lmd_EMC32006_abstracts.html

2006 .

F. Hourdin, I. Musat, S. Bony, P. Braconnot, F. Codron, J.-L. Dufresne, L. Fairhead, M.-A. Filiberti, P. Friedlingstein, J.-Y. Grandpeix, G. Krinner, P. Levan, Z.-X. Li, and F. Lott. The LMDZ4 general circulation model: climate performance and sensitivity to parametrized physics with emphasis on tropical convection. Climate Dynamics, 27:787-813, December 2006. [ bib | DOI | ADS link ]

The LMDZ4 general circulation model is the atmospheric component of the IPSL CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtkes convection scheme, used in previous versions, the Emanuels scheme improves the representation of the Hadley Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, these differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtkes parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model.

S. K. Deb, H. C. Upadhyaya, J. Y. Grandpeix, and O. P. Sharma. On convective entrainment in a mass flux cumulus parameterization. Meteorology and Atmospheric Physics, 94:145-152, November 2006. [ bib | DOI | ADS link ]

A new entrainment/detrainment formulation in the Tiedtkes mass flux cumulus parameterization is discussed here. Apart from validating it with observations both in one and three dimensional cases, it is also evaluated in the light of the results from the original Tiedtke scheme and another mass flux scheme due to Emanuel. The proposed analytical profiles of entrainment and detrainment, easier to implement in any mass flux scheme, give reasonable results in GCM experiments.

H. Brogniez, R. Roca, and L. Picon. A clear-sky radiance archive from Meteosat “water vapor” observations. Journal of Geophysical Research (Atmospheres), 111:21109, November 2006. [ bib | DOI | ADS link ]

A long-term archive of clear-sky Meteosat ”water vapor” observations, covering the July 1983 to February 1997 period with a 3 hourly time step and a spatial resolution of 0.625deg, is presented. Cloud clearing is performed using a scene selection procedure based on the International Satellite Cloud Climatology Project DX product. In this procedure low cloud scenes are kept because of the negligible contribution of the low atmospheric layer in this spectral band. Cloud contamination is shown to have little influence on the clear-sky radiance (CSR) field and is mainly confined to the continental Intertropical Convergence Zone with values less than 0.5 K. This scene selection yields to a significantly enhanced sampling with respect to pure clear-sky in the subtropical high regions. Homogenization of the 14 year database is performed in accordance with existing technique. A comparison to the operational radiosondes archive indicates a small bias of 0.3 K that is stable throughout the period. A first analysis of the CSR variability reveals that the intraseasonal variance over the subtropical dry regions has a strong seasonal cycle in the Northern Hemisphere that is not observed in the Southern Hemisphere. Such a data set completes the ones currently available to document the water vapor variability of the troposphere from climatic down to regional and daily scales.

G. Krinner, O. Boucher, and Y. Balkanski. Ice-free glacial northern Asia due to dust deposition on snow. Climate Dynamics, 27:613-625, November 2006. [ bib | DOI | ADS link ]

During the Last Glacial Maximum (LGM, 21 kyr BP), no large ice sheets were present in northern Asia, while northern Europe and North America (except Alaska) were heavily glaciated. We use a general circulation model with high regional resolution and a new parameterization of snow albedo to show that the ice-free conditions in northern Asia during the LGM are favoured by strong glacial dust deposition on the seasonal snow cover. Our climate model simulations indicate that mineral dust deposition on the snow surface leads to low snow albedo during the melt season. This, in turn, caused enhanced snow melt and therefore favoured snow-free peak summer conditions over almost the entire Asian continent during the LGM, whereas perennial snow cover is simulated over a large part of eastern Siberia when glacial dust deposition is not taken into account.

G. L. Liberti and F. Chéruy. Tropospheric dryness and clouds over tropical Indian Ocean. Atmospheric Research, 82:276-293, November 2006. [ bib | DOI | ADS link ]

Dry layers in the free troposphere over Tropical Oceans have been studied for their role in convective activity and for their effects in the radiation budget. Previous studies concentrated, mostly, on the Western Pacific region because of the abundance of observations during the TOGA-COARE. This study aims to document the occurrence of dry layers over the Indian Ocean and to investigate the cloudiness observed, but poorly documented, during such events. One month (March 1999, during INDOEX) of combined Visible/Infrared and Microwave data from the TRMM radiometers had been processed to classify observations in terms of total precipitable water vapour (TPWV), cloud occurrence and type. Soundings (1978-2004) from the Seychelles station have been analysed to validate the capability, through the analysis of TPWV, to detect dry layers. The study area (40degE-80degE, 30degS-30degN) had been portioned into 2.5deg × 2.5deg boxes to investigate the spatial distribution of occurrence of dry events with associated cloudiness. South of the ITCZ cloudiness associated with low TPWV is due to low-level clouds: probably generated by shallow convection trapped by the trade inversion. North of the ITCZ cirrus are mostly observed during relatively dry events. Further analyses, concentrating on possible links between occurrence of cirrus and TPWV, suggest, in addition to the obvious mechanism (i.e. the higher the moisture, the higher the convective activity and as a consequence the higher the occurrence of cirrus), a second one that would be responsible of relatively high occurrence of cirrus for low TPWV. A case (14th-15th March 1999) is studied in detail with TRMM observations, sounding data and METEOSAT imagery. The observed cirrus, generated by convection, migrate over relatively dry air of extra-tropical origin. Cirrus extend as filaments for more than 1000 km in length and about 50 to 100 km in width, and they last for 2-3 days. A retrieval method is designed and applied to the data giving as cirrus top pressure approximately 250 hPa while the retrieved effective radius value is consistent with what expected, from literature, for dissipating cirrus at that pressure. The vertical structure of the atmosphere, observed during such event suggests the hypothesis that a combination of presented radiative and dynamical mechanisms could be responsible, through the supply of moisture from lower levels, of increasing the cirrus lifetime and, as a consequence, increasing the occurrence of cirrus over relatively dry air columns. A larger data set is investigated to confirm the results based on the March 1999 data set analysis. The average vertical structure of the atmosphere, during such events, as obtained from the Seychelles sounding data set ( 7000 soundings) analysis confirms the occurrence of the features observed in the study case. Similarly, the analysis of 105 days of TRMM orbits over the box containing the Seychelles station, confirms the statistical features that indicate a possible relationship between the occurrence of dry air and associated cirrus. However, possible interaction between dry layer and cirrus occurrence should be investigated in a detailed modelling framework (beyond the scope of this study) such as the one offered by the cloud resolving models.

M. Schulz, C. Textor, S. Kinne, Y. Balkanski, S. Bauer, T. Berntsen, T. Berglen, O. Boucher, F. Dentener, S. Guibert, I. S. A. Isaksen, T. Iversen, D. Koch, A. Kirkevåg, X. Liu, V. Montanaro, G. Myhre, J. E. Penner, G. Pitari, S. Reddy, Ø. Seland, P. Stier, and T. Takemura. Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations. Atmospheric Chemistry & Physics, 6:5225-5246, November 2006. [ bib | ADS link ]

Nine different global models with detailed aerosol modules have independently produced instantaneous direct radiative forcing due to anthropogenic aerosols. The anthropogenic impact is derived from the difference of two model simulations with prescribed aerosol emissions, one for present-day and one for pre-industrial conditions. The difference in the solar energy budget at the top of the atmosphere (ToA) yields a new harmonized estimate for the aerosol direct radiative forcing (RF) under all-sky conditions. On a global annual basis RF is -0.22 Wm^-2, ranging from +0.04 to -0.41 Wm^-2, with a standard deviation of 0.16 Wm^-2. Anthropogenic nitrate and dust are not included in this estimate. No model shows a significant positive all-sky RF. The corresponding clear-sky RF is -0.68 Wm^-2. The cloud-sky RF was derived based on all-sky and clear-sky RF and modelled cloud cover. It was significantly different from zero and ranged between -0.16 and +0.34 Wm^-2. A sensitivity analysis shows that the total aerosol RF is influenced by considerable diversity in simulated residence times, mass extinction coefficients and most importantly forcing efficiencies (forcing per unit optical depth). The clear-sky forcing efficiency (forcing per unit optical depth) has diversity comparable to that for the all-sky/ clear-sky forcing ratio. While the diversity in clear-sky forcing efficiency is impacted by factors such as aerosol absorption, size, and surface albedo, we can show that the all-sky/clear-sky forcing ratio is important because all-sky forcing estimates require proper representation of cloud fields and the correct relative altitude placement between absorbing aerosol and clouds. The analysis of the sulphate RF shows that long sulphate residence times are compensated by low mass extinction coefficients and vice versa. This is explained by more sulphate particle humidity growth and thus higher extinction in those models where short-lived sulphate is present at lower altitude and vice versa. Solar atmospheric forcing within the atmospheric column is estimated at +0.820.17 Wm^-2. The local annual average maxima of atmospheric forcing exceed +5 Wm^-2 confirming the regional character of aerosol impacts on climate. The annual average surface forcing is -1.020.23 Wm^-2. With the current uncertainties in the modelling of the radiative forcing due to the direct aerosol effect we show here that an estimate from one model is not sufficient but a combination of several model estimates is necessary to provide a mean and to explore the uncertainty.

G. Ramillien, F. Frappart, A. Güntner, T. Ngo-Duc, A. Cazenave, and K. Laval. Time variations of the regional evapotranspiration rate from Gravity Recovery and Climate Experiment (GRACE) satellite gravimetry. Water Resources Research, 42:10403, October 2006. [ bib | DOI | ADS link ]

Since its launch in March 2002, the Gravity Recovery and Climate Experiment (GRACE) mission has been measuring the global time variations of the Earth's gravity field with a current resolution of 500 km. Especially over the continents, these measurements represent the integrated land water mass, including surface waters (lakes, wetlands and rivers), soil moisture, groundwater, and snow cover. In this study, we use the GRACE land water solutions computed by Ramillien et al. (2005a) through an iterative inversion of monthly geoids from April 2002 to May 2004 to estimate time series of basin-scale regional evapotranspiration rate and associated uncertainties. Evapotranspiration is determined by integrating and solving the water mass balance equation, which relates land water storage (from GRACE), precipitation data (from the Global Precipitation Climatology Centre), runoff (from a global land surface model), and evapotranspiration (the unknown). We further examine the sensibility of the computation when using different model runoff. Evapotranspiration results are compared to outputs of four different global land surface models. The overall satisfactory agreement between GRACE-derived and model-based evapotranspiration prove the ability of GRACE to provide realistic estimates of this parameter.

P. Stier, J. H. Seinfeld, S. Kinne, J. Feichter, and O. Boucher. Impact of nonabsorbing anthropogenic aerosols on clear-sky atmospheric absorption. Journal of Geophysical Research (Atmospheres), 111:18201, September 2006. [ bib | DOI | ADS link ]

Absorption of solar radiation by atmospheric aerosol has become recognized as important in regional and global climate. Nonabsorbing, hydrophilic aerosols, such as sulfate, potentially affect atmospheric absorption in opposing ways: first, decreasing absorption through aging initially hydrophobic black carbon (BC) to a hydrophilic state, enhancing its removal by wet scavenging, and consequently decreasing BC lifetime and abundance, and second, increasing absorption through enhancement of the BC absorption efficiency by internal mixing as well as through increasing the amount of diffuse solar radiation in the atmosphere. On the basis of General Circulation Model studies with an embedded microphysical aerosol module we systematically demonstrate the significance of these mechanisms both on the global and regional scales. In remote transport regions, the first mechanism prevails, reducing atmospheric absorption, whereas in the vicinity of source regions, despite enhanced wet scavenging, absorption is enhanced owing to the prevalence of the second mechanisms. Our findings imply that the sulfur to BC emission ratio plays a key role in aerosol absorption.

F. Dentener, S. Kinne, T. Bond, O. Boucher, J. Cofala, S. Generoso, P. Ginoux, S. Gong, J. J. Hoelzemann, A. Ito, L. Marelli, J. E. Penner, J.-P. Putaud, C. Textor, M. Schulz, G. R. van der Werf, and J. Wilson. Emissions of primary aerosol and precursor gases in the years 2000 and 1750 prescribed data-sets for AeroCom. Atmospheric Chemistry & Physics, 6:4321-4344, September 2006. [ bib | ADS link ]

Inventories for global aerosol and aerosol precursor emissions have been collected (based on published inventories and published simulations), assessed and prepared for the year 2000 (present-day conditions) and for the year 1750 (pre-industrial conditions). These global datasets establish a comprehensive source for emission input to global modeling, when simulating the aerosol impact on climate with state-of-the-art aerosol component modules. As these modules stratify aerosol into dust, sea-salt, sulfate, organic matter and soot, for all these aerosol types global fields on emission strength and recommendations for injection altitude and particulate size are provided. Temporal resolution varies between daily (dust and sea-salt), monthly (wild-land fires) and annual (all other emissions). These datasets benchmark aerosol emissions according to the knowledge in the year 2004. They are intended to serve as systematic constraints in sensitivity studies of the AeroCom initiative, which seeks to quantify (actual) uncertainties in aerosol global modeling.

D. Zurovac-Jevti, S. Bony, and K. Emanuel. On the Role of Clouds and Moisture in Tropical Waves: A Two-Dimensional Model Study. Journal of Atmospheric Sciences, 63:2140-2155, August 2006. [ bib | DOI | ADS link ]

Observations show that convective perturbations of the tropical atmosphere are associated with substantial variations of clouds and water vapor. Recent studies suggest that these variations may play an active role in the large-scale organization of the tropical atmosphere. The present study investigates that possibility by using a two-dimensional, nonrotating model that includes a set of physical parameterizations carefully evaluated against tropical data. In the absence of cloud radiation interactions, the model spontaneously generates fast upwind (eastward) moving planetary-scale oscillations through the wind-induced surface heat exchange mechanism. In the presence of cloud radiative effects, the model generates slower upwind (eastward) propagating modes in addition to small-scale disturbances advected downwind (westward) by the mean flow. Enhanced cloud radiative effects further slow down upwind propagating waves and make them more prominent in the spectrum. On the other hand, the model suggests that interactions between moisture and convection favor the prominence of moist Kelvin-like waves in tropical variability at the expense of small-scale advective disturbances. These numerical results, consistent with theoretical predictions, suggest that the interaction of water vapor and cloud variations with convection and radiation plays an active role in the large-scale organization of the tropical atmosphere.<BR />HR ALIGN=”center” WIDTH=”30%”<BR />

J. E. Penner, J. Quaas, T. Storelvmo, T. Takemura, O. Boucher, H. Guo, A. Kirkevåg, J. E. Kristjánsson, and Ø. Seland. Model intercomparison of indirect aerosol effects. Atmospheric Chemistry & Physics, 6:3391-3405, August 2006. [ bib | ADS link ]

Modeled differences in predicted effects are increasingly used to help quantify the uncertainty of these effects. Here, we examine modeled differences in the aerosol indirect effect in a series of experiments that help to quantify how and why model-predicted aerosol indirect forcing varies between models. The experiments start with an experiment in which aerosol concentrations, the parameterization of droplet concentrations and the autoconversion scheme are all specified and end with an experiment that examines the predicted aerosol indirect forcing when only aerosol sources are specified. Although there are large differences in the predicted liquid water path among the models, the predicted aerosol first indirect effect for the first experiment is rather similar, about -0.6 Wm^-2 to -0.7 Wm^-2. Changes to the autoconversion scheme can lead to large changes in the liquid water path of the models and to the response of the liquid water path to changes in aerosols. Adding an autoconversion scheme that depends on the droplet concentration caused a larger (negative) change in net outgoing shortwave radiation compared to the 1st indirect effect, and the increase varied from only 22% to more than a factor of three. The change in net shortwave forcing in the models due to varying the autoconversion scheme depends on the liquid water content of the clouds as well as their predicted droplet concentrations, and both increases and decreases in the net shortwave forcing can occur when autoconversion schemes are changed. The parameterization of cloud fraction within models is not sensitive to the aerosol concentration, and, therefore, the response of the modeled cloud fraction within the present models appears to be smaller than that which would be associated with model ”noise”. The prediction of aerosol concentrations, given a fixed set of sources, leads to some of the largest differences in the predicted aerosol indirect radiative forcing among the models, with values of cloud forcing ranging from -0.3 Wm^-2 to -1.4 Wm^-2. Thus, this aspect of modeling requires significant improvement in order to improve the prediction of aerosol indirect effects.

W. D. Collins, V. Ramaswamy, M. D. Schwarzkopf, Y. Sun, R. W. Portmann, Q. Fu, S. E. B. Casanova, J.-L. Dufresne, D. W. Fillmore, P. M. D. Forster, V. Y. Galin, L. K. Gohar, W. J. Ingram, D. P. Kratz, M.-P. Lefebvre, J. Li, P. Marquet, V. Oinas, Y. Tsushima, T. Uchiyama, and W. Y. Zhong. Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Journal of Geophysical Research (Atmospheres), 111:14317, July 2006. [ bib | DOI | ADS link ]

The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO₂, CH₄, N₂O, CFC-11, CFC-12, and the increased H₂O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH₄ and N₂O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.

S. Hallegatte, A. Lahellec, and J.-Y. Grandpeix. An Elicitation of the Dynamic Nature of Water Vapor Feedback in Climate Change Using a 1D Model. Journal of Atmospheric Sciences, 63:1878-1894, July 2006. [ bib | DOI | ADS link ]

The concept of feedback has been used by several authors in the field of climate science to describe the behavior of models and to assess the importance of the different mechanisms at stake. Here, a simple 1D model of climate has been built to analyze the water vapor feedback. Beyond a static quantification of the water feedback, a more general formal definition of feedback gain based on the tangent linear system is introduced. This definition reintroduces the dynamical aspect of the system response to perturbation from Bode's original concept.In the model here, it is found that, even though the water vapor static gain proves consistent with results from GCMs, it turns out to be negative for time scales below 4 yr and positive only for longer time scales. These results suggest two conclusions: (i) that the water vapor feedback may be fully active only in response to long-lived perturbations; and (ii) that the water vapor feedback could reduce the natural variability due to tropospheric temperature perturbations over short time scales, while enhancing it over longer time scales. This second conclusion would be consistent with studies investigating the influence of air sea coupling on variability on different time scales.<BR />HR ALIGN=”center” WIDTH=”30%”<BR />

M. J. Webb, C. A. Senior, D. M. H. Sexton, W. J. Ingram, K. D. Williams, M. A. Ringer, B. J. McAvaney, R. Colman, B. J. Soden, R. Gudgel, T. Knutson, S. Emori, T. Ogura, Y. Tsushima, N. Andronova, B. Li, I. Musat, S. Bony, and K. E. Taylor. On the contribution of local feedback mechanisms to the range of climate sensitivity in two GCM ensembles. Climate Dynamics, 27:17-38, July 2006. [ bib | DOI | ADS link ]

Global and local feedback analysis techniques have been applied to two ensembles of mixed layer equilibrium CO₂ doubling climate change experiments, from the CFMIP (Cloud Feedback Model Intercomparison Project) and QUMP (Quantifying Uncertainty in Model Predictions) projects. Neither of these new ensembles shows evidence of a statistically significant change in the ensemble mean or variance in global mean climate sensitivity when compared with the results from the mixed layer models quoted in the Third Assessment Report of the IPCC. Global mean feedback analysis of these two ensembles confirms the large contribution made by inter-model differences in cloud feedbacks to those in climate sensitivity in earlier studies; net cloud feedbacks are responsible for 66% of the inter-model variance in the total feedback in the CFMIP ensemble and 85% in the QUMP ensemble. The ensemble mean global feedback components are all statistically indistinguishable between the two ensembles, except for the clear-sky shortwave feedback which is stronger in the CFMIP ensemble. While ensemble variances of the shortwave cloud feedback and both clear-sky feedback terms are larger in CFMIP, there is considerable overlap in the cloud feedback ranges; QUMP spans 80% or more of the CFMIP ranges in longwave and shortwave cloud feedback. We introduce a local cloud feedback classification system which distinguishes different types of cloud feedbacks on the basis of the relative strengths of their longwave and shortwave components, and interpret these in terms of responses of different cloud types diagnosed by the International Satellite Cloud Climatology Project simulator. In the CFMIP ensemble, areas where low-top cloud changes constitute the largest cloud response are responsible for 59% of the contribution from cloud feedback to the variance in the total feedback. A similar figure is found for the QUMP ensemble. Areas of positive low cloud feedback (associated with reductions in low level cloud amount) contribute most to this figure in the CFMIP ensemble, while areas of negative cloud feedback (associated with increases in low level cloud amount and optical thickness) contribute most in QUMP. Classes associated with high-top cloud feedbacks are responsible for 33 and 20% of the cloud feedback contribution in CFMIP and QUMP, respectively, while classes where no particular cloud type stands out are responsible for 8 and 21%.

S. Kinne, M. Schulz, C. Textor, S. Guibert, Y. Balkanski, S. E. Bauer, T. Berntsen, T. F. Berglen, O. Boucher, M. Chin, W. Collins, F. Dentener, T. Diehl, R. Easter, J. Feichter, D. Fillmore, S. Ghan, P. Ginoux, S. Gong, A. Grini, J. Hendricks, M. Herzog, L. Horowitz, I. Isaksen, T. Iversen, A. Kirkevåg, S. Kloster, D. Koch, J. E. Kristjansson, M. Krol, A. Lauer, J. F. Lamarque, G. Lesins, X. Liu, U. Lohmann, V. Montanaro, G. Myhre, J. Penner, G. Pitari, S. Reddy, O. Seland, P. Stier, T. Takemura, and X. Tie. An AeroCom initial assessment - optical properties in aerosol component modules of global models. Atmospheric Chemistry & Physics, 6:1815-1834, May 2006. [ bib | ADS link ]

The AeroCom exercise diagnoses multi-component aerosol modules in global modeling. In an initial assessment simulated global distributions for mass and mid-visible aerosol optical thickness (aot) were compared among 20 different modules. Model diversity was also explored in the context of previous comparisons. For the component combined aot general agreement has improved for the annual global mean. At 0.11 to 0.14, simulated aot values are at the lower end of global averages suggested by remote sensing from ground (AERONET ca. 0.135) and space (satellite composite ca. 0.15). More detailed comparisons, however, reveal that larger differences in regional distribution and significant differences in compositional mixture remain. Of particular concern are large model diversities for contributions by dust and carbonaceous aerosol, because they lead to significant uncertainty in aerosol absorption (aab). Since aot and aab, both, influence the aerosol impact on the radiative energy-balance, the aerosol (direct) forcing uncertainty in modeling is larger than differences in aot might suggest. New diagnostic approaches are proposed to trace model differences in terms of aerosol processing and transport: These include the prescription of common input (e.g. amount, size and injection of aerosol component emissions) and the use of observational capabilities from ground (e.g. measurements networks) or space (e.g. correlations between aerosol and clouds).

C. Textor, M. Schulz, S. Guibert, S. Kinne, Y. Balkanski, S. Bauer, T. Berntsen, T. Berglen, O. Boucher, M. Chin, F. Dentener, T. Diehl, R. Easter, H. Feichter, D. Fillmore, S. Ghan, P. Ginoux, S. Gong, A. Grini, J. Hendricks, L. Horowitz, P. Huang, I. Isaksen, I. Iversen, S. Kloster, D. Koch, A. Kirkevåg, J. E. Kristjansson, M. Krol, A. Lauer, J. F. Lamarque, X. Liu, V. Montanaro, G. Myhre, J. Penner, G. Pitari, S. Reddy, Ø. Seland, P. Stier, T. Takemura, and X. Tie. Analysis and quantification of the diversities of aerosol life cycles within AeroCom. Atmospheric Chemistry & Physics, 6:1777-1813, May 2006. [ bib | ADS link ]

Simulation results of global aerosol models have been assembled in the framework of the AeroCom intercomparison exercise. In this paper, we analyze the life cycles of dust, sea salt, sulfate, black carbon and particulate organic matter as simulated by sixteen global aerosol models. The differences among the results (model diversities) for sources and sinks, burdens, particle sizes, water uptakes, and spatial dispersals have been established. These diversities have large consequences for the calculated radiative forcing and the aerosol concentrations at the surface. Processes and parameters are identified which deserve further research. P style=”line-height: 20px;” The AeroCom all-models-average emissions are dominated by the mass of sea salt (SS), followed by dust (DU), sulfate (SO₄), particulate organic matter (POM), and finally black carbon (BC). Interactive parameterizations of the emissions and contrasting particles sizes of SS and DU lead generally to higher diversities of these species, and for total aerosol. The lower diversity of the emissions of the fine aerosols, BC, POM, and SO₄, is due to the use of similar emission inventories, and does therefore not necessarily indicate a better understanding of their sources. The diversity of SO₄-sources is mainly caused by the disagreement on depositional loss of precursor gases and on chemical production. The diversities of the emissions are passed on to the burdens, but the latter are also strongly affected by the model-specific treatments of transport and aerosol processes. The burdens of dry masses decrease from largest to smallest: DU, SS, SO₄, POM, and BC. P style=”line-height: 20px;” The all-models-average residence time is shortest for SS with about half a day, followed by SO₄ and DU with four days, and POM and BC with six and seven days, respectively. The wet deposition rate is controlled by the solubility and increases from DU, BC, POM to SO₄ and SS. It is the dominant sink for SO₄, BC, and POM, and contributes about one third to the total removal of SS and DU species. For SS and DU we find high diversities for the removal rate coefficients and deposition pathways. Models do neither agree on the split between wet and dry deposition, nor on that between sedimentation and other dry deposition processes. We diagnose an extremely high diversity for the uptake of ambient water vapor that influences the particle size and thus the sink rate coefficients. Furthermore, we find little agreement among the model results for the partitioning of wet removal into scavenging by convective and stratiform rain. P style=”line-height: 20px;” Large differences exist for aerosol dispersal both in the vertical and in the horizontal direction. In some models, a minimum of total aerosol concentration is simulated at the surface. Aerosol dispersal is most pronounced for SO₄ and BC and lowest for SS. Diversities are higher for meridional than for vertical dispersal, they are similar for the individual species and highest for SS and DU. For these two components we do not find a correlation between vertical and meridional aerosol dispersal. In addition the degree of dispersals of SS and DU is not related to their residence times. SO₄, BC, and POM, however, show increased meridional dispersal in models with larger vertical dispersal, and dispersal is larger for longer simulated residence times.

T. S. Bates, T. L. Anderson, T. Baynard, T. Bond, O. Boucher, G. Carmichael, A. Clarke, C. Erlick, H. Guo, L. Horowitz, S. Howell, S. Kulkarni, H. Maring, A. McComiskey, A. Middlebrook, K. Noone, C. D. O'Dowd, J. Ogren, J. Penner, P. K. Quinn, A. R. Ravishankara, D. L. Savoie, S. E. Schwartz, Y. Shinozuka, Y. Tang, R. J. Weber, and Y. Wu. Aerosol direct radiative effects over the northwest Atlantic, northwest Pacific, and North Indian Oceans: estimates based on in-situ chemical and optical measurements and chemical transport modeling. Atmospheric Chemistry & Physics, 6:1657-1732, May 2006. [ bib | ADS link ]

The largest uncertainty in the radiative forcing of climate change over the industrial era is that due to aerosols, a substantial fraction of which is the uncertainty associated with scattering and absorption of shortwave (solar) radiation by anthropogenic aerosols in cloud-free conditions (IPCC, 2001). Quantifying and reducing the uncertainty in aerosol influences on climate is critical to understanding climate change over the industrial period and to improving predictions of future climate change for assumed emission scenarios. Measurements of aerosol properties during major field campaigns in several regions of the globe during the past decade are contributing to an enhanced understanding of atmospheric aerosols and their effects on light scattering and climate. The present study, which focuses on three regions downwind of major urban/population centers (North Indian Ocean (NIO) during INDOEX, the Northwest Pacific Ocean (NWP) during ACE-Asia, and the Northwest Atlantic Ocean (NWA) during ICARTT), incorporates understanding gained from field observations of aerosol distributions and properties into calculations of perturbations in radiative fluxes due to these aerosols. This study evaluates the current state of observations and of two chemical transport models (STEM and MOZART). Measurements of burdens, extinction optical depth (AOD), and direct radiative effect of aerosols (DRE - change in radiative flux due to total aerosols) are used as measurement-model check points to assess uncertainties. In-situ measured and remotely sensed aerosol properties for each region (mixing state, mass scattering efficiency, single scattering albedo, and angular scattering properties and their dependences on relative humidity) are used as input parameters to two radiative transfer models (GFDL and University of Michigan) to constrain estimates of aerosol radiative effects, with uncertainties in each step propagated through the analysis. Constraining the radiative transfer calculations by observational inputs increases the clear-sky, 24-h averaged AOD (348%), top of atmosphere (TOA) DRE (3212%), and TOA direct climate forcing of aerosols (DCF - change in radiative flux due to anthropogenic aerosols) (377%) relative to values obtained with ”a priori” parameterizations of aerosol loadings and properties (GFDL RTM). The resulting constrained clear-sky TOA DCF is -3.30.47, -142.6, -6.42.1 Wm^-2 for the NIO, NWP, and NWA, respectively. With the use of constrained quantities (extensive and intensive parameters) the calculated uncertainty in DCF was 25% less than the ”structural uncertainties” used in the IPCC-2001 global estimates of direct aerosol climate forcing. Such comparisons with observations and resultant reductions in uncertainties are essential for improving and developing confidence in climate model calculations incorporating aerosol forcing.

S. Verma, O. Boucher, C. Venkataraman, M. S. Reddy, D. Müller, P. Chazette, and B. Crouzille. Aerosol lofting from sea breeze during the Indian Ocean Experiment. Journal of Geophysical Research (Atmospheres), 111:7208, April 2006. [ bib | DOI | ADS link ]

This work was carried out to understand the mechanisms leading to lofting and large-scale advection of aerosols over the Indian Ocean region due to interaction of the sea breeze with the northeast monsoon winds along the west coast of India. European Centre for Medium-Range Weather Forecasts (ECMWF) wind fields for the months of February and March 1999 were analyzed at various times of day. Intense sea breeze activity was observed at 1200 UT (1730 local time) along the west coast of India with average intensity larger in March than in February. The sea breeze was seen to extend inland deeper in March than in February. Lofting of air observed as high as 800 hPa (approximately 2 km above sea level) could lead to entrainment of aerosols into the free troposphere and long-range transport. Upward motion of air was observed everywhere along the west coast of India (from 8deg to 20degN), on average higher in March than in February, because of convergence between the sea breeze and the synoptic-scale flow. A region of intense lofting of air and well-defined convergence was observed along the coast of the Karnataka region (12deg-16degN). A simulation with a general circulation model nudged with ECMWF data indicated that the intrusion of marine air masses with low concentrations of organic matter is seen as deep as 64 km inland in the evening (1500 UT). Intrusion of the sea-salt plume is seen to a maximum distance of around 200 km from 1500 until 2300 UT. A well-developed lofted layer of aerosols as high as 3 km was also simulated during sea breeze activity along the west coast of India. The general circulation model simulation shows a clear diurnal evolution of the vertical profile of the aerosol extinction coefficient at Goa but fails to reproduce several features of the lidar observations, for example, the marked diurnal variability of the upper layers between 1 and 3 km. However, the model simulates a diurnal cycle at the surface (0-0.7 km) that is not apparent in lidar measurements. The model simulates long-range transport and captures the lofted plume downwind of the west coast of India. However, there was a 1-2 day delay in the model transport of lofted aerosols at higher layers to Hulule, 700 km downwind of India, when compared to lidar observations.

K. Emanuel, S. Ravela, E. Vivant, and C. Risi. Supplement to A Statistical Deterministic Approach to Hurricane Risk Assessment. Bulletin of the American Meteorological Society, 87:1, March 2006. [ bib | DOI | ADS link ]

K. Emanuel, S. Ravela, E. Vivant, and C. Risi. A Statistical Deterministic Approach to Hurricane Risk Assessment. Bulletin of the American Meteorological Society, 87:299-314, March 2006. [ bib | DOI | ADS link ]

Hurricanes are lethal and costly phenomena, and it is therefore of great importance to assess the long-term risk they pose to society. Among the greatest threats are those associated with high winds and related phenomena, such as storm surges. Here we assess the probability that hurricane winds will affect any given point in space by combining an estimate of the probability that a hurricane will pass within some given radius of the point in question with an estimate of the spatial probability density of storm winds.To assess the probability that storms will pass close enough to a point of interest to affect it, we apply two largely independent techniques for generating large numbers of synthetic hurricane tracks. The first treats each track as a Markov chain, using statistics derived from observed hurricanetrack data. The second technique begins by generating a large class of synthetic, time-varying wind fields at 850 and 250 hPa whose variance, covariance, and monthly means match NCEP-NCAR reanalysis data and whose kinetic energy follows an ω^-3 geostrophic turbulence spectral frequency distribution. Hurricanes are assumed to move with a weighted mean of the 850- and 250-hPa flow plus a beta drift correction, after originating at points determined from historical genesis data. The statistical characteristics of tracks generated by these two means are compared.For a given point in space, many (10⁴) synthetic tracks are generated that pass within a specified distance of a point of interest, using both track generation methods. For each of these tracks, a deterministic, coupled, numerical simulation of the storm's intensity is carried out, using monthly mean upper-ocean and potential intensity climatologies, together with time-varying vertical wind shear generated from the synthetic time series of 850- and 250-hPa winds, as described above. For the case in which the tracks are generated using the synthetic environmental flow, the tracks and the shear are generated using the same wind fields and are therefore mutually consistent.The track and intensity data are finally used together with a vortex structure model to construct probability distributions of wind speed at fixed points in space. These are compared to similar estimates based directly on historical hurricane data for two coastal cities.

J. Quaas, O. Boucher, and U. Lohmann. Constraining the total aerosol indirect effect in the LMDZ and ECHAM4 GCMs using MODIS satellite data. Atmospheric Chemistry & Physics, 6:947-955, March 2006. [ bib | ADS link ]

Aerosol indirect effects are considered to be the most uncertain yet important anthropogenic forcing of climate change. The goal of the present study is to reduce this uncertainty by constraining two different general circulation models (LMDZ and ECHAM4) with satellite data. We build a statistical relationship between cloud droplet number concentration and the optical depth of the fine aerosol mode as a measure of the aerosol indirect effect using MODerate Resolution Imaging Spectroradiometer (MODIS) satellite data, and constrain the model parameterizations to match this relationship. We include here ”empirical” formulations for the cloud albedo effect as well as parameterizations of the cloud lifetime effect. When fitting the model parameterizations to the satellite data, consistently in both models, the radiative forcing by the combined aerosol indirect effect is reduced considerably, down to -0.5 and -0.3 Wm^-2, for LMDZ and ECHAM4, respectively.

K. D. Williams, M. A. Ringer, C. A. Senior, M. J. Webb, B. J. McAvaney, N. Andronova, S. Bony, J.-L. Dufresne, S. Emori, R. Gudgel, T. Knutson, B. Li, K. Lo, I. Musat, J. Wegner, A. Slingo, and J. F. B. Mitchell. Evaluation of a component of the cloud response to climate change in an intercomparison of climate models. Climate Dynamics, 26:145-165, February 2006. [ bib | DOI | ADS link ]

Most of the uncertainty in the climate sensitivity of contemporary general circulation models (GCMs) is believed to be connected with differences in the simulated radiative feedback from clouds. Traditional methods of evaluating clouds in GCMs compare time-mean geographical cloud fields or aspects of present-day cloud variability, with observational data. In both cases a hypothetical assumption is made that the quantity evaluated is relevant for the mean climate change response. Nine GCMs (atmosphere models coupled to mixed-layer ocean models) from the CFMIP and CMIP model comparison projects are used in this study to demonstrate a common relationship between the mean cloud response to climate change and present-day variability. Although atmosphere-mixed-layer ocean models are used here, the results are found to be equally applicable to transient coupled model simulations. When changes in cloud radiative forcing (CRF) are composited by changes in vertical velocity and saturated lower tropospheric stability, a component of the local mean climate change response can be related to present-day variability in all of the GCMs. This suggests that the relationship is not model specific and might be relevant in the real world. In this case, evaluation within the proposed compositing framework is a direct evaluation of a component of the cloud response to climate change. None of the models studied are found to be clearly superior or deficient when evaluated, but a couple appear to perform well on several relevant metrics. Whilst some broad similarities can be identified between the 60degN-60degS mean change in CRF to increased CO₂ and that predicted from present-day variability, the two cannot be quantitatively constrained based on changes in vertical velocity and stability alone. Hence other processes also contribute to the global mean cloud response to climate change.

N. Gedney, P. M. Cox, R. A. Betts, O. Boucher, C. Huntingford, and P. A. Stott. Detection of a direct carbon dioxide effect in continental river runoff records. Nature, 439:835-838, February 2006. [ bib | DOI | ADS link ]

Continental runoff has increased through the twentieth century despite more intensive human water consumption. Possible reasons for the increase include: climate change and variability, deforestation, solar dimming, and direct atmospheric carbon dioxide (CO₂) effects on plant transpiration. All of these mechanisms have the potential to affect precipitation and/or evaporation and thereby modify runoff. Here we use a mechanistic land-surface model and optimal fingerprinting statistical techniques to attribute observational runoff changes into contributions due to these factors. The model successfully captures the climate-driven inter-annual runoff variability, but twentieth-century climate alone is insufficient to explain the runoff trends. Instead we find that the trends are consistent with a suppression of plant transpiration due to CO₂-induced stomatal closure. This result will affect projections of freshwater availability, and also represents the detection of a direct CO₂ effect on the functioning of the terrestrial biosphere.

H. Yu, Y. J. Kaufman, M. Chin, G. Feingold, L. A. Remer, T. L. Anderson, Y. Balkanski, N. Bellouin, O. Boucher, S. Christopher, P. Decola, R. Kahn, D. Koch, N. Loeb, M. S. Reddy, M. Schulz, T. Takemura, and M. Zhou. A review of measurement-based assessments of the aerosol direct radiative effect and forcing. Atmospheric Chemistry & Physics, 6:613-666, February 2006. [ bib | ADS link ]

Aerosols affect the Earth's energy budget directly by scattering and absorbing radiation and indirectly by acting as cloud condensation nuclei and, thereby, affecting cloud properties. However, large uncertainties exist in current estimates of aerosol forcing because of incomplete knowledge concerning the distribution and the physical and chemical properties of aerosols as well as aerosol-cloud interactions. In recent years, a great deal of effort has gone into improving measurements and datasets. It is thus feasible to shift the estimates of aerosol forcing from largely model-based to increasingly measurement-based. Our goal is to assess current observational capabilities and identify uncertainties in the aerosol direct forcing through comparisons of different methods with independent sources of uncertainties. Here we assess the aerosol optical depth (τ), direct radiative effect (DRE) by natural and anthropogenic aerosols, and direct climate forcing (DCF) by anthropogenic aerosols, focusing on satellite and ground-based measurements supplemented by global chemical transport model (CTM) simulations. The multi-spectral MODIS measures global distributions of aerosol optical depth (τ) on a daily scale, with a high accuracy of 0.030.05τ over ocean. The annual average τ is about 0.14 over global ocean, of which about 21%7% is contributed by human activities, as estimated by MODIS fine-mode fraction. The multi-angle MISR derives an annual average AOD of 0.23 over global land with an uncertainty of ˜20% or 0.05. These high-accuracy aerosol products and broadband flux measurements from CERES make it feasible to obtain observational constraints for the aerosol direct effect, especially over global the ocean. A number of measurement-based approaches estimate the clear-sky DRE (on solar radiation) at the top-of-atmosphere (TOA) to be about -5.50.2 Wm^-2 (median standard error from various methods) over the global ocean. Accounting for thin cirrus contamination of the satellite derived aerosol field will reduce the TOA DRE to -5.0 Wm^-2. Because of a lack of measurements of aerosol absorption and difficulty in characterizing land surface reflection, estimates of DRE over land and at the ocean surface are currently realized through a combination of satellite retrievals, surface measurements, and model simulations, and are less constrained. Over the oceans the surface DRE is estimated to be -8.80.7 Wm^-2. Over land, an integration of satellite retrievals and model simulations derives a DRE of -4.90.7 Wm^-2 and -11.81.9 Wm^-2 at the TOA and surface, respectively. CTM simulations derive a wide range of DRE estimates that on average are smaller than the measurement-based DRE by about 30-40%, even after accounting for thin cirrus and cloud contamination. P style=”line-height: 20px;” A number of issues remain. Current estimates of the aerosol direct effect over land are poorly constrained. Uncertainties of DRE estimates are also larger on regional scales than on a global scale and large discrepancies exist between different approaches. The characterization of aerosol absorption and vertical distribution remains challenging. The aerosol direct effect in the thermal infrared range and in cloudy conditions remains relatively unexplored and quite uncertain, because of a lack of global systematic aerosol vertical profile measurements. A coordinated research strategy needs to be developed for integration and assimilation of satellite measurements into models to constrain model simulations. Enhanced measurement capabilities in the next few years and high-level scientific cooperation will further advance our knowledge.

S. Bony, R. Colman, V. M. Kattsov, R. P. Allan, C. S. Bretherton, J.-L. Dufresne, A. Hall, S. Hallegatte, M. M. Holland, W. Ingram, D. A. Randall, B. J. Soden, G. Tselioudis, and M. J. Webb. How Well Do We Understand and Evaluate Climate Change Feedback Processes? Journal of Climate, 19:3445, 2006. [ bib | DOI | ADS link ]

P. Rannou, F. Montmessin, F. Hourdin, and S. Lebonnois. The Latitudinal Distribution of Clouds on Titan. Science, 311:201-205, January 2006. [ bib | DOI | ADS link ]

Clouds have been observed recently on Titan, through the thick haze, using near-infrared spectroscopy and images near the south pole and in temperate regions near 40degS. Recent telescope and Cassini orbiter observations are now providing an insight into cloud climatology. To study clouds, we have developed a general circulation model of Titan that includes cloud microphysics. We identify and explain the formation of several types of ethane and methane clouds, including south polar clouds and sporadic clouds in temperate regions and especially at 40deg in the summer hemisphere. The locations, frequencies, and composition of these cloud types are essentially explained by the large-scale circulation.

F. Hourdin, O. Talagrand, and A. Idelkadi. Eulerian backtracking of atmospheric tracers. II: Numerical aspects. Quarterly Journal of the Royal Meteorological Society, 132:585-603, January 2006. [ bib | DOI | ADS link ]

In Part I of this paper, a mathematical equivalence was established between Eulerian backtracking or retro-transport, on the one hand, and adjoint transport with respect to an air-mass-weighted scalar product, on the other. The time symmetry which lies at the basis of this mathematical equivalence can however be lost through discretization. That question is studied, and conditions are explicitly identified under which discretization schemes possess the property of time symmetry. Particular consideration is given to the case of the LMDZ model. The linear schemes used for turbulent diffusion and subgrid-scale convection are symmetric. For the Van Leer advection scheme used in LMDZ, which is nonlinear, the question of time symmetry does not even make sense. Those facts are illustrated by numerical simulations performed in the conditions of the European Transport EXperiment (ETEX). For a model that is not time-symmetric, the question arises as to whether it is preferable, in practical applications, to use the exact numerical adjoint, or the retro-transport model. Numerical results obtained in the context of one-dimensional advection show that the presence of slope limiters in the Van Leer advection scheme can produce in some circumstances unrealistic (in particular, negative) adjoint sensitivities. The retro-transport equation, on the other hand, generally produces robust and realistic results, and always preserves the positivity of sensitivities. Retro-transport may therefore be preferable in sensitivity computations, even in the context of variational assimilation.

F. Hourdin and O. Talagrand. Eulerian backtracking of atmospheric tracers. I: Adjoint derivation and parametrization of subgrid-scale transport. Quarterly Journal of the Royal Meteorological Society, 132:567-583, January 2006. [ bib | DOI | ADS link ]

The problem of identification of sources of atmospheric tracers is most classically addressed through either Lagrangian backtracking or adjoint integration. On the basis of physical considerations, the retro-transport equation, which is at the basis of Lagrangian backtracking, can be derived in a Eulerian framework as well. Because of a fundamental time symmetry of fluid transport, Lagrangian or Eulerian backtracking can be used for inverting measurements of the concentration of an atmospheric tracer. The retro-transport equation turns out to be the adjoint of the direct transport equation, with respect to the scalar product defined by integration with respect to air mass. In the present paper, the exact equivalence between the physically-derived retro-transport and adjoint equations is proved. The transformation from the direct to the retro-transport equation requires only simple transformations. The sign of terms describing explicit advection is changed. Terms describing linear sources or sinks of tracers are kept unchanged. Terms representing diffusion by unresolved time-symmetric motions of the transporting air are also unchanged. This is rigorously shown for turbulent eddy-diffusion or mixing length theory. The case of subgrid-scale vertical transport by non-time-symmetric motions of air is studied using the example of the Tiedtke mass-flux scheme for cumulus convection. The retro-transport equation is then obtained by simply inverting the roles of updraughts and downdraughts, as well as of entrainment and detrainment. Conservation of mass of the transporting air is critical for all those properties to hold.

J.-L. Lin, G. N. Kiladis, B. E. Mapes, K. M. Weickmann, K. R. Sperber, W. Lin, M. C. Wheeler, S. D. Schubert, A. Del Genio, L. J. Donner, S. Emori, J.-F. Gueremy, F. Hourdin, P. J. Rasch, E. Roeckner, and J. F. Scinocca. Tropical Intraseasonal Variability in 14 IPCC AR4 Climate Models. Part I: Convective Signals. Journal of Climate, 19:2665, 2006. [ bib | DOI | ADS link ]

K. Emanuel, S. Ravela, E. Vivant, and C. Risi. Supplement to A Statistical Deterministic Approach to Hurricane Risk Assessment. Bulletin of the American Meteorological Society, 87:1, 2006. [ bib | DOI | ADS link ]

S. Verma, O. Boucher, H. C. Upadhyaya, and O. P. Sharma. Sulfate aerosols forcing: An estimate using a three-dimensional interactive chemistry scheme. Atmospheric Environment, 40:7953-7962, 2006. [ bib | DOI | ADS link ]

The tropospheric sulfate radiative forcing has been calculated using an interactive chemistry scheme in LMD-GCM. To estimate the radiative forcing of sulfate aerosol on climate, a consistent interaction between atmospheric circulation and radiation computation has been allowed in LMD-GCM. The model results indicate that the change in the sulfate aerosols number concentration is negatively correlated to the indirect radiative forcing. The model simulated annual mean direct radiative forcing ranges from -0.1 to -1.2 W m ^-2, and indirect forcing ranges from -0.4 to -1.6 W m ^-2. The global annual mean direct effect estimated by the model is -0.48 W m ^-2, and that of indirect is -0.68 W m ^-2.

G. Habib, C. Venkataraman, I. Chiapello, S. Ramachandran, O. Boucher, and M. Shekar Reddy. Seasonal and interannual variability in absorbing aerosols over India derived from TOMS: Relationship to regional meteorology and emissions. Atmospheric Environment, 40:1909-1921, 2006. [ bib | DOI | ADS link ]

The objective of this study is an analysis of the spatial, seasonal and interannual variability of regional-scale aerosol load over India, detected by TOMS during 1981-2000, with an evaluation of potential contributing factors, including estimated anthropogenic aerosol emission trends and regional meteorology (rainfall and circulation patterns). Spatial distributions in TOMS Ai were related to the emission densities of anthropogenic absorbing aerosols in April-May, but varied seasonally and were modified significantly by higher atmospheric dispersion in January-March and rainfall in June-September, both of which lead to low TOMS Ai, even in regions of high aerosol emissions. Dust emissions explain the high TOMS Ai over northwest region during April-May and June-September when rainfall is scanty and significant air-mass decent occurs in this region. The magnitude of TOMS Ai correlated with the anthropogenic absorbing aerosol (black carbon and inorganic matter) emission flux in five selected regions, dominated by biomass/biofuel burning and fossil fuel combustion, but not in a region with significant mineral dust emissions. The seasonal cycle in TOMS Ai was related to the seasonal variability in dust, biomass burning emissions and rainfall. Interannual variability in TOMS Ai was linked to that in forest burning emissions in the northeast, as evidenced by a correlation with ATSR fire-counts, both significantly enhanced in 1999. Trends in anthropogenic emissions during 1981-2000 potentially contribute to increases in the aerosol load detected by TOMS. This would need further investigation using analysis incorporating both trends in anthropogenic emissions and the interannual variability in natural sources of aerosols.