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Joyce E Penner - One of the best experts on this subject based on the ideXlab platform.
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satellite methods underestimate indirect Climate Forcing by aerosols
Proceedings of the National Academy of Sciences of the United States of America, 2011Co-Authors: Joyce E Penner, Li Xu, Minghuai WangAbstract:Satellite-based estimates of the aerosol indirect effect (AIE) are consistently smaller than the estimates from global aerosol models, and, partly as a result of these differences, the assessment of this Climate Forcing includes large uncertainties. Satellite estimates typically use the present-day (PD) relationship between observed cloud drop number concentrations (Nc) and aerosol optical depths (AODs) to determine the preindustrial (PI) values of Nc. These values are then used to determine the PD and PI cloud albedos and, thus, the effect of anthropogenic aerosols on top of the atmosphere radiative fluxes. Here, we use a model with realistic aerosol and cloud processes to show that empirical relationships for ln(Nc) versus ln(AOD) derived from PD results do not represent the atmospheric perturbation caused by the addition of anthropogenic aerosols to the preindustrial atmosphere. As a result, the model estimates based on satellite methods of the AIE are between a factor of 3 to more than a factor of 6 smaller than model estimates based on actual PD and PI values for Nc. Using ln(Nc) versus ln(AI) (Aerosol Index, or the optical depth times angstrom exponent) to estimate preindustrial values for Nc provides estimates for Nc and Forcing that are closer to the values predicted by the model. Nevertheless, the AIE using ln(Nc) versus ln(AI) may be substantially incorrect on a regional basis and may underestimate or overestimate the global average Forcing by 25 to 35%.
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Climate Forcing by carbonaceous and sulfate aerosols
Climate Dynamics, 1998Co-Authors: Joyce E Penner, C C Chuang, K E GrantAbstract:An atmospheric general circulation model is coupled to an atmospheric chemistry model to calculate the radiative Forcing by anthropogenic sulfate and carbonaceous aerosols. The latter aerosols result from biomass burning as well as fossil fuel burning. The black carbon associated with carbonaceous aerosols is absorbant and can decrease the amount of reflected radiation at the top-of-the-atmosphere. In contrast, sulfate aerosols are reflectant and the amount of reflected radiation depends nonlinearly on the relative humidity. We examine the importance of treating the range of optical properties associated with sulfate aerosol at high relative humidities and find that the direct Forcing by anthropogenic sulfate aerosols can decrease from −0.81 W m-2 to −0.55 Wm-2 if grid box average relative humidity is not allowed to increase above 90%. The Climate Forcing associated with fossil fuel emissions of carbonaceous aerosols is calculated to range from +0.16 to +0.20 Wm-2, depending on how much organic carbon is associated with the black carbon from fossil fuel burning. The direct Forcing of carbonaceous aerosols associated with biomass burning is calculated to range from −0.23 to −0.16 Wm-2. The pattern of Forcing by carbonaceous aerosols depends on both the surface albedo and the presence of clouds. Multiple scattering associated with clouds and high surface albedos can change the Forcing from negative to positive.
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quantifying and minimizing uncertainty of Climate Forcing by anthropogenic aerosols
Bulletin of the American Meteorological Society, 1994Co-Authors: Joyce E Penner, Robert J Charlson, Stephen E Schwartz, J M Hales, N S Laulainen, R Leifer, T Novakov, J A Ogren, Lawrence F Radke, Larry D TravisAbstract:The clear-sky Climate Forcing by anthropogenic aerosols has been shown to be of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Anthropogenic aerosols are composed of a variety of aerosol types including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), organic condensed species, elemental or black carbon, and mineral dust. Estimates of the clear-sky Forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols have been published previously. Here we estimate the uncertainty in the Forcing by these aerosol types. Estimates of the clear-sky Forcing by other anthropogenic aerosol types do not even exist though the Forcing by these aerosol types is thought to be smaller than that by sulfate and biomass burning aerosols.
Owen B Toon - One of the best experts on this subject based on the ideXlab platform.
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the impact of humidity above stratiform clouds on indirect aerosol Climate Forcing
Nature, 2004Co-Authors: Andrew S Ackerman, Michael P. Kirkpatrick, David E Stevens, Owen B ToonAbstract:Some of the global warming effect of anthropogenic greenhouse gases is offset by increased solar reflection from clouds with smaller droplets that form on increased numbers of cloud condensation nuclei in polluted air. The global magnitude of the resulting indirect aerosol Climate Forcing is estimated to be comparable (and opposed) to the anthropogenic carbon dioxide Forcing, but estimates are highly uncertain because of complexities in characterizing the physical process that determine global aerosol and cloud populations and their interactions. Beyond reflecting sunlight more effectively, smaller droplets are less efficient at producing precipitation, and decreased precipitation is expected to result in increased cloud water and cloud cover, further increasing the indirect Forcing. Yet polluted marine boundary-layer clouds are not generally observed to hold more water. Here we use model simulations of stratocumulus clouds to show that suppression of precipitation from increased droplet concentrations leads to increased cloud water only when sufficient precipitation reaches the surface, a condition favored when the overlying air is moist. Otherwise, aerosol induced suppression of precipitation enhances entrainment of overlying dry air, thereby reducing cloud water and diminishing the indirect Climate Forcing.
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The impact of humidity above stratiform clouds on indirect aerosol Climate Forcing
Nature, 2004Co-Authors: Andrew S Ackerman, Michael P. Kirkpatrick, David E Stevens, Owen B ToonAbstract:Some of the global warming from anthropogenic greenhouse gases is offset by increased reflection of solar radiation by clouds with smaller droplets that form in air polluted with aerosol particles that serve as cloud condensation nuclei. The resulting cooling tendency, termed the indirect aerosol Forcing, is thought to be comparable in magnitude to the Forcing by anthropogenic CO2, but it is difficult to estimate because the physical processes that determine global aerosol and cloud populations are poorly understood. Smaller cloud droplets not only reflect sunlight more effectively, but also inhibit precipitation, which is expected to result in increased cloud water. Such an increase in cloud water would result in even more reflective clouds, further increasing the indirect Forcing. Marine boundary-layer clouds polluted by aerosol particles, however, are not generally observed to hold more water. Here we simulate stratocumulus clouds with a fluid dynamics model that includes detailed treatments of cloud microphysics and radiative transfer. Our simulations show that the response of cloud water to suppression of precipitation from increased droplet concentrations is determined by a competition between moistening from decreased surface precipitation and drying from increased entrainment of overlying air. Only when the overlying air is humid or droplet concentrations are very low does sufficient precipitation reach the surface to allow cloud water to increase with droplet concentrations. Otherwise, the response of cloud water to aerosol-induced suppression of precipitation is dominated by enhanced entrainment of overlying dry air. In this scenario, cloud water is reduced as droplet concentrations increase, which diminishes the indirect Climate Forcing.
Robert J Charlson - One of the best experts on this subject based on the ideXlab platform.
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quantifying above cloud aerosol using spaceborne lidar for improved understanding of cloudy sky direct Climate Forcing
Journal of Geophysical Research, 2008Co-Authors: D Chand, Robert J Charlson, T L Anderson, Robert Wood, Yong X Hu, Mark A VaughanAbstract:[1] Estimates of global mean direct Climate Forcing by absorbing aerosols located above boundary layer clouds are large, uncertain, and almost entirely unconstrained by observations. Spaceborne lidar offers a new opportunity for global constraints. Here we examine techniques for using liquid water clouds as lidar targets, allowing aerosol optical depth and Angstrom exponent to be deduced directly from aerosol effects on light transmission. Two such techniques are examined using data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). The first is a previously reported method based on measurements of cloud depolarization ratio (DR) at 532-nm wavelength. The second is a new method using measurements of cloud color ratio (CR), which is the ratio of the signal from the cloud at 1064 nm to that at 532 nm. Optical depth retrievals from these two methods compare favorably over the eastern tropical Atlantic Ocean during August 2006, when biomass burning aerosols are frequently advected over marine stratiform clouds. The CR technique is mainly sensitive to fine-mode aerosols and essentially insensitive to clouds and coarse-mode dust. Because anthropogenic aerosol is predominantly found in the fine mode, the CR technique can be used to help identify situations where anthropogenic cloudy-sky direct radiative Forcing is occurring. We demonstrate this capability using 6 months data over the eastern tropical Atlantic Ocean.
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an a train strategy for quantifying direct Climate Forcing by anthropogenic aerosols
Bulletin of the American Meteorological Society, 2005Co-Authors: Theodore L Anderson, Robert J Charlson, Olivier Boucher, Nicolas Bellouin, Mian Chin, Sundar A Christopher, J M Haywood, Yoram J Kaufman, Stefan Kinne, J A OgrenAbstract:Abstract This document outlines a practical strategy for achieving an observationally based quantification of direct Climate Forcing by anthropogenic aerosols. The strategy involves a four-step program for shifting the current assumption-laden estimates to an increasingly empirical basis using satellite observations coordinated with suborbital remote and in situ measurements and with chemical transport models. Conceptually, the problem is framed as a need for complete global mapping of four parameters: clear-sky aerosol optical depth δ, radiative efficiency per unit optical depth E, fine-mode fraction of optical depth ff, and the anthropogenic fraction of the fine mode faf. The first three parameters can be retrieved from satellites, but correlative, suborbital measurements are required for quantifying the aerosol properties that control E, for validating the retrieval of ff, and for partitioning fine-mode δ between natural and anthropogenic components. The satellite focus is on the “A-Train,” a constella...
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Climate Forcing by aerosols a hazy picture
Science, 2003Co-Authors: Theodore L Anderson, Robert J Charlson, Stephen E Schwartz, Reto Knutti, Olivier Boucher, Henning Rodhe, Jost HeintzenbergAbstract:Anthropogenic aerosol emissions are believed to have counteracted the global-warming effect of greenhouse gases over the past century. However, the magnitude of this cooling effect is highly uncertain. In their Perspective, Anderson et al . argue that the magnitude and uncertainty of aerosol Forcing may be larger than is usually considered in models. This would have important implications for the total Climate Forcing by anthropogenic emissions, and hence for predicting future global warming.
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Coupling Satellite and Ground-Based Instruments to Map Climate Forcing by Anthropogenic Aerosols
2000Co-Authors: Robert J Charlson, Theodore L Anderson, Chris A. HostetlerAbstract:Climate Forcing by anthropogenic aerosols is a significant but highly uncertain factor in global Climate change. Only satellites can offer the global coverage essential to reducing this uncertainty; however, satellite measurements must be coupled with correlative, in situ measurements both to constrain the aerosol optical properties required in satellite retrieval algorithms and to provide chemical identification of aerosol sources. This grant funded the first two years of a three-year project which seeks to develop methodologies for combining spaceborne lidar with in-situ aerosol data sets to improve estimates of direct aerosol Climate Forcing. Progress under this two-year grant consisted in the development and deployment of a new in-situ capability for measuring aerosol 180' backscatter and the extinction-to-backscatter ratio. This new measurement capacity allows definitive lidar/in-situ comparisons and improves our ability to interpret lidar data in terms of climatically relevant quantities such as the extinction coefficient and optical depth. Measurements were made along the coast of Washington State, in Central Illinois, over the Indian Ocean, and in the Central Pacific. Thus, this research, combined with previous measurements by others, is rapidly building toward a global data set of extinction-to-backscatter ratio for key aerosol types. Such information will be critical to interpreting lidar data from the upcoming PICASSO-CENA, or P-C, satellite mission. Another aspect of this project is to investigate innovative ways to couple the lidar-satellite signal with targeted in-situ measurements toward a direct determination of aerosol Forcing. This aspect is progressing in collaboration with NASA Langley's P-C lidar simulator and radiative transfer modeling by the University of Lille, France.
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RELATIONSHIP BETWEEN ASYMMETRY PARAMETER AND HEMISPHERIC BACKSCATTER RATIO: IMPLICATIONS FOR Climate Forcing BY AEROSOLS
Applied Optics, 1995Co-Authors: Stephen F. Marshall, David S. Covert, Robert J CharlsonAbstract:Calculations of direct Climate Forcing by anthropogenic aerosols commonly use radiative transfer parameters, including asymmetry parameter g. One method of obtaining the asymmetry parameter of a particle population is to convert measured values of the hemispheric-to-total-scatter ratio (backscatter ratio b) into their corresponding g values. We compare a conversion derived from Mie calculations with one derived from the Henyey–Greenstein (HG) phase function to show that the HG method systematically overestimates g for typical size distributions of accumulation-mode aerosols. A delta-Eddington radiative transfer calculation is used to show that a 10% overestimation of g can systematically reduce Climate Forcing as a result of aerosols by 12% or more. Mie computations are used to derive an empirical relationship between backscatter ratio and asymmetry parameter for log-normal accumulation-mode aerosols. This relationship can be used to convert the backscatter ratio to the asymmetry parameter, independent of geometric mean diameter Dgv or complex refractive index m, but the conversion requires knowledge of the breadth σg of the size distribution.
John H Seinfeld - One of the best experts on this subject based on the ideXlab platform.
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global distribution and Climate Forcing of carbonaceous aerosols
Journal of Geophysical Research, 2002Co-Authors: S H Chung, John H SeinfeldAbstract:The global distribution of carbonaceous aerosols is simulated online in the Goddard Institute for Space Studies General Circulation Model II-prime (GISS GCM II-prime). Prognostic tracers include black carbon (BC), primary organic aerosol (POA), five groups of biogenic volatile organic compounds (BVOCs), and 14 semivolatile products of BVOC oxidation by O_3, OH, and NO_3, which condense to form secondary organic aerosols (SOA) based on an equilibrium partitioning model and experimental observations. Estimated global burdens of BC, organic carbon (OC), and SOA are 0.22, 1.2, and 0.19 Tg with lifetimes of 6.4, 5.3, and 6.2 days, respectively. The predicted global production of SOA is 11.2 Tg yr^(−1), with 91% due to O_3 and OH oxidation. Globally averaged, top of the atmosphere (TOA) radiative Forcing by anthropogenic BC is predicted as +0.51 to +0.8 W m^(−2), the former being for BC in an external mixture and the latter for BC in an internal mixture of sulfate, OC, and BC. Globally averaged, anthropogenic BC, OC, and sulfate are predicted to exert a TOA radiative Forcing of −0.39 to −0.78 W m^(−2), depending on the exact assumptions of aerosol mixing and water uptake by OC. Forcing estimates are compared with those published previously.
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sensitivity of direct Climate Forcing by atmospheric aerosols to aerosol size and composition
Journal of Geophysical Research, 1995Co-Authors: Christodoulos Pilinis, Spyros N Pandis, John H SeinfeldAbstract:We evaluate, using a box model, the sensitivity of direct Climate Forcing by atmospheric aerosols for a “global mean” aerosol that consists of fine and coarse modes to aerosol composition, aerosol size distribution, relative humidity (RH), aerosol mixing state (internal versus external mixture), deliquescence/crystallization hysteresis, and solar zenith angle. We also examine the dependence of aerosol upscatter fraction on aerosol size, solar zenith angle, and wavelength and the dependence of single scatter albedo on wavelength and aerosol composition. The single most important parameter in determining direct aerosol Forcing is relative humidity, and the most important process is the increase of the aerosol mass as a result of water uptake. An increase of the relative humidity from 40 to 80% is estimated for the global mean aerosol considered to result in an increase of the radiative Forcing by a factor of 2.1. Forcing is relatively insensitive to the fine mode diameter increase due to hygroscopic growth, as long as this mode remains inside the efficient scattering size region. The hysteresis/deliquescence region introduces additional uncertainty but, in general, errors less than 20% result by the use of the average of the two curves to predict Forcing. For fine aerosol mode mean diameters in the 0.2–0.5 μm range direct aerosol Forcing is relatively insensitive (errors less than 20%) to variations of the mean diameter. Estimation of the coarse mode diameter within a factor of 2 is generally sufficient for the estimation of the total aerosol radiative Forcing within 20%. Moreover, the coarse mode, which represents the nonanthropogenic fraction of the aerosol, is estimated to contribute less than 10% of the total radiative Forcing for all RHs of interest. Aerosol chemical composition is important to direct radiative Forcing as it determines (1) water uptake with RH, and (2) optical properties. The effect of absorption by aerosol components on Forcing is found to be significant even for single scatter albedo values of ω=0.93–0.97. The absorbing aerosol component reduces the aerosol Forcing from that in its absence by roughly 30% at 60% RH and 20% at 90% RH. The mixing state of the aerosol (internal versus external) for the particular aerosol considered here is found to be of secondary importance. While sulfate mass scattering efficiency (m2 (g SO42−)−1) and the normalized sulfate Forcing (W (g SO42−)−1) increase strongly with RH, total mass scattering efficiency (m2 g−1) and normalized Forcing (W g−1) are relatively insensitive to RH, wherein the mass of all species, including water, are accounted for. Following S. Nemesure et al. (Direct shortwave Forcing of Climate by anthropogenic sulfate aerosol: sensitivity to particle size, composition, and relative humidity, submitted to Journal of Geophysical Research, 1995), we find that aerosol feeing achieves a maximum at a particular solar zenith angle, reflecting a balance between increasing upscatter fraction with increasing solar zenith angle and decreasing solar flux (from Rayleigh scattering) with increasing solar zenith angle.
Andrew S Ackerman - One of the best experts on this subject based on the ideXlab platform.
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the impact of humidity above stratiform clouds on indirect aerosol Climate Forcing
Nature, 2004Co-Authors: Andrew S Ackerman, Michael P. Kirkpatrick, David E Stevens, Owen B ToonAbstract:Some of the global warming effect of anthropogenic greenhouse gases is offset by increased solar reflection from clouds with smaller droplets that form on increased numbers of cloud condensation nuclei in polluted air. The global magnitude of the resulting indirect aerosol Climate Forcing is estimated to be comparable (and opposed) to the anthropogenic carbon dioxide Forcing, but estimates are highly uncertain because of complexities in characterizing the physical process that determine global aerosol and cloud populations and their interactions. Beyond reflecting sunlight more effectively, smaller droplets are less efficient at producing precipitation, and decreased precipitation is expected to result in increased cloud water and cloud cover, further increasing the indirect Forcing. Yet polluted marine boundary-layer clouds are not generally observed to hold more water. Here we use model simulations of stratocumulus clouds to show that suppression of precipitation from increased droplet concentrations leads to increased cloud water only when sufficient precipitation reaches the surface, a condition favored when the overlying air is moist. Otherwise, aerosol induced suppression of precipitation enhances entrainment of overlying dry air, thereby reducing cloud water and diminishing the indirect Climate Forcing.
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The impact of humidity above stratiform clouds on indirect aerosol Climate Forcing
Nature, 2004Co-Authors: Andrew S Ackerman, Michael P. Kirkpatrick, David E Stevens, Owen B ToonAbstract:Some of the global warming from anthropogenic greenhouse gases is offset by increased reflection of solar radiation by clouds with smaller droplets that form in air polluted with aerosol particles that serve as cloud condensation nuclei. The resulting cooling tendency, termed the indirect aerosol Forcing, is thought to be comparable in magnitude to the Forcing by anthropogenic CO2, but it is difficult to estimate because the physical processes that determine global aerosol and cloud populations are poorly understood. Smaller cloud droplets not only reflect sunlight more effectively, but also inhibit precipitation, which is expected to result in increased cloud water. Such an increase in cloud water would result in even more reflective clouds, further increasing the indirect Forcing. Marine boundary-layer clouds polluted by aerosol particles, however, are not generally observed to hold more water. Here we simulate stratocumulus clouds with a fluid dynamics model that includes detailed treatments of cloud microphysics and radiative transfer. Our simulations show that the response of cloud water to suppression of precipitation from increased droplet concentrations is determined by a competition between moistening from decreased surface precipitation and drying from increased entrainment of overlying air. Only when the overlying air is humid or droplet concentrations are very low does sufficient precipitation reach the surface to allow cloud water to increase with droplet concentrations. Otherwise, the response of cloud water to aerosol-induced suppression of precipitation is dominated by enhanced entrainment of overlying dry air. In this scenario, cloud water is reduced as droplet concentrations increase, which diminishes the indirect Climate Forcing.