lmd_EMC31995_abstracts.html

1995 .

O. Boucher and T. L. Anderson. General circulation model assessment of the sensitivity of direct climate forcing by anthropogenic sulfate aerosols to aerosol size and chemistry. Journal of Geophysical Research, 100:26117, December 1995. [ bib | DOI | ADS link ]

Climate response to atmospheric changes brought about by human activity may depend strongly on the geographical and temporal pattern of radiative forcing [Taylor and Penner, 1994]. In the case of aerosols stemming from anthropogenic sulfur emissions, geographical and temporal variations are certainly caused by variations in local mass concentration [Charlson et el., 1991; Kiehl and Briegleb, 1993], but could also arise from variations in the optical properties of sulfate aerosols. Since optical properties (including their relative humidity (RH) variation) depend fundamentally on aerosol size and chemical form and since size and chemical form are features of the aerosol which are not likely to be modeled on the global scale in the near future, geographical and temporal variations in optical properties could represent a stumbling block to accurate climate change forecasts. While extensive measurements of aerosol optical properties are needed to fully assess this problem, a preliminary assessment can be gained by considering the sensitivity of climate forcing to realistic variations in sulfate aerosol size and chemical form. Within a plausible set of assumptions (sulfate aerosol resides in the accumulation mode size range and only interacts with water vapor and ammonia vapor), we show that this sensitivity is fairly small (20%). This low sensitivity derives from a number of compensating factors linking the three optical parameters identified by Charlson et al. [1991]. By implication, these optical parameters, low RH scattering efficiency, the ratio of hemispheric backscatter to total scatter, and the RH dependence of scattering efficiency, should not be treated independently in either theoretical or experimental investigations of direct climate forcing. A suggested logical focus for such investigations is the backscatter efficiency at high RH. If borne out by future research, low sensitivity to sulfate aerosol size and chemistry would mean that direct sulfate climate forcing can be incorporated in global climate models with only a knowledge of sulfate mass concentration. We emphasize, therefore, the need to study the extent to which our assumptions break down, in particular, the fraction of anthropogenic sulfate that forms on coarse mode particles (i.e., those with diameters 1 μm) and the extent and effects of sulfate interactions with other accumulation mode components. Finally, we find that a significant fraction of direct aerosol forcing occurs in cloud-covered regions, according to a simple bulk parameterization.

F. Hourdin, O. Talagrand, R. Sadourny, R. Courtin, D. Gautier, and C. P. Mckay. Numerical simulation of the general circulation of the atmosphere of Titan. Icarus, 117:358-374, October 1995. [ bib | DOI | ADS link ]

The atmospheric circulation of Titan is investigated with a general circulation model. The representation of the large-scale dynamics is based on a grid point model developed and used at Laboratoire de Météorologie Dynamique for climate studies. The code also includes an accurate representation of radiative heating and cooling by molecular gases and haze as well as a parametrization of the vertical turbulent mixing of momentum and potential temperature. Long-term simulations of the atmospheric circulation are presented. Starting from a state of rest, the model spontaneously produces a strong superrotation with prograde equatorial winds (i.e., in the same sense as the assumed rotation of the solid body) increasing from the surface to reach 100 m sec ^-1 near the 1-mbar pressure level. Those equatorial winds are in very good agreement with some indirect observations, especially those of the 1989 occultation of Star 28-Sgr by Titan. On the other hand, the model simulates latitudinal temperature contrasts in the stratosphere that are significantly weaker than those observed by Voyager 1 which, we suggest, may be partly due to the nonrepresentation of the spatial and temporal variations of the abundances of molecular species and haze. We present diagnostics of the simulated atmospheric circulation underlying the importance of the seasonal cycle and a tentative explanation for the creation and maintenance of the atmospheric superrotation based on a careful angular momentum budget.

O. Boucher, H. Le Treut, and M. B. Baker. Precipitation and radiation modeling in a general circulation model: Introduction of cloud microphysical processes. Journal of Geophysical Research, 100:16395, August 1995. [ bib | DOI | ADS link ]

Cloud microphysical processes are introduced in the precipitation parameterization of a general circulation model (GCM). Three microphysical processes are included in this representation of warm cloud precipitation: autoconversion of droplets, collection of droplets by falling raindrops, and evaporation of raindrops falling in clear sky. The mean droplet radius, r, is calculated from the cloud water mixing ratio, which is computed in the model, and the cloud droplet number concentration, N, which is prescribed. The autoconversion rate is set to zero for r r₀, a prescribed threshold mean droplet radius. We investigate the model sensitivity to r₀ and to N, the cloud droplet concentration, which is linked to the concentration of cloud condensation nuclei and is likely to vary. We find that an increase in N leads to an increase in the amount of cloud water stored in the atmosphere. In further experiments the mean droplet radius used in the parameterization of cloud optical properties is calculated in the same way as in the precipitation parameterization in order to bring more consistency between the different schemes. We again investigate the model sensitivity to r₀ and to N and we find that an increase in N significantly enhances cloud albedo.

L. Picon, S. Fongang, G. Seze, and M. Desbois. African and Atlantic short-term climatic variations described from Meteosat water vapor channel. Annales Geophysicae, 13:768-781, July 1995. [ bib | DOI | ADS link ]

Pluriannual series of Meteosat-2 water vapor (WV) images are used to build average maps of decadal and monthly brightness temperatures in the 6.3 m channel. This processing is applied to all the 3-hourly scenes, clear or cloudy, for July 1983 to July 1987. The ISCCP cloudiness analyses confirm that the warmest spots in the monthly WV images correspond to scenes either clear or covered with low clouds, whereas the coldest areas correspond to scenes where cloud tops above 440 hPa frequently occur. The WV statistics are then used to characterize seasonal and interannual variations of both the ITCZ (InterTropical Convergence Zone) and the warm (dry) areas, corresponding to subtropical subsidence. Thanks mainly to the seasonal variations, relationships between the variations in the ITCZ and in dry subtropical areas can be studied. It is shown that, for the Meteosat sector, a wetter subtropical high troposphere is associated with an enhanced activity of the ITCZ, and vice versa. For this area where the north-south assymetry is large, the negative water vapor feedback previously proposed seems not to occur.

O. Boucher and U. Lohmann. The sulfate-CCN-cloud albedo effect. Tellus Series B Chemical and Physical Meteorology B, 47:281, July 1995. [ bib | DOI | ADS link ]

Aerosol particles, such as sulfate aerosols, can act as cloud condensation nuclei (CCN). The CCN spectrum and the water vapour supply in a cloud determine the cloud droplet number concentration (CDNC) and hence the shortwave optical properties of low-level liquid clouds. The capability of anthropogenic aerosols to increase cloud reflectivity and thereby cool the Earth's surface is referred to as the indirect effect of anthropogenic aerosols. To obtain an estimate of this effect on climate, we empirically relate the CDNC, and thus the cloud optical properties, of two general circulation models (GCM) to the sulfate aerosol mass concentration derived from a chemical transport model. Based on a series of model experiments, the normalized globally averaged indirect forcing is about - 1W/m² and ranges from 0.5 to - 1.5W/m² in both GCMs for different experiments. However, it is argued that the total uncertainty of the forcing is certainly larger than this range. The overall agreement between the two climate models is good, although the geographical distributions of the forcing are somewhat different. The highest forcings occur in and off the coasts of the polluted regions of the Northern Hemisphere. The regional distribution of the forcing and the land/sea contrast are very sensitive to the choice of the CDNC-sulfate mass relationship. The general patterns of the forcing, and the appropriateness of the different CDNC-sulfate mass relationships, are assessed. We also examine the simulated droplet effective radii and compare them with satellite retrievals.

S. Bony, J.-P. Duvel, and H. Le Trent. Observed dependence of the water vapor and clear-sky greenhouse effect on sea surface temperature: comparison with climate warming experiments. Climate Dynamics, 11:307-320, July 1995. [ bib | DOI | ADS link ]

This study presents a comparison of the water vapor and clear-sky greenhouse effect dependence on sea surface temperature for climate variations of different types. Firstly, coincident satellite observations and meteorological analyses are used to examine seasonal and interannual variations and to evaluate the performance of a general circulation model. Then, this model is used to compare the results inferred from the analysis of observed climate variability with those derived from global climate warming experiments. One part of the coupling between the surface temperature, the water vapor and the clear-sky greenhouse effect is explained by the dependence of the saturation water vapor pressure on the atmospheric temperature. However, the analysis of observed and simulated fields shows that the coupling is very different according to the type of region under consideration and the type of climate forcing that is applied to the Earth-atmosphere system. This difference, due to the variability of the vertical structure of the atmosphere, is analyzed in detail by considering the temperature lapse rate and the vertical profile of relative humidity. Our results suggest that extrapolating the feedbacks inferred from seasonal and short-term interannual climate variability to longer-term climate changes requires great caution. It is argued that our confidence in climate models' predictions would be increased significantly if the basic physical processes that govern the variability of the vertical structure of the atmosphere, and its relation to the large-scale circulation, were better understood and simulated. For this purpose, combined observational and numerical studies focusing on physical processes are needed.

O. Boucher. GCM Estimate of the Indirect Aerosol Forcing Using Satellite-Retrieved Cloud Droplet Effective Radii. Journal of Climate, 8:1403-1409, May 1995. [ bib | DOI | ADS link ]

In a recent paper, Han et al. analyzed satellite data radiances to retrieve cloud droplet effective radii and reported significant interhemispheric differences for both maritime and continental clouds. The mean cloud droplet radius in the Northern Hemisphere is smaller than in the Southern Hemisphere by about 0.7 [mgr]m. This hemispheric contrast suggests the presence of an aerosol effect on cloud droplet size and is consistent with higher cloud condensation nuclei number concentration in the Northern Hemisphere due to anthropogenic production of aerosol precursors. In the present study, we constrain a climate model with the satellite retrievals of Han et al. and discuss the climate forcing that can be inferred from the observed distribution of cloud droplet radius. Based on two sets of experiments, this sensitivity study suggests that the indirect radiative forcing by anthropogenic aerosols could he about 0.6 or 1 W m² averaged in the 0deg-50degN latitude band. The uncertainty of these estimates is difficult to a assess but is at least 50%.

F. Hourdin, F. Forget, and O. Talagrand. The sensitivity of the Martian surface pressure and atmospheric mass budget to various parameters: A comparison between numerical simulations and Viking observations. Journal of Geophysical Research, 100:5501-5523, March 1995. [ bib | DOI | ADS link ]

The sensitvity of the Martian atmospheric circulation to a number of poorly known or strongly varying parameters (surface roughness length, atmospheric optical depth, CO2 ice albedo, and thermal emissivity) is investigated through experiments performed with the Martian version of the atmospheric general circulation model of Laboratoire de Meteorologie Dynamique, with a rather coarse horizontal resolution (a grid with 32 points in longitude and 24 points in latitude). The results are evaluated primarily on the basis of comparisons with the surface pressure records of the Viking mission. To that end, the records are decomposed into long-period seasonal variations due to mass exchange with the polar caps and latitudinal redistribution of mass, and short-period variations due to transient longitudinally propagating waves. The sensitivty experiments include a 5-year control simulation and shorter simulations (a little longer than 1 year) performed with 'perturbed' parameter values. The main conclusions are that (1) a change of horizontal resolution (twice as many points in each direction) mostly affects the transient waves, (2) surface roughness lengths have a significant impact on the near-suface wind and, as a matter of consequence, on the latitudinal redistribution of mass, (3) atmospheric dust optical depth has a significant impact on radiative balance and dynamics, and (4) CO2 ice albedo and thermal emissivity strongly influence mass exchange between the atmosphere and the polar caps. In view of this last conclusion, an automatic procedure is implemented through which the albedo and emissivity of each of the two polar caps are determined, together with the total (i.e., including the caps) atmospheric CO2 content, in such a way as to get the closest fit of the model to the Viking pressure measurements.

R. C. Raghava, K. Laval, R. Sadourny, and J. Polcher. Atmospheric response to tropical denuding of vegetation. Atmospheric Environment, 29:1963-2000, 1995. [ bib | DOI | ADS link ]

Two simulations of atmospheric circulations during June, July and August 1988 have been made with LMD Atmospheric General Circulation Model using a classified vegetation global cover with and without the tropical vegetation separately. The initial conditions prepared from ECMWF analysed data were used, while the Reynolds' monthly blended analysis, i.e., the blend of in situ, AVHRR satellite and ice data, were taken to prescibe the sea surface temperatures. The global charts of mean monthly precipitation and associated velocity potentials at 200 and 850 mb have been compared and analysed for June, July and August 1988. The temporal evolutions of precipitation averaged over a specific region of Indian summer monsoon during its regime from onset to retreat have also been discussed. Consequently, a pronounced impact of tropical vegetation on the precipitation has been observed so as to characterise a forest as one of the local rain inducing agents. Moreover, the tropical vegetation appears to modulate the Indian summer monsoon also for the contributive precipitation over India.

L. Picon and M. Desbois. High level moisture observations and derived parameters from METEOSAT and other geostationary satellites. Advances in Space Research, 16:73-86, 1995. [ bib | DOI | ADS link ]

This paper summarizes the interpretation and the main applications of the water vapor Meteosat channel. Some comparisons are made with GOES and GMS. The water vapor channel allows to detect the variations of the mid-tropospheric humidity between about 300 and 600 hPa over both sea and land. The upper tropospheric humidity operationaly producted by the ESOC is compared with humidity obtained from GOES data. The second kind of studies involves tropospheric dynamics. Results of the wind extraction method using the water vapor Meteosat data are shown. The usefulness of these data for the height assignment of cloud tracers and for the satellite rainfall estimation is also discussed. Finally some climatic studies performed with water vapor data are presented.