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2004 .

(7 publications)

J. Quaas, O. Boucher, J.-L. Dufresne, and H. Treut. Impacts of greenhouse gases and aerosol direct and indirect effects on clouds and radiation in atmospheric GCM simulations of the 1930 1989 period. Climate Dynamics, 23:779-789, December 2004. [ bib | DOI | ADS link ]

Among anthropogenic perturbations of the Earths atmosphere, greenhouse gases and aerosols are considered to have a major impact on the energy budget through their impact on radiative fluxes. We use three ensembles of simulations with the LMDZ general circulation model to investigate the radiative impacts of five species of greenhouse gases (CO2, CH4, N2O, CFC-11 and CFC-12) and sulfate aerosols for the period 1930 1989. Since our focus is on the atmospheric changes in clouds and radiation from greenhouse gases and aerosols, we prescribed sea-surface temperatures in these simulations. Besides the direct impact on radiation through the greenhouse effect and scattering of sunlight by aerosols, strong radiative impacts of both perturbations through changes in cloudiness are analysed. The increase in greenhouse gas concentration leads to a reduction of clouds at all atmospheric levels, thus decreasing the total greenhouse effect in the longwave spectrum and increasing absorption of solar radiation by reduction of cloud albedo. Increasing anthropogenic aerosol burden results in a decrease in high-level cloud cover through a cooling of the atmosphere, and an increase in the low-level cloud cover through the second aerosol indirect effect. The trend in low-level cloud lifetime due to aerosols is quantified to 0.5 min day-1 decade-1 for the simulation period. The different changes in high (decrease) and low-level (increase) cloudiness due to the response of cloud processes to aerosols impact shortwave radiation in a contrariwise manner, and the net effect is slightly positive. The total aerosol effect including the aerosol direct and first indirect effects remains strongly negative.

M. S. Reddy, O. Boucher, C. Venkataraman, S. Verma, J.-F. LéOn, N. Bellouin, and M. Pham. General circulation model estimates of aerosol transport and radiative forcing during the Indian Ocean Experiment. Journal of Geophysical Research (Atmospheres), 109:16205, August 2004. [ bib | DOI | ADS link ]

Aerosol sources, transport, and sinks are simulated, and aerosol direct radiative effects are assessed over the Indian Ocean for the Indian Ocean Experiment (INDOEX) Intensive Field Phase during January to March 1999 using the Laboratoire de Météorologie Dynamique (LMDZT) general circulation model. The model reproduces the latitudinal gradient in aerosol mass concentration and optical depth (AOD). The model-predicted aerosol concentrations and AODs agree reasonably well with measurements but are systematically underestimated during high-pollution episodes, especially in the month of March. The largest aerosol loads are found over southwestern China, the Bay of Bengal, and the Indian subcontinent. Aerosol emissions from the Indian subcontinent are transported into the Indian Ocean through either the west coast or the east coast of India. Over the INDOEX region, carbonaceous aerosols are the largest contributor to the estimated AOD, followed by sulfate, dust, sea salt, and fly ash. During the northeast winter monsoon, natural and anthropogenic aerosols reduce the solar flux reaching the surface by 25 W m-2, leading to 10-15% less insolation at the surface. A doubling of black carbon (BC) emissions from Asia results in an aerosol single-scattering albedo that is much smaller than in situ measurements, reflecting the fact that BC emissions are not underestimated in proportion to other (mostly scattering) aerosol types. South Asia is the dominant contributor to sulfate aerosols over the INDOEX region and accounts for 60-70% of the AOD by sulfate. It is also an important but not the dominant contributor to carbonaceous aerosols over the INDOEX region with a contribution of less than 40% to the AOD by this aerosol species. The presence of elevated plumes brings significant quantities of aerosols to the Indian Ocean that are generated over Africa and Southeast and east Asia.

N. Bellouin, O. Boucher, M. Vesperini, and D. E. Tanré. Estimating the direct aerosol radiative perturbation: Impact of ocean surface representation and aerosol non-sphericity. Quarterly Journal of the Royal Meteorological Society, 130:2217-2232, July 2004. [ bib | DOI | ADS link ]

Atmospheric aerosols are now actively studied, in particular because of their radiative and climate impacts. Estimations of the direct aerosol radiative perturbation, caused by extinction of incident solar radiation, usually rely on radiative transfer codes and involve simplifying hypotheses. This paper addresses two approximations which are widely used for the sake of simplicity and limiting the computational cost of the calculations. Firstly, it is shown that using a Lambertian albedo instead of the more rigorous bidirectional reflectance distribution function (BRDF) to model the ocean surface radiative properties leads to large relative errors in the instantaneous aerosol radiative perturbation. When averaging over the day, these errors cancel out to acceptable levels of less than 3% (except in the northern hemisphere winter). The other scope of this study is to address aerosol non-sphericity effects. Comparing an experimental phase function with an equivalent Mie-calculated phase function, we found acceptable relative errors if the aerosol radiative perturbation calculated for a given optical thickness is daily averaged. However, retrieval of the optical thickness of non-spherical aerosols assuming spherical particles can lead to significant errors. This is due to significant differences between the spherical and non-spherical phase functions. Discrepancies in aerosol radiative perturbation between the spherical and non-spherical cases are sometimes reduced and sometimes enhanced if the aerosol optical thickness for the spherical case is adjusted to fit the simulated radiance of the non-spherical case.

M. S. Reddy and O. Boucher. A study of the global cycle of carbonaceous aerosols in the LMDZT general circulation model. Journal of Geophysical Research (Atmospheres), 109:14202, July 2004. [ bib | DOI | ADS link ]

The global atmospheric cycle of carbonaceous aerosols is simulated in the Laboratoire de Météorologie Dynamique general circulation model, and the subsequent aerosol optical depth is estimated for the period 1997 to 1999. The seasonal and interannual variability in the open biomass burning emissions has been improved by combining existing emission inventories and satellite measured fire counts. The model performance has been thoroughly evaluated against measured aerosol mass concentrations and optical depth in different regions of the globe. At a majority of locations, the modeled mass concentrations of black carbon (BC) at the surface are within a factor of two of observed values. The concentrations of organic carbon (OC) are generally underestimated in comparison to measurements. The discrepancies between model predicted values and measurements are attributable to the difference in time periods between the measurements and model simulations and/or a real underestimation of aerosol emissions in the model. The atmospheric residence times of both BC and OC aerosols are about a week. The hydrophilic fraction of carbonaceous aerosols accounts for about 90% of the total burden. Organic matter (OM) and associated water dominate the optical depth by carbonaceous aerosols with a 86% contribution (global mean of 0.031 at 0.55 μm). Different sensitivity experiments on the transformation time for conversion of hydrophobic to hydrophilic aerosols and emission partitioning show significant changes in the distribution of aerosol burdens and optical depth. The globally averaged burdens change by 15% and residence times are shorter or longer by about 1 day in the various experiments as compared to the control simulation. In all of these experiments the largest sensitivity in aerosol concentrations is found in the remote regions and in the free troposphere (pressure range of 700-400 hPa). Emissions from biomass burning dominate the burden and optical depth of carbonaceous aerosols in the entire SH and NH tropics, while fossil fuel emissions dominate the NH extratropics. On the global scale biomass burning accounts for 78% of the total carbonaceous aerosol burden (BC + OM) followed by natural secondary organic aerosols (SOA)(14%) and fossil fuels (8%). The contributions to corresponding AOD are similar with the largest contribution from biomass burning (76%) followed by natural SOA (14%), and fossil fuels (10%).

J. Quaas, O. Boucher, and F.-M. BréOn. Aerosol indirect effects in POLDER satellite data and the Laboratoire de Météorologie Dynamique-Zoom (LMDZ) general circulation model. Journal of Geophysical Research (Atmospheres), 109:8205, April 2004. [ bib | DOI | ADS link ]

The POLDER-1 instrument was able to measure aerosol and cloud properties for eight months in 1996-1997. We use these observational data for aerosol concentration (the aerosol index), cloud optical thickness, and cloud droplet effective radius to establish statistical relationships among these parameters in order to analyze the first and second aerosol indirect effects. We also evaluate the representation of these effects as parameterized in the Laboratoire de Météorologie Dynamique-Zoom (LMDZ) general circulation model. We find a decrease in cloud top droplet radius with increasing aerosol index in both the model and the observations. Our results are only slightly changed if the analysis is done at fixed cloud liquid water path (LWP) instead of considering all LWP conditions. We also find a positive correlation between aerosol index and cloud liquid water path, which is particularly pronounced over the Northern Hemisphere midlatitudes. This may be interpreted as observational evidence for the second aerosol indirect effect on a large scale. The model-simulated relationship agrees well with that derived from POLDER data. Model simulations show a rather small change in the two relationships if preindustrial rather than present-day aerosol distributions are used. However, when entirely switching off the second aerosol indirect effect in our model, we find a much steeper slope than we do when including it.

F. Parol, J. C. Buriez, C. Vanbauce, J. Riedi, L. C. Labonnote, M. Doutriaux-Boucher, M. Vesperini, G. Sèze, P. Couvert, M. Viollier, and F. M. Bréon. Review of capabilities of multi-angle and polarization cloud measurements from POLDER. Advances in Space Research, 33:1080-1088, January 2004. [ bib | DOI | ADS link ]

Polarization and directionality of the Earth's reflectances (POLDER) is a multispectral imaging radiometer-polarimeter with a wide field-of-view, a moderate spatial resolution, and a multi-angle viewing capability. It functioned nominally aboard ADEOS1 from November 1996 to June 1997. When the satellite passes over a target, POLDER allows to observe it under up to 14 different viewing directions and in several narrow spectral bands of the visible and near-infrared spectrum (443-910 nm). This new type of multi-angle instruments offers new opportunity for deriving cloud parameters at global scale. The aim of this short overview paper is to point out the main contributions of such an instrument for cloud study through its original instrumental capabilities (multidirectionality, multipolarization, and multispectrality). This is mainly illustrated by using ADEOS 1-POLDER derived cloud parameters which are operationally processed by CNES and are available since the beginning of 1999.

O. Boucher, G. Myhre, and A. Myhre. Direct human influence of irrigation on atmospheric water vapour and climate. Climate Dynamics, 22:597-603, 2004. [ bib | DOI | ADS link ]

Human activity increases the atmospheric water vapour content in an indirect way through climate feedbacks. We conclude here that human activity also has a direct influence on the water vapour concentration through irrigation. In idealised simulations we estimate a global mean radiative forcing in the range of 0.03 to +0.1 Wm-2 due to the increase in water vapour from irrigation. However, because the water cycle is embodied in the climate system, irrigation has a more complex influence on climate. We also simulate a change in the temperature vertical profile and a large surface cooling of up to 0.8 K over irrigated land areas. This is of opposite sign than expected from the radiative forcing alone, and this questions the applicability of the radiative forcing concept for such a climatic perturbation. Further, this study shows stronger links than previously recognised between climate change and freshwater scarcity which are environmental issues of paramount importance for the twenty first century.

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