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1998 .

(17 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.

F. Chevallier, F. Chéruy, N. A. Scott, and A. Chédin. A Neural Network Approach for a Fast and Accurate Computation of a Longwave Radiative Budget. Journal of Applied Meteorology, 37:1385--1397, November 1998. [ bib | DOI | ADS link ]

The authors have investigated the possibility of elaborating a new generation of radiative transfer models for climate studies based on the neural network technique. The authors show that their neural network-based model, NeuroFlux, can be used successfully for accurately deriving the longwave radiative budget from the top of the atmosphere to the surface. The reliable sampling of the earth's atmospheric situations in the new version of the TIGR (Thermodynamic Initial Guess Retrieval) dataset, developed at the Laboratoire de Météorologie Dynamique, allows for an efficient learning of the neural networks. Two radiative transfer models are applied to the computation of the radiative part of the dataset: a line-by-line model and a band model. These results have been used to infer the parameters of two neural network-based radiative transfer codes. Both of them achieve an accuracy comparable to, if not better than, the current general circulation model radiative transfer codes, and they are much faster. The dramatic saving of computing time based on the neural network technique (22 times faster compared with the band model), 106 times faster compared with the line-by-line model, allows for an improved estimation of the longwave radiative properties of the atmosphere in general circulation model simulations.

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.

O. P. Sharma, H. Le Treut, G. Sèze, L. Fairhead, and R. Sadourny. Interannual Variations of Summer Monsoons: Sensitivity to Cloud Radiative Forcing. Journal of Climate, 11:1883--1905, August 1998. [ bib | DOI | ADS link ]

The sensitivity of the interannual variations of the summer monsoons to imposed cloudiness has been studied with a general circulation model using the initial conditions prepared from the European Centre for Medium-Range Forecasts analyses of 1 May 1987 and 1988. The cloud optical properties in this global model are calculated from prognostically computed cloud liquid water. The model successfully simulates the contrasting behavior of these two successive monsoons. However, when the optical properties of the observed clouds are specified in the model runs, the simulations show some degradation over India and its vicinity. The main cause of this degradation is the reduced land-sea temperature contrast resulting from the radiative effects of the observed clouds imposed in such simulations. It is argued that the high concentration of condensed water content of clouds over the Indian land areas will serve to limit heating of the land, thereby reducing the thermal contrast that gives rise to a weak Somali jet. A countermonsoon circulation is, therefore, simulated in the vector difference field of 850-hPa winds from the model runs with externally specified clouds. This countermonsoon circulation is associated with an equatorial heat source that is the response of the model to the radiative effects of the imposed clouds. Indeed, there are at least two clear points that can be made: 1) the cloud-SST patterns, together, affect the interannual variability; and 2) with both clouds and SST imposed, the model simulation is less sensitive to initial conditions. Additionally, the study emphasizes the importance of dynamically consistent clouds developing in response to the dynamical, thermal, and moist state of the atmosphere during model integrations.

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.

C. Herbaut, F. Codron, and M. Crépon. Separation of a Coastal Current at a Strait Level: Case of the Strait of Sicily. Journal of Physical Oceanography, 28:1346--1362, July 1998. [ bib | DOI | ADS link ]

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.

P. Barthelet, S. Bony, P. Braconnot, A. Braun, D. Cariolle, E. Cohen-Solal, J.-L. Dufresne, P. Delecluse, M. Déqué, L. Fairhead, M.-A. Filiberti, M. Forichon, J.-Y. Grandpeix, E. Guilyardi, M.-N. Hqussais, M. Imbard, H. Le Treut, C. Lévy, Z. Xin Li, G. Madec, P. Marquet, O. Marti, S. Planton, L. Terray, O. Thual, and S. Valcke. Simulations couplées globales des changements climatiques associés à une augmentation de la teneur atmosphérique en CO 2. Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science, 326:677--684, May 1998. [ bib | DOI | ADS link ]

Two transient CO 2 experiments using two coupled general circulation models developed by the French GASTON group have been realized using the same methodology. No flux corrections at the air-sea interface were used in these experiments. The main features of the present climate are reasonably well captured by both coupled models in the control simulations, although the biases are not the same, The transient CO 2 simulations show a global warming, ranging between 1.6 and 2.0 degC at the time of CO 2 doubling (+ 70 years). These values, and the main geographical characteristics of climate change, are in agreement with previous studies published by other research groups, using either flux corrected or non-flux corrected models.

H. Teitelbaum and R. Sadourny. The rôle of planetary waves in the formation of polar stratospheric clouds. Tellus Series A, 50:302, May 1998. [ bib | DOI | ADS link ]

Several recent works attribute the formation of polar stratospheric clouds (PSCs) to the occurrence of localized orographic waves. Using ECMWF analyses, we investigate the large scale stratospheric flow conditions in a number of cases where PSCs have been detected, both in the Arctic and in the Antarctic. We show that PSCs appear within strong planetary scale uplifts of isentropic surfaces. The adiabatic cooling of air parcels travelling within such planetary scale uplifts while conserving their humidity and trace constituents, seems to be the main mechanism for PSC formation. The PSC distribution would then follow a planetary structure, even though local orographic waves could still play an additional role when planetary scale conditions are met.

F. Billebaud, J. Rosenqvist, E. Lellouch, J.-P. Maillard, T. Encrenaz, and F. Hourdin. Observations of CO in the atmosphere of Mars in the (2-0) vibrational band at 2.35 microns. Astronomy Astrophysics, 333:1092--1099, May 1998. [ bib | ADS link ]

Following our high-resolution infrared observations of CO in the atmosphere of Mars in 1988 and 1989 at 4.7mu m (Billebaud et al., 1992), we recorded new spectra of CO: one covering the whole disk of the planet in 1990 and 4 spectra corresponding to 4 different locations on the planet in 1991. All these spectra were recorded in the (2-0) vibrational band at 2.35mu m. These data allow us to measure the CO abundance and to search for possible middle-scale spatial variations of this abundance in the case of the 1991 spectra. The CO mixing ratio derived from the 1990 data is in good agreement with the values we obtained in 1988 and 1989 (Billebaud et al., 1992), showing a great stability over a period of 3 years, with a value of the CO mixing ratio remaining in the range of 4.2-8.5 x 10(-4) . The results we obtained with the 1991 data also seem to comfort the stability of the CO mixing ratio, although the possible range is somewhat larger (5.5-11.5 x 10(-4) ). This common CO mixing ratio range for the four locations on the planet then tends to exclude the presence of any significant horizontal variations of the CO mixing ratio, even if, from our data, we cannot firmly rule them out.

Z. X. Li. Measurements of interannual variation of the vertical at Jozefoslaw by astrometric and gravimetric observations. , 129:353--355, April 1998. [ bib | DOI | ADS link ]

Jozefoslaw astronomical and geodetical observatory at Warsaw is the place where parallel observations of astrometric latitude (since 1959) and meridional plumb line variations from gravimetric methods (since 1976) have been carried out continuously over the past 20 years. The observational data at this observatory have been analysed to confirm the reality of the plumb line variation results derived from the astrometric latitude residuals. Cross correlation analyses between the results of the two techniques have demonstrated that the interannual plumb line variations along the meridian detected by the astrometric technique is in good accordance with those from the gravimetric technique. The results shown in the paper can be considered as evidence of the existence of non-tidal plumb line variations at interannual time scales, of which the scale is about 0.02” in the case of Jozefoslaw observatory, and the possibility in measuring them by astrometric techniques.

F. Forget, F. Hourdin, and O. Talagrand. CO 2Snowfall on Mars: Simulation with a General Circulation Model. Icarus, 131:302--316, February 1998. [ bib | DOI | ADS link ]

Although CO 2snowfall has never been directly observed on Mars, it has been suggested that such precipitation may explain the puzzling infrared measurements obtained by Mariner 9 and Viking during the polar night in each hemisphere. The radiative effect of the snow would strongly alter the radiative balance of the condensing polar caps and thus the CO 2cycle and the global climate. We have simulated this phenomenon with a general circulation model (GCM). For that purpose, a new parameterization of CO 2condensation in the atmosphere and on the ground has been developed, paying particular attention to mass and energy conservation and allowing for the possible sublimation of sedimenting CO 2ice particles. Atmospheric condensation may result from radiative cooling on the one hand (especially when the atmosphere is dust laden) and from adiabatic cooling in upward motions on the other hand. This latter process can be very efficient locally. On this basis, we have modeled the effect of the CO 2snowfall on the infrared emission by decreasing the local emissivities when atmospheric condensation was predicted by the model. This parameterization is based on physical considerations (radiative transfer through the CO 2ice particles, snow metamorphism on the ground). Without tuning the model parameters, we have been able to accurately reproduce the general behavior of the features observed by Viking in the thermal infrared. These modeling results support the CO 2snowfall scenario suggested from the observations. Overall, this new parameterization, used in combination with the digital terrain model topography and with allowance for a varying atmospheric dust content, allows the GCM to simulate the CO 2condensation-sublimation cycle realistically. In particular, the seasonal variations of the surface pressure recorded by the Viking Landers can now be reproduced without artificially decreasing the condensation rate as was done in previous studies.

J. Dufresne, R. Fournier, and J. Grandpeix. Méthode de Monte-Carlo par échanges pour le calcul des bilans radiatifs au sein d'une cavité 2D remplie de gaz. Academie des Sciences Paris Comptes Rendus Serie B Sciences Physiques, 326:33--38, January 1998. [ bib | DOI | ADS link ]

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.

C. Vanbauce, J. C. Buriez, F. Parol, B. Bonnel, G. Sèze, and P. Couvert. Apparent pressure derived from ADEOS-POLDER observations in the oxygen A-band over ocean. Geophysical Research Letters, 25:3159--3162, 1998. [ bib | DOI | ADS link ]

The POLDER radiometer was on board the ADEOS satellite from August 1996 to June 1997. This instrument measures radiances in eight narrow spectral bands of the visible and near infrared spectrum. Two of them are centered on the O2 A-band in order to infer cloud pressure. By assuming the atmosphere behaves as a pure absorbing medium overlying a perfect reflector, an apparent pressure Papp is derived from POLDER data. For validation purposes, Papp is first compared to the sea-surface pressure Ps for clear-sky conditions; Papp is found to be close to Ps (within ~30 hPa) for measurements in the sunglint region. For overcast conditions, Papp differs from the cloud-top pressure mainly because of multiple scattering inside the cloud. When Papp is compared to the cloud pressure determined from brightness temperature measurements, large differences are observed (typically 180 hPa).

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.

A. Ducharne, K. Laval, and J. Polcher. Sensitivity of the hydrological cycle to the parametrization of soil hydrology in a GCM. Climate Dynamics, 14:307--327, 1998. [ bib | DOI | ADS link ]

The sensitivity of the hydrological cycle to soil hydrology is investigated with the LMD GCM. The reference simulation includes the land-surface scheme SECHIBA, with a two-reservoir scheme for soil water storage and runoff at saturation. We studied a non-linear drainage parametrization, and a distributed surface runoff parametrization, accounting for the subgrid scale variability (SSV) of soil moisture capacity, through a distribution where the shape parameter was b. GCM results show that the drainage parametrization induces significant reductions in soil moisture and evaporation rate compared to the reference simulation. They are related to changes in moisture convergence in the tropics, and to a precipitation decrease in the extratropics. When drainage is implemented, the effect of the SSV parametrization (b=0.2) is also to reduce soil moisture and evaporation rates compared to the simulation with drainage only. These changes are much smaller than the former, but the sensitivity of the hydrological cycle to the SSV parametrization is shown to be larger in dry periods, and to be enhanced by an increase of the shape parameter b. The comparison of simulated total runoffs with observed data shows that the soil hydrological parametrizations does not reduce the GCM systematic errors in the annual water balance, but that they can improve the representation of the total runoff's annual cycle.

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