lmd_EMC32005_abstracts.html

2005 .

N. Bellouin, O. Boucher, J. Haywood, and M. S. Reddy. Global estimate of aerosol direct radiative forcing from satellite measurements. Nature, 438:1138-1141, December 2005. [ bib | DOI | ADS link ]

Atmospheric aerosols cause scattering and absorption of incoming solar radiation. Additional anthropogenic aerosols released into the atmosphere thus exert a direct radiative forcing on the climate system. The degree of present-day aerosol forcing is estimated from global models that incorporate a representation of the aerosol cycles. Although the models are compared and validated against observations, these estimates remain uncertain. Previous satellite measurements of the direct effect of aerosols contained limited information about aerosol type, and were confined to oceans only. Here we use state-of-the-art satellite-based measurements of aerosols and surface wind speed to estimate the clear-sky direct radiative forcing for 2002, incorporating measurements over land and ocean. We use a Monte Carlo approach to account for uncertainties in aerosol measurements and in the algorithm used. Probability density functions obtained for the direct radiative forcing at the top of the atmosphere give a clear-sky, global, annual average of -1.9Wm^-2 with standard deviation, +/- 0.3Wm^-2. These results suggest that present-day direct radiative forcing is stronger than present model estimates, implying future atmospheric warming greater than is presently predicted, as aerosol emissions continue to decline.

F. Vimeux, R. Gallaire, S. Bony, G. Hoffmann, and J. C. H. Chiang. What are the climate controls on @dD in precipitation in the Zongo Valley (Bolivia)? Implications for the Illimani ice core interpretation [rapid communication]. Earth and Planetary Science Letters, 240:205-220, December 2005. [ bib | DOI | ADS link ]

Controversy has surrounded the interpretation of the water isotopic composition ( δD or δ¹⁸O) in tropical and subtropical ice cores in South America. Although recent modeling studies using AGCM have provided useful constraints at interannual time scales, no direct calibration based on modern observations has been achieved. In the context of the recent ice core drilling at Nevado Illimani (16deg39'S-67deg47'W) in Bolivia, we examine the climatic controls on the modern isotopic composition of precipitation in the Zongo Valley, located on the northeast side of the Cordillera Real, at about 55 km from Nevado Illimani. Monthly precipitation samples were collected from September 1999 to August 2004 at various altitudes along this valley. First we examine the local and regional controls on the common δD signal measured along this valley. We show that (1) local temperature has definitely no control on δD variations, and (2) local rainout is a poor factor to explain δD variations. We thus seek regional controls upstream the Valley potentially affecting air masses distillation. Based on backtrajectory calculations and using satellite data (TRMM precipitation, NOAA OLR) and direct observations of precipitation (IAEA/GNIP), we show that moisture transport history and the degree of rainout upstream are more important factors explaining seasonal δD variations. Analysis of a 92-yr simulation from the ECHAM-4 model (T30 version) implemented with water stable isotopes confirms our observations at seasonal time scale and emphasize the role of air masses distillation upstream as a prominent factor controlling interannual δD variations. Lastly, we focus on the isotopic depletion along the valley when air masses are lifted up. Our results suggest that, if the temperature gradient between the base and the top of the Andes was higher by a few degrees during the Last Glacial Maximum (LGM), less than 10% of the glacial to interglacial isotopic variation recorded in the Illimani ice core could be accounted for by this temperature change. It implies that the rest of the variation would originate from wetter conditions along air masses trajectory during LGM.

F. Lott, L. Fairhead, F. Hourdin, and P. Levan. The stratospheric version of LMDz: dynamical climatologies, arctic oscillation, and impact on the surface climate. Climate Dynamics, 25:851-868, December 2005. [ bib | DOI | ADS link ]

A climatology of the stratosphere is determined from a 20-year integration with the stratospheric version of the Atmospheric General Circulation Model LMDz. The model has an upper boundary at near 65 km, uses a Doppler spread non-orographic gravity waves drag parameterization and a subgrid-scale orography parameterization. It also has a Rayleigh damping layer for resolved waves only (not the zonal mean flow) over the top 5 km. This paper describes the basic features of the model and some aspects of its radiative-dynamical climatology. Standard first order diagnostics are presented but some emphasis is given to the models ability to reproduce the low frequency variability of the stratosphere in the winter northern hemisphere. In this model, the stratospheric variability is dominated at each altitudes by patterns which have some similarities with the arctic oscillation (AO). For those patterns, the signal sometimes descends from the stratosphere to the troposphere. In an experiment where the parameterized orographic gravity waves that reach the stratosphere are exaggerated, the model stratosphere in the NH presents much less variability. Although the stratospheric variability is still dominated by patterns that resemble to the AO, the downward influence of the stratosphere along these patterns is near entirely lost. In the same time, the persistence of the surface AO decreases, which is consistent with the picture that this persistence is linked to the descent of the AO signal from the stratosphere to the troposphere. A comparison between the stratospheric version of the model, and its routinely used tropospheric version is also done. It shows that the introduction of the stratosphere in a model that already has a realistic AO persistence can lead to overestimate the actual influence of the stratospheric dynamics onto the surface AO. Although this result is certainly model dependent, it suggests that the introduction of the stratosphere in a GCM also call for a new adjustment of the model parameters that affect the tropospheric variability.

T. L. Anderson, R. J. Charlson, N. Bellouin, O. Boucher, M. Chin, S. A. Christopher, J. Haywood, Y. J. Kaufman, S. Kinne, J. A. Ogren, L. A. Remer, T. Takemura, D. Tanré, O. Torres, C. R. Trepte, B. A. Wielicki, D. M. Winker, and H. Yu. An ”A-Train” Strategy for Quantifying Direct Climate Forcing by Anthropogenic Aerosols. Bulletin of the American Meteorological Society, 86:1795-1809, December 2005. [ bib | DOI | ADS link ]

This document outlines a practical strategy for achieving an observationally based quantification of direct climate forcing by anthropogenic aerosols. The strategy involves a four-step program for shifting the current assumption-laden estimates to an increasingly empirical basis using satellite observations coordinated with suborbital remote and in situ measurements and with chemical transport models. Conceptually, the problem is framed as a need for complete global mapping of four parameters: clear-sky aerosol optical depth δ, radiative efficiency per unit optical depth E, fine-mode fraction of optical depth f_f, and the anthropogenic fraction of the fine mode f_af. The first three parameters can be retrieved from satellites, but correlative, suborbital measurements are required for quantifying the aerosol properties that control E, for validating the retrieval of f_f, and for partitioning fine-mode δ between natural and anthropogenic components. The satellite focus is on the ”A-Train,” a constellation of six spacecraft that will fly in formation from about 2005 to 2008. Key satellite instruments for this report are the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and the Earth's Radiant Energy System (CERES) radiometers on Aqua, the Ozone Monitoring Instrument (OMI) radiometer on Aura, the Polarization and Directionality of Earth's Reflectances (POLDER) polarimeter on the Polarization and Anistropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar (PARASOL), and the Cloud and Aerosol Lider with Orthogonal Polarization (CALIOP) lidar on the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). This strategy is offered as an initial frameworksubject to improvement over timefor scientists around the world to participate in the A-Train opportunity. It is a specific implementation of the Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) program, presented earlier in this journal, which identified the integration of diverse data as the central challenge to progress in quantifying global-scale aerosol effects. By designing a strategy around this need for integration, we develop recommendations for both satellite data interpretation and correlative suborbital activities that represent, in many respects, departures from current practice.

J.-L. Dufresne, J. Quaas, O. Boucher, S. Denvil, and L. Fairhead. Contrasts in the effects on climate of anthropogenic sulfate aerosols between the 20th and the 21st century. Geophysical Research Letters, 32:21703, November 2005. [ bib | DOI | ADS link ]

In this study, we examine the time evolution of the relative contribution of sulfate aerosols and greenhouse gases to anthropogenic climate change. We use the new IPSL-CM4 coupled climate model for which the first indirect effect of sulfate aerosols has been calibrated using POLDER satellite data. For the recent historical period the sulfate aerosols play a key role on the temperature increase with a cooling effect of 0.5 K, to be compared to the 1.4 K warming due to greenhouse gas increase. In contrast, the projected temperature change for the 21st century is remarkably independent of the effects of anthropogenic sulfate aerosols for the SRES-A2 scenario. Those results are interpreted comparing the different radiative forcings, and can be extended to other scenarios. We also highlight that the first indirect effect of aerosol strongly depends on the land surface model by changing the cloud cover.

P. Peylin, P. J. Rayner, P. Bousquet, C. Carouge, F. Hourdin, P. Heinrich, P. Ciais, and A. Contributors. Daily CO₂ flux estimates over Europe from continuous atmospheric measurements: 1, inverse methodology. Atmospheric Chemistry & Physics, 5:3173-3186, November 2005. [ bib | ADS link ]

This paper presents an inverse method for inferring trace gas fluxes at high temporal (daily) and spatial (model grid) resolution from continuous atmospheric concentration measurements. The method is designed for regional applications and for use in intensive campaigns. We apply the method to a one month inversion of fluxes over Europe. We show that the information added by the measurements depends critically on the smoothness constraint assumed among the source components. We show that the initial condition affects the inversion for 20 days, provided one has enough observing sites to constrain regional fluxes. We show that the impact of the far-field fluxes grows throughout the inversion and hence a reasonable global flux field is a prerequisite for a regional inversion.

S. Bony and J.-L. Dufresne. Marine boundary layer clouds at the heart of tropical cloud feedback uncertainties in climate models. Geophysical Research Letters, 32:20806, October 2005. [ bib | DOI | ADS link ]

The radiative response of tropical clouds to global warming exhibits a large spread among climate models, and this constitutes a major source of uncertainty for climate sensitivity estimates. To better interpret the origin of that uncertainty, we analyze the sensitivity of the tropical cloud radiative forcing to a change in sea surface temperature that is simulated by 15 coupled models simulating climate change and current interannual variability. We show that it is in regimes of large-scale subsidence that the model results (1) differ the most in climate change and (2) disagree the most with observations in the current climate (most models underestimate the interannual sensitivity of clouds albedo to a change in temperature). This suggests that the simulation of the sensitivity of marine boundary layer clouds to changing environmental conditions constitutes, currently, the main source of uncertainty in tropical cloud feedbacks simulated by general circulation models.

H. Brogniez, R. Roca, and L. Picon. Evaluation of the distribution of subtropical free tropospheric humidity in AMIP-2 simulations using METEOSAT water vapor channel data. Geophysical Research Letters, 32:19708, October 2005. [ bib | DOI | ADS link ]

In the framework of the Atmospheric Model Intercomparison Project (AMIP) phase 2, we have established a diagnostic of the free tropospheric humidity (FTH) distribution using METEOSAT data over the 1984-1995 period for 14 climate models. The methodology of evaluation follows a two step “model-to-satellite” approach. First the raw METEOSAT “Water Vapor” radiances are simulated from the model profiles of temperature and humidity using the RTTOV-7 radiative transfer model. Second, the radiances are converted into FTH using the same coefficients as in the satellite product offering a direct comparison. The analysis is focused on the dry subtropical areas observed by METEOSAT: the Eastern Mediterranean and the tropical South Atlantic Ocean. Most of the models reproduce the observed seasonal cycle both in terms of phasing and magnitude, despite an overall moist bias. A few models are in close agreement with the satellite data. The magnitude of the satellite estimated inter-annual variability is also generally captured by models. Again, a small subset of models shows close agreement with the observations. This comparison suggests general improvements of the models with respect to the AMIP-1 simulations.

J.-L. Dufresne, R. Fournier, C. Hourdin, and F. Hourdin. Net Exchange Reformulation of Radiative Transfer in the CO₂ 15-μm Band on Mars. Journal of Atmospheric Sciences, 62:3303-3319, September 2005. [ bib | DOI | ADS link ]

The net exchange formulation (NEF) is an alternative to the usual radiative transfer formulation. It was proposed by two authors in 1967, but until now, this formulation has been used only in a very few cases for atmospheric studies. The aim of this paper is to present the NEF and its main advantages and to illustrate them in the case of planet Mars.In the NEF, the radiative fluxes are no longer considered. The basic variables are the net exchange rates between each pair of atmospheric layers i, j. NEF offers a meaningful matrix representation of radiative exchanges, allows qualification of the dominant contributions to the local heating rates, and provides a general framework to develop approximations satisfying reciprocity of radiative transfer as well as the first and second principles of thermodynamics. This may be very useful to develop fast radiative codes for GCMs.A radiative code developed along those lines is presented for a GCM of Mars. It is shown that computing the most important optical exchange factors at each time step and the other exchange factors only a few times a day strongly reduces the computation time without any significant precision lost. With this solution, the computation time increases proportionally to the number N of the vertical layers and no longer proportionally to its square N². Some specific points, such as numerical instabilities that may appear in the high atmosphere and errors that may be introduced if inappropriate treatments are performed when reflection at the surface occurs, are also investigated.

R. Roca, S. Louvet, L. Picon, and M. Desbois. A study of convective systems, water vapor and top of the atmosphere cloud radiative forcing over the Indian Ocean using INSAT-1B and ERBE data. Meteorology and Atmospheric Physics, 90:49-65, September 2005. [ bib | DOI | ADS link ]

The distribution of cloud radiative forcing (CRF) at the top of the atmosphere over the Indian Ocean is investigated using satellite observations. Two key regions are considered: The eastern Indian Ocean and the Bay of Bengal which experience maximum upper-level cloudiness in winter and summer respectively. It is found that longwave CRF in the Bay of Bengal during summer is similar to that over the eastern Indian Ocean during winter. On the other hand shortwave CRF magnitude is larger in the Bay of Bengal. These differences explain the net CRF difference between the two regions. The stronger shortwave forcing seems to be related to the Upper-Level Cloudiness being larger over the Bay than over the eastern Indian Ocean. The reasons for the longwave CRF similarities are analysed in more details. Using the results from a convective system classification method, it is first shown that the longwave radiative properties of the individual systems do not vary much from one region to another. The distribution of the different kind of systems, a proxy for the vertical cloudiness structure, does not either indicate strong difference between the regions. It is then proposed that the substantial precipitable water vapour amount observed over the Bay of Bengal damps the effects of the upper-level cloudiness on radiation compared to the relatively dryer eastern Indian Ocean area; yielding to similar LW CRF in both region despite more Upper-Level Cloudiness over the Bay of Bengal. These observations are supported by idealised radiative transfer computations. The distribution of cloudiness and radiative forcing is then analysed over the whole tropical Indian Ocean for each season. July is characterized by a low longwave CRF regime (relative to January) over the most convectively active part of the Ocean. The non linear damping effect of water vapor on longwave CRF is also shown to contribute to this regime. Overall, this study reaffirms the need for simultaneous documentation of the cloud systems properties together with their moist environment in order to understand the overall net radiative signature of tropical convection at the top of the atmosphere (TOA).

M. Yoshioka, N. Mahowald, J.-L. Dufresne, and C. Luo. Simulation of absorbing aerosol indices for African dust. Journal of Geophysical Research (Atmospheres), 110:18, September 2005. [ bib | DOI | ADS link ]

It has been speculated that the vegetation change and human land use have modulated the dust sources in North Africa and contributed to the observed increase of desert dust since 1960s. However, the roles of surface disturbances on dust generation are not well constrained because of limitations in the available data and models. This study addresses this issue by simulating the Total Ozone Mapping Spectrometer (TOMS) Absorbing Aerosol Indices (AAIs) for model-predicted dust and comparing them with the observations. Model simulations are conducted for natural topographic depression sources with and without adding sources due to vegetation change and cultivation over North Africa. The simulated AAIs capture the previously reported properties of TOMS AAI as well as observed magnitude and spatial distribution reasonably well, although there are some important disagreements with observations. Statistical analyses of spatial and temporal patterns of simulated AAI suggest that simulations using only the natural topographic source capture the observed patterns better than those using 50% of surface disturbance sources. The AAI gradients between Sahara (north) and Sahel (south) suggest that the best mixture of surface disturbance sources is 20-25%, while spatial and temporal correlations suggest that the optimum mixture is 0-15% with the upper bound of 25-40%. However, sensitivity studies show that uncertainties associated with meteorology and source parameterization are large and may undermine the findings derived from the simulations. Additional uncertainties will arise because of model errors in sources, transport, and deposition. Such uncertainties in the model simulations need to be reduced in order to constrain the roles of different types of dust sources better using AAI simulation.

J. Quaas and O. Boucher. Constraining the first aerosol indirect radiative forcing in the LMDZ GCM using POLDER and MODIS satellite data. Geophysical Research Letters, 32:17814, September 2005. [ bib | DOI | ADS link ]

The indirect effects of anthropogenic aerosols are expected to cause a significant radiative forcing of the Earth's climate whose magnitude, however, is still uncertain. Most climate models use parameterizations for the aerosol indirect effects based on so-called “empirical relationships” which link the cloud droplet number concentration to the aerosol concentration. New satellite datasets such as those from the POLDER and MODIS instruments are well suited to evaluate and improve such parameterizations at a global scale. We derive statistical relationships of cloud-top droplet radius and aerosol index (or aerosol optical depth) from satellite retrievals and fit an empirical parameterization in a general circulation model to match the relationships. When applying the fitted parameterizations in the model, the simulated radiative forcing by the first aerosol indirect effect is reduced by 50% as compared to our baseline simulation (down to -0.3 and -0.4 Wm^-2 when using MODIS and POLDER satellite data, respectively).

Y. J. Kaufman, O. Boucher, D. Tanré, M. Chin, L. A. Remer, and T. Takemura. Aerosol anthropogenic component estimated from satellite data. Geophysical Research Letters, 32:17804, September 2005. [ bib | DOI | ADS link ]

Satellite instruments do not measure the aerosol chemical composition needed to discriminate anthropogenic from natural aerosol components. However the ability of new satellite instruments to distinguish fine (submicron) from coarse (supermicron) aerosols over the oceans, serves as a signature of the anthropogenic component and can be used to estimate the fraction of anthropogenic aerosols with an uncertainty of +/-30%. Application to two years of global MODIS data shows that 21 +/- 7% of the aerosol optical thickness over the oceans has an anthropogenic origin. We found that three chemical transport models, used for global estimates of the aerosol forcing of climate, calculate a global average anthropogenic optical thickness over the ocean between 0.030 and 0.036, in line with the present MODIS assessment of 0.033. This increases our confidence in model assessments of the aerosol direct forcing of climate. The MODIS estimated aerosol forcing over cloud free oceans is therefore -1.4 +/- 0.4 W/m².

M. H. Zhang, W. Y. Lin, S. A. Klein, J. T. Bacmeister, S. Bony, R. T. Cederwall, A. D. Del Genio, J. J. Hack, N. G. Loeb, U. Lohmann, P. Minnis, I. Musat, R. Pincus, P. Stier, M. J. Suarez, M. J. Webb, J. B. Wu, S. C. Xie, M.-S. Yao, and J. H. Zhang. Comparing clouds and their seasonal variations in 10 atmospheric general circulation models with satellite measurements. Journal of Geophysical Research (Atmospheres), 110:15, August 2005. [ bib | DOI | ADS link ]

To assess the current status of climate models in simulating clouds, basic cloud climatologies from ten atmospheric general circulation models are compared with satellite measurements from the International Satellite Cloud Climatology Project (ISCCP) and the Clouds and Earth's Radiant Energy System (CERES) program. An ISCCP simulator is employed in all models to facilitate the comparison. Models simulated a four-fold difference in high-top clouds. There are also, however, large uncertainties in satellite high thin clouds to effectively constrain the models. The majority of models only simulated 30-40% of middle-top clouds in the ISCCP and CERES data sets. Half of the models underestimated low clouds, while none overestimated them at a statistically significant level. When stratified in the optical thickness ranges, the majority of the models simulated optically thick clouds more than twice the satellite observations. Most models, however, underestimated optically intermediate and thin clouds. Compensations of these clouds biases are used to explain the simulated longwave and shortwave cloud radiative forcing at the top of the atmosphere. Seasonal sensitivities of clouds are also analyzed to compare with observations. Models are shown to simulate seasonal variations better for high clouds than for low clouds. Latitudinal distribution of the seasonal variations correlate with satellite measurements at 0.9, 0.6-0.9, and -0.2-0.7 levels for high, middle, and low clouds, respectively. The seasonal sensitivities of cloud types are found to strongly depend on the basic cloud climatology in the models. Models that systematically underestimate middle clouds also underestimate seasonal variations, while those that overestimate optically thick clouds also overestimate their seasonal sensitivities. Possible causes of the systematic cloud biases in the models are discussed.

S. Bony and K. A. Emanuel. On the Role of Moist Processes in Tropical Intraseasonal Variability: Cloud-Radiation and Moisture-Convection Feedbacks. Journal of Atmospheric Sciences, 62:2770-2789, August 2005. [ bib | DOI | ADS link ]

Recent observations of the tropical atmosphere reveal large variations of water vapor and clouds at intraseasonal time scales. This study investigates the role of these variations in the large-scale organization of the tropical atmosphere, and in intraseasonal variability in particular. For this purpose, the influence of feedbacks between moisture (water vapor, clouds), radiation, and convection that affect the growth rate and the phase speed of unstable modes of the tropical atmosphere is investigated.Results from a simple linear model suggest that interactions between moisture and tropospheric radiative cooling, referred to as moist-radiative feedbacks, play a significant role in tropical intraseasonal variability. Their primary effect is to reduce the phase speed of large-scale tropical disturbances: by cooling the atmosphere less efficiently during the rising phase of the oscillations (when the atmosphere is moister) than during episodes of large-scale subsidence (when the atmosphere is drier), the atmospheric radiative heating reduces the effective stratification felt by propagating waves and slows down their propagation. In the presence of significant moist-radiative feedbacks, planetary disturbances are characterized by an approximately constant frequency. In addition, moist-radiative feedbacks excite small-scale disturbances advected by the mean flow. The interactions between moisture and convection exert a selective damping effect upon small-scale disturbances, thereby favoring large-scale propagating waves at the expense of small-scale advective disturbances. They also weaken the ability of radiative processes to slow down the propagation of planetary-scale disturbances. This study suggests that a deficient simulation of cloud radiative interactions or of convection-moisture interactions may explain some of the difficulties experienced by general circulation models in simulating tropical intraseasonal oscillations.

R. N. Halthore, D. Crisp, S. E. Schwartz, G. P. Anderson, A. Berk, B. Bonnel, O. Boucher, F.-L. Chang, M.-D. Chou, E. E. Clothiaux, P. Dubuisson, B. Fomin, Y. Fouquart, S. Freidenreich, C. Gautier, S. Kato, I. Laszlo, Z. Li, J. H. Mather, A. Plana-Fattori, V. Ramaswamy, P. Ricchiazzi, Y. Shiren, A. Trishchenko, and W. Wiscombe. Intercomparison of shortwave radiative transfer codes and measurements. Journal of Geophysical Research (Atmospheres), 110:11206, June 2005. [ bib | DOI | ADS link ]

Computation of components of shortwave (SW) or solar irradiance in the surface-atmospheric system forms the basis of intercomparison between 16 radiative transfer models of varying spectral resolution ranging from line-by-line models to broadband and general circulation models. In order of increasing complexity the components are: direct solar irradiance at the surface, diffuse irradiance at the surface, diffuse upward flux at the surface, and diffuse upward flux at the top of the atmosphere. These components allow computation of the atmospheric absorptance. Four cases are considered from pure molecular atmospheres to atmospheres with aerosols and atmosphere with a simple uniform cloud. The molecular and aerosol cases allow comparison of aerosol forcing calculation among models. A cloud-free case with measured atmospheric and aerosol properties and measured shortwave radiation components provides an absolute basis for evaluating the models. For the aerosol-free and cloud-free dry atmospheres, models agree to within 1% (root mean square deviation as a percentage of mean) in broadband direct solar irradiance at surface; the agreement is relatively poor at 5% for a humid atmosphere. A comparison of atmospheric absorptance, computed from components of SW radiation, shows that agreement among models is understandably much worse at 3% and 10% for dry and humid atmospheres, respectively. Inclusion of aerosols generally makes the agreement among models worse than when no aerosols are present, with some exceptions. Modeled diffuse surface irradiance is higher than measurements for all models for the same model inputs. Inclusion of an optically thick low-cloud in a tropical atmosphere, a stringent test for multiple scattering calculations, produces, in general, better agreement among models for a low solar zenith angle (SZA = 30deg) than for a high SZA (75deg). All models show about a 30% increase in broadband absorptance for 30deg SZA relative to the clear-sky case and almost no enhancement in absorptance for a higher SZA of 75deg, possibly due to water vapor line saturation in the atmosphere above the cloud.

M. S. Reddy, O. Boucher, Y. Balkanski, and M. Schulz. Aerosol optical depths and direct radiative perturbations by species and source type. Geophysical Research Letters, 32:12803, June 2005. [ bib | DOI | ADS link ]

We have used the Laboratoire de Météorologie Dynamique General Circulation Model (LMDZT GCM) to estimate the relative contributions of different aerosol source types (i.e., fossil fuels, biomass burning, and “natural”) and aerosol species to the aerosol optical depth (AOD) and direct aerosol radiative perturbation (DARP) at the top-of-atmosphere. The largest estimated contribution to the global annual average AOD (0.12 at 550 nm) is from natural (58%), followed by fossil fuel (26%), and biomass burning (16%) sources. The global annual mean all-sky DARP in the shortwave (SW) spectrum by sulfate, black carbon (BC), organic matter (OM), dust, and sea salt are -0.62, +0.55, -0.33, -0.28, and -0.30 Wm^-2, respectively. The all-sky DARP in the longwave spectrum (LW) is not negligible and is a bit less than half of the SW DARP. The net (i.e., SW+LW) DARP distribution is predominantly negative with patches of positive values over the dust source regions, and off the west coasts of Southern Africa and South and North America. For dust aerosols the SW effect is partially offset by LW greenhouse effect.

M. S. Reddy, O. Boucher, N. Bellouin, M. Schulz, Y. Balkanski, J.-L. Dufresne, and M. Pham. Estimates of global multicomponent aerosol optical depth and direct radiative perturbation in the Laboratoire de Météorologie Dynamique general circulation model. Journal of Geophysical Research (Atmospheres), 110:10, May 2005. [ bib | DOI | ADS link ]

The global cycle of multicomponent aerosols including sulfate, black carbon (BC), organic matter (OM), mineral dust, and sea salt is simulated in the Laboratoire de Météorologie Dynamique general circulation model (LMDZT GCM). The seasonal open biomass burning emissions for simulation years 2000-2001 are scaled from climatological emissions in proportion to satellite detected fire counts. The emissions of dust and sea salt are parameterized online in the model. The comparison of model-predicted monthly mean aerosol optical depth (AOD) at 500 nm with Aerosol Robotic Network (AERONET) shows good agreement with a correlation coefficient of 0.57(N = 1324) and 76% of data points falling within a factor of 2 deviation. The correlation coefficient for daily mean values drops to 0.49 (N = 23,680). The absorption AOD (τ_a at 670 nm) estimated in the model is poorly correlated with measurements (r = 0.27, N = 349). It is biased low by 24% as compared to AERONET. The model reproduces the prominent features in the monthly mean AOD retrievals from Moderate Resolution Imaging Spectroradiometer (MODIS). The agreement between the model and MODIS is better over source and outflow regions (i.e., within a factor of 2). There is an underestimation of the model by up to a factor of 3 to 5 over some remote oceans. The largest contribution to global annual average AOD (0.12 at 550 nm) is from sulfate (0.043 or 35%), followed by sea salt (0.027 or 23%), dust (0.026 or 22%), OM (0.021 or 17%), and BC (0.004 or 3%). The atmospheric aerosol absorption is predominantly contributed by BC and is about 3% of the total AOD. The globally and annually averaged shortwave (SW) direct aerosol radiative perturbation (DARP) in clear-sky conditions is -2.17 Wm^-2 and is about a factor of 2 larger than in all-sky conditions (-1.04 Wm^-2). The net DARP (SW + LW) by all aerosols is -1.46 and -0.59 Wm^-2 in clear- and all-sky conditions, respectively. Use of realistic, less absorbing in SW, optical properties for dust results in negative forcing over the dust-dominated regions.

T. Ngo-Duc, K. Laval, J. Polcher, and A. Cazenave. Contribution of continental water to sea level variations during the 1997-1998 El Niño-Southern Oscillation event: Comparison between Atmospheric Model Intercomparison Project simulations and TOPEX/Poseidon satellite data. Journal of Geophysical Research (Atmospheres), 110:9103, May 2005. [ bib | DOI | ADS link ]

Satellite altimetry from TOPEX/Poseidon (T/P) is used to estimate the variation of the global sea level. This signal, once corrected for steric effects, reflects water mass exchange with the atmosphere and land reservoirs (mainly ice caps, soils and snowpack). It can thus be used to test the capacity of general circulations models (GCMs) to estimate change in land water storage. In this study, we compare the land hydrology contribution to global mean sea level variations during the major 1997-1998 El Niño-Southern Oscillation event from two data sets: (1) the results of the Organizing Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface scheme, developed at the Institute Pierre Simon Laplace, coupled to the Laboratoire de Météorologie Dynamique Atmospheric General Circulation Model (LMD AGCM) and (2) the T/P-based estimates. We show that the seasonal variation of the continental water storage is well represented in the model. The drastic amplitude change between the two contrasted years, 1997 and 1998, observed from satellite altimetry, is also simulated. We analyze the role of each component of simulated water fluxes (precipitation, evaporation, and runoff) in determining the range of annual continental water mass variation and its interannual variability. The difference between the two years, 1997 and 1998, is, for an essential part, due to land precipitation in the 20degN-20degS domain. This analysis emphasizes the important role of tropical regions in interannual variability of climate.

T. Ngo-Duc, K. Laval, J. Polcher, A. Lombard, and A. Cazenave. Effects of land water storage on global mean sea level over the past half century. Geophysical Research Letters, 32:9704, May 2005. [ bib | DOI | ADS link ]

The output of the ORCHIDEE Land Surface Model, driven by a 53-yr (1948-2000) atmospheric forcing data set, was used to estimate the effects of land water storage on global mean sea level. Over the past half century, no significant trend was detected but there is a strong decadal variability in the land water storage, driven by precipitation and originating principally in the tropics. The land water contribution to sea level change over the past 50 yr appears highly anti-correlated with thermal expansion of the oceans. This result suggests that change in ocean heat content influences the global water cycle. It also shows that, at decadal time scale, there is partial compensation in sea level changes between thermal expansion and ocean water mass change due to changes in land water storage.

X. B. Wang, J. C. Tuo, Z. X. Li, and H. Yan. Distribution of radioactive uranium and radon in sedimentary environments. Geochimica et Cosmochimica Acta Supplement, 69:480, May 2005. [ bib | ADS link ]

S. Fueglistaler, M. Bonazzola, P. H. Haynes, and T. Peter. Stratospheric water vapor predicted from the Lagrangian temperature history of air entering the stratosphere in the tropics. Journal of Geophysical Research (Atmospheres), 110:8107, April 2005. [ bib | DOI | ADS link ]

We present results of Lagrangian troposphere-to-stratosphere transport (TST) in the tropics based on trajectory calculations for the period 1979-2001. The trajectories and corresponding temperature histories are calculated from wind and temperature fields provided by the reanalysis data ERA-40 of the European Centre for Medium-Range Weather Forecasts (ECMWF). The water vapor mixing ratio of air entering the tropical stratosphere is calculated from the minimum saturation mixing ratio over ice encountered by each trajectory. We show that this Lagrangian approach, which considers the global-scale to synoptic-scale dynamics of tropical TST but neglects mesoscale dynamics and details of cloud microphysics, substantially improves estimates of stratospheric humidity compared to calculations based on Eulerian mean tropical tropopause temperatures. For the period 1979-2001 we estimate from the Lagrangian calculation that the mean water mixing ratio of air entering the stratosphere is 3.5 ppmv, which is in good agreement with measurements during the same period, ranging from 3.3 ppmv to 4 ppmv, whereas an estimate based on an Eulerian mean calculation is about 6 ppmv. The amplitude of the annual cycle in water vapor mixing ratio at a potential temperature of 400 K in the tropics estimated from the Lagrangian calculation is compared with measurements of water vapor from the Halogen Occultation Experiment (HALOE). For the period 1992-2001, when HALOE measurements and ERA-40 data overlap, we calculate a peak-to-peak amplitude of 1.7 ppmv, in good agreement with 1.6 ppmv seen in HALOE data. On average, the Lagrangian calculations have a moist bias of 0.2 ppmv, equivalent to a warm bias of the Lagrangian cold point of about 0.5 K. We conclude that the Lagrangian calculation based on synoptic-scale velocity and temperature fields yields estimates for stratospheric water vapor in good agreement with observations and that mesoscale and cloud microphysical processes need not be invoked, at first order, to explain annual mean and seasonal variation of water vapor mixing ratios in the tropical lower stratosphere.

T. Ngo-Duc, J. Polcher, and K. Laval. A 53-year forcing data set for land surface models. Journal of Geophysical Research (Atmospheres), 110:6116, March 2005. [ bib | DOI | ADS link ]

As most variables describing the state of the surface are not directly observable, we have to use land surface models in order to reconstruct an estimate of their evolution. These large-scale land surface models often require high-quality forcing data with a subdiurnal sampling. Building these data sets is a major challenge but an essential step for estimating the land surface water budget, which is a crucial part of climate change prediction. To study the interannual variability of surface conditions over the last half century, we have built a 53-year forcing data set, named NCC. NCC has a 6-hourly time step from 1948 to 2000 and a spatial resolution of 1deg × 1deg. It is based on the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project and a number of independent in situ observations. In this study we show the adjustments which need to be applied to the reanalysis and how they impact the simulated continental water balance. The model outputs are validated with the observed discharges of the world's 10 largest rivers to estimate the combined errors of the forcing data and the land surface model. The seasonal and interannual variations of these discharges are used for this validation. Five numerical experiments have been carried out. They used the forcing data sets obtained after each step of data adjustment and the forcing of the Global Soil Wetness Project 2 as inputs for the Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) land surface model. The quality of forcing data is improved after each adjustment. The precipitation correction gives the most important improvement in the simulated river discharges, while the temperature correction has a significant effect only at high latitudes. The radiation correction also improves the forcing quality, especially in term of discharge amplitude. The NCC forcing data set can be used to study the water budget over many areas and catchment basins that have not been yet analyzed in this study. With its period of 53 years, NCC can also be used to evaluate the trends of terrestrial water storage in particular regions.

M. Pham, O. Boucher, and D. Hauglustaine. Changes in atmospheric sulfur burdens and concentrations and resulting radiative forcings under IPCC SRES emission scenarios for 1990-2100. Journal of Geophysical Research (Atmospheres), 110:6112, March 2005. [ bib | DOI | ADS link ]

Simulations of the global sulfur cycle under the IPCC SRES scenarios have been performed. Sulfur dioxide and sulfate burdens, as well as the direct and first indirect radiative forcing (RF) by sulfate aerosols only, are presented for the period 1990 to 2100. By 2100, global sulfur emission rates decline everywhere in all scenarios. At that time, the anthropogenic sulfate burden ranges from 0.34 to 1.03 times the 1990 value of 0.47 Tg S. Direct and indirect global and annually mean RFs relative to the year 1990 are near 0 or positive (range of -0.07 to 0.28 Wm^-2 and 0.01 to 0.38 Wm^-2 for the direct and indirect effects, respectively). For reference these forcings amount respectively to -0.42 and -0.79 Wm^-2 in 1990 relative to preindustrial conditions (around 1750). Sulfur aerosols will therefore induce a smaller cooling effect in 2100 than in 1990 relative to preindustrial conditions. For the period 1990 to 2100, the forcing efficiencies (computed relatively to 1990) are fairly constant for the direct effect (around -160 W (g sulfate)^-1). The forcing efficiencies for the indirect effect are around -200 and -100 W (g sulfate)^-1 for negative and positive burden differences, respectively. This is due to a shift in regional patterns of emissions and a saturation in the indirect effect. The simulated annually averaged SO₂ concentrations for A1B scenario in 2020 are close to air quality objectives for public health in some parts of Africa and exceed these objectives in some parts of China and Korea. Moreover, sulfate deposition rates are estimated to increase by 200% from the present level in East and Southeast Asia. This shows that Asia may experience in the future sulfur-related environmental and human health problems as important as Europe and the United States did in the 1970s.

P. Stier, J. Feichter, S. Kinne, S. Kloster, E. Vignati, J. Wilson, L. Ganzeveld, I. Tegen, M. Werner, Y. Balkanski, M. Schulz, O. Boucher, A. Minikin, and A. Petzold. The aerosol-climate model ECHAM5-HAM. Atmospheric Chemistry & Physics, 5:1125-1156, March 2005. [ bib | ADS link ]

The aerosol-climate modelling system ECHAM5-HAM is introduced. It is based on a flexible microphysical approach and, as the number of externally imposed parameters is minimised, allows the application in a wide range of climate regimes. ECHAM5-HAM predicts the evolution of an ensemble of microphysically interacting internally- and externally-mixed aerosol populations as well as their size-distribution and composition. The size-distribution is represented by a superposition of log-normal modes. In the current setup, the major global aerosol compounds sulfate (SU), black carbon (BC), particulate organic matter (POM), sea salt (SS), and mineral dust (DU) are included. The simulated global annual mean aerosol burdens (lifetimes) for the year 2000 are for SU: 0.80 Tg(S) (3.9 days), for BC: 0.11 Tg (5.4 days), for POM: 0.99 Tg (5.4 days), for SS: 10.5 Tg (0.8 days), and for DU: 8.28 Tg (4.6 days). An extensive evaluation with in-situ and remote sensing measurements underscores that the model results are generally in good agreement with observations of the global aerosol system. The simulated global annual mean aerosol optical depth (AOD) is with 0.14 in excellent agreement with an estimate derived from AERONET measurements (0.14) and a composite derived from MODIS-MISR satellite retrievals (0.16). Regionally, the deviations are not negligible. However, the main patterns of AOD attributable to anthropogenic activity are reproduced.

M. Haeffelin, L. Barthès, O. Bock, C. Boitel, S. Bony, D. Bouniol, H. Chepfer, M. Chiriaco, J. Cuesta, J. Delanoë, P. Drobinski, J.-L. Dufresne, C. Flamant, M. Grall, A. Hodzic, F. Hourdin, F. Lapouge, Y. Lemaître, A. Mathieu, Y. Morille, C. Naud, V. Noël, W. O'Hirok, J. Pelon, C. Pietras, A. Protat, B. Romand, G. Scialom, and R. Vautard. SIRTA, a ground-based atmospheric observatory for cloud and aerosol research. Annales Geophysicae, 23:253-275, February 2005. [ bib | DOI | ADS link ]

Ground-based remote sensing observatories have a crucial role to play in providing data to improve our understanding of atmospheric processes, to test the performance of atmospheric models, and to develop new methods for future space-borne observations. Institut Pierre Simon Laplace, a French research institute in environmental sciences, created the Site Instrumental de Recherche par Télédétection Atmosphérique (SIRTA), an atmospheric observatory with these goals in mind. Today SIRTA, located 20km south of Paris, operates a suite a state-of-the-art active and passive remote sensing instruments dedicated to routine monitoring of cloud and aerosol properties, and key atmospheric parameters. Detailed description of the state of the atmospheric column is progressively archived and made accessible to the scientific community. This paper describes the SIRTA infrastructure and database, and provides an overview of the scientific research associated with the observatory. Researchers using SIRTA data conduct research on atmospheric processes involving complex interactions between clouds, aerosols and radiative and dynamic processes in the atmospheric column. Atmospheric modellers working with SIRTA observations develop new methods to test their models and innovative analyses to improve parametric representations of sub-grid processes that must be accounted for in the model. SIRTA provides the means to develop data interpretation tools for future active remote sensing missions in space (e.g. CloudSat and CALIPSO). SIRTA observation and research activities take place in networks of atmospheric observatories that allow scientists to access consistent data sets from diverse regions on the globe.

E. Cosme, F. Hourdin, C. Genthon, and P. Martinerie. Origin of dimethylsulfide, non-sea-salt sulfate, and methanesulfonic acid in eastern Antarctica. Journal of Geophysical Research (Atmospheres), 110:3302, February 2005. [ bib | DOI | ADS link ]

Ignoring the origin of atmospheric chemicals is often a strong limitation to the full interpretation of their measurement. In this article, this question is addressed in the case of the sulfur species in Antarctica, with an original method of retrotransport of tracers. The retrotransport model is derived from the Laboratoire de Météorologie Dynamique Zoom-Tracers (LMD-ZT) atmospheric general circulation model, optimized for polar climate and expanded to simulate atmospheric sulfur chemistry. For two East Antarctic scientific stations (Dumont d'Urville and Vostok) the effects of transport and chemistry and the influence of oceanic, volcanic, and anthropogenic sources on dimethylsulfide (DMS), non-sea-salt (nss) sulfate, and methanesulfonic acid (MSA) concentrations are evaluated in summer and winter. The oceanic source largely dominates, but other sources can episodically be significant. The meridional origin and the age of DMS, MSA, and biogenic nss sulfate are also estimated. The latitudes of origin of MSA and nss sulfate are similar in summer, but they differ markedly in winter. This is a signature of their different chemical production scheme. Also, the interannual variability of the origin of the sulfur species at Vostok is weak compared to that at Dumont d'Urville. Acknowledging that the DMS concentrations in the ocean have no interannual variability in the model, this result suggests unsurprisingly that inland Antarctic stations may be better observation sites to monitor large-scale DMS bioproductivity variability than coastal sites are. The combination of slower chemistry and more intense atmospheric circulation in winter leads to unexpected results, such as a younger DMS in winter than in summer at Vostok.

F. Codron. Relation between Annular Modes and the Mean State: Southern Hemisphere Summer. Journal of Climate, 18:320-330, January 2005. [ bib | DOI | ADS link ]

The annular modes emerge as the leading mode of extratropical month-to-month climate variability in both hemispheres. Here the influence of the background state on the structure and dynamics of the Southern Hemisphere annular mode (SAM) during the austral summer when the climatology is characterized by a single, well-defined, eddy-driven jet is studied. Subsets of the climatology are constructed for early and late summer, and for contrasting polarities of the ENSO cycle. The analysis is based both on observations and on perpetual-state GCM experiments. The main differences between the subsets involve variations of the latitude of the mean jet.It is found that in all the cases, the SAM is characterized by latitudinal shifts of the jet about its mean position, reinforced by a positive momentum flux feedback from baroclinic waves. This result is consistent with previous studies of the dynamics of the zonally averaged circulation but is found here to hold over all longitudes and for different positions of the mean jet. The low frequency eddies exert a weaker negative feedback upon the mean flow, with a less zonally symmetric structure.The strong differences in the amplitude of the SAM among the various climatologies seem to be determined by a combination of 1) the variance of the ”random” forcing by transient eddies and 2) the strength of the positive feedback component of this forcing. The latter mechanism increases the variance at low frequencies only and lengthens the decorrelation time of zonal-mean zonal wind anomalies. It tends to become stronger when the mean jet moves equatorward.

P. Rannou, S. Lebonnois, F. Hourdin, and D. Luz. Titan atmosphere database. Advances in Space Research, 36:2194-2198, 2005. [ bib | DOI | ADS link ]

We have developed in the last decade a two-dimensional version of the Titan global circulation model LMDZ. This model accounts for multiple coupling occuring on Titan between dynamics, haze, chemistry and radiative transfer. It was successful at explaining many observed features related to atmosphere state (wind, temperature), haze structure and chemical species distributions, recently, an important step in our knowledge about Titan has been done with Cassini and Huygens visits to Titan. In this context, we want to make the results of our model available for the scientific community which is involved in the study of Titan. Such a tool should be useful to give a global frame (spatial and time behaviour of physical quantities) for interpreting ground based telescope observations.

J.-L. Bertaux, O. Korablev, D. Fonteyn, S. Guibert, E. Chassefière, F. Lefèvre, E. Dimarellis, J. P. Dubois, A. Hauchecorne, M. Cabane, P. Rannou, A. C. Levasseur-Regourd, G. Cernogora, E. Quémerais, C. Hermans, G. Kockarts, C. Lippens, M. de Maziere, D. Moreau, C. Muller, E. Neefs, P. C. Simon, F. Forget, F. Hourdin, O. Talagrand, V. I. Moroz, A. Rodin, B. Sandel, and A. Stern. Global structure and composition of the martian atmosphere with SPICAM on Mars express. Advances in Space Research, 35:31-36, 2005. [ bib | DOI | ADS link ]

SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) Light, a light-weight (4.7 kg) UV-IR instrument to be flown on Mars Express orbiter, is dedicated to the study of the atmosphere and ionosphere of Mars. A UV spectrometer (118-320 nm, resolution 0.8 nm) is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation (3.8 kg). It addresses key issues about ozone, its coupling with H₂O, aerosols, atmospheric vertical temperature structure and ionospheric studies. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO⁺, and CO2+, and its direct interaction with the solar wind. An IR spectrometer (1.0-1.7 μm, resolution 0.5-1.2 nm) is dedicated primarily to nadir measurements of H₂O abundances simultaneously with ozone measured in the UV, and to vertical profiling during solar occultation of H₂O, CO₂, and aerosols. The SPICAM Light near-IR sensor employs a pioneering technology acousto-optical tunable filter (AOTF), leading to a compact and light design. Overall, SPICAM Light is an ideal candidate for future orbiter studies of Mars, after Mars Express, in order to study the interannual variability of martian atmospheric processes. The potential contribution to a Mars International Reference Atmosphere is clear.