Johann Feichter

Transient Climate Change Simulations with a Coupled Atmosphere–Ocean GCM Including the Tropospheric Sulfur Cycle

Abstract The time-dependent climate response to changing concentrations of greenhouse gases and sulfate aerosols is studied using a coupled general circulation model of the atmosphere and the ocean (ECHAM4/OPYC3). The concentrations of the well-mixed greenhouse gases like CO2, CH4, N2O, and CFCs are prescribed for the past (1860–1990) and projected into the future according to International Panel on Climate Change (IPCC) scenario IS92a. In addition, the space–time distribution of tropospheric ozone is prescribed, and the tropospheric sulfur cycle is calculated within the coupled model using sulfur emissions of the past and projected into the future (IS92a). The radiative impact of the aerosols is considered via both the direct and the indirect (i.e., through cloud albedo) effect. It is shown that the simulated trend in sulfate deposition since the end of the last century is broadly consistent with ice core measurements, and the calculated radiative forcings from preindustrial to present time are within th...

Simulation of the tropospheric sulfur cycle in a global climate model

Abstract Emission, transport, chemistry and rainout of the sulfur species DMS, SO2 and sulfate are calculated on-line with the meteorology in a global atmospheric circulation model. The model simulates the main components of the hydrological cycle, including the liquid water content of clouds, and hence it allows an explicit treatment of cloud transformation processes and precipitation scavenging. The importance of the different oxidation pathways of DMS and SO2 is estimated. About 2 3 of the sulfate is produced within clouds, with H2O2 being the most efficient pathway (59%) and with a minor contribution due to oxidation with O3 (7%). Predicted atmospheric surface concentrations of SO2 and sulfate and the deposition fluxes are compared with the observations. Over most parts of the globe the agreement between simulated and observed annual averages is within a factor of 2. A significant underestimate of the simulated sulfate concentrations was found in high latitudes in winter. This bias may be attributed to a too slow oxidation in clouds. The calculated global mean turn-over times for DMS (2.2 d), SO2 (1.6 d) and sulfate (4.4 d) are within the range of previous estimates.

Prediction of the number of cloud droplets in the ECHAM GCM

In this paper a prognostic equation for the number of cloud droplets (CDNC) is introduced into the ECHAM general circulation model. The initial CDNC is based on the mechanistic model of Chuang and Penner [1995], providing a more realistical prediction of CDNC than the empirical method previously used. Cloud droplet nucleation is parameterized as a function of total aerosol number concentration, updraft velocity, and a shape parameter, which takes the aerosol composition and size distribution into account. The total number of aerosol particles is obtained as the sum of marine sulfate aerosols produced from dimethyl sulfide, hydrophylic organic and black carbon, submicron dust, and sea-salt aerosols. Anthropogenic sulfate aerosols only add mass to the preexisting aerosols but do not form new particles. The simulated annual mean liquid water path, column CDNC, and effective radius agree well with observations, as does the frequency distributions of column CDNC for clouds over oceans and the variations of cloud optical depth with effective radius.

Evaluation and intercomparison of global atmospheric transport models using 222Rn and other short‐lived tracers

Simulations of 222Rn and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean 222Rn concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of 222Rn concentrations in the upper troposphere, and none reproduces the high 222Rn concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high-222Rn episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of 222Rn in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.

Volcanic sulfur emissions: Estimates of source strength and its contribution to the global sulfate distribution

Anthropogenic emission of SO 2 and conversion into SO 4 2- is argued to be the most important factor damping and modulating the global greenhouse effect. Recent estimates of the relative strength of the three important sources of volatile sulfur (SO 2 from fossil fuel combustion ∼78 Tg S/yr, from biomass burning ∼2 Tg S/yr, and from natural sources ∼25 Tg S/yr) suggest an overwhelming effect of the anthropogenic emissions for climate forcing. However, the radiatively relevant product SO 4 2- may have different patterns due to the distribution of the sources (some very dense areas near the surface for anthropogenic SO 2 , formation of SO 2 from dimethylsulfide in the marine boundary layer, and emission of volcanic SO 2 mostly in the free atmosphere in rural areas). In this paper we study the relative contribution of volcanic SO 2 emissions to the atmospheric sulfur budget applying an atmospheric general circulation model including a full sulfur cycle and prescribed source distributions. An off-line analysis tool is applied to determine the radiative forcing of sulfate aerosols. The results show that natural S sources are at least as important as the anthropogenic ones, even though their source strength is much smaller. The reasons are different lifetimes due to different production and emission processes. Therefore, we should improve our knowledge about the volcanic volatile sources and their time-space variability.

Indirect effect of sulfate and carbonaceous aerosols: A mechanistic treatment

The indirect effect of anthropogenic aerosols, whereby aerosol particles change cloud optical properties, is the most uncertain component of climate forcing over the past 100 years. Here we use a mechanistic treatment of droplet nucleation and a prognostic treatment of the number of cloud droplets to study the indirect aerosol effect from changes in carbonaceous and sulfate aerosols. Cloud droplet nucleation is parameterized as a function of total aerosol number concentration, updraft velocity, and an activation parameter, which takes into account the mechanism of sulfate aerosol formation. Where previous studies focussed either on sulfate aerosols or carbonaceous aerosols only, here we estimate the combined effect. The combined indirect aerosol effect amounts to −1.1 W m−2 for an internally mixed aerosol and −1.5 W m−2 for an externally mixed aerosol compared to −1.4 W m−2, which we obtained by empirically relating sulfate mass to cloud droplet number. In the case of an internally mixed aerosol, the contribution from increasing carbonaceous and sulfate aerosols is close to being additive as the individual simulations yield an indirect effect of −0.4 W m−2 due to anthropogenic sulfate aerosols and −0.9 W m−2 due to anthropogenic carbonaceous aerosols. The contribution of anthropogenic sulfate to the indirect effect is close to zero if an externally mixed aerosol is assumed, while the contribution of carbonaceous aerosols increases to −1.3 W m−2. The effect of sulfate in the external mixture approach is much smaller than that of carbonaceous aerosols because its burden only increases by a third of that of carbonaceous aerosols and because the mode radius of sulfate is much larger than that of black and organic carbon.

Impact of sulfate aerosols on albedo and lifetime of clouds: A sensitivity study with the ECHAM4 GCM

A coupled sulfur chemistry-cloud microphysics scheme (COUPL) is used to study the impact of sulfate aerosols on cloud lifetime and albedo. The cloud microphysics scheme includes precipitation formation, which depends on the cloud droplet number concentration (CDNC) and on the liquid water content. On the basis of different observational data sets, CDNC is proportional to the sulfate aerosol mass, which is calculated by the model. Cloud cover is a function of relative humidity only. Additional sensitivity experiments with another cloud cover parameterization (COUPL-CC), which also depends on cloud water, and with a different autoconversion rate of cloud droplets (COUPL-CC-Aut) are conducted to investigate the range of the indirect effect due to uncertainties in cloud physics. For each experiment, two simulations, one using present-day and one using preindustrial sulfur emissions are carried out. The increase in liquid water path, cloud cover, and shortwave cloud forcing due to anthropogenic sulfur emissions depends crucially upon the parameterization of cloud cover and autoconversion of cloud droplets. In COUPL the liquid water path increases by 17% and cloud cover increases by 1% because of anthropogenic sulfur emissions, yielding an increase in shortwave cloud forcing of -1.4 W m -2 . In COUPL-CC the liquid water path increases by 32%, cloud cover increases by 3% and thus shortwave cloud forcing increases by -4.8 W m -2 . This large effect is caused by the strong dependence of cloud cover on cloud water and of the autoconversion rate on CDNC, cloud water, and cloud cover. Choosing a different autoconversion rate (COUPL-CC-Aut) with a reduced dependence on CDNC and cloud water results in an increase of liquid water path by only 11% and of cloud cover by 1%, and the increase in shortwave cloud forcing amounts to -2.2 W m -2 . These results clearly show that the uncertainties linked to the indirect aerosol effect are higher than was previously suggested.

Can the direct and semi‐direct aerosol effect compete with the indirect effect on a global scale?

During the Indian Ocean Experiment (INDOEX) Ackerman et al. [2000] found that black carbon (BC) aerosols can reduce cloud cover and liquid water path (“semi-direct” aerosol effect). Absorption of solar radiation by BC leads to a heating of the air which can result in an evaporation of cloud droplets. This warming can partially offset the cooling due to the indirect aerosol effect. We use the ECHAM4 general circulation model to investigate whether this effect is discernible on a global scale. The results of this sensitivity study, where the absorption of BC only takes place in clear air and interstitially, indicate that the zonal mean reduction in cloud cover and liquid water path in highly polluted regions of the Northern Hemisphere can amount to 4% and 10 g m−2, respectively. However, the indirect aerosol effects of increasing cloud lifetime and cloud albedo dominate.

Three-dimensional simulation of 7Be in a global climate model

In the upper troposphere and lower stratosphere, cosmic rays create Beryllium 7 atoms, which subsequently attach to submicron dust particles, so that wet deposition ultimately removes most 7Be from the troposphere. Because this source is weil known and because there is a large climatological data set for 7Be concentration in surface air and deposition on the surface, simulating 7Be provides a good test of the wet scavenging parameterization in a global climate simulation model, such as ECHAM2, which is the European Center for Medium Range Weather Forecasting (ECMWF) model with new physics introduced by the University of Hamburg. A simple parameterization in which the simulated condensation rate determines the scavenging frequency in each grid cell is used in the tracer transport model GLOMAC1, which is embedded in the meteorological model ECHAM2. In this paper we compare observed and model-calculated values of monthly average and annual average surface concentration at a global network of 79 stations. The average absolute value of the error in simulated surface concentration is 1.4 mBq m−3 compared with an average observed concentration of 3.5 mBq m−3. At most stations and in most regions the simulated surface concentration has about the correct magnitude and seasonal cycle, although there is a bias so that the modeled concentration is high at mountain stations in the tropics and low at sea level in polar regions. There are less climatological deposition data than there are climatological concentration data, but the model basically simulates the correct latitudinal variation of the zonally averaged deposition, which has a peak at the polar front (30°–50°N), although the model also has a peak produced by convective precipitation in the intertropical convergence zone, a peak that is not observed. We think that 7Be, used in conjunction with other species such as 210Pb, provides an excellent test of wet scavenging in a global model.

Nonlinear Aspects of the Climate Response to Greenhouse Gas and Aerosol Forcing

Abstract In a series of equilibrium experiments the climate response to present-day radiative forcings of anthropogenic greenhouse gases and aerosol particles is calculated. The study was performed with a model system consisting of the ECHAM4 atmospheric general circulation model coupled to a slab ocean and thermodynamic sea ice model. The model includes transport of the relevant chemical constituents, a sulfur chemistry model that calculates sulfate production in the gas and aqueous phase, and an aerosol model that accounts for source and sink processes. The aerosol cycle, the hydrological cycle, and the atmospheric dynamics are fully interactive. The climate response to aerosol forcing is not just a mirror image of the response to greenhouse forcing. This applies to the temperature changes, which are regionally more uniform for greenhouse forcing than for aerosol forcing as is already well known, and, in particular, to the hydrological cycle: the global hydrological sensitivity (Δprecip/Δtemp) to a 1-K ...