2004 .

(23 publications)

J. Quaas, O. Boucher, J.-L. Dufresne, and H. Treut. Impacts of greenhouse gases and aerosol direct and indirect effects on clouds and radiation in atmospheric GCM simulations of the 1930 1989 period. Climate Dynamics, 23:779-789, December 2004. [ bib | DOI | ADS link ]

Among anthropogenic perturbations of the Earths atmosphere, greenhouse gases and aerosols are considered to have a major impact on the energy budget through their impact on radiative fluxes. We use three ensembles of simulations with the LMDZ general circulation model to investigate the radiative impacts of five species of greenhouse gases (CO2, CH4, N2O, CFC-11 and CFC-12) and sulfate aerosols for the period 1930 1989. Since our focus is on the atmospheric changes in clouds and radiation from greenhouse gases and aerosols, we prescribed sea-surface temperatures in these simulations. Besides the direct impact on radiation through the greenhouse effect and scattering of sunlight by aerosols, strong radiative impacts of both perturbations through changes in cloudiness are analysed. The increase in greenhouse gas concentration leads to a reduction of clouds at all atmospheric levels, thus decreasing the total greenhouse effect in the longwave spectrum and increasing absorption of solar radiation by reduction of cloud albedo. Increasing anthropogenic aerosol burden results in a decrease in high-level cloud cover through a cooling of the atmosphere, and an increase in the low-level cloud cover through the second aerosol indirect effect. The trend in low-level cloud lifetime due to aerosols is quantified to 0.5 min day-1 decade-1 for the simulation period. The different changes in high (decrease) and low-level (increase) cloudiness due to the response of cloud processes to aerosols impact shortwave radiation in a contrariwise manner, and the net effect is slightly positive. The total aerosol effect including the aerosol direct and first indirect effects remains strongly negative.

F. Hourdin, S. Lebonnois, D. Luz, and P. Rannou. Titan's stratospheric composition driven by condensation and dynamics. Journal of Geophysical Research (Planets), 109:E12005, December 2004. [ bib | DOI | ADS link ]

Atmospheric transport of chemical compounds and organic haze in the stratosphere of Titan is investigated with an axisymmetric general circulation model. It has been shown previously that the meridional circulation, dominated by global Hadley cells, is responsible both for the creation of an intense stratospheric zonal flow and for the accumulation of chemical compounds and haze in high latitudes. The modified composition in turn intensifies the meridional circulation and equator-to-pole thermal contrasts. This paper analyzes in detail the transport processes responsible for the observed vertical and latitudinal variations of atmospheric composition. It is shown that the competition between rapid sinking of air from the upper stratosphere in the winter polar vortex and latitudinal mixing by barotropic planetary waves (parameterized in the model) controls the vertical gradient of chemical compounds. The magnitude of polar enrichment (of a factor 1.4 to 20 depending on the particular species) with respect to low latitudes is mostly controlled by the way the meridional advection increases the concentrations of chemical compounds in the clean air which is rising from the troposphere, where most of the chemical compounds are removed by condensation (the temperature at the tropopause being close to 70 K). The agreement between the observed and simulated contrasts provides an indirect but strong validation of the simulated dynamics, thus confirming the explanation put forward for atmospheric superrotation. It is shown also that by measuring the atmospheric composition, the Cassini-Huygens mission will provide a strong constraint about Titan's atmospheric circulation.

F. Chèruy, A. Speranza, A. Sutera, and N. Tartaglione. Surface winds in the Euro-Mediterranean area: the real resolution of numerical grids. Annales Geophysicae, 22:4043-4048, December 2004. [ bib | DOI | ADS link ]

Surface wind is a variable of great importance in forcing marine waves and circulations, modulating surface fluxes, etc. Surface wind defined on numerical grids is currently used in forecast-analysis, as well as in climatology. Gridded fields, however, suffer for systematic errors associated with the numerical procedures adopted in computing them. In this paper the climatology of surface wind produced by three different numerical models in the European-Mediterranean area is analyzed. The systematic loss of power at the smallest grid-scales appears in the power spectrum of all the different models. Some prototype numerical integrations show that this systematic over-smoothing is due to numerical stabilization operators that represent the main source of the diagnosed error; the error progression in space and time is also analyzed.

V. Eymet, J. L. Dufresne, P. Ricchiazzi, R. Fournier, and S. Blanco. Long-wave radiative analysis of cloudy scattering atmospheres using a net exchange formulation. Atmospheric Research, 72:239-261, November 2004. [ bib | DOI | arXiv | ADS link ]

The Net Exchange Formulation (NEF) is an alternative to the usual radiative transfer equation. It was proposed in 1967 by Green [Q. J. R. Meteorol. Soc. 93 (1967) 371] for atmospheric sciences and by Hottel [H.C. Hottel, A.F. Sarofim. Radiative Transfer McGraw Hill, New York, 1967] for engineering sciences. Until now, the NEF has been used only in a very few cases for atmospheric studies. Recently we have developed a long-wave radiative code based on this formulation for a GCM of the Mars planet. Here, we will present results for the Earth atmosphere, obtained with a Monte Carlo Method based on the NEF. In this method, fluxes are not addressed any more. The basic variables are the net exchange rates (NER) between each pair of atmospheric layer ( i, j), i.e. the radiative power emitted by i and absorbed by j minus the radiative power emitted by j and absorbed by i. The graphical representation of the NER matrix highlights the radiative exchanges that dominate the radiative budget of the atmosphere and allows one to have a very good insight of the radiative exchanges. Results will be presented for clear sky atmospheres with Mid-Latitude Summer and Sub-Arctic Winter temperature profiles, and for the same atmospheres with three different types of clouds. The effect of scattering on long-wave radiative exchanges will also be analysed.

R. Tailleux and J. Y. Grandpeix. On the seemingly incompatible parcel and globally integrated views of the energetics of triggered atmospheric deep convection over land. Quarterly Journal of the Royal Meteorological Society, 130:3223-3243, October 2004. [ bib | DOI | ADS link ]

The energetics of the diurnal cycle of atmospheric deep convection over land remain difficult to understand and simulate accurately with current cumulus parametrizations. Furthermore, a proper formulation has remained elusive owing to seeming incompatibilities between, on the one hand, the parcel view of energetics which relies on such concepts as Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN), and, on the other hand, the globally integrated view, which relies on such concepts as Moist Available Energy (MAE), reference states, and energy conversion terms. While the MAE is intuitively the global counterpart of the parcel-defined CAPE, there seems to be no global analogue to the parcel-defined concept of energy barrier attached to CIN. To gain insights into this issue, a new cost function PE is introduced to quantify the amount of positive or negative energy required for a given sounding to undergo an arbitrary adiabatic rearrangement of mass, and which encompasses both the parcel-defined and global energy concepts as particular cases. The function PE offers a complementary view of the stability and energy characteristics of atmospheric soundings, whose local minima are naturally associated with the reference states of the system. It is established that: (a) MAE is essentially equivalent to CAPE multiplied by a mass conversion factor Mb which scales as the amount of convectively unstable boundary-layer air. Using the available summer 1997 IOP data from the ARM- SGP site, Mb is found to correlate with CAPE, which suggests the existence of a functional relationship between CAPE and MAE; if further confirmed, this result would considerably simplify the computation of MAE. (b) A global counterpart to the parcel-defined concept of energy barrier can only be defined if the system considered admits several reference states, and not one as is classically assumed. In that case, energy barriers naturally arise as the amount of energy required to switch from one reference state to another. In the context of triggered deep convection, there must be at least two reference states: a shallow one, which is the actual state (or a slightly modified one if there is boundary layer CAPE), and a deep one associated with the release of MAE/CAPE; the energy barrier separating these two reference states naturally defines a generalized CIN. In the limited context of the above-mentioned IOP ARM data, it is further shown that: (c) Spatially averaged conditions exhibiting potential instability to deep convection may be associated with individual soundings having widely different stability characteristics, including absolute stability, potential instability, and absolute instability; this suggests that triggered deep convection may not necessarily be the result of a parcel's vertical kinetic energy exceeding its negative buoyancy, but rather from the destruction of convective inhibition (i.e. production of absolute instability) in a local region. (d) A few local soundings exhibit multiple reference states, corresponding roughly to multiple levels of neutral buoyancy. These may allow for convective clouds with cloud-top heights significantly lower than those classically predicted by the undiluted ascent of a boundary-layer air parcel up to its highest level of neutral buoyancy, even in the absence of complex entrainment scenarios.

J. Y. Grandpeix, V. Phillips, and R. Tailleux. Improved mixing representation in Emanuel's convection scheme. Quarterly Journal of the Royal Meteorological Society, 130:3207-3222, October 2004. [ bib | DOI | ADS link ]

Recent empirical and modelling studies suggest that mid-tropospheric relative humidity (RH) is an important controlling factor of deep atmospheric convection, which appears to be underestimated in present cumulus parametrizations. This indicates the possible presence of shortcomings in the way that entrainment is represented in such parametrizations. This matter was explored in the European Cloud Systems project (EUROCS) by means of an idealized humidity experiment in which the main controlling parameter is RH. In the latter study, cloud-resolving model (CRM) experiments suggested that a shallow/deep convection transition occurs when RH crosses a threshold value that ranges from about RH = 50% to RH = 60%. In this paper, we seek to increase the responsiveness of Emanuel's convection scheme to RH, and to reproduce the threshold behaviour of the idealized humidity case, by replacing the original uniform probability density function (PDF) for mixing fractions by a more flexible two-parameter bell-shaped function that allows a wider range of behaviour. The main result is that the parameters of this PDF can be tuned to allow a regime transition to occur near a threshold value of RH 55%. In contrast to CRM results, however, this transition is between two different regimes of deep convection rather than between a shallow and deep regime. Possible ways to obtain a shallow-to-deep transition with Emanuel's scheme are discussed.

S. H. Derbyshire, I. Beau, P. Bechtold, J.-Y. Grandpeix, J.-M. Piriou, J. L. Redelsperger, and P. M. M. Soares. Sensitivity of moist convection to environmental humidity. Quarterly Journal of the Royal Meteorological Society, 130:3055-3079, October 2004. [ bib | DOI | ADS link ]

As part of the EUROCS (EUROpean Cloud Systems study) project, cloud-resolving model (CRM) simulations and parallel single-column model (SCM) tests of the sensitivity of moist atmospheric convection to mid-tropospheric humidity are presented. This sensitivity is broadly supported by observations and some previous model studies, but is still poorly quantified. Mixing between clouds and environment is a key mechanism, central to many of the fundamental differences between convection schemes. Here, we define an idealized quasi-steady 'testbed', in which the large-scale environment is assumed to adjust the local mean profiles on a timescale of one hour. We then test sensitivity to the target profiles at heights above 2 km. Two independent CRMs agree reasonably well in their response to the different background profiles and both show strong deep precipitating convection in the more moist cases, but only shallow convection in the driest case. The CRM results also appear to be numerically robust. All the SCMs, most of which are one-dimensional versions of global climate models (GCMs), show sensitivity to humidity but differ in various ways from the CRMs. Some of the SCMs are improved in the light of these comparisons, with GCM improvements documented elsewhere.

M. Bonazzola and P. H. Haynes. A trajectory-based study of the tropical tropopause region. Journal of Geophysical Research (Atmospheres), 109:D20112, October 2004. [ bib | DOI | ADS link ]

Large ensembles of 90-day backward trajectory calculations from the tropical lower stratosphere are performed for Northern Hemisphere (NH) winters 1997-1998 and 1998-1999 and summer 1999 on the basis of European Center for Medium-Range Weather Forecasts operational analysis data. The calculated trajectories are analyzed to determine patterns of transport and encountered temperatures and implications for lower stratospheric water vapor. For each set of back-trajectories, a troposphere-to-stratosphere (TS) ensemble, originating below 355 K, is identified. Trajectories in the TS ensemble sample the coldest regions of the tropical tropopause region very efficiently. Corresponding water vapor concentrations are calculated using two simple dehydration models, one (model 1) assuming instantaneous dehydration and the other (model 2) taking some account of time delays associated with microphysical processes. Model 1 predicts average concentrations for the TS ensembles of 1.5 and 2.0 ppmv in the two NH winters and 3.8 ppmv in NH summer. Model 2 predicts concentrations that are about 0.5 ppmv larger. The effect of temperature variability along the trajectories is considered and is shown to arise primarily through horizontal advection through strong gradients rather than through temporal variability. A quantitative method is described to assess the efficiency of sampling of cold regions, the roles played by different transport processes, and differences between seasons or years. Both vertical transport (the “stratospheric fountain” effect) and horizontal transport are shown to play important roles in dehydration, with the former more important in NH winter and the latter more important in NH summer. Differences in predicted water vapor between NH winter 1997-1998 (El Niño) and 1998-1999 (La Niña) are due to the warmer region of coldest temperatures in 1997-1998 than in 1998-1999 and to the less efficient sampling of cold temperatures by both horizontal and vertical circulations during the former.

M. S. Reddy, O. Boucher, C. Venkataraman, S. Verma, J.-F. LéOn, N. Bellouin, and M. Pham. General circulation model estimates of aerosol transport and radiative forcing during the Indian Ocean Experiment. Journal of Geophysical Research (Atmospheres), 109:D16205, August 2004. [ bib | DOI | ADS link ]

Aerosol sources, transport, and sinks are simulated, and aerosol direct radiative effects are assessed over the Indian Ocean for the Indian Ocean Experiment (INDOEX) Intensive Field Phase during January to March 1999 using the Laboratoire de Météorologie Dynamique (LMDZT) general circulation model. The model reproduces the latitudinal gradient in aerosol mass concentration and optical depth (AOD). The model-predicted aerosol concentrations and AODs agree reasonably well with measurements but are systematically underestimated during high-pollution episodes, especially in the month of March. The largest aerosol loads are found over southwestern China, the Bay of Bengal, and the Indian subcontinent. Aerosol emissions from the Indian subcontinent are transported into the Indian Ocean through either the west coast or the east coast of India. Over the INDOEX region, carbonaceous aerosols are the largest contributor to the estimated AOD, followed by sulfate, dust, sea salt, and fly ash. During the northeast winter monsoon, natural and anthropogenic aerosols reduce the solar flux reaching the surface by 25 W m-2, leading to 10-15% less insolation at the surface. A doubling of black carbon (BC) emissions from Asia results in an aerosol single-scattering albedo that is much smaller than in situ measurements, reflecting the fact that BC emissions are not underestimated in proportion to other (mostly scattering) aerosol types. South Asia is the dominant contributor to sulfate aerosols over the INDOEX region and accounts for 60-70% of the AOD by sulfate. It is also an important but not the dominant contributor to carbonaceous aerosols over the INDOEX region with a contribution of less than 40% to the AOD by this aerosol species. The presence of elevated plumes brings significant quantities of aerosols to the Indian Ocean that are generated over Africa and Southeast and east Asia.

P. Rannou, F. Hourdin, C. P. McKay, and D. Luz. A coupled dynamics-microphysics model of Titan's atmosphere. Icarus, 170:443-462, August 2004. [ bib | DOI | ADS link ]

We have developed a coupled general circulation model of Titan's atmosphere in which the aerosol haze is treated with a microphysical model and is advected by the winds. The radiative transfer accounts for the non uniform haze distribution and, in turn, drives the dynamics. We analyze the GCM results, especially focusing on the difference between a uniform haze layer and a haze layer coupled to the dynamics. In the coupled simulation the aerosols tend to accumulate at the poles, at latitudes higher than 60deg. During winter, aerosols strongly radiate at thermal infrared wavelengths enhancing the cooling rate near the pole. Since this tends to increase the latitudinal gradients of temperature the direct effect of this cooling excess, in contrast to the uncoupled haze case, is to increase the strength of the meridional cells as well as the strength of the zonal winds and profile. This is a positive feedback of the haze on dynamics. The coupled model reproduces observations about the state of the atmosphere better than the uniform haze model, and in addition, the northern polar hood and the detached haze are qualitatively reproduced.

N. Bellouin, O. Boucher, M. Vesperini, and D. E. Tanré. Estimating the direct aerosol radiative perturbation: Impact of ocean surface representation and aerosol non-sphericity. Quarterly Journal of the Royal Meteorological Society, 130:2217-2232, July 2004. [ bib | DOI | ADS link ]

Atmospheric aerosols are now actively studied, in particular because of their radiative and climate impacts. Estimations of the direct aerosol radiative perturbation, caused by extinction of incident solar radiation, usually rely on radiative transfer codes and involve simplifying hypotheses. This paper addresses two approximations which are widely used for the sake of simplicity and limiting the computational cost of the calculations. Firstly, it is shown that using a Lambertian albedo instead of the more rigorous bidirectional reflectance distribution function (BRDF) to model the ocean surface radiative properties leads to large relative errors in the instantaneous aerosol radiative perturbation. When averaging over the day, these errors cancel out to acceptable levels of less than 3% (except in the northern hemisphere winter). The other scope of this study is to address aerosol non-sphericity effects. Comparing an experimental phase function with an equivalent Mie-calculated phase function, we found acceptable relative errors if the aerosol radiative perturbation calculated for a given optical thickness is daily averaged. However, retrieval of the optical thickness of non-spherical aerosols assuming spherical particles can lead to significant errors. This is due to significant differences between the spherical and non-spherical phase functions. Discrepancies in aerosol radiative perturbation between the spherical and non-spherical cases are sometimes reduced and sometimes enhanced if the aerosol optical thickness for the spherical case is adjusted to fit the simulated radiance of the non-spherical case.

M. S. Reddy and O. Boucher. A study of the global cycle of carbonaceous aerosols in the LMDZT general circulation model. Journal of Geophysical Research (Atmospheres), 109:D14202, July 2004. [ bib | DOI | ADS link ]

The global atmospheric cycle of carbonaceous aerosols is simulated in the Laboratoire de Météorologie Dynamique general circulation model, and the subsequent aerosol optical depth is estimated for the period 1997 to 1999. The seasonal and interannual variability in the open biomass burning emissions has been improved by combining existing emission inventories and satellite measured fire counts. The model performance has been thoroughly evaluated against measured aerosol mass concentrations and optical depth in different regions of the globe. At a majority of locations, the modeled mass concentrations of black carbon (BC) at the surface are within a factor of two of observed values. The concentrations of organic carbon (OC) are generally underestimated in comparison to measurements. The discrepancies between model predicted values and measurements are attributable to the difference in time periods between the measurements and model simulations and/or a real underestimation of aerosol emissions in the model. The atmospheric residence times of both BC and OC aerosols are about a week. The hydrophilic fraction of carbonaceous aerosols accounts for about 90% of the total burden. Organic matter (OM) and associated water dominate the optical depth by carbonaceous aerosols with a 86% contribution (global mean of 0.031 at 0.55 μm). Different sensitivity experiments on the transformation time for conversion of hydrophobic to hydrophilic aerosols and emission partitioning show significant changes in the distribution of aerosol burdens and optical depth. The globally averaged burdens change by 15% and residence times are shorter or longer by about 1 day in the various experiments as compared to the control simulation. In all of these experiments the largest sensitivity in aerosol concentrations is found in the remote regions and in the free troposphere (pressure range of 700-400 hPa). Emissions from biomass burning dominate the burden and optical depth of carbonaceous aerosols in the entire SH and NH tropics, while fossil fuel emissions dominate the NH extratropics. On the global scale biomass burning accounts for 78% of the total carbonaceous aerosol burden (BC + OM) followed by natural secondary organic aerosols (SOA)(14%) and fossil fuels (8%). The contributions to corresponding AOD are similar with the largest contribution from biomass burning (76%) followed by natural SOA (14%), and fossil fuels (10%).

A. Sima, A. Paul, and M. Schulz. The Younger Dryasan intrinsic feature of late Pleistocene climate change at millennial timescales. Earth and Planetary Science Letters, 222:741-750, June 2004. [ bib | DOI | ADS link ]

A box model of the North Atlantic Ocean exhibits self-sustained oscillations of the large-scale ocean circulation, which are reminiscent of Dansgaard-Oeschger (DO)-style oscillations. The freshwater forcing of this ocean model depends on mean climate state, represented by global ice volume. This is computed by a one-dimensional ice-sheet model, subject to changes in high-latitude northern hemisphere summer insolation. At low/large ice volume, the ocean-ice system stays in a permanent interstadial/stadial mode. Millennial scale DO-style oscillations result at intermediate ice-volume values, which thus define a “DO window”. During an interglacial-to-glacial transition, this DO window is crossed sufficiently slow to allow for sustained DO-style oscillations to develop. In contrast, during a glacial termination, the system moves relatively rapid through the DO window, resulting in an intermittent re-appearance of DO-style oscillations, which resemble the sequence of events surrounding the Younger Dryas (YD). When forced with modeled ice-volume evolution over the last 800 thousand years, our model predicts Younger-Dryas-type cooling events for each major glacial termination. Accordingly, the Younger Dryas does not appear to be a one-time event.

J. Quaas, O. Boucher, and F.-M. BréOn. Aerosol indirect effects in POLDER satellite data and the Laboratoire de Météorologie Dynamique-Zoom (LMDZ) general circulation model. Journal of Geophysical Research (Atmospheres), 109:D08205, April 2004. [ bib | DOI | ADS link ]

The POLDER-1 instrument was able to measure aerosol and cloud properties for eight months in 1996-1997. We use these observational data for aerosol concentration (the aerosol index), cloud optical thickness, and cloud droplet effective radius to establish statistical relationships among these parameters in order to analyze the first and second aerosol indirect effects. We also evaluate the representation of these effects as parameterized in the Laboratoire de Météorologie Dynamique-Zoom (LMDZ) general circulation model. We find a decrease in cloud top droplet radius with increasing aerosol index in both the model and the observations. Our results are only slightly changed if the analysis is done at fixed cloud liquid water path (LWP) instead of considering all LWP conditions. We also find a positive correlation between aerosol index and cloud liquid water path, which is particularly pronounced over the Northern Hemisphere midlatitudes. This may be interpreted as observational evidence for the second aerosol indirect effect on a large scale. The model-simulated relationship agrees well with that derived from POLDER data. Model simulations show a rather small change in the two relationships if preindustrial rather than present-day aerosol distributions are used. However, when entirely switching off the second aerosol indirect effect in our model, we find a much steeper slope than we do when including it.

F. Sahraoui, J. L. PinçOn, G. Belmont, L. Rezeau, N. Cornilleau-Wehrlin, P. Robert, L. Mellul, J. M. Bosqued, A. Balogh, P. Canu, and G. Chanteur. Correction to “ULF wave identification in the magnetosheath: The k-filtering technique applied to Cluster II data”. Journal of Geophysical Research (Space Physics), 109:A04222, April 2004. [ bib | DOI | ADS link ]

Z. X. Li, D. A. D. Evans, and S. Zhang. A 90deg spin on Rodinia: possible causal links between the Neoproterozoic supercontinent, superplume, true polar wander and low-latitude glaciation. Earth and Planetary Science Letters, 220:409-421, April 2004. [ bib | DOI | ADS link ]

We report here new geochronological and paleomagnetic data from the 80210 Ma Xiaofeng dykes in South China. Together with existing data, these results suggest that Rodinia probably spread from the equator to the polar region at ca. 800 Ma, followed by a rapid ca. 90deg rotation around an axis near Greenland that brought the entire supercontinent to a low-latitude position by ca. 750 Ma. We propose that it was the initiation of a mantle superplume under the polar end of Rodinia that triggered an episode of true polar wander (TPW) which brought the entire supercontinent into equatorial latitudes. An unusually extensive emerged land area at the equator increased both atmospheric CO 2 drawdown and global albedo, which, along with waning plume volcanism led directly to the low-latitude Sturtian glaciation at ca. 750-720 Ma.

V. Barabash, S. Kirkwood, A. Feofilov, and A. Kutepov. Polar mesosphere summer echoes during the July 2000 solar protonevent. Annales Geophysicae, 22:759-771, March 2004. [ bib | DOI | ADS link ]

Available from <A href=“http://www.copernicus.org/site/EGU/annales/22/3/759.htm?FrameEngine=false;”>http://www.copernicus.org/site/EGU/annales/22/3/759.htm?FrameEngine=false;</A>

D. A. Hauglustaine, F. Hourdin, L. Jourdain, M.-A. Filiberti, S. Walters, J.-F. Lamarque, and E. A. Holland. Interactive chemistry in the Laboratoire de Météorologie Dynamique general circulation model: Description and background tropospheric chemistry evaluation. Journal of Geophysical Research (Atmospheres), 109:D04314, February 2004. [ bib | DOI | ADS link ]

We provide a description and evaluation of LMDz-INCA, which couples the Laboratoire de Météorologie Dynamique general circulation model (LMDz) and the Interaction with Chemistry and Aerosols (INCA) model. In this first version of the model a CH4-NOx-CO-O3 chemical scheme representative of the background chemistry of the troposphere is considered. We derive rapid interhemispheric exchange times of 1.13-1.38 years and 0.70-0.82 years, based on surface and pressure-weighted mixing ratios of inert tracers, respectively. The general patterns of the nitrogen deposition are correctly reproduced by the model. However, scavenging processes remain a major source of uncertainty in current models, with convective precipitation playing a key role in the global distribution of soluble species. The global and annual mean methane (7.9 years) and methylchloroform (4.6 years) chemical lifetimes suggest that OH is too high by about 19-25% in the model. This disagreement with previous estimates is attributed to the missing nonmethane hydrocarbons in this version of the model. The model simulates quite satisfactorily the distribution and seasonal cycle of CO at most stations. At several tropical sites and in the Northern Hemisphere during summer, the OH overestimate leads, however, to a too intense CO chemical destruction. LMDz-INCA reproduces fairly well the distribution of ozone throughout most of the troposphere. A main disagreement appears in the Northern Hemisphere upper troposphere during summer, due to a too high tropopause in the GCM. When the GCM winds are relaxed toward assimilated meteorology, a much higher variability is obtained for ozone in the upper troposphere, reflecting more frequent stratospheric intrusions. The stratospheric influx of ozone increases from 523 Tg/yr in the base case simulation to 783 Tg/yr in the nudged version.

N. M. Mahowald and J.-L. Dufresne. Sensitivity of TOMS aerosol index to boundary layer height: Implications for detection of mineral aerosol sources. Geophysical Research Letters, 31:L03103, February 2004. [ bib | DOI | ADS link ]

The TOMS aerosol index (AI) is proposed as a powerful tool in determining the sources of mineral aerosols. The sensitivity of the AI to the height of the aerosol layer has been noted previously, but the implications of this sensitivity for deducing sources has not been explicitly considered. Here, we present a methodology and sensitivity test to show the importance of spatial and temporal variations of the planetary boundary layer height to deducing sources using the AI. These results suggest that while dry topographic low sources may be large sources of desert dust, conclusions eliminating other sources may be premature, especially when these sources occur on the edges of deserts, where boundary layer heights are lower, and human influences potentially more important. The compounding problem of differentiating downwind transport and local sources suggests it may not currently be possible to use the AI to conclusively determine mineral aerosol source regions.

S. Bony, J.-L. Dufresne, H. Le Treut, J.-J. Morcrette, and C. Senior. On dynamic and thermodynamic components of cloud changes. Climate Dynamics, 22:71-86, 2004. [ bib | DOI | ADS link ]

Clouds are sensitive to changes in both the large-scale circulation and the thermodynamic structure of the atmosphere. In the tropics, temperature changes that occur on seasonal to decadal time scales are often associated with circulation changes. Therefore, it is difficult to determine the part of cloud variations that results from a change in the dynamics from the part that may result from the temperature change itself. This study proposes a simple framework to unravel the dynamic and non-dynamic (referred to as thermodynamic) components of the cloud response to climate variations. It is used to analyze the contrasted response, to a prescribed ocean warming, of the tropically-averaged cloud radiative forcing (CRF) simulated by the ECMWF, LMD and UKMO climate models. In each model, the dynamic component largely dominates the CRF response at the regional scale, but this is the thermodynamic component that explains most of the average CRF response to the imposed perturbation. It is shown that this component strongly depends on the behaviour of the low-level clouds that occur in regions of moderate subsidence (e.g. in the trade wind regions). These clouds exhibit a moderate sensitivity to temperature changes, but this is mostly their huge statistical weight that explains their large influence on the tropical radiation budget. Several propositions are made for assessing the sensitivity of clouds to changes in temperature and in large-scale motions using satellite observations and meteorological analyses on the one hand, and mesoscale models on the other hand.

F. Parol, J. C. Buriez, C. Vanbauce, J. Riedi, L. C. Labonnote, M. Doutriaux-Boucher, M. Vesperini, G. Sèze, P. Couvert, M. Viollier, and F. M. Bréon. Review of capabilities of multi-angle and polarization cloud measurements from POLDER. Advances in Space Research, 33:1080-1088, January 2004. [ bib | DOI | ADS link ]

Polarization and directionality of the Earth's reflectances (POLDER) is a multispectral imaging radiometer-polarimeter with a wide field-of-view, a moderate spatial resolution, and a multi-angle viewing capability. It functioned nominally aboard ADEOS1 from November 1996 to June 1997. When the satellite passes over a target, POLDER allows to observe it under up to 14 different viewing directions and in several narrow spectral bands of the visible and near-infrared spectrum (443-910 nm). This new type of multi-angle instruments offers new opportunity for deriving cloud parameters at global scale. The aim of this short overview paper is to point out the main contributions of such an instrument for cloud study through its original instrumental capabilities (multidirectionality, multipolarization, and multispectrality). This is mainly illustrated by using ADEOS 1-POLDER derived cloud parameters which are operationally processed by CNES and are available since the beginning of 1999.

A. Mathieu, A. Lahellec, and A. Weill. Evaluation of a Numerical Weather Forecast Model Using Boundary Layer Cloud-Top Temperature Retrieved from AVHRR. Monthly Weather Review, 132:915, 2004. [ bib | DOI | ADS link ]

O. Boucher, G. Myhre, and A. Myhre. Direct human influence of irrigation on atmospheric water vapour and climate. Climate Dynamics, 22:597-603, 2004. [ bib | DOI | ADS link ]

Human activity increases the atmospheric water vapour content in an indirect way through climate feedbacks. We conclude here that human activity also has a direct influence on the water vapour concentration through irrigation. In idealised simulations we estimate a global mean radiative forcing in the range of 0.03 to +0.1 Wm-2 due to the increase in water vapour from irrigation. However, because the water cycle is embodied in the climate system, irrigation has a more complex influence on climate. We also simulate a change in the temperature vertical profile and a large surface cooling of up to 0.8 K over irrigated land areas. This is of opposite sign than expected from the radiative forcing alone, and this questions the applicability of the radiative forcing concept for such a climatic perturbation. Further, this study shows stronger links than previously recognised between climate change and freshwater scarcity which are environmental issues of paramount importance for the twenty first century.