lmd_EMC32002_abstracts.html

2002 .

E. Cosme, C. Genthon, P. Martinerie, O. Boucher, and M. Pham. The sulfur cycle at high-southern latitudes in the LMD-ZT General Circulation Model. Journal of Geophysical Research (Atmospheres), 107:4690, December 2002. [ bib | DOI | ADS link ]

This modeling study was motivated by the recent publication of year-round records of dimethylsulfide (DMS) and dimethylsulfoxide (DMSO) in Antarctica, completing the available series of sulfate and methanesulfonic acid (MSA). Sulfur chemistry has been incorporated in the Laboratoire de Météorologie Dynamique-Zoom Tracers (LMD-ZT) Atmospheric General Circulation Model (AGCM), with high-resolution and improved physics at high-southern latitudes. The model predicts the concentration of six major sulfur species through emissions, transport, wet and dry deposition, and chemistry in both gas and aqueous phases. Model results are broadly realistic when compared with measurements in air and snow or ice, as well as to results of other modeling studies, at high- and middle-southern latitudes. Atmospheric MSA concentrations are underestimated and DMSO concentrations are overestimated in summer, reflecting the lack of a DMSO heterogeneous sink leading to MSA. Experiments with various recently published estimates of the rate of this sink are reported. Although not corrected in this work, other defects are identified and discussed: DMS concentrations are underestimated in winter, MSA and non-sea-salt (nss) sulfate concentrations may be underestimated at the South Pole, the deposition scheme used in the model may not be adapted to polar regions, and the model does not adequately reproduces interannual variability. Oceanic DMS sources have a major contribution to the variability of sulfur in these regions. The model results suggest that in a large part of central Antarctica ground-level atmospheric DMS concentrations are larger in winter than in summer. At high-southern latitudes, high loads of DMS and DMSO are found and the main chemical sink of sulfur dioxide (SO₂) is aqueous oxidation by ozone (O₃), whereas oxidation by hydrogen peroxide (H₂O₂) dominates at the global scale. A comprehensive modeled sulfur budget of Antarctica is provided.

P. Formenti, O. Boucher, T. Reiner, D. Sprung, M. O. Andreae, M. Wendisch, H. Wex, D. Kindred, M. Tzortziou, A. Vasaras, and C. Zerefos. STAAARTE-MED 1998 summer airborne measurements over the Aegean Sea 2. Aerosol scattering and absorption, and radiative calculations. Journal of Geophysical Research (Atmospheres), 107:4551, November 2002. [ bib | DOI | ADS link ]

Chemical, physical, and optical measurements of aerosol particle properties within an aged biomass-burning plume were performed on board a research aircraft during a profile descent over a ground-based site in northeastern Greece (40deg24'N, 23deg57'E 170 m asl) where continuous measurements of the spectral downwelling solar irradiance (global, direct, and diffuse) are being made. The aerosol optical depth measured at the ground during the time of overflight was significantly enhanced (0.39 at a wavelength of 500 nm) due to a haze layer between 1 and 3.5 km altitude. The dry particle scattering coefficient within the layer was around 80 Mm^-1, and the particle absorption coefficient was around 15 Mm^-1, giving a single scattering albedo of 0.89 at 500 nm (dry state). The black carbon fraction is estimated to account for 6-9% of the total accumulation mode particle mass (1 μm diameter). The increase of the particle scattering coefficient with increasing relative humidity at 500 nm is of the order of 40% for a change in relative humidity from 30 to 80%. The dry, altitude-dependent, particle number size distribution is used as input parameter for radiative transfer calculations of the spectral short-wave, downwelling irradiance at the surface. The agreement between the calculated irradiances and the experimental results from the ground-based radiometer is within 10%, both for the direct and the diffuse components (at 415, 501, and 615 nm). Calculations of the net radiative forcing at the surface and at the top of the atmosphere (TOA) show that due to particle absorption the effect of aerosols is much stronger at the surface than at the TOA. Over sea the net short-wave radiative forcing (daytime average) between 280 nm and 4 μm is up to -64 W m^-2 at the surface and up to -22 W m^-2 at the TOA.

P. Formenti, O. Boucher, T. Reiner, D. Sprung, M. O. Andreae, M. Wendisch, H. Wex, D. Kindred, M. Tzortziou, A. Vasaras, and C. Zerefos. STAAARTE-MED 1998 summer airborne measurements over the Aegean Sea 2. Aerosol scattering and absorption, and radiative calculations. Journal of Geophysical Research (Atmospheres), 107:4451, November 2002. [ bib | DOI | ADS link ]

Chemical, physical, and optical measurements of aerosol particle properties within an aged biomass-burning plume were performed on board a research aircraft during a profile descent over a ground-based site in northeastern Greece (40deg24'N, 23deg57'E; 170 m asl) where continuous measurements of the spectral downwelling solar irradiance (global, direct, and diffuse) are being made. The aerosol optical depth measured at the ground during the time of overflight was significantly enhanced (0.39 at a wavelength of 500 nm) due to a haze layer between 1 and 3.5 km altitude. The dry particle scattering coefficient within the layer was around 80 Mm^-1, and the particle absorption coefficient was around 15 Mm^-1, giving a single scattering albedo of 0.89 at 500 nm (dry state). The black carbon fraction is estimated to account for 6-9% of the total accumulation mode particle mass (1 μm diameter). The increase of the particle scattering coefficient with increasing relative humidity at 500 nm is of the order of 40% for a change in relative humidity from 30 to 80%. The dry, altitude-dependent, particle number size distribution is used as input parameter for radiative transfer calculations of the spectral short-wave, downwelling irradiance at the surface. The agreement between the calculated irradiances and the experimental results from the ground-based radiometer is within 10%, both for the direct and the diffuse components (at 415, 501, and 615 nm). Calculations of the net radiative forcing at the surface and at the top of the atmosphere (TOA) show that due to particle absorption the effect of aerosols is much stronger at the surface than at the TOA. Over sea the net short-wave radiative forcing (daytime average) between 280 nm and 4 μm is up to -64 W m^-2 at the surface and up to -22 W m^-2 at the TOA.

Y. J. Kaufman, D. Tanré, and O. Boucher. A satellite view of aerosols in the climate system. Nature, 419:215-223, September 2002. [ bib | ADS link ]

Anthropogenic aerosols are intricately linked to the climate system and to the hydrologic cycle. The net effect of aerosols is to cool the climate system by reflecting sunlight. Depending on their composition, aerosols can also absorb sunlight in the atmosphere, further cooling the surface but warming the atmosphere in the process. These effects of aerosols on the temperature profile, along with the role of aerosols as cloud condensation nuclei, impact the hydrologic cycle, through changes in cloud cover, cloud properties and precipitation. Unravelling these feedbacks is particularly difficult because aerosols take a multitude of shapes and forms, ranging from desert dust to urban pollution, and because aerosol concentrations vary strongly over time and space. To accurately study aerosol distribution and composition therefore requires continuous observations from satellites, networks of ground-based instruments and dedicated field experiments. Increases in aerosol concentration and changes in their composition, driven by industrialization and an expanding population, may adversely affect the Earth's climate and water supply.

F. Codron and R. Sadourny. Saturation limiters for water vapour advection schemes: impact on orographic precipitation. Tellus Series A, 54:338, August 2002. [ bib | DOI | ADS link ]

P. Rannou, F. Hourdin, and C. P. McKay. A wind origin for Titan's haze structure. Nature, 418:853-856, August 2002. [ bib | ADS link ]

Titan, the largest moon of Saturn, is the only satellite in the Solar System with a dense atmosphere. Titan's atmosphere is mainly nitrogen with a surface pressure of 1.5atmospheres and a temperature of 95K (ref. 1). A seasonally varying haze, which appears to be the main source of heating and cooling that drives atmospheric circulation, shrouds the moon. The haze has numerous features that have remained unexplained. There are several layers, including a `polar hood', and a pronounced hemispheric asymmetry. The upper atmosphere rotates much faster than the surface of the moon, and there is a significant latitudinal temperature asymmetry at the equinoxes. Here we describe a numerical simulation of Titan's atmosphere, which appears to explain the observed features of the haze. The critical new factor in our model is the coupling of haze formation with atmospheric dynamics, which includes a component of strong positive feedback between the haze and the winds.

R. Roca, M. Viollier, L. Picon, and M. Desbois. A multisatellite analysis of deep convection and its moist environment over the Indian Ocean during the winter monsoon. Journal of Geophysical Research (Atmospheres), 107:8012, August 2002. [ bib | DOI | ADS link ]

The aim of this paper is to characterize the deep convective systems over the Indian Ocean during Indian Ocean Experiment (INDOEX) and their relationship to cloudiness and to the Upper Tropospheric Humidity (UTH) of their environment together with the relevant longwave radiation fields. Multisatellite analyses are performed (Meteosat, Scanner for Radiation Budget (ScaRaB), and Special Sensor Microwave Imager (SSM/I)) to measure these environmental parameters. The use of Meteosat water vapor (WV) channel appears very efficient not only for estimating UTH but also for separating high level cloudiness, including thin cirrus, from clear sky and low clouds. The Meteosat infrared (IR) and WV channels are also used for correlating Meteosat and ScaRaB measurements, allowing to retrieve continuously the longwave radiative flux. The longwave flux is used to compute the cloud radiative forcing as well as the clear-sky greenhouse effect. Spatial relationships between upper level cloudiness and UTH are established. A strong positive linear relationship is found suggesting a local moistening of the upper troposphere by convection. The temporal analysis reveals that during the active phase of the intraseasonal oscillation, the longwave cloud radiative forcing reaches a mean value up to 40 W m^-2 over a large region in the open ocean, while the average clear-sky greenhouse effect is in excess of 180 W m^-2. These radiative parameters are strongly correlated with the upper level cloudiness and upper level moisture, respectively. The temporal variability of UTH explains up to 80% of the greenhouse effect variability. The structure of the convective cloud systems is then studied. The observed population of systems spans a wide spectrum of area from 100 to 1,000,000 km². The contribution to the high level cloudiness of the systems with a strong vertical development is dominant. These systems, with at least one convective cell reaching the highest levels (below 210 K), present indices of overshooting tops and are the most horizontally extended. The largest system exhibits an average longwave radiative forcing of around 100 W m^-2. Their contribution to the cloud forcing over the Indian Ocean is overwhelming. The spatial and temporal variability of the systems is finally related to the UTH and to the clear-sky greenhouse effect. Strong correlations are found indicating that these organized convective systems at mesoscale play a leading role in the Indian Ocean climate. The analysis suggests that deeper convection is associated with larger cloud desks with larger cloud radiative forcing. It is also associated with a moister upper troposphere and a larger clear-sky greenhouse effect. These two effects would provide a positive feedback on the surface conditions.

E. Van den Acker, T. Van Hoolst, O. de Viron, P. Defraigne, F. Forget, F. Hourdin, and V. Dehant. Influence of the seasonal winds and the CO₂ mass exchange between atmosphere and polar caps on Mars' rotation. Journal of Geophysical Research (Planets), 107:5055, July 2002. [ bib | DOI | ADS link ]

The Martian atmosphere and the CO₂ polar ice caps exchange mass. This exchange, together with the atmospheric response to solar heating, induces variations of the rotation of Mars. Using the angular momentum budget equation of the system solid-Mars-atmosphere-polar ice caps, the variations of Mars' rotation can be deduced from the variations of the angular momentum of the superficial layer; this later is associated with the winds, that is, the motion term, and with the mass redistribution, that is, the matter term. For the “mean” Martian atmosphere, without global dust storms, total amplitudes of 10 cm on the surface are obtained for both the annual and semiannual polar motion excited by the atmosphere and ice caps. The atmospheric pressure variations are the dominant contribution to these amplitudes. Length-of-day (lod) variations have amplitudes of 0.253 ms for the annual signal and of 0.246 ms for the semiannual signal. The lod variations are mainly associated with changes in the atmospheric contribution to the mass term, partly compensated by the polar ice cap contribution. We computed lod variations and polar motion for three scenarios having different atmospheric dust contents. The differences between the three sets of results for lod variations are about one order of magnitude larger than the expected accuracy of the NEtlander Ionosphere and Geodesy Experiment (NEIGE) for lod. It will thus be possible to constrain the global atmospheric circulation models from the NEIGE measurements.

P. de Rosnay, J. Polcher, M. Bruen, and K. Laval. Impact of a physically based soil water flow and soil-plant interaction representation for modeling large-scale land surface processes. Journal of Geophysical Research (Atmospheres), 107:4118, June 2002. [ bib | DOI | ADS link ]

The aim of this paper is to improve our understanding and the representation of hydrological and energetic exchanges between the soil, the vegetation, and the atmosphere at the continental scale. The soil hydrology of the land surface scheme Schématisation des Echanges Hydriques l'Interface entre la Biosphère et l'Atmosphère (SECHIBA) is improved. It is derived from the physically based hydrological model of the Centre for Water Resources Research and is adapted to the representation of soil-plant-atmosphere interactions at large scale and to the coupling with an atmospheric model. In the new model, soil-plant interactions result from root-soil moisture profile interactions, represented on a fine vertical resolution. This allows a better control of land evapotranspiration by soil-vegetation systems. SECHIBA in this new version takes into account a subgrid-scale variability of soil texture. Different possibilities of interactions between soil and vegetation variabilities allow the representation of various soil-plant-atmosphere systems. It is shown that the distribution of vegetation on the soil and the soil texture influence the way in which soil, plants, and atmosphere interact.

J.-L. Dufresne, C. Gautier, P. Ricchiazzi, and Y. Fouquart. Longwave Scattering Effects of Mineral Aerosols. Journal of Atmospheric Sciences, 59:1959-1966, June 2002. [ bib | DOI | ADS link ]

Scattering in the longwave domain has been neglected in the first generation of radiative codes and is still neglected in most current GCMs. Scattering in the longwave domain does not play any significant role for clear-sky conditions but recent works have shown that it is not negligible for cloudy conditions. This paper highlights the importance of scattering by mineral aerosols in the longwave domain for a wide range of conditions commonly encountered during dust events. The authors show that neglecting scattering may lead to an underestimate of longwave aerosol forcing. This underestimate may reach 50% of the longwave forcing at the top of atmosphere and 15% at the surface for aerosol effective radius greater than a few tenths of a micron. For an aerosol optical thickness of one and for typical atmospheric conditions, the longwave forcing at the top of the atmosphere increases to 8 W m² when scattering effects are included. In contrast, the heating rate inside the atmosphere is only slightly affected by aerosol scattering: neglecting it leads to an underestimate by no more than 10% of the cooling caused by aerosols.

J.-L. Dufresne, L. Fairhead, H. Le Treut, M. Berthelot, L. Bopp, P. Ciais, P. Friedlingstein, and P. Monfray. On the magnitude of positive feedback between future climate change and the carbon cycle. Geophysical Research Letters, 29:1405, May 2002. [ bib | DOI | ADS link ]

We use an ocean-atmosphere general circulation model coupled to land and ocean carbon models to simulate the evolution of climate and atmospheric CO₂ from 1860 to 2100. Our model reproduces the observed global mean temperature changes and the growth rate of atmospheric CO₂ for the period 1860-2000. For the future, we simulate that the climate change due to CO₂ increase will reduce the land carbon uptake, leaving a larger fraction of anthropogenic CO₂ in the atmosphere. By 2100, we estimate that atmospheric CO₂ will be 18% higher due to the climate change impact on the carbon cycle. Such a positive feedback has also been found by Cox et al. [2000]. However, the amplitude of our feedback is three times smaller than the one they simulated. We show that the partitioning between carbon stored in the living biomass or in the soil, and their respective sensitivity to increased CO₂ and climate change largely explain this discrepancy.

O. Boucher and M. Pham. History of sulfate aerosol radiative forcings. Geophysical Research Letters, 29:1308, May 2002. [ bib | DOI | ADS link ]

The history of the global sulfur cycle has been simulated using an emission inventory of SO₂ for 1990 and previously published historical trends in emission on a per country basis. The global- annual-mean radiative forcings due to sulfate aerosols increase (in absolute values) from near-zero and -0.17 Wm^-2 up to -0.4 and -1 Wm^-2 between 1850 and 1990, for the direct and indirect effects, respectively. The forcing efficiency (defined as the ratio of the radiative forcing to the anthropogenic sulfate burden) is fairly constant for the direct effect at -150 W(g sulfate)^-1 but decreases significantly for the indirect effect with increasing sulfate burden. The model results are compared with long-term observations for the period 1980 to 1998 in the U.S. and Europe.

F. Hourdin, F. Couvreux, and L. Menut. Parameterization of the Dry Convective Boundary Layer Based on a Mass Flux Representation of Thermals. Journal of Atmospheric Sciences, 59:1105-1123, March 2002. [ bib | DOI | ADS link ]

Presented is a mass flux parameterization of vertical transport in the convective boundary layer. The formulation of the new parameterization is based on an idealization of thermal cells or rolls. The parameterization is validated by comparison to large eddy simulations (LES). It is also compared to classical boundary layer schemes on a documented case of a well-developed convective boundary layer observed in the Paris area during the Étude et Simulation de la Qualité de l'air en Ile de France (ESQUIF) campaign. For both LES and observations, the new scheme performs better at simulating entrainment fluxes at the top of the convective boundary layer and at near-surface conditions. The explicit representation of mass fluxes allows a direct comparison with campaign observations and opens interesting possibilities for coupling with clouds and deep convection schemes.

T.-J. Zhou and Z.-X. Li. Simulation of the east asian summer monsoon using a variable resolution atmospheric GCM. Climate Dynamics, 19:167-180, February 2002. [ bib | DOI | ADS link ]

The East Asia summer monsoon (EASM) is simulated with a variable resolution global atmospheric general circulation model (GCM) developed at the Laboratoire de Météorologie Dynamique, France. The version used has a local zoom centered on China. This study validates the model's capability in reproducing the fundamental features of the EASM. The monsoon behaviors over East Asia revealed by the ECMWF reanalysis data are also addressed systematically, providing as observational evidence. The mean state of the EASM is generally portrayed well in the model, including the large-scale monsoon airflows, the monsoonal meridional circulation, the cross-equatorial low-level jets, the monsoon trough in the South China Sea, the surface cold high in Australia, and the upper-level northeasterly return flow. While the performance of simulating large-scale monsoonal climate is encouraging, the model's main deficiency lies in the rainfall. The marked rainbelt observed along the Yangtze River Valley is missed in the simulation. This is due to the weakly reproduced monsoonal components in essence and is directly related to the weak western Pacific subtropical high, which leads to a fragile subtropical southwest monsoon on its western flank and results in a weaker convergence of the southwest monsoon flow with the midlatitude westerlies. The excessively westward extension of the high, together with the distorted Indian low, also makes the contribution of the tropical southwest monsoon to the moisture convergence over the Yangtze River Valley too weak in the model. The insufficient plateau heating and the resulting weak land-sea thermal contrast are responsible for the weakly reproduced monsoon. It is the deficiency of the model in handling the low-level cloud cover over the plateau rather than the horizontal resolution and the associated depiction of plateau topography that results in the insufficient plateau heating. Comparison with the simulation employing regular coarser mesh model reveals that the local zoom technique improves, in a general manner, the EASM simulation.

F. M. Bréon, J. C. Buriez, P. Couvert, P. Y. Deschamps, J. L. Deuzé, M. Herman, P. Goloub, M. Leroy, A. Lifermann, C. Moulin, F. Parol, G. Sèze, D. Tanré, C. Vanbauce, and M. Vesperini. Scientific results from the Polarization and Directionality of the Earth's Reflectances (POLDER). Advances in Space Research, 30:2383-2386, 2002. [ bib | DOI | ADS link ]

The POLDER (POlarization and Directionality of the Earth's Reflectances) instrument, developed by the French Space Agency (CNES) has flown on board the ADEOS-1/NASDA platform from November 1996 until June 1997. The sensor has a wide field of view (2400km swath) for collecting global daily data and has multi-angle viewing capability. It measures the solar radiation reflected by the Earth in eight spectral bands. For three of these bands (0.443, 0.670 and 0.865 μm), measurements include the polarization ratio by the use of 3 polarizers. This measurement strategy provides unique information on aerosols, clouds and surfaces.