scholarly journals Biases in modeled surface snow BC mixing ratios in prescribed-aerosol climate model runs

2014 ◽  
Vol 14 (21) ◽  
pp. 11697-11709 ◽  
Author(s):  
S. J. Doherty ◽  
C. M. Bitz ◽  
M. G. Flanner
Keyword(s):  
Black Carbon ◽  
Radiative Forcing ◽  
Climate Model ◽  
Aerosol Deposition ◽  
Deposition Flux ◽  
Snow Layer ◽  
Mixing Ratios ◽  
Mass Fluxes ◽  
Surface Snow ◽  
Snow Water

Abstract. Black carbon (BC) in snow lowers its albedo, increasing the absorption of sunlight, leading to positive radiative forcing, climate warming and earlier snowmelt. A series of recent studies have used prescribed-aerosol deposition flux fields in climate model runs to assess the forcing by black carbon in snow. In these studies, the prescribed mass deposition flux of BC to surface snow is decoupled from the mass deposition flux of snow water to the surface. Here we compare prognostic- and prescribed-aerosol runs and use a series of offline calculations to show that the prescribed-aerosol approach results, on average, in a factor of about 1.5–2.5 high bias in annual-mean surface snow BC mixing ratios in three key regions for snow albedo forcing by BC: Greenland, Eurasia and North America. These biases will propagate directly to positive biases in snow and surface albedo reduction by BC. The bias is shown be due to coupling snowfall that varies on meteorological timescales (daily or shorter) with prescribed BC mass deposition fluxes that are more temporally and spatially smooth. The result is physically non-realistic mixing ratios of BC in surface snow. We suggest that an alternative approach would be to prescribe BC mass mixing ratios in snowfall, rather than BC mass fluxes, and we show that this produces more physically realistic BC mixing ratios in snowfall and in the surface snow layer.

scholarly journals Biases in modeled surface snow BC mixing ratios in prescribed aerosol climate model runs

2014 ◽  
Vol 14 (9) ◽  
pp. 13167-13196 ◽  
Author(s):  
S. J. Doherty ◽  
C. M. Bitz ◽  
M. G. Flanner
Keyword(s):  
Climate Model ◽  
Aerosol Deposition ◽  
Deposition Flux ◽  
Snow Layer ◽  
Mixing Ratios ◽  
Mass Fluxes ◽  
Alternative Approach ◽  
Surface Snow ◽  
Snow Water ◽  
High Bias

Abstract. A series of recent studies have used prescribed aerosol deposition flux fields in climate model runs to assess forcing by black carbon in snow. In these studies, the prescribed mass deposition flux of BC to surface snow is decoupled from the mass deposition flux of snow water to the surface. Here we use a series of offline calculations to show that this approach results, on average, in a~factor of about 1.5–2.5 high bias in annual-mean surface snow BC mixing ratios in three key regions for snow albedo forcing by BC: Greenland, Eurasia and North America. These biases will propagate directly to positive biases in snow and surface albedo reduction by BC. The bias is shown to be due to coupling snowfall that varies on meteorological timescales (daily or shorter) with prescribed BC mass deposition fluxes that are more temporally and spatially smooth. The result is physically non-realistic mixing ratios of BC in surface snow. We suggest that an alternative approach would be to prescribe BC mass mixing ratios in snowfall, rather than BC mass fluxes, and we show that this produces more physically realistic BC mixing ratios in snowfall and in the surface snow layer.

scholarly journals Combining atmospheric and snow radiative transfer models to assess the solar radiative effects of black carbon in the Arctic

2020 ◽  
Vol 20 (13) ◽  
pp. 8139-8156
Author(s):  
Tobias Donth ◽  
Evelyn Jäkel ◽  
André Ehrlich ◽  
Bernd Heinold ◽  
Jacob Schacht ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Arctic Ocean ◽  
Black Carbon ◽  
Heating Rate ◽  
Radiative Forcing ◽  
Mass Concentration ◽  
Early Summer ◽  
The Arctic ◽  
Snow Layer ◽  
Surface Snow

Abstract. The magnitude of solar radiative effects (cooling or warming) of black carbon (BC) particles embedded in the Arctic atmosphere and surface snow layer was explored on the basis of case studies. For this purpose, combined atmospheric and snow radiative transfer simulations were performed for cloudless and cloudy conditions on the basis of BC mass concentrations measured in pristine early summer and more polluted early spring conditions. The area of interest is the remote sea-ice-covered Arctic Ocean in the vicinity of Spitsbergen, northern Greenland, and northern Alaska typically not affected by local pollution. To account for the radiative interactions between the black-carbon-containing snow surface layer and the atmosphere, an atmospheric and snow radiative transfer model were coupled iteratively. For pristine summer conditions (no atmospheric BC, minimum solar zenith angles of 55∘) and a representative BC particle mass concentration of 5 ng g−1 in the surface snow layer, a positive daily mean solar radiative forcing of +0.2 W m−2 was calculated for the surface radiative budget. A higher load of atmospheric BC representing early springtime conditions results in a slightly negative mean radiative forcing at the surface of about −0.05 W m−2, even when the low BC mass concentration measured in the pristine early summer conditions was embedded in the surface snow layer. The total net surface radiative forcing combining the effects of BC embedded in the atmosphere and in the snow layer strongly depends on the snow optical properties (snow specific surface area and snow density). For the conditions over the Arctic Ocean analyzed in the simulations, it was found that the atmospheric heating rate by water vapor or clouds is 1 to 2 orders of magnitude larger than that by atmospheric BC. Similarly, the daily mean total heating rate (6 K d−1) within a snowpack due to absorption by the ice was more than 1 order of magnitude larger than that of atmospheric BC (0.2 K d−1). Also, it was shown that the cooling by atmospheric BC of the near-surface air and the warming effect by BC embedded in snow are reduced in the presence of clouds.

scholarly journals Historical greenhouse gas concentrations

2016 ◽  
Author(s):  
Malte Meinshausen ◽  
Elisabeth Vogel ◽  
Alexander Nauels ◽  
Katja Lorbacher ◽  
Nicolai Meinshausen ◽  
...  
Keyword(s):  
Climate Change ◽  
Greenhouse Gas ◽  
Radiative Forcing ◽  
Sulfur Hexafluoride ◽  
Climate Model ◽  
Future Climate Change ◽  
Large Set ◽  
Mixing Ratios ◽  
Surface Mixing ◽  
Cooling Effects

Abstract. Atmospheric greenhouse gas concentrations are at unprecedented, record-high levels compared to pre-industrial reconstructions over the last 800,000 years. Those elevated greenhouse gas concentrations warm the planet and together with net cooling effects by aerosols, they are the reason of observed climate change over the past 150 years. An accurate representation of those concentrations is hence important to understand and model recent and future climate change. So far, community efforts to create composite datasets with seasonal and latitudinal information have focused on marine boundary layer conditions and recent trends since 1980s. Here, we provide consolidated data sets of historical atmospheric (volume) mixing ratios of 43 greenhouse gases specifically for the purpose of climate model runs. The presented datasets are based on AGAGE and NOAA networks and a large set of literature studies. In contrast to previous intercomparisons, the new datasets are latitudinally resolved, and include seasonality over the period between year 0 to 2014. We assimilate data for CO2, methane (CH4) and nitrous oxide (N2O), 5 chlorofluorocarbons (CFCs), 3 hydrochlorofluorocarbons (HCFCs), 16 hydrofluorocarbons (HFCs), 3 halons, methyl bromide (CH3Br), 3 perfluorocarbons (PFCs), sulfur hexafluoride (SF6), nitrogen triflouride (NF3) and sulfuryl fluoride (SO2F2). We estimate 1850 annual and global mean surface mixing ratios of CO2 at 284.3 ppmv, CH4 at 808.2 ppbv and N2O at 273.0 ppbv and quantify the seasonal and hemispheric gradients of surface mixing ratios. Compared to earlier intercomparisons, the stronger implied radiative forcing in the northern hemisphere winter (due to the latitudinal gradient and seasonality) may help to improve the skill of climate models to reproduce past climate and thereby reduce uncertainty in future projections.

scholarly journals Sensitivity of aerosol radiative effects to different mixing assumptions in the AEROPT 1.0 submodel of the EMAC atmospheric-chemistry–climate model

2014 ◽  
Vol 7 (5) ◽  
pp. 2503-2516 ◽  
Author(s):  
K. Klingmüller ◽  
B. Steil ◽  
C. Brühl ◽  
H. Tost ◽  
J. Lelieveld
Keyword(s):  
Optical Properties ◽  
Radiative Transfer ◽  
Black Carbon ◽  
Atmospheric Chemistry ◽  
Radiative Forcing ◽  
Climate Model ◽  
Mixing Rule ◽  
Aerosol Optical Properties ◽  
Internal Mixing ◽  
Aerosol Radiative

Abstract. The modelling of aerosol radiative forcing is a major cause of uncertainty in the assessment of global and regional atmospheric energy budgets and climate change. One reason is the strong dependence of the aerosol optical properties on the mixing state of aerosol components, such as absorbing black carbon and, predominantly scattering sulfates. Using a new column version of the aerosol optical properties and radiative-transfer code of the ECHAM/MESSy atmospheric-chemistry–climate model (EMAC), we study the radiative transfer applying various mixing states. The aerosol optics code builds on the AEROPT (AERosol OPTical properties) submodel, which assumes homogeneous internal mixing utilising the volume average refractive index mixing rule. We have extended the submodel to additionally account for external mixing, partial external mixing and multilayered particles. Furthermore, we have implemented the volume average dielectric constant and Maxwell Garnett mixing rule. We performed regional case studies considering columns over China, India and Africa, corroborating much stronger absorption by internal than external mixtures. Well-mixed aerosol is a good approximation for particles with a black-carbon core, whereas particles with black carbon at the surface absorb significantly less. Based on a model simulation for the year 2005, we calculate that the global aerosol direct radiative forcing for homogeneous internal mixing differs from that for external mixing by about 0.5 W m−2.

scholarly journals Impact of methane and black carbon mitigation on forcing and temperature: a multi-model scenario analysis

2020 ◽  
Vol 163 (3) ◽  
pp. 1427-1442 ◽  
Author(s):  
Steven J Smith ◽  
Jean Chateau ◽  
Kalyn Dorheim ◽  
Laurent Drouet ◽  
Olivier Durand-Lasserve ◽  
...  
Keyword(s):  
Black Carbon ◽  
Radiative Forcing ◽  
Climate Model ◽  
Economic Structure ◽  
Time Frame ◽  
Focused Attention ◽  
Carbon Mitigation ◽  
Global Mean Temperature ◽  
Model Parameter Uncertainty ◽  
Global Mean

AbstractThe relatively short atmospheric lifetimes of methane (CH4) and black carbon (BC) have focused attention on the potential for reducing anthropogenic climate change by reducing Short-Lived Climate Forcer (SLCF) emissions. This paper examines radiative forcing and global mean temperature results from the Energy Modeling Forum (EMF)-30 multi-model suite of scenarios addressing CH4 and BC mitigation, the two major short-lived climate forcers. Central estimates of temperature reductions in 2040 from an idealized scenario focused on reductions in methane and black carbon emissions ranged from 0.18–0.26 °C across the nine participating models. Reductions in methane emissions drive 60% or more of these temperature reductions by 2040, although the methane impact also depends on auxiliary reductions that depend on the economic structure of the model. Climate model parameter uncertainty has a large impact on results, with SLCF reductions resulting in as much as 0.3–0.7 °C by 2040. We find that the substantial overlap between a SLCF-focused policy and a stringent and comprehensive climate policy that reduces greenhouse gas emissions means that additional SLCF emission reductions result in, at most, a small additional benefit of ~ 0.1 °C in the 2030–2040 time frame.

scholarly journals Numerical framework and performance of the new multiple-phase cloud microphysics scheme in RegCM4.5: precipitation, cloud microphysics, and cloud radiative effects

2016 ◽  
Vol 9 (7) ◽  
pp. 2533-2547 ◽  
Author(s):  
Rita Nogherotto ◽  
Adrian Mark Tompkins ◽  
Graziano Giuliani ◽  
Erika Coppola ◽  
Filippo Giorgi
Keyword(s):  
Radiative Forcing ◽  
Climate Model ◽  
Regional Climate ◽  
Cloud Microphysics ◽  
Microphysics Scheme ◽  
Additional Information ◽  
Mixing Ratios ◽  
Multiple Phase ◽  
Realistic Representation ◽  
And Performance

Abstract. We implement and evaluate a new parameterization scheme for stratiform cloud microphysics and precipitation within regional climate model RegCM4. This new parameterization is based on a multiple-phase one-moment cloud microphysics scheme built upon the implicit numerical framework recently developed and implemented in the ECMWF operational forecasting model. The parameterization solves five prognostic equations for water vapour, cloud liquid water, rain, cloud ice, and snow mixing ratios. Compared to the pre-existing scheme, it allows a proper treatment of mixed-phase clouds and a more physically realistic representation of cloud microphysics and precipitation. Various fields from a 10-year long integration of RegCM4 run in tropical band mode with the new scheme are compared with their counterparts using the previous cloud scheme and are evaluated against satellite observations. In addition, an assessment using the Cloud Feedback Model Intercomparison Project (CFMIP) Observational Simulator Package (COSP) for a 1-year sub-period provides additional information for evaluating the cloud optical properties against satellite data. The new microphysics parameterization yields an improved simulation of cloud fields, and in particular it removes the overestimation of upper level cloud characteristics of the previous scheme, increasing the agreement with observations and leading to an amelioration of a long-standing problem in the RegCM system. The vertical cloud profile produced by the new scheme leads to a considerably improvement of the representation of the longwave and shortwave components of the cloud radiative forcing.

Investigation of strongly enhanced methane Part I: Chemical feedbacks and rapid adjustments.

2020 ◽  
Author(s):  
Franziska Winterstein ◽  
Patrick Jöckel ◽  
Martin Dameris ◽  
Michael Ponater ◽  
Fabian Tanalski ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Radiative Forcing ◽  
Climate Model ◽  
Numerical Study ◽  
Stratospheric Ozone ◽  
Lower Boundary ◽  
Tropospheric Chemistry ◽  
Substantial Part ◽  
Mixing Ratios ◽  
The Impact

<p>Methane (CH<sub>4</sub>) is the second most important greenhouse gas, which atmospheric concentration is influenced by human activities and currently on a sharp rise. We present a study with numerical simulations using a Chemistry-Climate-Model (CCM), which are performed to assess possible consequences of strongly enhanced CH<sub>4</sub> concentrations in the Earth's atmosphere for the climate.</p><p>Our analysis includes experiments with 2xCH<sub>4</sub> and 5xCH<sub>4</sub> present day (2010) lower boundary mixing ratios using the CCM EMAC. The simulations are conducted with prescribed oceanic conditions, mimicking present day tropospheric temperatures as its changes are largely suppressed. By doing so we are able to investigate the quasi-instantaneous chemical impact on the atmosphere. We find that the massive increase in CH<sub>4</sub> strongly influences the tropospheric chemistry by reducing the OH abundance and thereby extending the tropospheric CH<sub>4</sub> lifetime as well as the residence time of other chemical pollutants. The region above the tropopause is impacted by a substantial rise in stratospheric water vapor (SWV). The stratospheric ozone (O<sub>3</sub>) column increases overall, but SWV induced stratospheric cooling also leads to enhanced ozone depletion in the Antarctic lower stratosphere. Regional  patterns of ozone change are affected by modification of stratospheric dynamics, i.e. increased tropical up-welling and stronger meridional transport  towards the polar regions. We calculate the net radiative impact (RI) of the 2xCH<sub>4</sub> experiment to be 0.69 W m<sup>-2</sup> and for the 5xCH<sub>4</sub> experiment to be 1.79 W m<sup>-2</sup>. A substantial part of the RI is contributed by chemically induced O<sub>3</sub> and SWV changes, in line with previous radiative forcing estimates and is for the first time splitted and spatially asigned to its chemical contributors.</p><p>This numerical study using a CCM with prescibed oceanic conditions shows the rapid responses to significantly enhanced CH<sub>4</sub> mixing ratios, which is the first step towards investigating the impact of possible strong future CH<sub>4</sub> emissions on atmospheric chemistry and its feedback on climate.</p>

scholarly journals Simulation of black carbon in snow and its climate impact in the Canadian Global Climate Model

2015 ◽  
Vol 15 (13) ◽  
pp. 18839-18882 ◽  
Author(s):  
M. Namazi ◽  
K. von Salzen ◽  
J. N. S. Cole
Keyword(s):  
Black Carbon ◽  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Surface Air Temperature ◽  
Climate Impact ◽  
Radiative Forcings ◽  
Mixing Ratios ◽  
Emission Changes ◽  
The Impact

Abstract. A new physically-based parameterization of black carbon (BC) in snow was developed and implemented in the Canadian Atmospheric Global Climate Model (CanAM4.2). Simulated BC snow mixing ratios and BC snow radiative forcings are in good agreement with measurements and results from other models. Simulations with the improved model yield considerable trends in regional BC concentrations in snow and BC snow radiative forcings during the time period from 1950–1959 to 2000–2009. Increases in radiative forcings for Asia and decreases for Europe and North America are found to be associated with changes in BC emissions. Additional sensitivity simulations were performed in order to study the impact of BC emission changes between 1950–1959 and 2000–2009 on surface albedo, snow cover fraction, and surface air temperature. Results from these simulations indicate that impacts of BC emission changes on snow albedos between these two decades are small and not significant. Overall, changes in BC concentrations in snow have much smaller impacts on the cryosphere than the net warming surface air temperatures during the second half of the 20th century.

scholarly journals Black carbon absorption effects on cloud cover, review and synthesis

2010 ◽  
Vol 10 (3) ◽  
pp. 7323-7346 ◽  
Author(s):  
D. Koch ◽  
A. Del Genio
Keyword(s):  
Black Carbon ◽  
Radiative Forcing ◽  
Cloud Cover ◽  
Climate Model ◽  
Deep Convection ◽  
Temperature Structure ◽  
Low Level ◽  
Model Studies ◽  
Stratocumulus Clouds ◽  
Cloud Response

Abstract. Absorbing aerosols (AA's) such as black carbon (BC) or dust absorb incoming solar radiation, perturb the temperature structure of the atmosphere, and influence cloud cover. Previous studies have described conditions where AA's either increase or decrease cloud cover. The effect depends on several factors, including the altitude of the AA relative to the cloud and on the cloud type. Cloud cover is decreased if the AA's are embedded in the cloud layer. AA's below cloud may enhance convection and cloud cover. AA's over cloud-level stabilize the underlying layer and tend to enhance stratocumulus clouds but may reduce cumulus clouds. AA's can also promote cloud cover in convergent regions as they enhance deep convection and low level convergence as it draws in moisture from ocean to land regions. Most global model studies indicate a regional variation in the cloud response but generally increased cloud cover over oceans and some land regions, with net increased low-level and/or reduced upper level cloud cover. The result is net negative radiative forcing from cloud response to AA's. In some of these climate model studies, the cooling effect of BC due to cloud changes was strong enough to essentially cancel the warming direct effects.

scholarly journals Enhanced solar energy absorption by internally-mixed black carbon in snow grains

2012 ◽  
Vol 12 (1) ◽  
pp. 2057-2113 ◽  
Author(s):  
M. G. Flanner ◽  
X. Liu ◽  
C. Zhou ◽  
J. E. Penner
Keyword(s):  
Sea Ice ◽  
Black Carbon ◽  
Radiative Forcing ◽  
Climate Model ◽  
Volume Fraction ◽  
Global Climate ◽  
Global Climate Model ◽  
Absorption Enhancement ◽  
Liquid Droplets ◽  
Vapor Transfer

Abstract. Here we explore light absorption by snowpack containing black carbon (BC) particles residing within ice grains. Basic considerations of particle volumes and BC/snow mass concentrations show that there are generally 0.05–109 BC particles for each ice grain. This suggests that internal BC is likely distributed as multiple inclusions within ice grains, and thus the dynamic effective medium approximation (DEMA) (Chýlek and Srivastava, 1983) is a more appropriate optical representation for BC/ice composites than coated-sphere or standard mixing approximations. DEMA calculations show that the 460 nm absorption cross-section of BC/ice composites, normalized to the mass of BC, is typically enhanced by factors of 1.8–2.1 relative to interstitial BC. BC effective radius is the dominant cause of variation in this enhancement, compared with ice grain size and BC volume fraction. We apply two atmospheric aerosol models that simulate interstitial and within-hydrometeor BC lifecycles. Although only ~2% of the atmospheric BC burden is cloud-borne, 71–83% of the BC deposited to global snow and sea-ice surfaces occurs within hydrometeors. Key processes responsible for within-snow BC deposition are development of hydrophilic coatings on BC, activation of liquid droplets, and subsequent snow formation through riming or ice nucleation by other species and aggregation/accretion of ice particles. Applying deposition fields from these aerosol models in offline snow and sea-ice simulations, we calculate that 32–73% of BC in global surface snow resides within ice grains. This fraction is smaller than the within-hydrometeor deposition fraction because meltwater flux preferentially removes internal BC, while sublimation and freezing within snowpack expose internal BC. Incorporating the DEMA into a global climate model, we simulate increases in BC/snow radiative forcing of 43–86%, relative to scenarios that apply external optical properties to all BC. We show that snow metamorphism driven by diffusive vapor transfer likely proceeds too slowly to alter the mass of internal BC while it is radiatively active, but neglected processes like wind pumping and convection may play much larger roles. These results suggest that a large portion of BC in surface snowpack may reside within ice grains and increase BC/snow radiative forcing, although measurements to evaluate this are lacking. Finally, previous studies of BC/snow forcing that neglected this absorption enhancement are not necessarily biased low, because of application of absorption-enhancing sulfate coatings to hydrophilic BC, neglect of coincident absorption by dust in snow, and implicit treatment of cloud-borne BC resulting in longer-range transport.

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