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1998 .

(6 publications)

T. Claquin, M. Schulz, Y. Balkanski, and O. Boucher. Uncertainties in assessing radiative forcing by mineral dust. Tellus Series B Chemical and Physical Meteorology B, 50:491, November 1998. [ bib | DOI | ADS link ]

The assessment of the climatic effects of an aerosol with a large variability like mineral dust requires some approximations whose validity is investigated in this paper. Calculations of direct radiative forcing by mineral dust (short-wave, long-wave and net) are performed with a single-column radiation model for two standard cases in clear sky condition: a desert case and an oceanic case. Surface forcing result from a large diminution of the short-wave fluxes and of the increase in down-welling long-wave fluxes. Top of the atmosphere (TOA) forcing is negative when short-wave backscattering dominates, for instance above the ocean, and positive when short-wave or long-wave absorption dominates, which occurs above deserts. We study here the sensitivity of these mineral forcings to different treatments of the aerosol complex refractive index and size distribution. We also describe the importance of the dust vertical profile, ground temperature, emissivity and albedo. Among these parameters, the aerosol complex refractive index has been identified as a critical parameter given the paucity and the incertitude associated with it. Furthermore, the imaginary part of the refractive index is inadequate if spectrally averaged. Its natural variability (linked to mineralogical characteristics) lead to variations of up to 40% in aerosol forcing calculations. A proper representation of the size distribution when modelling mineral aerosols is required since dust optical properties are very sensitive to the presence of small particles. In addition we demonstrate that LW forcing imply a non-negligible sensitivity to the vertical profiles of temperature and dust, the latter being an important constraint for dust effect calculations.

M. Doutriaux-Boucher, J. Pelon, V. Trouillet, G. SèZe, H. Le Treut, P. Flamant, and M. Desbois. Simulation of satellite lidar and radiometer retrievals of a general circulation model three-dimensional cloud data set. Journal of Geophysical Research, 103:26025, October 1998. [ bib | DOI | ADS link ]

The inclusion of a backscatter lidar on a space platform for a radiation mission, as proposed by various space agencies, aims to bring new information on three-dimensional cloud distribution, with a special emphasis on optically thin cirrus clouds, which are presently poorly detected by passive sensors. Key issues for such cloud observational studies are the detection of multilayered cloud systems, thin cirrus, and fractional cloud cover, knowledge that would improve our understanding of the global radiation budget. To assess the impact of such lidar measurements on cloud climatology, a 1 month cloud data set has been simulated with a general circulation model (GCM). The cloud detection capability of a spaceborne scanning backscatter lidar is assessed with the use of two detection schemes, one based on limitations in the detected cloud optical depth and the other based on lidar signal-to-noise ratio. The cloud information retrieved from passive radiometric measurements using a procedure like that used in the International Satellite Cloud Climatology Project is also simulated from the same GCM cloud data set. It is shown that a spaceborne backscatter lidar can improve significantly the retrieval of thin cirrus clouds as well as underlying cloud layers. High-level cloud retrieval from a spaceborne lidar therefore appears as a powerful complement to radiometric measurements for improving our knowledge of actual cloud climatology.

H. Le Treut, M. Forichon, O. Boucher, and Z.-X. Li. Sulfate Aerosol Indirect Effect and CO2 Greenhouse Forcing: EquilibriumResponse of the LMD GCM and Associated Cloud Feedbacks. Journal of Climate, 11:1673-1684, July 1998. [ bib | DOI | ADS link ]

The climate sensitivity to various forcings, and in particular to changes in CO2 and sulfate aerosol concentrations, imposed separately or in a combined manner, is studied with an atmospheric general circulation model coupled to a simple slab oceanic model. The atmospheric model includes a rather detailed treatment of warm cloud microphysics and takes the aerosol indirect effects into account explicitly, although in a simplified manner. The structure of the model response appears to be organized at a global scale, with a partial independence from the geographical structure of the forcing. Atmospheric and surface feedbacks are likely to explain this feature. In particular the cloud feedbacks play a very similar role in the CO2 and aerosol experiments, but with opposite sign. These results strengthen the idea, already apparent from other studies, that, in spite of their different nature and their different geographical and vertical distributions, aerosol may have substantially counteracted the climate effect of greenhouse gases, at least in the Northern Hemisphere, during the twentieth century. When the effects of the two forcings are added, the model response is not symmetric between the two hemispheres. This feature is also consistent with the findings of other modeling groups and has implications for the detection of future climate changes.

O. Boucher, S. E. Schwartz, T. P. Ackerman, T. L. Anderson, B. Bergstrom, B. Bonnel, P. Chýlek, A. Dahlback, Y. Fouquart, Q. Fu, R. N. Halthore, J. M. Haywood, T. Iversen, S. Kato, S. Kinne, A. KirkevâG, K. R. Knapp, A. Lacis, I. Laszlo, M. I. Mishchenko, S. Nemesure, V. Ramaswamy, D. L. Roberts, P. Russell, M. E. Schlesinger, G. L. Stephens, R. Wagener, M. Wang, J. Wong, and F. Yang. Intercomparison of models representing direct shortwave radiative forcing by sulfate aerosols. Journal of Geophysical Research, 103:16979, July 1998. [ bib | DOI | ADS link ]

The importance of aerosols as agents of climate change has recently been highlighted. However, the magnitude of aerosol forcing by scattering of shortwave radiation (direct forcing) is still very uncertain even for the relatively well characterized sulfate aerosol. A potential source of uncertainty is in the model representation of aerosol optical properties and aerosol influences on radiative transfer in the atmosphere. Although radiative transfer methods and codes have been compared in the past, these comparisons have not focused on aerosol forcing (change in net radiative flux at the top of the atmosphere). Here we report results of a project involving 12 groups using 15 models to examine radiative forcing by sulfate aerosol for a wide range of values of particle radius, aerosol optical depth, surface albedo, and solar zenith angle. Among the models that were employed were high and low spectral resolution models incorporating a variety of radiative transfer approximations as well as a line-by-line model. The normalized forcings (forcing per sulfate column burden) obtained with the several radiative transfer models were examined, and the discrepancies were characterized. All models simulate forcings of comparable amplitude and exhibit a similar dependence on input parameters. As expected for a non-light-absorbing aerosol, forcings were negative (cooling influence) except at high surface albedo combined with small solar zenith angle. The relative standard deviation of the zenith-angle-averaged normalized broadband forcing for 15 models was 8% for particle radius near the maximum in this forcing (0.2 μm) and at low surface albedo. Somewhat greater model-to-model discrepancies were exhibited at specific solar zenith angles. Still greater discrepancies were exhibited at small particle radii, and much greater discrepancies were exhibited at high surface albedos, at which the forcing changes sign; in these situations, however, the normalized forcing is quite small. Discrepancies among the models arise from inaccuracies in Mie calculations, differing treatment of the angular scattering phase function, differing wavelength and angular resolution, and differing treatment of multiple scattering. These results imply the need for standardized radiative transfer methods tailored to the direct aerosol forcing problem. However, the relatively small spread in these results suggests that the uncertainty in forcing arising from the treatment of radiative forcing of a well-characterized aerosol at well-specified surface albedo is smaller than some of the other sources of uncertainty in estimates of direct forcing by anthropogenic sulfate aerosols and anthropogenic aerosols generally.

M. Doutriaux-Boucher and G. Sèze. Significant changes between the ISCCP C and D cloud climatologies. Geophysical Research Letters, 25:4193-4196, 1998. [ bib | DOI | ADS link ]

We analyse one year of cloud data from the ISCCP C and D datasets. The two datasets differ by their retrieval algorithms and their definitions of the cloud types defined from the cloud top pressure and cloud optical depth. The differences between the two datasets are first described in terms of the total cloud cover, as well as its repartition in low, middle, and high level cloudiness. We also project the ISCCP C cloud classes into the ISCCP D cloud types to circumvent the problem of different cloud type definitions in the two datasets. The differences between the two datasets are then also investigated in terms of the most frequent cloud type.

O. Boucher. On Aerosol Direct Shortwave Forcing and the Henyey-Greenstein Phase Function. Journal of Atmospheric Sciences, 55:128-134, January 1998. [ bib | DOI | ADS link ]

This technical note extends previous Mie calculations to show that there are complex relationships between the asymmetry parameter g and the upscatter fractions for monodirectional incident radiation (0). Except for intermediate zenith angles and for the upscatter fraction for diffuse radiation, there are significant differences between (0) predicted by the Mie theory and that approximated by a Henyey-Greenstein phase function. While the Henyey-Greenstein phase function is widely used in radiative transfer calculations to characterize aerosol or cloud droplet scattering, it may cause important discrepancies in the computation of the aerosol direct radiative forcing, depending on solar zenith angle, aerosol size, and refractive index. The implications of this work for aerosol and climate-related studies are also discussed.

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