Estimates of global multicomponent aerosol optical depth and direct radiative perturbation in the Laboratoire de Météorologie Dynamique general circulation model (original) (raw)
Related papers
Journal of Geophysical Research, 2003
1] New aerosol modules of global (circulation and chemical transport) models are evaluated. These new modules distinguish among at least five aerosol components: sulfate, organic carbon, black carbon, sea salt, and dust. Monthly and regionally averaged predictions for aerosol mass and aerosol optical depth are compared. Differences among models are significant for all aerosol types. The largest differences were found near expected source regions of biomass burning (carbon) and dust. Assumptions for the permitted water uptake also contribute to optical depth differences (of sulfate, organic carbon, and sea salt) at higher latitudes. The decline of mass or optical depth away from recognized sources reveals strong differences in aerosol transport or removal among models. These differences are also a function of altitude, as transport biases of dust do not always extend to other aerosol types. Ratios of optical depth and mass demonstrate large differences in the mass extinction efficiency, even for hydrophobic aerosol. This suggests that efforts of good mass simulations could be wasted or that conversions are misused to cover for poor mass simulations. In an attempt to provide an absolute measure for model skill, simulated total optical depths (when adding contributions from all five aerosol types) are compared to measurements from ground and space. Comparisons to the Aerosol Robotic Network (AERONET) suggest a source strength underestimate in many models, most frequently for (subtropical) tropical biomass or dust. Comparisons to the combined best of Moderate-Resolution Imaging Spectroradiometer (MODIS) and Total Ozone Mapping Spectrometer (TOMS) indicate that away from sources, model simulations are usually smaller. Particularly large are discrepancies over tropical oceans and oceans of the Southern Hemisphere, raising issues on the treatment of sea salt in models. Totals for mass or optical depth in many models are defined by the absence or dominance of only one aerosol component. With appropriate corrections to that component (e.g., to removal, to source strength, or to seasonality) a much better model performance can be expected. Still, many important modeling issues remain inconclusive as the combined result of poor coordination (different emissions and meteorology), insufficient model output (vertical distributions, water uptake by aerosol type), and unresolved measurement issues (retrieval assumptions and temporal or spatial sampling biases).
Multi-decadal variations of atmospheric aerosols and their effects on surface radiation trends
2010
Aerosol variations and trends over different land and ocean regions from 1980 to 2009 are analyzed with the Goddard Chemistry Aerosol Radiation and Transport (GO-CART) model and observations from multiple satellite sensors and available ground-based networks. Excluding time periods with large volcanic influence, aerosol optical depth (AOD) and surface concentration over polluted land regions generally vary with anthropogenic emissions, but the magnitude of this association can be dampened by the presence of natural aerosols, especially dust. Over the 30-year period in this study, the largest reduction in aerosol levels occurs over Europe, where AOD has decreased by 40-60 % on average and surface sulfate concentrations have declined by a factor of up to 3-4. In contrast, East Asia and South Asia show AOD increases, but the relatively high level of dust aerosols in Asia reduces the correlation between AOD and pollutant emission trends. Over major dust source regions, model analysis indicates that the change of dust emissions over the Sa-hara and Sahel has been predominantly driven by the change of near-surface wind speed, but over Central Asia it has been largely influenced by the change of the surface wetness. The decreasing dust trend in the North African dust outflow region of the tropical North Atlantic and the receptor sites of Barbados and Miami is closely associated with an increase of the sea surface temperature in the North Atlantic. This temperature increase may drive the decrease of the wind velocity over North Africa, which reduces the dust emission, and the increase of precipitation over the tropical North Atlantic, which enhances dust removal during transport. Despite significant trends over some major continental source regions, the model-calculated global annual average AOD shows little change over land and ocean in the past three decades, because opposite trends in different land regions cancel each other out in the global average, and changes over large open oceans are negligible. This highlights the necessity for regional-scale Published by Copernicus Publications on behalf of the European Geosciences Union. 3658 Mian Chin et al.: Multi-decadal aerosol variations from 1980 to 2009 assessment of aerosols and their climate impacts, as globalscale average values can obscure important regional changes. Mian Chin et al.: Multi-decadal aerosol variations from 1980 to 2009 Table 2. Emissions used in the present study *. Emission source Emitted species Emission data sets/Methods Reference Fossil fuel/biofuel SO 2 , BC, OC, sulfate A2-ACCMIP (ACCMIP 1980-2000, Lamarque et al. (2010); combustion, land-based RCP8.5 2000-2009, linearly Riahi et al. (2011); interpolated from the decadal/ Granier et al. (2011); half decadal increments) Diehl et al. (2012) Fuel combustion, SO 2 , BC, OC A2-MAP (EDGAR 32FT2000, Diehl et al. (2012) international shipping interpolated) and references therein Fuel combustion, aircraft SO 2 , BC, OC A2-MAP (constructed and Diehl et al. (2012) interpolated from several data sets) and references therein Biomass burning SO 2 , BC, OC
Journal of Applied Meteorology, 2004
A method based on the synergistic use of low earth orbit and geostationary earth orbit satellite data for aerosoltype characterization and aerosol optical thickness (AOT: a ) retrieval and monitoring over the ocean is presented in Part I of this paper. The method is now applied to a strong dust outbreak over the Atlantic Ocean in June 1997 and to two other relevant transport events of biomass burning and desert dust aerosol that occurred in 2000 over the Atlantic and Indian Oceans, respectively. The retrievals of the aerosol optical properties are checked against retrievals from sun and sky radiance measurements from the ground-based Aerosol Robotic Network (AERONET). The single-scattering albedo values obtained from AERONET are always within the error bars presented for Global Ozone Monitoring Experiment (GOME) retrievals, resulting in differences lower than 0.041. The retrieved AOT values are compared with the independent space-time-collocated measurements from the AERONET, as well as to the satellite aerosol official products of the Polarization and Directionality of the Earth Reflectances (POLDER) and the Moderate Resolution Imaging Spectroradiometer (MODIS). A first estimate of the AOT accuracy derived from comparisons with AERONET data leads to Ϯ0.02 Ϯ 0.22 a when all AOT values are retained or to Ϯ0.02 Ϯ 0.16 a for aerosol transport events (AOT Ͼ 0.4). The upwelling flux at the top of the atmosphere (TOA) was computed with radiative transfer calculations and used to estimate the TOA direct shortwave aerosol radiative forcing; a comparison with space-time-collocated measurements from the Clouds and the Earth's Radiant Energy System (CERES) TOA flux product was also done. It was found that more than 90% of the values differ from CERES fluxes by less than Ϯ15%.
Aerosol optical depths and direct radiative perturbations by species and source type
Geophysical Research Letters, 2005
1] We have used the Laboratoire de Météorologie Dynamique General Circulation Model (LMDZT GCM) to estimate the relative contributions of different aerosol source types (i.e., fossil fuels, biomass burning, and ''natural'') and aerosol species to the aerosol optical depth (AOD) and direct aerosol radiative perturbation (DARP) at the top-of-atmosphere. The largest estimated contribution to the global annual average AOD (0.12 at 550 nm) is from natural (58%), followed by fossil fuel (26%), and biomass burning (16%) sources. The global annual mean all-sky DARP in the shortwave (SW) spectrum by sulfate, black carbon (BC), organic matter (OM), dust, and sea salt are À0.62, +0.55, À0.33, À0.28, and À0.30 Wm À2 , respectively. The all-sky DARP in the longwave spectrum (LW) is not negligible and is a bit less than half of the SW DARP. The net (i.e., SW+LW) DARP distribution is predominantly negative with patches of positive values over the dust source regions, and off the west coasts of Southern Africa and South and North America. For dust aerosols the SW effect is partially offset by LW greenhouse effect. Citation: Reddy, M. S., O. Boucher, Y. Balkanski, and M. Schulz (2005), Aerosol optical depths and direct radiative perturbations by species and source type, Geophys. Res. Lett., 32, L12803,
Atmospheric Chemistry and Physics, 2007
A global estimate of the seasonal direct radiative effect (DRE) of natural plus anthropogenic aerosols on solar radiation under all-sky conditions is obtained by combining satellite measurements and reanalysis data with a spectral radiative transfer model and spectral aerosol optical properties taken from the Global Aerosol Data Set (GADS). The estimates are obtained with detailed spectral model computations separating the ultraviolet (UV), visible and near-infrared wavelengths. The global distribution of spectral aerosol optical properties was taken from GADS whereas data for clouds, water vapour, ozone, carbon dioxide, methane and surface albedo were taken from various satellite and reanalysis datasets. Using these aerosol properties and other related variables, we generate climatological (for the 12-year period 1984-1995) monthly mean aerosol DREs. The global annual mean DRE on the outgoing SW radiation at the top of atmosphere (TOA, F TOA ) is −1.62 W m −2 (with a range of −15 to 10 W m −2 , negative values corresponding to planetary cooling), the effect on the atmospheric absorption of SW radiation ( F atmab ) is 1.6 W m −2 (values up to 35 W m −2 , corresponding to atmospheric warming), and the effect on the surface downward and absorbed SW radiation ( F surf , and F surfnet , respectively) is −3.93 and −3.22 W m −2 (values up to −45 and −35 W m −2 , respectively, corresponding to surface cooling). According to our results, aerosols decrease/increase the plan-Correspondence to: N. Hatzianastassiou (nhatzian@cc.uoi.gr) etary albedo by −3 to 13% at the local scale, whereas on planetary scale the result is an increase of 1.5%. Aerosols can warm locally the atmosphere by up to 0.98 K day −1 , whereas they can cool the Earth's surface by up to −2.9 K day −1 . Both these effects, which can significantly modify atmospheric dynamics and the hydrological cycle, can produce significant planetary cooling on a regional scale, although planetary warming can arise over highly reflecting surfaces. The aerosol DRE at the Earth's surface compared to TOA can be up to 15 times larger at the local scale. The largest aerosol DRE takes place in the northern hemisphere both at the surface and the atmosphere, arising mainly at ultraviolet and visible wavelengths.
A method based on the synergistic use of low earth orbit and geostationary earth orbit satellite data for aerosoltype characterization and aerosol optical thickness (AOT: a ) retrieval and monitoring over the ocean is presented in Part I of this paper. The method is now applied to a strong dust outbreak over the Atlantic Ocean in June 1997 and to two other relevant transport events of biomass burning and desert dust aerosol that occurred in 2000 over the Atlantic and Indian Oceans, respectively. The retrievals of the aerosol optical properties are checked against retrievals from sun and sky radiance measurements from the ground-based Aerosol Robotic Network (AERONET). The single-scattering albedo values obtained from AERONET are always within the error bars presented for Global Ozone Monitoring Experiment (GOME) retrievals, resulting in differences lower than 0.041. The retrieved AOT values are compared with the independent space-time-collocated measurements from the AERONET, as well as to the satellite aerosol official products of the Polarization and Directionality of the Earth Reflectances (POLDER) and the Moderate Resolution Imaging Spectroradiometer (MODIS). A first estimate of the AOT accuracy derived from comparisons with AERONET data leads to Ϯ0.02 Ϯ 0.22 a when all AOT values are retained or to Ϯ0.02 Ϯ 0.16 a for aerosol transport events (AOT Ͼ 0.4). The upwelling flux at the top of the atmosphere (TOA) was computed with radiative transfer calculations and used to estimate the TOA direct shortwave aerosol radiative forcing; a comparison with space-time-collocated measurements from the Clouds and the Earth's Radiant Energy System (CERES) TOA flux product was also done. It was found that more than 90% of the values differ from CERES fluxes by less than Ϯ15%.
Aerosol Properties and Their Impacts on Climate
2008
This report critically reviews current knowledge about global distributions and properties of atmospheric aerosols, as they relate to aerosol impacts on climate. It assesses possible next steps aimed at substantially reducing uncertainties in aerosol radiative forcing estimates. Current measurement techniques and modeling approaches are summarized, providing context. As a part of the Synthesis and Assessment Product in the Climate Change Science Program, this assessment builds upon recent related assessments, including the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4, 2007) and other Climate Change Science Program reports. The result provides a synthesis and integration of current knowledge about climate-relevant anthropogenic aerosol impacts for policy makers, policy analysts, as well as the general public. ES 1. Aerosols and Their Climate Effects ES 1.1. Atmospheric Aerosols Atmospheric aerosols are suspensions of solid and/or liquid particles in air. Aerosols are ubiquitous in air and are often observable as dust, smoke, and haze. Both natural and human processes contribute to aerosol concentrations. On a global basis, aerosol mass derives predominantly from natural sources, mainly sea-salt and dust. However, anthropogenic (manmade) aerosols, arising primarily from a variety of combustion sources, can dominate in and downwind of highly populated and industrialized regions, and in areas of intense agricultural burning. The term "atmospheric aerosol" encompasses a wide range of particle types having different compositions, sizes, shapes, and optical properties. Aerosol loading, or amount in the atmosphere, is usually quantified by mass concentration or by an optical measure, aerosol optical depth (AOD). AOD is the vertical integral through the entire height of the atmosphere of the fraction of incident light either scattered or absorbed by airborne particles per unit length. Usually numerical models and in situ observations use mass concentration as the primary measure of aerosol loading, whereas most remote sensing methods retrieve AOD. ES 1.2. Radiative Forcing of Aerosols Aerosols affect Earth's energy budget by scattering and absorbing radiation (the "direct effect") and by modifying amounts and microphysical and radiative properties of clouds (the "indirect effects"). Aerosols influence cloud properties through their role as cloud condensation nuclei (CCN) and/or ice nuclei. Increases in aerosol particle concentrations may increase the ambient concentration of CCN and ice nuclei, affecting cloud properties. A CCN increase can lead to of-atmosphere (TOA). Applying various methods using MODIS, MISR and the Clouds and Earth's Radiant Energy System (CERES), the aerosol direct RF derived above ocean converges to-5.5±0.2 W m-2. Here, the uncertainty is the standard deviation of the various methods, indicating close agreement between the methods and satellite data sets. However, regional comparisons of the various methods show greater spread than the global mean. Estimates of direct radiative forcing at the ocean surface, and at top and bottom of the atmosphere over land, are also reported, but are much less certain. These measurement-based estimates are calculated for cloud-free conditions and comparing to an aerosol-free atmosphere. Although there are no proven methods exist for measuring the anthropogenic component of the observed aerosol over broad geographic regions, satellite retrievals are able to qualitatively determine aerosol type under some conditions. From observations of aerosol type, the best estimates indicate approximately 20% of the AOD over the global oceans is a result of human activities. Following from these estimates of anthropogenic fraction, the cloud-free anthropogenic direct radiative forcing at top of atmosphere is approximated to be-1.1±0.4 Wm-2 over the global ocean, representing the anthropogenic perturbation to today's natural aerosol. ES 2.2. Assessments of Aerosol Indirect Radiative Forcing Remote sensing estimates of aerosol indirect forcing are still very uncertain. Even on small spatial scales, remote sensing of aerosol effects on cloud albedo do not match in situ observations, due to a variety of difficulties with the remote sensing of cloud properties at fine scales, the inability of satellites to observe aerosol properties beneath cloud base, and the difficulty of making aerosol retrievals in cloud fields. Key quantities such as liquid water path, cloud updraft velocity and detailed aerosol size distributions are rarely constrained by coincident observations. Most remote sensing observations of aerosol-cloud interactions and aerosol indirect forcing are based on simple correlations among variables, which do not establish cause-and-effect relationships. Inferring aerosol effects on clouds from the observed relationships is complicated further because aerosol loading and meteorology are often correlated, making it difficult to distinguish aerosol from meteorological effects. As in the case of direct forcing, the regional nature of indirect forcing is especially important for understanding actual climate impact. ES 3. Model Estimated Aerosol Radiative Forcing and Its Climate Impact Just as different types of aerosol observations serve similar purposes, diverse types of models provide a variety of approaches to understanding aerosol forcing of climate. Large-scale Chemistry and transport models (CTMs) are used to test current understanding of the processes controlling aerosol spatial and temporal distributions, including aerosol and precursor emissions, chemical and microphysical transformations, transport, and removal. CTMs are used to describe the global aerosol system and to make estimates of direct aerosol radiative forcing. In general, CTMs do not explore the climate response to this forcing. General Circulation Models (GCMs), sometimes called Global Climate Models, have the capability of including aerosol processes as a part of the climate system to estimate aerosol climate forcing, including aerosol-cloud interactions, and the climate response to this forcing. Another type of model represents atmospheric processes on much smaller scales, such as cloud resolving and large Eddy simulation models. These small-scale models are the primary tools for improving understanding of aerosol-cloud processes, although they are not used to make estimates of aerosol-cloud radiative forcing on regional or global scales. ES 3.1. The Importance of Aerosol Radiative Forcing in Climate Models The IPCC AR4 reported estimates of surface temperature change due to anthropogenic forcing by greenhouse gases and aerosols from more than 20 participating global climate models. Despite a wide range of climate sensitivity (i.e. the amount of surface temperature increase due to a change in radiative forcing, such as an increase of CO 2) employed by the models, they all yield a global average temperature change similar to the observed change. This agreement across models appears to be a result of the use of very different aerosol forcing values, which compensate for the range of climate sensitivity. For example, the direct cooling effect of sulfate aerosol varied by a factor of six among the models. Even greater disparity was found in the model treatment of black carbon and organic carbon. Some models ignored aerosol indirect effects whereas others included large indirect effects. In addition, the aerosol effect on cloud brightness (reflectivity) varied by up to a factor of nine among models for those models that included the effect. ES 3.2. Modeling Atmospheric Aerosols Simulations of the global aerosol distribution by different models show good agreement in their representation of the global mean AOD, which in general also agrees with satellite-observed values. However, large differences exist in model simulations of regional and seasonal distributions of AOD, and in the proportion of aerosol mass attributed to individual species. Each model uses its own estimates of aerosol and precursor emissions and configurations for chemical transformations, microphysical properties, transport, and deposition. Multi-model experiments indicate that differences in the models' atmospheric processes play a more important role than differences in emissions in creating the diversity among model results. Although aerosol mass concentration is the basic measure of aerosol loading in the models, this quantity is translated to AOD via mass extinction efficiency in order to compare with observations and then to estimate aerosol direct RF. Each model employs its own mass extinction efficiency based on assumed optical and physical properties of each aerosol type. Thus, it is possible for the models to produce different distributions of aerosol loading as mass concentrations but agree in their distributions of AOD, and vice-versa. Model calculated total global mean direct anthropogenic aerosol RF at TOA, based on the difference between pre-industrial and current aerosol fields, is-0.22 W m-2 , with a range from-0.63 to +0.04 W m-2. This estimate does not include man-made contributions of nitrate and dust, which could add another-0.2 W m-2 estimated by IPCC AR4. The mean value is much smaller than the estimates of total greenhouse gas forcing of +2.9 W m-2 , but the comparison of global average values does not take into account immense regional variability. Over the major sources and their downwind regions, the model-calculated negative forcing from aerosols can be comparable to or even larger than the positive forcing by greenhouse gases. ES 3.3. Aerosol Effects on Clouds Large-scale models are increasingly incorporating aerosol indirect effects into their calculations. Published large-scale model studies report calculated global cloud albedo effect RF at top-ofatmosphere ranging from-0.22 to-1.85 W m-2 with a central value of-0.7 W m-2 .. Numerical experiments have shown that the...
2006
The largest uncertainty in the radiative forcing of climate change over the industrial era is that due to aerosols, a substantial fraction of which is the uncertainty associated with scattering and absorption of shortwave (solar) radiation by anthropogenic aerosols in cloud-free conditions [IPCC, 2001]. Quantifying and reducing the uncertainty in aerosol influences on climate is critical to understanding climate change over the industrial period and to improving predictions of future climate change for assumed emission scenarios. Measurements of aerosol properties during major field campaigns in several regions of the globe during the past decade are contributing to an enhanced understanding of atmospheric aerosols and their effects on light scattering and climate. The present study, which focuses on three regions downwind of major urban/population centers (North Indian Ocean (NIO) during INDOEX, the Northwest Pacific Ocean (NWP) during ACE-Asia, and the Northwest Atlantic Ocean (NWA) during ICARTT), incorporates understanding gained from field observations of aerosol distributions and properties into calculations of perturbations in radiative fluxes due to these aerosols. This study evaluates the current state of observations and of two chemical transport models (STEM and MOZART). Measurements of burdens, extinction optical depth (AOD), and direct radiative effect of aerosols (DRE-change in radiative flux due to total aerosols) are used as measurement-model check points to assess uncertainties. In-situ measured and remotely sensed aerosol properties for each region (mixing state, mass scattering efficiency, single scattering albedo, and angular scattering properties and their dependences on relative humidity) are used as input parameters to two radiative transfer models (GFDL and University of Michigan) to constrain estimates of aerosol radiative effects, with uncertainties in each step propagated through the analysis. Constraining the radiative transfer calculations by observational inputs increases the AOD (34±8%), top of atmosphere (TOA) DRE (32±12%), and TOA direct climate forcing of aerosols (DCFchange in radiative flux due to anthropogenic aerosols) (37±7%) relative to values obtained with "a priori" parameterizations of aerosol loadings and properties (GFDL RTM). The resulting constrained TOA DCF is-3.3±0.47,-14±2.6,-6.4±2.1 W m-2 for the NIO, NWP, and NWA, respectively. Constraining the radiative transfer calculations by observational inputs reduces the uncertainty range in the DCF in these regions relative to global IPCC [2001] estimates by a factor of approximately 3. Such comparisons with observations and resultant reductions in uncertainties are essential for improving and developing confidence in climate model calculations incorporating aerosol forcing. ΔF = ε A τ A + ε B τ B (2) is expected to hold. This relation is the basis of use of forcing efficiency as a measurable aerosol property that can be compared with observations and used to constrain estimates of DRE and DCF. We note, however, that non-linearities can be important in global-mean calculations. Aerosol properties have been intensely measured over several regions of the globe in major international field campaigns conducted during the past decade [Yu et al., 2005]. These measurements provide in-situ and remotely sensed aerosol data that can be used in calculations of aerosol distributions and their radiative effects. The present study examines DRE and DCF over the North Indian, northwestern Pacific, and northwestern Atlantic Oceans (Figure 2 and Table 1). These regions are selected because of the large anthropogenic aerosol sources upwind of these ocean basins and the availability of suitable measurement data sets: North Indian Ocean (1999-INDOEX); northwestern Pacific Ocean (2001-ACE-Asia and TRACE-P]); and northwestern Atlantic Ocean (2002-NEAQS; 2004-ICARTT). Aerosol concentrations and their radiative impacts are particularly large in these regions, with diurnally averaged clear-sky surface DRE as great as-30 Wm-2 [Russell et al., 1999; Ramanathan et al., 2001; Conant et al., 2003]; here the negative sign denotes a decrease in the net incoming radiative flux to Earth. Restriction of the examination to ocean areas, which are characterized by low surface reflectance, minimizes the influence of uncertainty in this reflectance. This study summarizes in-situ data from these regions from the above named campaigns (Sections 3), compares the data from these campaigns with available longer term monitoring data (Sections 3), compares the chemical data from the intensive campaigns with results of CTM calculations (Section 4), and uses the CTM distributions and in-situ measured aerosol optical properties in RTMs to calculate regional aerosol optical depth, DRE, DCF, and aerosol radiative efficiency (forcing per unit optical depth) (section 5). This analysis is one of three aerosol-related studies being prepared for the Climate Change