lmd_Boucher2005_abstracts.html

2005 .

N. Bellouin, O. Boucher, J. Haywood, and M. S. Reddy. Global estimate of aerosol direct radiative forcing from satellite measurements. Nature, 438:1138-1141, December 2005. [ bib | DOI | ADS link ]

Atmospheric aerosols cause scattering and absorption of incoming solar radiation. Additional anthropogenic aerosols released into the atmosphere thus exert a direct radiative forcing on the climate system. The degree of present-day aerosol forcing is estimated from global models that incorporate a representation of the aerosol cycles. Although the models are compared and validated against observations, these estimates remain uncertain. Previous satellite measurements of the direct effect of aerosols contained limited information about aerosol type, and were confined to oceans only. Here we use state-of-the-art satellite-based measurements of aerosols and surface wind speed to estimate the clear-sky direct radiative forcing for 2002, incorporating measurements over land and ocean. We use a Monte Carlo approach to account for uncertainties in aerosol measurements and in the algorithm used. Probability density functions obtained for the direct radiative forcing at the top of the atmosphere give a clear-sky, global, annual average of -1.9Wm^-2 with standard deviation, +/- 0.3Wm^-2. These results suggest that present-day direct radiative forcing is stronger than present model estimates, implying future atmospheric warming greater than is presently predicted, as aerosol emissions continue to decline.

T. L. Anderson, R. J. Charlson, N. Bellouin, O. Boucher, M. Chin, S. A. Christopher, J. Haywood, Y. J. Kaufman, S. Kinne, J. A. Ogren, L. A. Remer, T. Takemura, D. Tanré, O. Torres, C. R. Trepte, B. A. Wielicki, D. M. Winker, and H. Yu. An ”A-Train” Strategy for Quantifying Direct Climate Forcing by Anthropogenic Aerosols. Bulletin of the American Meteorological Society, 86:1795-1809, December 2005. [ bib | DOI | ADS link ]

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 f_f, and the anthropogenic fraction of the fine mode f_af. 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 f_f, and for partitioning fine-mode δ between natural and anthropogenic components. The satellite focus is on the ”A-Train,” a constellation of six spacecraft that will fly in formation from about 2005 to 2008. Key satellite instruments for this report are the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and the Earth's Radiant Energy System (CERES) radiometers on Aqua, the Ozone Monitoring Instrument (OMI) radiometer on Aura, the Polarization and Directionality of Earth's Reflectances (POLDER) polarimeter on the Polarization and Anistropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar (PARASOL), and the Cloud and Aerosol Lider with Orthogonal Polarization (CALIOP) lidar on the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). This strategy is offered as an initial frameworksubject to improvement over timefor scientists around the world to participate in the A-Train opportunity. It is a specific implementation of the Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) program, presented earlier in this journal, which identified the integration of diverse data as the central challenge to progress in quantifying global-scale aerosol effects. By designing a strategy around this need for integration, we develop recommendations for both satellite data interpretation and correlative suborbital activities that represent, in many respects, departures from current practice.

J.-L. Dufresne, J. Quaas, O. Boucher, S. Denvil, and L. Fairhead. Contrasts in the effects on climate of anthropogenic sulfate aerosols between the 20th and the 21st century. Geophysical Research Letters, 32:21703, November 2005. [ bib | DOI | ADS link ]

In this study, we examine the time evolution of the relative contribution of sulfate aerosols and greenhouse gases to anthropogenic climate change. We use the new IPSL-CM4 coupled climate model for which the first indirect effect of sulfate aerosols has been calibrated using POLDER satellite data. For the recent historical period the sulfate aerosols play a key role on the temperature increase with a cooling effect of 0.5 K, to be compared to the 1.4 K warming due to greenhouse gas increase. In contrast, the projected temperature change for the 21st century is remarkably independent of the effects of anthropogenic sulfate aerosols for the SRES-A2 scenario. Those results are interpreted comparing the different radiative forcings, and can be extended to other scenarios. We also highlight that the first indirect effect of aerosol strongly depends on the land surface model by changing the cloud cover.

J. Quaas and O. Boucher. Constraining the first aerosol indirect radiative forcing in the LMDZ GCM using POLDER and MODIS satellite data. Geophysical Research Letters, 32:17814, September 2005. [ bib | DOI | ADS link ]

The indirect effects of anthropogenic aerosols are expected to cause a significant radiative forcing of the Earth's climate whose magnitude, however, is still uncertain. Most climate models use parameterizations for the aerosol indirect effects based on so-called “empirical relationships” which link the cloud droplet number concentration to the aerosol concentration. New satellite datasets such as those from the POLDER and MODIS instruments are well suited to evaluate and improve such parameterizations at a global scale. We derive statistical relationships of cloud-top droplet radius and aerosol index (or aerosol optical depth) from satellite retrievals and fit an empirical parameterization in a general circulation model to match the relationships. When applying the fitted parameterizations in the model, the simulated radiative forcing by the first aerosol indirect effect is reduced by 50% as compared to our baseline simulation (down to -0.3 and -0.4 Wm^-2 when using MODIS and POLDER satellite data, respectively).

Y. J. Kaufman, O. Boucher, D. Tanré, M. Chin, L. A. Remer, and T. Takemura. Aerosol anthropogenic component estimated from satellite data. Geophysical Research Letters, 32:17804, September 2005. [ bib | DOI | ADS link ]

Satellite instruments do not measure the aerosol chemical composition needed to discriminate anthropogenic from natural aerosol components. However the ability of new satellite instruments to distinguish fine (submicron) from coarse (supermicron) aerosols over the oceans, serves as a signature of the anthropogenic component and can be used to estimate the fraction of anthropogenic aerosols with an uncertainty of +/-30%. Application to two years of global MODIS data shows that 21 +/- 7% of the aerosol optical thickness over the oceans has an anthropogenic origin. We found that three chemical transport models, used for global estimates of the aerosol forcing of climate, calculate a global average anthropogenic optical thickness over the ocean between 0.030 and 0.036, in line with the present MODIS assessment of 0.033. This increases our confidence in model assessments of the aerosol direct forcing of climate. The MODIS estimated aerosol forcing over cloud free oceans is therefore -1.4 +/- 0.4 W/m².

R. N. Halthore, D. Crisp, S. E. Schwartz, G. P. Anderson, A. Berk, B. Bonnel, O. Boucher, F.-L. Chang, M.-D. Chou, E. E. Clothiaux, P. Dubuisson, B. Fomin, Y. Fouquart, S. Freidenreich, C. Gautier, S. Kato, I. Laszlo, Z. Li, J. H. Mather, A. Plana-Fattori, V. Ramaswamy, P. Ricchiazzi, Y. Shiren, A. Trishchenko, and W. Wiscombe. Intercomparison of shortwave radiative transfer codes and measurements. Journal of Geophysical Research (Atmospheres), 110:11206, June 2005. [ bib | DOI | ADS link ]

Computation of components of shortwave (SW) or solar irradiance in the surface-atmospheric system forms the basis of intercomparison between 16 radiative transfer models of varying spectral resolution ranging from line-by-line models to broadband and general circulation models. In order of increasing complexity the components are: direct solar irradiance at the surface, diffuse irradiance at the surface, diffuse upward flux at the surface, and diffuse upward flux at the top of the atmosphere. These components allow computation of the atmospheric absorptance. Four cases are considered from pure molecular atmospheres to atmospheres with aerosols and atmosphere with a simple uniform cloud. The molecular and aerosol cases allow comparison of aerosol forcing calculation among models. A cloud-free case with measured atmospheric and aerosol properties and measured shortwave radiation components provides an absolute basis for evaluating the models. For the aerosol-free and cloud-free dry atmospheres, models agree to within 1% (root mean square deviation as a percentage of mean) in broadband direct solar irradiance at surface; the agreement is relatively poor at 5% for a humid atmosphere. A comparison of atmospheric absorptance, computed from components of SW radiation, shows that agreement among models is understandably much worse at 3% and 10% for dry and humid atmospheres, respectively. Inclusion of aerosols generally makes the agreement among models worse than when no aerosols are present, with some exceptions. Modeled diffuse surface irradiance is higher than measurements for all models for the same model inputs. Inclusion of an optically thick low-cloud in a tropical atmosphere, a stringent test for multiple scattering calculations, produces, in general, better agreement among models for a low solar zenith angle (SZA = 30deg) than for a high SZA (75deg). All models show about a 30% increase in broadband absorptance for 30deg SZA relative to the clear-sky case and almost no enhancement in absorptance for a higher SZA of 75deg, possibly due to water vapor line saturation in the atmosphere above the cloud.

M. S. Reddy, O. Boucher, Y. Balkanski, and M. Schulz. Aerosol optical depths and direct radiative perturbations by species and source type. Geophysical Research Letters, 32:12803, June 2005. [ bib | DOI | ADS link ]

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 -0.62, +0.55, -0.33, -0.28, and -0.30 Wm^-2, 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.

M. S. Reddy, O. Boucher, N. Bellouin, M. Schulz, Y. Balkanski, J.-L. Dufresne, and M. Pham. Estimates of global multicomponent aerosol optical depth and direct radiative perturbation in the Laboratoire de Météorologie Dynamique general circulation model. Journal of Geophysical Research (Atmospheres), 110:10, May 2005. [ bib | DOI | ADS link ]

The global cycle of multicomponent aerosols including sulfate, black carbon (BC), organic matter (OM), mineral dust, and sea salt is simulated in the Laboratoire de Météorologie Dynamique general circulation model (LMDZT GCM). The seasonal open biomass burning emissions for simulation years 2000-2001 are scaled from climatological emissions in proportion to satellite detected fire counts. The emissions of dust and sea salt are parameterized online in the model. The comparison of model-predicted monthly mean aerosol optical depth (AOD) at 500 nm with Aerosol Robotic Network (AERONET) shows good agreement with a correlation coefficient of 0.57(N = 1324) and 76% of data points falling within a factor of 2 deviation. The correlation coefficient for daily mean values drops to 0.49 (N = 23,680). The absorption AOD (τ_a at 670 nm) estimated in the model is poorly correlated with measurements (r = 0.27, N = 349). It is biased low by 24% as compared to AERONET. The model reproduces the prominent features in the monthly mean AOD retrievals from Moderate Resolution Imaging Spectroradiometer (MODIS). The agreement between the model and MODIS is better over source and outflow regions (i.e., within a factor of 2). There is an underestimation of the model by up to a factor of 3 to 5 over some remote oceans. The largest contribution to global annual average AOD (0.12 at 550 nm) is from sulfate (0.043 or 35%), followed by sea salt (0.027 or 23%), dust (0.026 or 22%), OM (0.021 or 17%), and BC (0.004 or 3%). The atmospheric aerosol absorption is predominantly contributed by BC and is about 3% of the total AOD. The globally and annually averaged shortwave (SW) direct aerosol radiative perturbation (DARP) in clear-sky conditions is -2.17 Wm^-2 and is about a factor of 2 larger than in all-sky conditions (-1.04 Wm^-2). The net DARP (SW + LW) by all aerosols is -1.46 and -0.59 Wm^-2 in clear- and all-sky conditions, respectively. Use of realistic, less absorbing in SW, optical properties for dust results in negative forcing over the dust-dominated regions.

M. Pham, O. Boucher, and D. Hauglustaine. Changes in atmospheric sulfur burdens and concentrations and resulting radiative forcings under IPCC SRES emission scenarios for 1990-2100. Journal of Geophysical Research (Atmospheres), 110:6112, March 2005. [ bib | DOI | ADS link ]

Simulations of the global sulfur cycle under the IPCC SRES scenarios have been performed. Sulfur dioxide and sulfate burdens, as well as the direct and first indirect radiative forcing (RF) by sulfate aerosols only, are presented for the period 1990 to 2100. By 2100, global sulfur emission rates decline everywhere in all scenarios. At that time, the anthropogenic sulfate burden ranges from 0.34 to 1.03 times the 1990 value of 0.47 Tg S. Direct and indirect global and annually mean RFs relative to the year 1990 are near 0 or positive (range of -0.07 to 0.28 Wm^-2 and 0.01 to 0.38 Wm^-2 for the direct and indirect effects, respectively). For reference these forcings amount respectively to -0.42 and -0.79 Wm^-2 in 1990 relative to preindustrial conditions (around 1750). Sulfur aerosols will therefore induce a smaller cooling effect in 2100 than in 1990 relative to preindustrial conditions. For the period 1990 to 2100, the forcing efficiencies (computed relatively to 1990) are fairly constant for the direct effect (around -160 W (g sulfate)^-1). The forcing efficiencies for the indirect effect are around -200 and -100 W (g sulfate)^-1 for negative and positive burden differences, respectively. This is due to a shift in regional patterns of emissions and a saturation in the indirect effect. The simulated annually averaged SO₂ concentrations for A1B scenario in 2020 are close to air quality objectives for public health in some parts of Africa and exceed these objectives in some parts of China and Korea. Moreover, sulfate deposition rates are estimated to increase by 200% from the present level in East and Southeast Asia. This shows that Asia may experience in the future sulfur-related environmental and human health problems as important as Europe and the United States did in the 1970s.

P. Stier, J. Feichter, S. Kinne, S. Kloster, E. Vignati, J. Wilson, L. Ganzeveld, I. Tegen, M. Werner, Y. Balkanski, M. Schulz, O. Boucher, A. Minikin, and A. Petzold. The aerosol-climate model ECHAM5-HAM. Atmospheric Chemistry & Physics, 5:1125-1156, March 2005. [ bib | ADS link ]

The aerosol-climate modelling system ECHAM5-HAM is introduced. It is based on a flexible microphysical approach and, as the number of externally imposed parameters is minimised, allows the application in a wide range of climate regimes. ECHAM5-HAM predicts the evolution of an ensemble of microphysically interacting internally- and externally-mixed aerosol populations as well as their size-distribution and composition. The size-distribution is represented by a superposition of log-normal modes. In the current setup, the major global aerosol compounds sulfate (SU), black carbon (BC), particulate organic matter (POM), sea salt (SS), and mineral dust (DU) are included. The simulated global annual mean aerosol burdens (lifetimes) for the year 2000 are for SU: 0.80 Tg(S) (3.9 days), for BC: 0.11 Tg (5.4 days), for POM: 0.99 Tg (5.4 days), for SS: 10.5 Tg (0.8 days), and for DU: 8.28 Tg (4.6 days). An extensive evaluation with in-situ and remote sensing measurements underscores that the model results are generally in good agreement with observations of the global aerosol system. The simulated global annual mean aerosol optical depth (AOD) is with 0.14 in excellent agreement with an estimate derived from AERONET measurements (0.14) and a composite derived from MODIS-MISR satellite retrievals (0.16). Regionally, the deviations are not negligible. However, the main patterns of AOD attributable to anthropogenic activity are reproduced.