clouds and precipitation are subject to large uncertainties. State of the art global aerosol-cloud climate models allow estimates of aerosol-cloud interactions. However, limitations in the underlying cloud and aerosol microphysics and in particular the requirement to reduce their complexity for the implementation in global climate models introduce significant uncertainties. Previous comparisons of conceptually different approaches were often of limited explanatory power due to the usage of varying models and setups. In this study we investigate uncertainties in estimates of indirect aerosol effects through studies with the ECHAM5-HAM aerosol-climate model. We employ parameterizations of the aerosol-cloud interactions of various complexities in an identical model setup to quantify the contribution of the process parameterization to the uncertainty in the simulated aerosol-cloud interactions. The consideration of the direct and semi-direct effects as well as the indirect cloud albedo and lifetime effects allows for a comprehensive investigation of the aerosol effects on precipitation. A detailed evaluation of the results with satellite observations of aerosol and cloud parameters will provide observational constraints on the simulated aerosol-cloud-precipitation interactions in the different model setups. Our results help to understand and quantify uncertainties in estimates of aerosol-cloud interactions and yield valuable information about the necessary level of detail of the process representation in global climate models.03/2009; 11:2778. -
Article: Aerosol indirect effects – general circulation model intercomparison and evaluation with satellite data
[show abstract] [hide abstract]
ABSTRACT: 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 inter-actions 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 in-clude ice nucleation effects, and none of the model explicitly parameterises aerosol effects on convective clouds. We com-pute 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 Correspondence to: J. Quaas (johannes.quaas@zmaw.de)Atmospheric Chemistry and Physics 01/2009; 9:8697-8717. -
Article: Aerosol indirect effects – general circulation model intercomparison and evaluation with satellite data
[show abstract] [hide abstract]
ABSTRACT: 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 inter-actions 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 in-clude ice nucleation effects, and none of the model explicitly parameterises aerosol effects on convective clouds. We com-pute 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 Correspondence to: J. Quaas (johannes.quaas@zmaw.de)ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2009; 9:8697-8717. · 5.52 Impact Factor -
Article: The global influence of dust mineralogical composition on heterogeneous ice nucleation in mixed-phase clouds
[show abstract] [hide abstract]
ABSTRACT: Mineral dust is the dominant natural ice nucleating aerosol. Its ice nucleation efficiency depends on the mineralogical composition. We show the first sensitivity studies with a global climate model and a three-dimensional dust mineralogy. Results show that, depending on the dust mineralogical composition, coating with soluble material from anthropogenic sources can lead to quasi-deactivation of natural dust ice nuclei. This effect counteracts the increased cloud glaciation by anthropogenic black carbon particles. The resulting aerosol indirect effect through the glaciation of mixed-phase clouds by black carbon particles is small (+0.1 W m−2 in the shortwave top-of-the-atmosphere radiation in the northern hemisphere).Environmental Research Letters 05/2008; 3(2):025003. · 3.63 Impact Factor -
Article: Modeling of the Wegener–Bergeron–Findeisen process—implications for aerosol indirect effects
[show abstract] [hide abstract]
ABSTRACT: A new parameterization of the Wegener–Bergeron–Findeisen (WBF) process has been developed, and implemented in the general circulation model CAM-Oslo. The new parameterization scheme has important implications for the process of phase transition in mixed-phase clouds. The new treatment of the WBF process replaces a previous formulation, in which the onset of the WBF effect depended on a threshold value of the mixing ratio of cloud ice. As no observational guidance for such a threshold value exists, the previous treatment added uncertainty to estimates of aerosol effects on mixed-phase clouds. The new scheme takes subgrid variability into account when simulating the WBF process, allowing for smoother phase transitions in mixed-phase clouds compared to the previous approach. The new parameterization yields a model state which gives reasonable agreement with observed quantities, allowing for calculations of aerosol effects on mixed-phase clouds involving a reduced number of tunable parameters. Furthermore, we find a significant sensitivity to perturbations in ice nuclei concentrations with the new parameterization, which leads to a reversal of the traditional cloud lifetime effect.Environ. Res. Lett. 01/2008; 3:45001-10. -
Chapter: Technical Summary
01/2007; -
Chapter: How well do aerosol retrievals from satellites and representation in global circulation models match ground-based AERONET aerosol statistics?
[show abstract] [hide abstract]
ABSTRACT: Statistics from sky/sunphotometers at AERONET sites throughout the world provide the background for a comparison of monthly or seasonally averaged aerosol optical depths to retrievals by operational satellites and to representations in global models. Available data-sets, however, rarely relate to the same year(s). With strong year-to-year variations even for monthly averaged aerosol optical depths and open issues on sampling biases and regional representation by local measurements only larger discrepancies are investigated. Aerosol optical depths retrievals of five different satellites and five different global models are compared. Quantitative accurate satellite retrievals over land remain a challenge and even their relative difference cannot provide clear answers on regional representation. Model predicted aerosol optical depth averages are usually smaller than AERONET. The behavior of models is further explored on a component basis. For sulfate, dust, carbon and sea-salt optical depths, mass and assumed aerosol sizes are compared. For the conversion of the column (dry) component mass in optical depth in models, assumptions for component aerosol size and aerosol humidification are critical. Statistical comparisons to ground-based monitoring will be more useful, if temporal differences are removed. This requires data from the same time-period and the use of sampling screens, to accommodate less frequent measurements. For the understanding of regional representation by local measurements, satellite data play a key role. Necessities to validate critical aerosol assumptions in models or satellite retrievals require field-experiments that focus on individual aerosol components plus continued and additional monitoring (e.g. AERONET) at sites, where a particular aerosol component dominates.11/2006: pages 103-158; -
Article: Solid ammonium sulfate aerosols as ice nuclei: a pathway for cirrus cloud formation.
[show abstract] [hide abstract]
ABSTRACT: Laboratory measurements support a cirrus cloud formation pathway involving heterogeneous ice nucleation by solid ammonium sulfate aerosols. Ice formation occurs at low ice-saturation ratios consistent with the formation of continental cirrus and an interhemispheric asymmetry observed for cloud onset. In a climate model, this mechanism provides a widespread source of ice nuclei and leads to fewer but larger ice crystals as compared with a homogeneous freezing scenario. This reduces both the cloud albedo and the longwave heating by cirrus. With the global ammonia budget dominated by agricultural practices, this pathway might further couple anthropogenic activity to the climate system.Science 10/2006; 313(5794):1770-3. · 31.20 Impact Factor -
Article: Monthly averages of aerosol properties: A global comparison among models, satellite data, and AERONET ground data
Journal of Geophysical Research 10/2003; 108(D20-4634). · 3.02 Impact Factor -
Article: A comparison of large‐scale atmospheric sulphate aerosol models (COSAM): overview and highlights
[show abstract] [hide abstract]
ABSTRACT: The comparison of large-scale sulphate aerosol models study (COSAM) compared the performance of atmospheric models with each other and observations. It involved: (i) design of a standard model experiment for the world wide web, (ii) 10 model simulations of the cycles of sulphur and 222Rn/210Pb conforming to the experimental design, (iii) assemblage of the best available observations of atmospheric SO=4, SO2 and MSA and (iv) a workshop in Halifax, Canada to analyze model performance and future model development needs. The analysis presented in this paper and two companion papers by Roelofs, and Lohmann and co-workers examines the variance between models and observations, discusses the sources of that variance and suggests ways to improve models. Variations between models in the export of SOx from Europe or North America are not sufficient to explain an order of magnitude variation in spatial distributions of SOx downwind in the northern hemisphere. On average, models predicted surface level seasonal mean SO=4 aerosol mixing ratios better (most within 20%) than SO2 mixing ratios (over-prediction by factors of 2 or more). Results suggest that vertical mixing from the planetary boundary layer into the free troposphere in source regions is a major source of uncertainty in predicting the global distribution of SO=4 aerosols in climate models today. For improvement, it is essential that globally coordinated research efforts continue to address emissions of all atmospheric species that affect the distribution and optical properties of ambient aerosols in models and that a global network of observations be established that will ultimately produce a world aerosol chemistry climatology.Tellus B 03/2003; 53(5):615 - 645. · 4.38 Impact Factor -
Article: Evaluating aerosol/cloud/radiation process parameterizations with single column models and Second Aerosol Characterization Experiment (ACE-2) cloudy column observations
Journal of Geophysical Research 01/2003; 108:4762. · 3.02 Impact Factor -
Article: Vertical distributions of sulfur species simulated by large scale atmospheric models in COSAM: Comparison with observations
[show abstract] [hide abstract]
ABSTRACT: A comparison of large-scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on wet deposition measurements and sulfate aerosol concentrations in source regions at the surface. They found that different models simulated the observed sulfate surface concentrations mostly within a factor of two, but that the simulated column burdens and vertical profiles were very different amongst different models. In the COSAM exercise, one aspect is the comparison of sulfate aerosol and precursor gases above the surface. Vertical profiles of SO2, SO2−4, oxidants and cloud properties were measured by aircraft during the North Atlantic Regional Experiment (NARE) experiment in August/September 1993 off the coast of Nova Scotia and during the Second Eulerian Model Evaluation Field Study (EMEFSII), in central Ontario in March/April 1990. While no single model stands out as being best or worst, the general tendency is that those models simulating the full oxidant chemistry tend to agree best with observations although differences in transport and treatment of clouds are important as well.Tellus B 10/2001; 53(5):646 - 672. · 4.38 Impact Factor -
Article: Analysis of regional budgets of sulfur species modeled for the COSAM exercise
[show abstract] [hide abstract]
ABSTRACT: The COSAM intercomparison exercise (comparison of large-scale sulfur models) was organized to compare and evaluate the performance of global sulfur cycle models. Eleven models participated, and from these models the simulated surface concentrations, vertical profiles and budget terms were submitted. This study focuses on simulated budget terms for the sources and sinks of SO2 and sulfate in three polluted regions in the Northern Hemisphere, i.e., eastern North America, Europe, and Southeast Asia. Qualitatively, features of the sulfur cycle are modeled quite consistently between models, such as the relative importance of dry deposition as a removal mechanism for SO2, the importance of aqueous phase oxidation over gas phase oxidation for SO2, and the importance of wet over dry deposition for removal of sulfate aerosol. Quantitatively, however, models may show large differences, especially for cloud-related processes, i.e., aqueous phase oxidation of SO2 and sulfate wet deposition. In some cases a specific behavior can be related to the treatment of oxidants for aqueous phase SO2 oxidation, or the vertical resolution applied in models. Generally, however, the differences between models appear to be related to simulated cloud (micro-)physics and distributions, whereas differences in vertical transport efficiencies related to convection play an additional rôle. The estimated sulfur column burdens, lifetimes and export budgets vary between models by about a factor of 2 or 3. It can be expected that uncertainties in related effects which are derived from global sulfur model calculations, such as direct and indirect climate forcing estimates by sulfate aerosol, are at least of similar magnitude.Tellus B 10/2001; 53(5):673 - 694. · 4.38 Impact Factor -
Article: An Intercomparison of Single-Column Model Simulations of Summertime Midlatitude Continental Convection
[show abstract] [hide abstract]
ABSTRACT: Introduction The ultimate goal of the Atmospheric Radiation Measurement (ARM) Program is the improvement of parameterizations of clouds and radiation used in climate models. This goal is being achieved through the use of field measurements to evaluate the parameterizations. One common parameterization testbed is the single-column model (SCM), which is essentially an isolated column of a global climate model. In this study, we have brought together a collection of 11 SCMs and one Cloud Ensemble Model (CEM), and subjected each of them to several alternative analyses of the large-scale forcing observed over the ARM Southern Great Plains (SGP) site during the period July 18 to August 3, 1995. Our goals were as follows: Ninth ARM Science Team Meeting Proceedings, San Antonio, Texas, March 22-26, 1999 2 1. To explore the influence of alternative objective analyses on the SCM and CEM simulations. 2. To investigate alternative strategies for using the objective ana03/2000; -
Article: The atmospheric sulfur cycle in ECHAM-4 and its impact on the shortwave radiation
[show abstract] [hide abstract]
ABSTRACT: The atmospheric general circulation model ECHAM-4 is coupled to a chemistry model to calculate sulfate mass distribution and the radiative forcing due to sulfate aerosol particles. The model simulates the main components of the hydrological cycle and, hence, it allows an explicit treatment of cloud transformation processes and precipitation scavenging. Two experiments are performed, one with pre-industrial and one with present-day sulfur emissions. In the pre-industrial emission scenario SO2 is oxidized faster to sulfate and the in-cloud oxidation via the reaction with ozone is more important than in the present-day scenario. The atmospheric sulfate mass due to anthropogenic emissions is estimated as 0.38 Tg sulfur. The radiative forcing due to anthropogenic sulfate aerosols is calculated diagnostically. The backscattering of shortwave radiation (direct effect) as well as the impact of sulfate aerosols on the cloud albedo (indirect effect) is estimated. The model predicts a direct forcing of −0.35 W m-2 and an indirect forcing of −0.76 W m-2. Over the continents of the Northern Hemisphere the direct forcing amounts to −0.64 W m-2. The geographical distribution of the direct and indirect effect is very different. Whereas the direct forcing is strongest over highly polluted continental regions, the indirect forcing over sea exceeds that over land. It is shown that forcing estimates based on monthly averages rather than on instantaneous sulfate pattern overestimate the indirect effect but have little effect on the direct forcing.Climate Dynamics 03/1997; 13(4):235-246. · 4.60 Impact Factor -
Article: Design and performance of a new cloud microphysics scheme developed for the ECHAM general circulation model
[show abstract] [hide abstract]
ABSTRACT: A new cloud microphysics scheme including a prognostic treatment of cloud ice (PCI) is developed to yield a more physically based representation of the components of the atmospheric moisture budget in the general circulation model ECHAM. The new approach considers cloud water and cloud ice as separate prognostic variables. The precipitation formation scheme for warm clouds distinguishes between maritime and continental clouds by considering the cloud droplet number concentration, in addition to the liquid water content. Based on several observational data sets, the cloud droplet number concentration is derived from the sulfate aerosol mass concentration as given from the sulfur cycle simulated by ECHAM. Results obtained with the new scheme are compared to satellite observations and in situ measurements of cloud physical and radiative properties. In general, the standard model ECHAM4 and also PCI capture the overall features, and the simulated results usually lie within the range of observed uncertainty. As compared to ECHAM4, only slight improvements are achieved with the new scheme. For example, the overestimated liquid water path and total cloud cover over convectively active regions are reduced in PCI. On the other hand, some shortcomings of the standard model such as underestimated shortwave cloud forcing over the extratropical oceans of the respective summer hemisphere are more pronounced in PCI.Climate Dynamics 05/1996; 12(8):557-572. · 4.60 Impact Factor -
Article: Sensitivity studies of the effect of increased aerosol concentrations and snow crystal shape on the snowfall rate in the Arctic
[show abstract] [hide abstract]
ABSTRACT: 1] The mesoscale model GESIMA is used to simulate microphysical properties of Arctic clouds and their effect on radiation. Different case studies during the FIRE ACE/SHEBA project show that GESIMA is able to simulate the cloud boundaries, ice and liquid water content, and effective radii in good agreement with observations. For two different aerosol scenarios, the simulation results show that the anthropogenic aerosol can alter microphysical properties of Arctic clouds, and consequently modify surface precipitation. Borys et al. [2000] proposed that anthropogenically induced decreases in cloud droplet size inhibit the riming process. On the contrary, we find that the accretion of snow crystals with cloud droplets is increased in the polluted cloud owing to its higher cloud droplet number concentration. Instead, the autoconversion rate of cloud droplets and accretion of drizzle by snow decreases caused by the shutdown of the collision-coalescence process in the polluted cloud. The amount of precipitation reaching the surface as snow depends crucially on the crystal shape. If aggregates are assumed, then a tenfold increase in aerosol concentration leads to an increase in accumulated snow by 40% after 7 hours of simulation whereas the snow amount decreases by 30% when planar crystals are assumed because of the larger accretion efficiency of snow crystals with cloud droplets in case of aggregates. Citation: Lohmann, U., J. Zhang, and J. Pi, Sensitivity studies of the effect of increased aerosol concentrations and snow crystal shape on the snowfall rate in the Arctic, J. Geophys. Res., 108(D11), 4341, doi:10.1029/2003JD003377, 2003.01/1854; 3354.
Top co-authors
Top Journals
Institutions
-
2006–2012
-
ETH Zurich
- Institute of Atmosphere and Climate Science
Zürich, ZH, Switzerland
-
-
1854–2006
-
Dalhousie University
- Department of Physics and Atmospheric Science
Halifax, Nova Scotia, Canada -
University of Nebraska at Omaha
Omaha, NE, USA
-
-
1997
-
Max-Planck-Institut für Meteorologie
Hamburg, Hamburg, Germany
-
