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2001 .

(16 publications)

M. Bonazzola, L. Picon, H. Laurent, F. Hourdin, G. SèZe, H. Pawlowska, and R. Sadourny. Retrieval of large-scale wind divergences from infrared Meteosat-5 brightness temperatures over the Indian Ocean. Journal of Geophysical Research, 106:28113, November 2001. [ bib | DOI | ADS link ]

Over the tropics the atmospheric general circulation models usually fail in predicting horizontal wind divergence, which is closely related to atmospheric heating and to the vertical exchanges associated with convection. With the aim of forcing atmospheric models we present here a reconstruction of wind divergences based on the links between infrared brightness temperatures, convective activity, and large-scale divergence. In practice, wind divergences are reconstructed from brightness temperatures using correlations obtained from numerical simulations performed with a general circulation model. When building those correlations, a distinction must be made between the brightness temperatures of opaque clouds and those of semitransparent clouds, only the former being directly associated with convection. In order to filter out semitransparent clouds we use radiative thresholds in the water vapor channel in addition to the window channel. We apply our approach to Meteosat-5 data over the Indian Ocean. Comparison with wind divergences reconstructed independently from Meteosat water vapor winds partially validates our retrieval. Comparison with European Center for Medium-Range Weather Forecasts analyses indicates that much can be gained by adding information on the wind divergence in the tropics to force an atmospheric model.

J.-F. Leon, P. Chazette, F. Dulac, J. Pelon, C. Flamant, M. Bonazzola, G. Foret, S. C. Alfaro, H. Cachier, S. Cautenet, E. Hamonou, A. Gaudichet, L. Gomes, J.-L. Rajot, F. Lavenu, S. R. Inamdar, P. R. Sarode, and J. S. Kadadevarmath. Large-scale advection of continental aerosols during INDOEX. Journal of Geophysical Research, 106:28427, November 2001. [ bib | DOI | ADS link ]

In this paper, we present passive and active remote sensing measurements of atmospheric aerosols over the North Indian Ocean (NIO) during the Intensive Field Phase (IFP, January to March 1999) of the Indian Ocean Experiment. The variability of the aerosol load over NIO is discussed based on three-dimentional numerical simulations made at a local scale by use of Regional Atmospheric Modeling System (RAMS) and at a regional scale using the zoomed Laboratoire de Météorologie Dynamique global circulation model (LMD-Z version 3.3). Ground-based measurements of the columnar aerosol optical thickness (AOT) and of surface black carbon (BC) concentration were carried out at two different sites in India: Goa University on the NIO coast and Dharwar 150 km inland. Local-scale investigations point out that the trend in BC concentration at the ground is not correlated with AOT. Lidar profiles obtained both from the surface at Goa and in the NIO from the Mystere-20 research aircraft indicate that a significant contribution to the total AOT (more than 50%) is due to a turbid monsoon layer located between 1 and 3 km height. RAMS simulation shows that the advection of aerosols in the monsoon layer is due to the conjunction of land-sea breeze and topography. We present the regional-scale extent of the aerosol plume in terms of AOT derived from the visible channel of Meteosat-5. During March, most of the Bay of Bengal is overcast by a haze with a monthly average AOT of 0.610.18, and a spatially well-defined aerosol plume is spreading from the Indian west coast to the Intertropical Convergence Zone with an average AOT of 0.490.08. Those values are bigger than in February with AOT at 0.350.18 and 0.370.09 for the Bay of Bengal and the Arabian Sea, respectively. One of the principal findings of this paper is that a significant contribution to the aerosol load over the NIO is due to the advection of continental aerosols from India in a well-identified monsoon layer above the marine boundary layer. Moreover, it is suggested that the increase in biomass burning plays a significant role in the increasing trend in AOT during the winter dry monsoon season.

G. SèZe and H. Pawlowska. Cloud cover analysis with METEOSAT-5 during INDOEX. Journal of Geophysical Research, 106:28415, November 2001. [ bib | DOI | ADS link ]

During the Indian Ocean Experiment (INDOEX), METEOSAT-5 positioned at 63degE provided observation of the visible and infrared radiance field over the Indian Ocean. A cloud classification process using the dynamic cluster method is applied to these data. For the 3 months of the experiment (January-March 1999), daily maps of the cloud cover type are built for 0730 and 0900 UTC. The occurrence frequency of clear sky, low- and high-level cloud cover is examined. These frequencies are compared to the International Satellite Cloud Climatology Program (ISCCP) D1 data set for the period 1984 to 1994. The Indian Ocean region can be classified in three zones. In the north subtropics, clear sky and small cumulus occur at least 90% of the time. Near the coast of India, clear sky is as frequent as 80 to 100%. The Intertropical Convergence Zone, characterized by the occurrence frequency of high-level clouds greater than 30%, spreads from Indonesia to North Madagascar. Near Indonesia, high-level cloud cover occurs more than 55% of the time. In the south subtropics, low cloud cover is the most frequent. In the eastern part the occurrence frequency reaches 80%. This percentage decreases along the western side of the ocean where low clouds break up. Between the African coast and Madagascar, high-level clouds are frequent. The mean spatial features found are in agreement with the ISCCP climatology, except for the eastern part of the south subtropics. A regional comparison shows the difficulty of making the analysis of interannual variations of cloud cover obtained from various cloud cover retrievals applied to different satellite data sets. This difficulty arises from the nonneglectable percentage of satellite pixels which can contain some very small low clouds.

S. Bony and K. A. Emanuel. A Parameterization of the Cloudiness Associated with Cumulus Convection; Evaluation Using TOGA COARE Data. Journal of Atmospheric Sciences, 58:3158-3183, November 2001. [ bib | DOI | ADS link ]

A new parameterization of the cloudiness associated with cumulus convection is proposed for use in climate models. It is based upon the idea that the convection scheme predicts the local concentration of condensed water (the in-cloud water content) produced at the subgrid scale, and that a statistical cloud scheme predicts how this condensed water is spatially distributed within the domain. The cloud scheme uses a probability distribution function (PDF) of the total water whose variance and skewness coefficient are diagnosed from the amount of condensed water produced at the subgrid scale by cumulus convection and at the large scale by supersaturation, from the degree of saturation of the environment, and from the lower bound of the total water distribution that is taken equal to zero.This parameterization is used in a single-column model forced by the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) data, and including the cumulus convection scheme of Emanuel whose humidity prediction has been optimized using these data. Simulations are carried out during the 120 days of operation of the TOGA COARE intensive observation period. The model is able to reproduce some of the main characteristics of the cloudiness observed over the warm pool. This includes the occurrence of different populations of clouds (shallow, midlevel, and deep convective), a minimum cloud cover between 600 and 800 hPa, some relationship between the distribution of cloud tops and the presence of stable atmospheric layers, the formation of long-lasting upper-tropospheric anvils associated with the maturation of the convective cloud systems, and the presence of an extensive layer of thin cirrus clouds just below the tropopause. Nevertheless, shallow-level clouds are likely to be underestimated. The behavior of the predicted cloud fields is consistent with some statistical features suggested by cloud-resolving model simulations of tropical cloud systems over oceans. The radiative fluxes calculated interactively by the model from the predicted profiles of humidity, temperature, and clouds are in reasonable agreement with satellite data. Sea surface temperatures predicted by the model using its own radiative and turbulent fluxes calculated at the ocean surface differ from observations by a few tenths of a degree.Sensitivity tests show that the performance of the cloudiness parameterization does not critically depend upon the choice of the PDF. On the other hand, they show that the prediction of radiative fluxes is improved when the statistical moments of the PDF are predicted from both large-scale variables and subgrid-scale convective activity rather than from large-scale variables only.

S. Lebonnois, D. Toublanc, F. Hourdin, and P. Rannou. Seasonal Variations of Titan's Atmospheric Composition. Icarus, 152:384-406, August 2001. [ bib | DOI | ADS link ]

In order to investigate seasonal variations of the composition of Titan's low stratosphere, we developed a two-dimensional (latitude-altitude) photochemical and transport model. Large-scale advection, hidden in the vertical eddy diffusion for one-dimensional models, is accounted for explicitly. Atmospheric dynamics is prescribed using results of independent numerical simulations of the atmospheric general circulation. Both the mean meridional transport and latitudinal mixing by transient planetary waves are taken into account. Chemistry is based on 284 reactions involving 40 hydrocarbons and nitriles. Photodissociation rates are based on a three-dimensional description of the ultraviolet flux. For most species, the model fits well the latitudinal variations observed by Voyager I giving for the first time a full and self-consistent interpretation of these observations. In particular, the enrichment of the high northern latitudes is attributed to subsidence during the winter preceeding the Voyager encounter. Discrepancies are observed for C 2H 4, HC 3N, and C 2N 2 and are attributed to problems in the chemical scheme. Sensitivity to dynamical parameters is investigated. The vertical eddy diffusion coefficient keeps an important role for the upper atmosphere. The wind strength and horizontal eddy diffusion strongly control the latitudinal behavior of the composition in the low stratosphere, while mean concentrations appear to be essentially controlled by chemistry.

X. K. Qiang, Z. X. Li, C. M. Powell, and H. B. Zheng. Magnetostratigraphic record of the Late Miocene onset of the East Asian monsoon, and Pliocene uplift of northern Tibet. Earth and Planetary Science Letters, 187:83-93, April 2001. [ bib | DOI | ADS link ]

Widespread eolian red clay underlying the Plio-Pleistocene loess-palaeosol succession in northern China has been dated magnetostratigraphically back to 8.35 Ma, indicating that the East Asian monsoon started at about the same time as the Indian monsoon. An initial sedimentation rate of 11 m/Myr increased gradually to 17.5 m/Myr by 6 Ma, and then decreased to 6 m/Myr between 5 Ma and 3.5 Ma. A marked increase in sedimentation rate and grain size beginning between 3.5 Ma and 3.1 Ma indicates that the East Asian winter monsoon strengthened at this time, and intensified further after 2.6 Ma. The temporal coincidence of the stronger winter monsoon and the Pliocene uplift of northwestern Tibet just before the onset of the Northern Hemisphere glaciation indicate that the three events could be causally linked.

F. Codron, A. Vintzileos, and R. Sadourny. Influence of Mean State Changes on the Structure of ENSO in a Tropical Coupled GCM. Journal of Climate, 14:730-742, March 2001. [ bib | DOI | ADS link ]

This study examines the response of the climate simulated by the Institut Pierre Simon Laplace tropical Pacific coupled general circulation model to two changes in parameterization: an improved coupling scheme at the coast, and the introduction of a saturation mixing ratio limiter in the water vapor advection scheme, which improves the rainfall distribution over and around orography. The main effect of these modifications is the suppression of spurious upwelling off the South American coast in Northern Hemisphere summer. Coupled feedbacks then extend this warming over the whole basin in an El Niño-like structure, with a maximum at the equator and in the eastern part of the basin. The mean precipitation pattern widens and moves equatorward as the trade winds weaken.This warmer mean state leads to a doubling of the standard deviation of interannual SST anomalies, and to a longer ENSO period. The structure of the ENSO cycle also shifts from westward propagation in the original simulation to a standing oscillation. The simulation of El Niño thus improves when compared to recent observed events. The study of ENSO spatial structure and lagged correlations shows that these changes of El Niño characteristics are caused by both the increase of amplitude and the modification of the spatial structure of the wind stress response to SST anomalies.These results show that both the mean state and variability of the tropical ocean can be very sensitive to biases or forcings, even geographically localized. They may also give some insight into the mechanisms responsible for the changes in ENSO characteristics due to decadal variability or climate change.

P. Friedlingstein, L. Bopp, P. Ciais, J.-L. Dufresne, L. Fairhead, H. LeTreut, P. Monfray, and J. Orr. Positive feedback between future climate change and the carbon cycle. Geophysical Research Letters, 28:1543-1546, 2001. [ bib | DOI | ADS link ]

Future climate change due to increased atmospheric CO2 may affect land and ocean efficiency to absorb atmospheric CO2. Here, using climate and carbon three-dimensional models forced by a 1% per year increase in atmospheric CO2, we show that there is a positive feedback between the climate system and the carbon cycle. Climate change reduces land and ocean uptake of CO2, respectively by 54% and 35% at 4 × CO2. This negative impact implies that for prescribed anthropogenic CO2 emissions, the atmospheric CO2 would be higher than the level reached if climate change does not affect the carbon cycle. We estimate the gain of this climate-carbon cycle feedback to be 10% at 2 × CO2 and 20% at 4 × CO2. This translates into a 15% higher mean temperature increase.

Z. X. Li. Thermodynamic Air-Sea Interactions and Tropical Atlantic SST Dipole Pattern. Physics and Chemistry of the Earth B, 26:155-157, January 2001. [ bib | DOI | ADS link ]

G. L. Liberti, F. Chéruy, and M. Desbois. Land Effect on the Diurnal Cycle of Clouds over the TOGA COARE Area, as Observed from GMS IR Data. Monthly Weather Review, 129:1500, 2001. [ bib | DOI | ADS link ]

O. Boucher and J. Haywood. On summing the components of radiative forcing of climate change. Climate Dynamics, 18:297-302, 2001. [ bib | DOI | ADS link ]

Radiative forcing is a useful concept in determining the potential influence of a particular mechanism of climate change. However, due to the increased number of forcing agents identified over the past decade, the total radiative forcing is difficult to assess. By assigning a range of probability distribution functions to the individual radiative forcings and using a Monte-Carlo approach, we estimate the total radiative forcing since pre-industrial times including all quantitative radiative forcing estimates to date. The resulting total radiative forcing has a 75-97% probability of being positive (or similarly a 3-25% probability of being negative), with mean radiative forcing ranging from +0.68 to +1.34Wm-2, and median radiative forcing ranging from +0.94 to +1.39Wm-2.

K. Ide, H. Le Treut, Z.-X. Li, and M. Ghil. Atmospheric radiative equilibria. Part II: bimodal solutions for atmospheric optical properties. Climate Dynamics, 18:29-49, 2001. [ bib | DOI | ADS link ]

A simple theoretical model of atmospheric radiative equilibrium is solved analytically to help understand the energetics of maintaining Earth's tropical and subtropical climate. The model climate is constrained by energy balance between shortwave (SW) and longwave (LW) radiative fluxes. Given a complete set of SW and LW optical properties in each atmospheric layer, the model yields a unique equilibrium-temperature profile. In contrast, if the atmospheric temperature profile and SW properties are prescribed, the model yields essentially two distinct LW transmissivity profiles. This bimodality is due to a nonlinear competition between the ascending and descending energy fluxes, as well as to their local conversion to sensible heat in the atmosphere. Idealized slab models that are often used to describe the greenhouse effect are shown to be a special case of our model when this nonlinearity is suppressed. In this special case, only one solution for LW transmissivity is possible. Our model's bimodality in LW transmissivity for given SW fluxes and temperature profile may help explain certain features of Earth's climate: at low latitudes the temperature profiles are fairly homogeneous, while the humidity profiles exhibit a bimodal distribution; one mode is associated with regions of moist-and-ascending, the other with dry-and-subsiding air. The model's analytical results show good agreement with the European Centre for Medium-Range Weather Forecasts' reanalysis data. Sensitivity analysis of the temperature profile with respect to LW transmissivity changes leads to an assessment of the low-latitude climate's sensitivity to the “runaway greenhouse” effect.

M. Webb, C. Senior, S. Bony, and J.-J. Morcrette. Combining ERBE and ISCCP data to assess clouds in the Hadley Centre, ECMWF and LMD atmospheric climate models. Climate Dynamics, 17:905-922, 2001. [ bib | DOI | ADS link ]

This study compares radiative fluxes and cloudiness fields from three general circulation models (the HadAM4 version of the Hadley Centre Unified model, cycle 16r2 of the ECMWF model and version LMDZ 2.0 of the LMD GCM), using a combination of satellite observations from the Earth Radiation Budget Experiment (ERBE) and the International Satellite Cloud Climatology Project (ISCCP). To facilitate a meaningful comparison with the ISCCP C1 data, values of column cloud optical thickness and cloud top pressure are diagnosed from the models in a manner consistent with the satellite view from space. Decomposing the cloud radiative effect into contributions from low-medium- and high-level clouds reveals a tendency for the models' low-level clouds to compensate for underestimates in the shortwave cloud radiative effect caused by a lack of high-level or mid-level clouds. The low clouds fail to compensate for the associated errors in the longwave. Consequently, disproportionate errors in the longwave and shortwave cloud radiative effect in models may be taken as an indication that compensating errors are likely to be present. Mid-level cloud errors in the mid-latitudes appear to depend as much on the choice of the convection scheme as on the cloud scheme. Convective and boundary layer mixing schemes require as much consideration as cloud and precipitation schemes when it comes to assessing the simulation of clouds by models. Two distinct types of cloud feedback are discussed. While there is reason to doubt that current models are able to simulate potential `cloud regime' type feedbacks with skill, there is hope that a model capable of simulating potential `cloud amount' type feedbacks will be achievable once the reasons for the remaining differences between the models are understood.

C. G. Menéndez, A. C. Saulo, and Z.-X. Li. Simulation of South American wintertime climate with a nesting system. Climate Dynamics, 17:219-231, 2001. [ bib | DOI | ADS link ]

A numerical nesting system is developed to simulate wintertime climate of the eastern South Pacific-South America-western South Atlantic region, and preliminary results are presented. The nesting system consists of a large-scale global atmospheric general circulation model (GCM) and a regional climate model (RCM). The latter is driven at its boundaries by the GCM. The particularity of this nesting system is that the GCM itself has a variable horizontal resolution (stretched grid). Our main purpose is to assess the plausibility of such a technique to improve climate representation over South America. In order to evaluate how this nesting system represents the main features of the regional circulation, several mean fields have been analyzed. The global model, despite its relatively low resolution, could simulate reasonably well the more significant large-scale circulation patterns. The use of the regional model often results in improvements, but not universally. Many of the systematic errors of the global model are also present in the regional model, although the biases tend to be rectified. Our preliminary results suggest that nesting technique is a computationally low-cost alternative for simulating regional climate features. However, additional simulations, parametrizations tuning and further diagnosis are clearly needed to represent local patterns more precisely.

F. Codron. Sensitivity of the tropical Pacific to a change of orbital forcing in two versions of a coupled GCM. Climate Dynamics, 17:187-203, 2001. [ bib | DOI | ADS link ]

The changes of the variability of the tropical Pacific ocean forced by a shift of six months in the date of the perihelion are studied using a coupled tropical Pacific ocean/global atmosphere GCM. The sensitivity experiments are conducted with two versions of the atmospheric model, varied by two parametrization changes. The first one concerns the interpolation scheme between the atmosphere and ocean models grids near the coasts, the second one the advection of water vapor in the presence of downstream negative temperature gradients, as encountered in the vicinity of mountains. In the tropical Pacific region, the parametrization differences only have a significant direct effect near the coasts; but coupled feedbacks lead to a 1degC warming of the equatorial cold tongue in the modified (version 2) model, and a widening of the western Pacific large-scale convergence area. The sensitivity of the seasonal cycle of equatorial SST is very different between the two experiments. In both cases, the response to the solar flux forcing is strongly modified by coupled interactions between the SST, wind stress response and ocean dynamics. In the first version, the main feedback is due to anomalous upwelling and leads to westward propagation of SST anomalies; whereas the version 2 model is dominated by an eastward-propagating thermocline mode. The main reason diagnosed for these different behaviors is the atmospheric response to SST anomalies. In the warmer climate simulated by the second version, the wind stress response in the western Pacific is enhanced, and the off-equatorial curl is reduced, both effects favoring eastward propagation through thermocline depth anomalies. The modifications of the simulated seasonal cycle in version 2 lead to a change in ENSO behavior. In the control climate, the interannual variability in the eastern Pacific is dominated by warm events, whereas cold events tend to be the more extreme ones with a shifted perihelion.

E. Chassefière, F. Forget, F. Hourdin, F. Vial, H. Rème, C. Mazelle, D. Vignes, J.-A. Sauvaud, P.-L. Blelly, D. Toublanc, J.-J. Berthelier, J.-C. Cerisier, G. Chanteur, L. Duvet, M. Menvielle, J. Lilensten, O. Witasse, P. Touboul, E. Quèmerais, J.-L. Bertaux, G. Hulot, Y. Cohen, P. Lognonné, J. P. Barriot, G. Balmino, M. Blanc, P. Pinet, M. Parrot, J.-G. Trotignon, M. Moncuquet, J.-L. Bougeret, K. Issautier, E. Lellouch, N. Meyer, C. Sotin, O. Grasset, F. Barlier, C. Berger, P. Tarits, J. Dyment, D. Breuer, T. Spohn, M. Pätzold, K. Sperveslage, P. Gough, A. Buckley, K. Szego, S. Sasaki, S. Smrekar, D. Lyons, M. Acuna, J. Connerney, M. Purucker, R. Lin, J. Luhmann, D. Mitchell, F. Leblanc, R. Johnson, J. Clarke, A. Nagy, D. Young, S. Bougher, G. Keating, R. Haberle, B. Jakosky, R. Hodges, M. Parmentier, H. Waite, and D. Bass. Scientific objectives of the DYNAMO mission. Advances in Space Research, 27:1851-1860, 2001. [ bib | DOI | ADS link ]

DYNAMO is a small Mars orbiter planned to be launched in 2005 or 2007, in the frame of the NASA/ CNES Mars exploration program. It is aimed at improving gravity and magnetic field resolution, in order to better understand the magnetic, geologic and thermal history of Mars, and at characterizing current atmospheric escape, which is still poorly constrained. These objectives are achieved by using a low periapsis orbit, similar to the one used by the Mars Global Surveyor spacecraft during its aerobraking phases. The proposed periapsis altitude for DYNAMO of 120-130 km, coupled with the global distribution of periapses to be obtained during one Martian year of operation, through about 5000 low passes, will produce a magnetic/gravity field data set with approximately five times the spatial resolution of MGS. Additional data on the internal structure will be obtained by mapping the electric conductivity. Low periapsis provides a unique opportunity to investigate the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, therefore atmospheric escape, which may have played a crucial role in removing atmosphere and water from the planet.

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