Journal: Atmospheric Chemistry and Physics Discussions

Abbreviation

Atmos. chem. phys. discuss.

Publisher

Copernicus

Journal Volumes

ISSN

1680-7375, 1680-7367

Description

Filters

2003 - 2009172010 - 201926
Reset filters

Search Results

Publications 1 - 10 of 43
  • Dust ice nuclei effects on cirrus clouds
    Item type: Working Paper
    Kuebbeler, Miriam; Lohmann, Ulrike; Hendricks, Johannes; et al. (2013)
    Atmospheric Chemistry and Physics Discussions
    In order to study aerosol-cloud interactions in cirrus clouds we apply a new multiple-mode ice microphysical scheme to the general circulation model ECHAM5-HAM. Themultiple-mode ice microphysical scheme allows to analyse the competition betweenhomogeneous freezing of solution droplets, deposition nucleation of pure dust particles, immersion freezing of coated dust particles and pre-existing ice. We base thefreezing efficiencies of coated and pure dust particles on most recent laboratory data.The effect of pre-existing ice, which was neglected in previous ice nucleation param-eterizations, is to deplete water vapour by depositional growth and thus prevent ho-mogeneous and heterogeneous freezing from occurring. In a first step, we extensively tested the model and validated the results against in-situ measurements from vari-ous aircraft campaigns. The results compare well with observations; properties like icecrystal size and number concentration as well as supersaturation are predicted withinthe observational spread.We find that heterogeneous nucleation on mineral dust particles and the consideration of pre-existing ice in the nucleation process may lead to significant effects: globally,ice crystal number and mass are reduced by 10% and 5%, whereas the ice crystalssize is increased by 3%. The reductions in ice crystal number are most pronouncedin the tropics and mid-latitudes on the Northern Hemisphere. While changes in the mi-crophysical and radiative properties of cirrus clouds in the tropics are mostly driven by considering pre-existing ice, changes in the northern hemispheric mid-latitudes mainlyresult from heterogeneous nucleation. The so called negative Twomey-effect in cirrusclouds is represented in ECHAM5-HAM. The net change in the radiation budget is−0.94 Wm−2, implying that both, heterogeneous nucleation on dust and pre-existingice have the potential to modulate cirrus properties in climate simulations and thus should be considered in future studies.
  • Impact of parametric uncertainties on the present-day climate and on the anthropogenic aerosol effect
    Item type: Working Paper
    Lohmann, Ulrike; Ferrachat, Sylvaine (2010)
    Atmospheric Chemistry and Physics Discussions
    Clouds constitute a large uncertainty in global climate modeling and climate changeprojections as many clouds are smaller than the size of a model grid box. Some pro-cesses, such as the rates of rain and snow formation that have a large impact onclimate, cannot be observed. These processes are thus used as tuning parameters in order to achieve radiation balance. Here we systematically investigate the impactof various tunable parameters within the convective and stratiform cloud schemes andof the ice cloud optical properties on the present-day climate in terms of clouds, radi-ation and precipitation. The total anthropogenic aerosol effect between pre-industrialand present-day times amounts to−1.00 Wm−2obtained as an average over all simulations as compared to−1.02 Wm−2from those simulations where the global annualmean top-of-the atmosphere radiation balance is within±1 Wm−2. The parametric un-certainty when taking all simulations into account has an uncertainty range of 25%between the minimum and maximum value. It is reduced to 11% when only the simu-lations with a balanced top-of-the atmosphere radiation are considered.
  • Soot microphysical effects on liquid clouds, a multi-model investigation
    Item type: Working Paper
    Koch, Dorothy; Balkanski, Yves; Bauer, Susanne E.; et al. (2010)
    Atmospheric Chemistry and Physics Discussions
    We use global models to explore the microphysical effects of carbonaceous aerosolson clouds. Although absorption of solar radiation by soot warms the atmosphere, sootmay 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 cloudradiative response to biofuel soot (black and organic carbon), including both indirectand semi-direct effects, is−0.11 Wm−2, comparable in size but opposite in sign tothe 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 andevolution of viable CCN. Biofuel soot particles were also typically assumed to be largerand more hygroscopic than for fossil fuel soot and therefore caused more negativeforcing, as also found in previous studies. Diesel soot (black and organic carbon)experiments had relatively smaller cloud impacts with five of the models<±0.06 Wm−2 from clouds. The results are subject to the caveats that variability among models,and regional and interrannual variability for each model, are large. This comparisontogether with previously published results stresses the need to further constrain aerosolmicrophysical schemes. The non-linearities resulting from the competition of opposingeffects on the CCN population make it difficult to extrapolate from idealized experiments to likely impacts of realistic potential emission changes.
  • Validation of farm-scale methane emissions using nocturnal boudary layer budgets
    Item type: Working Paper
    Stieger, Jacqueline; Bamberger, Ines; Buchmann, Nina; et al. (2015)
    Atmospheric Chemistry and Physics Discussions
  • Validation of the Swiss methane emission inventory by atmospheric observations and inverse modelling
    Item type: Working Paper
    Henne, Stephan; Brunner, Dominik; Oney, Brian; et al. (2015)
    Atmospheric Chemistry and Physics Discussions
    Atmospheric inverse modelling has the potential to provide observation-based esti-mates of greenhouse gas emissions at the country scale, thereby allowing for an inde-pendent validation of national emission inventories. Here, we present a regional scaleinverse modelling study to quantify the emissions of methane (CH4) from Switzerland, making use of the newly established CarboCount-CH measurement network and a highresolution Lagrangian transport model. Overall we estimate national CH4emissions tobe 196±18 Gg yr−1for the year 2013 (1σuncertainty). This result is in close agree-ment with the recently revised “bottom-up” estimate of 206±33 Gg yr−1published bythe Swiss Federal Office for the Environment as part of the Swiss Greenhouse Gas Inventory (SGHGI). Results from sensitivity inversions using alternative prior emissions,covariance settings, baseline treatments, two different inverse algorithms (Bayesianand extended Kalman Filter), and two different transport models confirms the robust-ness and independent character of our estimate. According to the latest “bottom-up”inventory the main CH4source categories in Switzerland are agriculture (78 %), waste handling (15 %) and natural gas distribution and combustion (6 %). The spatial distri-bution and seasonal variability of our posterior emissions suggest an overestimation ofagricultural CH4emissions by 10 to 20 % in the most recent national inventory, whichis likely due to an overestimation of emissions from manure handling. Urban areas donot appear as emission hotspots in our posterior results suggesting that leakages from natural gas disribution are only a minor source of CH4in Switzerland. This is consis-tent with rather low emissions of 8.4 Gg yr−1reported by the SGHGI but inconsistentwith the much higher value of 32 Gg yr−1implied by the EDGARv4.2 inventory for thissector. Increased CH4emissions (up to 30 % compared to the prior) were deduced forthe north-eastern parts of Switzerland. This feature was common to most sensitivity inversions, which rules out an artefact of the transport model and the inversion system.However, it was not possible to assign an unambiguous source process to the region.The observations of the CarboCount-CH network provided invaluable and independent information for the validation of the national bottom-up inventory. Similar systems needto be sustained to provide independent monitoring of future climate agreements.
  • Online coupled regional meteorology chemistry models in Europe: current status and prospects
    Item type: Working Paper
    Baklanov, Alexander; Schlünzen, Katharina H.; Suppan, Peter; et al. (2013)
    Atmospheric Chemistry and Physics Discussions
    The simulation of the coupled evolution of atmospheric dynamics, pollutant transport,chemical reactions and atmospheric composition is one of the most challenging tasksin environmental modelling, climate change studies, and weather forecasting for thenext decades as they all involve strongly integrated processes. Weather strongly influences air quality (AQ) and atmospheric transport of hazardous materials, while atmo-spheric composition can influence both weather and climate by directly modifying theatmospheric radiation budget or indirectly affecting cloud formation. Until recently, how-ever, due to the scientific complexities and lack of computational power, atmosphericchemistry and weather forecasting have developed as separate disciplines, leading to the development of separate modelling systems that are only loosely coupled.The continuous increase in computer power has now reached a stage that enablesus to perform online coupling of regional meteorological models with atmosphericchemical transport models. The focus on integrated systems is timely, since recentresearch has shown that meteorology and chemistry feedbacks are important in the context of many research areas and applications, including numerical weather predic-tion (NWP), AQ forecasting as well as climate and Earth system modelling. However,the relative importance of online integration and its priorities, requirements and lev-els of detail necessary for representing different processes and feedbacks can greatlyvary for these related communities: (i) NWP, (ii) AQ forecasting and assessments, (iii) climate and earth system modelling. Additional applications are likely to benefit fromonline modelling, e.g.: simulation of volcanic ash or forest fire plumes, pollen warnings,dust storms, oil/gas fires, geo-engineering tests involving changes in the radiation bal-ance.The COST Action ES1004 – European framework for online integrated air quality and meteorology modelling (EuMetChem) – aims at paving the way towards a new gener-ation of online integrated atmospheric chemical transport and meteorology modellingwith two-way interactions between different atmospheric processes including dynamics, chemistry, clouds, radiation, boundary layer and emissions. As its first task, wesummarise the current status of European modelling practices and experience withonline coupled modelling of meteorology with atmospheric chemistry including feed-back mechanisms and attempt reviewing the various issues connected to the differentmodules of such online coupled models but also providing recommendations for coping with them for the benefit of the modelling community at large.
  • Aerosol indirect effects – general circulation model intercomparison and evaluation with satellite data
    Item type: Working Paper
    Quaas, Johannes; Ming, Yi; Menon, Surabi; et al. (2009)
    Atmospheric Chemistry and Physics Discussions
    Aerosol indirect effects continue to constitute one of the most important uncertaintiesfor anthropogenic climate perturbations. Within the international AEROCOM initiative,the representation of aerosol-cloud-radiation interactions in ten different general cir-culation 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 models explicitly parameterizes aerosol effects on convective clouds.We compute statistical relationships between aerosol optical depth (τa) and variouscloud and radiation quantities in a manner that is consistent between the models andthe satellite data. It is found that the model-simulated influence of aerosols on cloud droplet number concentration (Nd) compares relatively well to the satellite data at leastover the ocean. The relationship betweenτaand liquid water path is simulated muchtoo strongly by the models. It is shown that this is partly related to the representationof the second aerosol indirect effect in terms of autoconversion. A positive relationshipbetween total cloud fraction (fcld) andτaas found in the satellite data is simulated by the majority of the models, albeit less strongly than that in the satellite data in most of them.In a discussion of the hypotheses proposed in the literature to explain the satellite-derived strongfcld–τarelationship, our results indicate that none can be identified asunique explanation. Relationships similar to the ones found in satellite data betweenτaand cloud top temperature or outgoing long-wave radiation (OLR) are simulated by only a few GCMs. The GCMs that simulate a negative OLR –τarelationship show a strongpositive correlation betweenτaandfcld. The short-wave total aerosol radiative forc-ing as simulated by the GCMs is strongly influenced by the simulated anthropogenicfraction ofτa, and parameterisation assumptions such as a lower bound onNd. Never-theless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred fromthe combination of these predictors for the modelled forcings with the satellite-derivedstatistical relationships yields a global annual mean value of−1.5±0.5 Wm−2. An alternative estimate obtained by scaling the simulated clear- and cloudy-sky forcingswith estimates of anthropogenicτaand satellite-retrievedNd–τaregression slopes,respectively, yields a global annual mean clear-sky (aerosol direct effect) estimate of−0.4±0.2 Wm−2and a cloudy-sky (aerosol indirect effect) estimate of−0.7±0.5 Wm−2, with a total estimate of−1.2±0.4 Wm−2.
  • Single ice crystal measurements during nucleation experiments with the depolarization detector IODE
    Item type: Working Paper
    Nicolet, Mathieu; Stetzer, Olaf; Lohmann, Ulrike; et al. (2008)
    Atmospheric Chemistry and Physics Discussions
    In order to determine the efficiency of aerosol particles of several types to nucleate ice,an Ice Optical DEpolarization detector (IODE) was developed to distinguish betweenwater droplets and ice crystals in ice nucleation chambers. A laser beam polarizedlinearly (power: 50 mW, wavelength: 407 nm) is directed through the chamber. The scattered light intensity from particles is measured at a scattering angle ofΘ=175◦inboth polarization components (parallel and perpendicular). The ratio between the per-pendicular intensity over the total one gives the depolarization ratioδ. Single particledetection is possible, using a peak detection algorithm. For high particle concentra-tions, a real-time signal averaging method can also be run simultaneously. The IODE detector was used in connection with the Zurich ice nucleation chamber during the ICIS2007 workshop where ice nucleation experiments were performed with several aerosoltypes. In presence of ice crystals, peaks were detected in both channels, generatingdepolarization signals. Mean values ofδranged from 0.24 to 0.37.
  • Sensitivity estimations for cloud droplet formation in the vicinity of the high alpine research station Jungfraujoch (3580 m a.s.l.)
    Item type: Working Paper
    Hammer, E.; Bukowiecki, Nicolas; Luo, Beiping P.; et al. (2014)
    Atmospheric Chemistry and Physics Discussions
    Aerosol radiative forcing estimates suffer from large uncertainties as a result of insuffi-cient understanding of aerosol–cloud interactions. The main source of these uncertain-ties are dynamical processes such as turbulence and entrainment but also key aerosolparameters such as aerosol number concentration and size distribution, and to a much lesser extent, the composition. From June to August 2011 a Cloud and Aerosol Char-acterization Experiment (CLACE) was performed at the high-alpine research stationJungfraujoch (Switzerland, 3580 m a.s.l.) focusing on the activation of aerosol to formliquid-phase clouds (in the cloud base temperature range of−8 to 5◦C). With a boxmodel the sensitivity of the effective peak supersaturation (SSpeak), an important parameter for cloud activation, to key aerosol and dynamical parameters was investigated.It was found that the updraft velocity, defining the cooling rate of an air parcel, is the pa-rameter with the largest influence on SSpeak. Small-scale variations in the cooling ratewith large amplitudes can significantly alter CCN activation. Thus, an accurate knowl-edge of the air parcel history is required to estimate SSpeak. The results show that the cloud base updraft velocities estimated from the horizontal wind measurements madeat the Jungfraujoch can be divided by a factor of approximately 4 to get the updraftvelocity required for the model to reproduce the observed SSpeak.
Publications 1 - 10 of 43