A. Sima, D.-D. Rousseau, M. Kageyama, G. Ramstein, M. Schulz, Y. Balkanski, P. Antoine, F. Dulac, and C. Hatté. Imprint of North-Atlantic abrupt climate changes on western European loess deposits as viewed in a dust emission model. Quaternary Science Reviews, 28:2851-2866, December 2009. [ bib | DOI | ADS link ]
Western European loess sequences of the last glaciation (100,000-15,000 years BP) exhibit strong, cyclic variations of the sedimentation rate, which are coeval to the Greenland stadial/interstadial cycles and the Heinrich events. These North-Atlantic rapid climate changes appear, thus, as a potential cause for the sedimentation variations, via changes in dust intensity cycle. Here we make a first step in testing this hypothesis, by modelling the impact of the North-Atlantic abrupt climate variations on dust emission. Our dust emission calculations use meteorological fields generated by the LMDZ atmospheric general circulation model at a resolution down to 60 km over Western Europe. Three numerical experiments are run, representing a Greenland stadial, an interstadial and a Heinrich event. Orbital parameters and ice-sheet configuration correspond to conditions from Marine Isotope Stage 3 (60,000-25,000 years BP), a period characterized by strong millennial-scale climate variability. The only differences we impose in the boundary conditions regard the North-Atlantic surface temperature and sea-ice cover in the latitudinal band 30deg-63degN. The changes in wind, precipitation, soil moisture and snow cover from one simulated state to another result in small differences in dust emission intensity. In contrast, when the inhibition of the aeolian erosion by vegetation is taken into account, the dust fluxes for the cold climate states (Greenland stadial and Heinrich event) become generally more than twice higher than those for the relatively warmer Greenland interstadial, in agreement with the loess data. These results support the hypothesis that the North-Atlantic millennial-scale variability is imprinted in Western European loess profiles, and point to vegetation changes as the main factor responsible for millennial-scale sedimentation variations. An analysis for the English Channel and southern North Sea areas, major potential dust sources, shows that the seasonality of dust emission is not controlled by the wind speed, as in modern large deserts, but by the surface conditions. Consequently, the dusty season lasts from late winter to early summer, with maximum activity in April-May, and is shifted towards summer when the climate is colder.
V. Eymet, R. Fournier, J.-L. Dufresne, S. Lebonnois, F. Hourdin, and M. A. Bullock. Net exchange parameterization of thermal infrared radiative transfer in Venus' atmosphere. Journal of Geophysical Research (Planets), 114:E11008, November 2009. [ bib | DOI | ADS link ]
Thermal radiation within Venus atmosphere is analyzed in close details. Prominent features are identified, which are then used to design a parameterization (a highly simplified and yet accurate enough model) to be used in General Circulation Models. The analysis is based on a net exchange formulation, using a set of gaseous and cloud optical data chosen among available referenced data. The accuracy of the proposed parameterization methodology is controlled against Monte Carlo simulations, assuming that the optical data are exact. Then, the accuracy level corresponding to our present optical data choice is discussed by comparison with available observations, concentrating on the most unknown aspects of Venus thermal radiation, namely the deep atmosphere opacity and the cloud composition and structure.
N. Huneeus, O. Boucher, and F. Chevallier. Simplified aerosol modeling for variational data assimilation. Geoscientific Model Development, 2:213-229, November 2009. [ bib | ADS link ]
We have developed a simplified aerosol model together with its tangent linear and adjoint versions for the ultimate aim of optimizing global aerosol and aerosol precursor emission using variational data assimilation. The model was derived from the general circulation model LMDz; it groups together the 24 aerosol species simulated in LMDz into 4 species, namely gaseous precursors, fine mode aerosols, coarse mode desert dust and coarse mode sea salt. The emissions have been kept as in the original model. Modifications, however, were introduced in the computation of aerosol optical depth and in the processes of sedimentation, dry and wet deposition and sulphur chemistry to ensure consistency with the new set of species and their composition. <BR /><BR /> The simplified model successfully manages to reproduce the main features of the aerosol distribution in LMDz. The largest differences in aerosol load are observed for fine mode aerosols and gaseous precursors. Differences between the original and simplified models are mainly associated to the new deposition and sedimentation velocities consistent with the definition of species in the simplified model and the simplification of the sulphur chemistry. Furthermore, simulated aerosol optical depth remains within the variability of monthly AERONET observations for all aerosol types and all sites throughout most of the year. Largest differences are observed over sites with strong desert dust influence. In terms of the daily aerosol variability, the model is less able to reproduce the observed variability from the AERONET data with larger discrepancies in stations affected by industrial aerosols. The simplified model however, closely follows the daily simulation from LMDz. <BR /><BR /> Sensitivity analyses with the tangent linear version show that the simplified sulphur chemistry is the dominant process responsible for the strong non-linearity of the model.
D. Koch, M. Schulz, S. Kinne, C. McNaughton, J. R. Spackman, Y. Balkanski, S. Bauer, T. Berntsen, T. C. Bond, O. Boucher, M. Chin, A. Clarke, N. de Luca, F. Dentener, T. Diehl, O. Dubovik, R. Easter, D. W. Fahey, J. Feichter, D. Fillmore, S. Freitag, S. Ghan, P. Ginoux, S. Gong, L. Horowitz, T. Iversen, A. Kirkevåg, Z. Klimont, Y. Kondo, M. Krol, X. Liu, R. Miller, V. Montanaro, N. Moteki, G. Myhre, J. E. Penner, J. Perlwitz, G. Pitari, S. Reddy, L. Sahu, H. Sakamoto, G. Schuster, J. P. Schwarz, Ø. Seland, P. Stier, N. Takegawa, T. Takemura, C. Textor, J. A. van Aardenne, and Y. Zhao. Evaluation of black carbon estimations in global aerosol models. Atmospheric Chemistry & Physics, 9:9001-9026, November 2009. [ bib | ADS link ]
We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by considering the diversity among year 2000 model simulations and comparing model predictions with available measurements. These model-measurement intercomparisons include BC surface and aircraft concentrations, aerosol absorption optical depth (AAOD) retrievals from AERONET and Ozone Monitoring Instrument (OMI) and BC column estimations based on AERONET. In regions other than Asia, most models are biased high compared to surface concentration measurements. However compared with (column) AAOD or BC burden retreivals, the models are generally biased low. The average ratio of model to retrieved AAOD is less than 0.7 in South American and 0.6 in African biomass burning regions; both of these regions lack surface concentration measurements. In Asia the average model to observed ratio is 0.7 for AAOD and 0.5 for BC surface concentrations. Compared with aircraft measurements over the Americas at latitudes between 0 and 50N, the average model is a factor of 8 larger than observed, and most models exceed the measured BC standard deviation in the mid to upper troposphere. At higher latitudes the average model to aircraft BC ratio is 0.4 and models underestimate the observed BC loading in the lower and middle troposphere associated with springtime Arctic haze. Low model bias for AAOD but overestimation of surface and upper atmospheric BC concentrations at lower latitudes suggests that most models are underestimating BC absorption and should improve estimates for refractive index, particle size, and optical effects of BC coating. Retrieval uncertainties and/or differences with model diagnostic treatment may also contribute to the model-measurement disparity. Largest AeroCom model diversity occurred in northern Eurasia and the remote Arctic, regions influenced by anthropogenic sources. Changing emissions, aging, removal, or optical properties within a single model generated a smaller change in model predictions than the range represented by the full set of AeroCom models. Upper tropospheric concentrations of BC mass from the aircraft measurements are suggested to provide a unique new benchmark to test scavenging and vertical dispersion of BC in global models.
J. Quaas, Y. Ming, S. Menon, T. Takemura, M. Wang, J. E. Penner, A. Gettelman, U. Lohmann, N. Bellouin, O. Boucher, A. M. Sayer, G. E. Thomas, A. McComiskey, G. Feingold, C. Hoose, J. E. Kristjánsson, X. Liu, Y. Balkanski, L. J. Donner, P. A. Ginoux, P. Stier, B. Grandey, J. Feichter, I. Sednev, S. E. Bauer, D. Koch, R. G. Grainger, A. Kirkevåg, T. Iversen, Ø. Seland, R. Easter, S. J. Ghan, P. J. Rasch, H. Morrison, J.-F. Lamarque, M. J. Iacono, S. Kinne, and M. Schulz. Aerosol indirect effects - general circulation model intercomparison and evaluation with satellite data. Atmospheric Chemistry & Physics, 9:8697-8717, November 2009. [ bib | ADS link ]
Aerosol indirect effects continue to constitute one of the most important uncertainties for anthropogenic climate perturbations. Within the international AEROCOM initiative, the representation of aerosol-cloud-radiation interactions in ten different general circulation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects, and none of the model explicitly parameterises aerosol effects on convective clouds. We compute statistical relationships between aerosol optical depth (τa) and various cloud and radiation quantities in a manner that is consistent between the models and the satellite data. It is found that the model-simulated influence of aerosols on cloud droplet number concentration (Nd) compares relatively well to the satellite data at least over the ocean. The relationship between τa and liquid water path is simulated much too strongly by the models. This suggests that the implementation of the second aerosol indirect effect mainly in terms of an autoconversion parameterisation has to be revisited in the GCMs. A positive relationship between total cloud fraction (fcld) and τa as found in the satellite data is simulated by the majority of the models, albeit less strongly than that in the satellite data in most of them. In a discussion of the hypotheses proposed in the literature to explain the satellite-derived strong fcld-τa relationship, our results indicate that none can be identified as a unique explanation. Relationships similar to the ones found in satellite data between τa and cloud top temperature or outgoing long-wave radiation (OLR) are simulated by only a few GCMs. The GCMs that simulate a negative OLR-τa relationship show a strong positive correlation between τa and fcld. The short-wave total aerosol radiative forcing as simulated by the GCMs is strongly influenced by the simulated anthropogenic fraction of τa, and parameterisation assumptions such as a lower bound on Nd. Nevertheless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred from the combination of these predictors for the modelled forcings with the satellite-derived statistical relationships yields a global annual mean value of -1.50.5 Wm-2. In an alternative approach, the radiative flux perturbation due to anthropogenic aerosols can be broken down into a component over the cloud-free portion of the globe (approximately the aerosol direct effect) and a component over the cloudy portion of the globe (approximately the aerosol indirect effect). An estimate obtained by scaling these simulated clear- and cloudy-sky forcings with estimates of anthropogenic τa and satellite-retrieved Nd-τa regression slopes, respectively, yields a global, annual-mean aerosol direct effect estimate of -0.40.2 Wm-2 and a cloudy-sky (aerosol indirect effect) estimate of -0.70.5 Wm-2, with a total estimate of -1.20.4 Wm-2.
J. Quaas, O. Boucher, A. Jones, G. P. Weedon, J. Kieser, and H. Joos. Exploiting the weekly cycle as observed over Europe to analyse aerosol indirect effects in two climate models. Atmospheric Chemistry & Physics, 9:8493-8501, November 2009. [ bib | ADS link ]
A weekly cycle in aerosol pollution and some meteorological quantities is observed over Europe. In the present study we exploit this effect to analyse aerosol-cloud-radiation interactions. A weekly cycle is imposed on anthropogenic emissions in two general circulation models that include parameterizations of aerosol processes and cloud microphysics. It is found that the simulated weekly cycles in sulfur dioxide, sulfate, and aerosol optical depth in both models agree reasonably well with those observed indicating model skill in simulating the aerosol cycle. A distinct weekly cycle in cloud droplet number concentration is demonstrated in both observations and models. For other variables, such as cloud liquid water path, cloud cover, top-of-the-atmosphere radiation fluxes, precipitation, and surface temperature, large variability and contradictory results between observations, model simulations, and model control simulations without a weekly cycle in emissions prevent us from reaching any firm conclusions about the potential aerosol impact on meteorology or the realism of the modelled second aerosol indirect effects.
J.-B. Madeleine, F. Forget, J. W. Head, B. Levrard, F. Montmessin, and E. Millour. Amazonian northern mid-latitude glaciation on Mars: A proposed climate scenario. Icarus, 203:390-405, October 2009. [ bib | DOI | ADS link ]
Recent geological observations in the northern mid-latitudes of Mars show evidence for past glacial activity during the late Amazonian, similar to the integrated glacial landsystems in the Dry Valleys of Antarctica. The large accumulation of ice (many hundreds of meters) required to create the observed glacial deposits points to significant atmospheric precipitation, snow and ice accumulation, and glacial flow. In order to understand the climate scenario required for these conditions, we used the LMD (Laboratoire de Météorologie Dynamique) Mars GCM (General Circulation Model), which is able to reproduce the present-day water cycle, and to predict past deposition of ice consistent with geological observations in many cases. Prior to this analysis, however, significant mid-latitude glaciation had not been simulated by the model, run under a range of parameters. In this analysis, we studied the response of the GCM to a wider range of orbital configurations and water ice reservoirs, and show that during periods of moderate obliquity ( ε = 25-35deg) and high dust opacity ( τdust = 1.5-2.5), broad-scale glaciation in the northern mid-latitudes occurs if water ice deposited on the flanks of the Tharsis volcanoes at higher obliquity is available for sublimation. We find that high dust contents of the atmosphere increase its water vapor holding capacity, thereby moving the saturation region to the northern mid-latitudes. Precipitation events are then controlled by topographic forcing of stationary planetary waves and transient weather systems, producing surface ice distribution and amounts that are consistent with the geological record. Ice accumulation rates of 10 mm yr -1 lead to the formation of a 500-1000 m thick regional ice sheet that will produce glacial flow patterns consistent with the geological observations.
O. Boucher, P. Friedlingstein, B. Collins, and K. P. Shine. The indirect global warming potential and global temperature change potential due to methane oxidation. Environmental Research Letters, 4(4):044007, October 2009. [ bib | DOI | ADS link ]
Methane is the second most important anthropogenic greenhouse gas in the atmosphere next to carbon dioxide. Its global warming potential (GWP) for a time horizon of 100 years is 25, which makes it an attractive target for climate mitigation policies. Although the methane GWP traditionally includes the methane indirect effects on the concentrations of ozone and stratospheric water vapour, it does not take into account the production of carbon dioxide from methane oxidation. We argue here that this CO2-induced effect should be included for fossil sources of methane, which results in slightly larger GWP values for all time horizons. If the global temperature change potential is used as an alternative climate metric, then the impact of the CO2-induced effect is proportionally much larger. We also discuss what the correction term should be for methane from anthropogenic biogenic sources.
X. Jin, T. Wu, L. Li, and C. Shi. Cloudiness characteristics over Southeast Asia from satellite FY-2C and their comparison to three other cloud data sets. Journal of Geophysical Research (Atmospheres), 114:D17207, September 2009. [ bib | DOI | ADS link ]
Fengyun-2C (FY-2C), launched in October 2004, is the first operational geostationary meteorological satellite in China. It can provide 1-h interval cloudiness products with a spatial resolution of 0.04deg latitude × 0.04deg longitude. The main characteristics of the regional-scale clouds from a 2-year FY-2C data set (from July 2005 to June 2007) are presented, including the spatial distribution and the annual and diurnal cycles of cloudiness. The reliability of FY-2C cloud products over Southeast Asia is investigated through comparisons to cloud cover from the International Satellite Cloud Climatology Project, the Moderate Resolution Imaging Spectroradiometer on board Terra and Aqua satellites, and conventional ground observations. It is shown that the FY-2C cloud mask performs consistently with other cloud mask products over Southeast Asia. In the boreal winter, the whole domain is dry with little cloudiness. More extensive cloudiness can be observed over the Sichuan Basin, in the East China Sea and the South China Sea, along the northwestern border of China, and around the ITCZ in the Southern Hemisphere. In the boreal summer, the summer monsoon is the dominant system for the studied domain, which is generally humid with extensive cloudiness, corresponding to zones of strong convective activities. Results also reveal considerable discrepancies among different cloud products over extended areas of north China and Mongolia. The Sichuan Basin is another region of large discrepancies among the four cloud products. Diurnal cycles of FY-2C cloudiness for the four seasons of a year are analyzed. The diurnal range of cloudiness is generally larger over land than over ocean. Remarkable diurnal variation is found over the Tibetan Plateau, the northern part of the Indian Peninsula, and central Asia where there is generally less precipitation. The peaks of diurnal cycle of cloudiness appear around local noon over the subtropical land, in the morning over the Indian Peninsula, and in the afternoon near the equator.
A. Hannart, J.-L. Dufresne, and P. Naveau. Why climate sensitivity may not be so unpredictable. Geophysical Research Letters, 36:L16707, August 2009. [ bib | DOI | ADS link ]
Different explanations have been proposed as to why the range of climate sensitivity predicted by GCMs has not lessened substantially in the last decades, and subsequently if it can be reduced. One such study (Why is climate sensitivity so unpredictable?) addressed these questions using rather simple theoretical considerations and reached the conclusion that reducing uncertainties on climate feedbacks and underlying climate processes will not yield a large reduction in the envelope of climate sensitivity. In this letter, we revisit the premises of this conclusion. We show that it results from a mathematical artifact caused by a peculiar definition of uncertainty used by these authors. Applying standard concepts and definitions of descriptive statistics to the exact same framework of analysis as Roe and Baker, we show that within this simple framework, reducing inter-model spread on feedbacks does in fact induce a reduction of uncertainty on climate sensitivity, almost proportionally. Therefore, following Roe and Baker assumptions, climate sensitivity is actually not so unpredictable.
H. Bellenger, J.-P. Duvel, M. Lengaigne, and P. Levan. Impact of organized intraseasonal convective perturbations on the tropical circulation. Geophysical Research Letters, 36:L16703, August 2009. [ bib | DOI | ADS link ]
Using an atmospheric Global Circulation Model (GCM), we study the dynamical response of the tropical atmosphere to large-scale convective perturbations. We hypothesize that such perturbations can be triggered by intraseasonal SST anomalies. We thus compare two ensembles of simulations forced either with actual daily SST fields (high frequency, HF) or with low-pass filtered SST fields (T 90 days; low frequency, LF). In HF simulations, there is a small increase of seasonal mean precipitation, but a strong and statistically significant increase of the low-level westerly flow. It is argued that it is mostly the aggregation of the convection into a few large-scale organized intraseasonal events, and not the intensification of the seasonal mean precipitation, that acts to intensify the low-level westerly flow in the HF simulations. This non-linear behavior shows that a correct representation of intraseasonal variability is required for a correct simulation of the average tropical circulation.
A. Benedetti, J.-J. Morcrette, O. Boucher, A. Dethof, R. J. Engelen, M. Fisher, H. Flentje, N. Huneeus, L. Jones, J. W. Kaiser, S. Kinne, A. Mangold, M. Razinger, A. J. Simmons, and M. Suttie. Aerosol analysis and forecast in the European Centre for Medium-Range Weather Forecasts Integrated Forecast System: 2. Data assimilation. Journal of Geophysical Research (Atmospheres), 114:D13205, July 2009. [ bib | DOI | ADS link ]
This study presents the new aerosol assimilation system, developed at the European Centre for Medium-Range Weather Forecasts, for the Global and regional Earth-system Monitoring using Satellite and in-situ data (GEMS) project. The aerosol modeling and analysis system is fully integrated in the operational four-dimensional assimilation apparatus. Its purpose is to produce aerosol forecasts and reanalyses of aerosol fields using optical depth data from satellite sensors. This paper is the second of a series which describes the GEMS aerosol effort. It focuses on the theoretical architecture and practical implementation of the aerosol assimilation system. It also provides a discussion of the background errors and observations errors for the aerosol fields, and presents a subset of results from the 2-year reanalysis which has been run for 2003 and 2004 using data from the Moderate Resolution Imaging Spectroradiometer on the Aqua and Terra satellites. Independent data sets are used to show that despite some compromises that have been made for feasibility reasons in regards to the choice of control variable and error characteristics, the analysis is very skillful in drawing to the observations and in improving the forecasts of aerosol optical depth.
H. Bellenger and J.-P. Duvel. An Analysis of Tropical Ocean Diurnal Warm Layers. Journal of Climate, 22:3629-3646, July 2009. [ bib | DOI | ADS link ]
Z. Ke, P. Zhang, W. Dong, and L. Li. A New Way to Improve Seasonal Prediction by Diagnosing and Correcting the Intermodel Systematic Errors. Monthly Weather Review, 137:1898-1907, June 2009. [ bib | DOI | ADS link ]
A. Lenton, F. Codron, L. Bopp, N. Metzl, P. Cadule, A. Tagliabue, and J. Le Sommer. Stratospheric ozone depletion reduces ocean carbon uptake and enhances ocean acidification. Geophysical Research Letters, 36:L12606, June 2009. [ bib | DOI | ADS link ]
Observational and atmospheric inversion studies find that the strength of the Southern Ocean carbon dioxide (CO2) sink is not increasing, despite rising atmospheric CO2. However, this is yet to be captured by contemporary coupled-climate-carbon-models used to predict future climate. We show that by accounting for stratospheric ozone depletion in a coupled-climate-carbon-model, the ventilation of carbon rich deep water is enhanced through stronger winds, increasing surface water CO2 at a rate in good agreement with observed trends. We find that Southern Ocean uptake is reduced by 2.47 PgC (1987-2004) and is consistent with atmospheric inversion studies. The enhanced ventilation also accelerates ocean acidification, despite lesser Southern Ocean CO2 uptake. Our results link two important anthropogenic changes: stratospheric ozone depletion and greenhouse gas increases; and suggest that studies of future climate that neglect stratospheric ozone depletion likely overestimate regional and global oceanic CO2 uptake and underestimate the impact of ocean acidification.
X. C. Wang, X. H. Li, J. Li, Z. X. Li, and W. X. Li. Nature of mantle lithological heterogenity and its role in generation of ca. 825Ma komatiitic basalts, South China. Geochimica et Cosmochimica Acta Supplement, 73:A1414, June 2009. [ bib | ADS link ]
J.-I. Yano, S. Mulet, and M. Bonazzola. Tropical large-scale circulations: asymptotically non-divergent? Tellus Series A, 61:417-427, May 2009. [ bib | DOI | ADS link ]
Although the large-scale tropical atmospheric circulations are often considered as primarily divergent, a simple scale analysis, originally presented by Charney (1963), suggests otherwise-a dominance of vorticity over divergence. The present paper quantitatively documents the asymptotic non-divergence of the large-scale tropical atmosphere, in association with Madden-Julian oscillations, with use of the Tropical Ocean Global Atmosphere-Coupled Ocean Atmosphere Response Experiment, Large-Scale Array (TOGA-COARE LSA) data set.
The vorticity is larger than the divergence at the majority (70%-80%) of points at any instant for the levels between 850 and 250 hPa, and the vorticity is more than 10 times stronger than the divergence both at 850 and 500 hPa more than half of the time. The root mean square (rms) ratio between the transient components of divergence and vorticity, which is defined as the deviation from the mean for the whole data period, decreases substantially with increasing horizontal scales from 100 to 2000 km, over an intraseasonal timescale (20-100 d). The analysis suggests that the Madden-Julian oscillations are dominated more by vorticity than divergence and more so than at the smaller scales.
The analysis as a whole suggests the feasibility of constructing an asymptotically non-divergent theory for large-scale tropical circulations. A brief sketch of the formulation is presented.
A. Jones, J. Haywood, and O. Boucher. Climate impacts of geoengineering marine stratocumulus clouds. Journal of Geophysical Research (Atmospheres), 114:D10106, May 2009. [ bib | DOI | ADS link ]
Theoretical potential geoengineering solutions to the global warming problem have recently been proposed. Here, we present an idealized study of the climate response to deliberately seeding large-scale stratocumulus cloud decks in the North Pacific, South Pacific, and South Atlantic, thereby inducing cooling via aerosol indirect effects. Atmosphere-only, atmosphere/mixed-layer ocean, and fully coupled atmosphere/ocean versions of the Met Office Hadley Centre model are used to investigate the radiative forcing, climate efficacy, and regional response of temperature, precipitation, and net primary productivity to such geoengineering. The radiative forcing simulations indicate that, for our parameterization of aerosol indirect effects, up to 35% of the radiative forcing due to current levels of greenhouse gases could be offset by stratocumulus modification. Equilibrium simulations with the atmosphere/mixed-layer ocean model, wherein each of the three stratocumulus sheets is modified in turn, reveal that the most efficient cooling per unit radiative forcing occurs when the South Pacific stratocumulus sheet is modified. Transient coupled model simulations suggest that geoengineering all three stratocumulus areas delays the simulated global warming by about 25 years. These simulations also indicate that, while some areas experience increases in precipitation and net primary productivity, sharp decreases are simulated in South America, with particularly detrimental impacts on the Amazon rain forest. These results show that, while some areas benefit from geoengineering, there are significant areas where the response could be very detrimental with implications for the practical applicability of such a scheme.
C. Rio, F. Hourdin, J.-Y. Grandpeix, and J.-P. Lafore. Shifting the diurnal cycle of parameterized deep convection over land. Geophysical Research Letters, 36:L07809, April 2009. [ bib | DOI | ADS link ]
In most atmospheric circulation models used for climate projections, cloud and convective processes are not explicitly resolved but parameterized. Such models are known to produce a diurnal cycle of continental thunderstorms in phase with insolation, while observed precipitation peaks in late afternoon. We propose a new approach which corrects this long standing bias of parameterized convection. In this approach, deep convection triggering and intensity are controlled by sub-cloud processes: here boundary layer thermals and gust fronts, and potentially orography or surface heterogeneities. The representation of the diurnal cycle of deep convection is greatly improved in 1D mode, with rainfall maximum delayed from midday to late afternoon, provided parameterizations account for the key role played by shallow cumulus in preconditioning deep convection and by gust fronts in the self-sustaining of thunderstorms in the afternoon.
J.-J. Morcrette, O. Boucher, L. Jones, D. Salmond, P. Bechtold, A. Beljaars, A. Benedetti, A. Bonet, J. W. Kaiser, M. Razinger, M. Schulz, S. Serrar, A. J. Simmons, M. Sofiev, M. Suttie, A. M. Tompkins, and A. Untch. Aerosol analysis and forecast in the European Centre for Medium-Range Weather Forecasts Integrated Forecast System: Forward modeling. Journal of Geophysical Research (Atmospheres), 114:D06206, March 2009. [ bib | DOI | ADS link ]
This paper presents the aerosol modeling now part of the ECMWF Integrated Forecasting System (IFS). It includes new prognostic variables for the mass of sea salt, dust, organic matter and black carbon, and sulphate aerosols, interactive with both the dynamics and the physics of the model. It details the various parameterizations used in the IFS to account for the presence of tropospheric aerosols. Details are given of the various formulations and data sets for the sources of the different aerosols and of the parameterizations describing their sinks. Comparisons of monthly mean and daily aerosol quantities like optical depths against satellite and surface observations are presented. The capability of the forecast model to simulate aerosol events is illustrated through comparisons of dust plume events. The ECMWF IFS provides a good description of the horizontal distribution and temporal variability of the main aerosol types. The forecast-only model described here generally gives the total aerosol optical depth within 0.12 of the relevant observations and can therefore provide the background trajectory information for the aerosol assimilation system described in part 2 of this paper.
S. Lebonnois, P. Rannou, and F. Hourdin. The coupling of winds, aerosols and chemistry in Titan's atmosphere. Philosophical Transactions of the Royal Society of London Series A, 367:665-682, February 2009. [ bib | DOI | ADS link ]
O. Boucher, A. Jones, and R. A. Betts. Climate response to the physiological impact of carbon dioxide on plants in the Met Office Unified Model HadCM3. Climate Dynamics, 32:237-249, February 2009. [ bib | DOI | ADS link ]
The concentration of carbon dioxide in the atmosphere acts to control the stomatal conductance of plants. There is observational and modelling evidence that an increase in the atmospheric concentration of CO2 would suppress the evapotranspiration (ET) rate over land. This process is known as CO2 physiological forcing and has been shown to induce changes in surface temperature and continental runoff. We analyse two transient climate simulations for the twenty-first century to isolate the climate response to the CO2 physiological forcing. The land surface warming associated with the decreased ET rate is accompanied by an increase in the atmospheric lapse rate, an increase in specific humidity, but a decrease in relative humidity and stratiform cloud over land. We find that the water vapour feedback more than compensates for the decrease in latent heat flux over land as far as the budget of atmospheric water vapour is concerned. There is evidence that surface snow, water vapour and cloudiness respond to the CO2 physiological forcing and all contribute to further warm the climate system. The climate response to the CO2 physiological forcing has a quite different signature to that from the CO2 radiative forcing, especially in terms of the changes in the temperature vertical profile and surface energy budget over land.
M. Doutriaux-Boucher, M. J. Webb, J. M. Gregory, and O. Boucher. Carbon dioxide induced stomatal closure increases radiative forcing via a rapid reduction in low cloud. Geophysical Research Letters, 36:L02703, January 2009. [ bib | DOI | ADS link ]
We performed an ensemble of twelve five-year experiments using a coupled climate-carbon-cycle model with scenarios of prescribed atmospheric carbon dioxide concentration; CO2 was instantaneously doubled or quadrupled at the start of the experiments. Within these five years, climate feedback is not significantly influenced by the effects of climate change on the carbon system. However, rapid changes take place, within much less than a year, due to the physiological effect of CO2 on plant stomatal conductance, leading to adjustment in the shortwave cloud radiative effect over land, due to a reduction in low cloud cover. This causes a 10% enhancement to the radiative forcing due to CO2, which leads to an increase in the equilibrium warming of 0.4 and 0.7 K for doubling and quadrupling. The implications for calibration of energy-balance models are discussed.
F. Chéruy and F. Aires. Cluster Analysis of Cloud Properties over the Southern European Mediterranean Area in Observations and a Model. Monthly Weather Review, 137:3161, 2009. [ bib | DOI | ADS link ]
H. Brogniez, R. Roca, and L. Picon. A Study of the Free Tropospheric Humidity Interannual Variability Using Meteosat Data and an Advection-Condensation Transport Model. Journal of Climate, 22:6773, 2009. [ bib | DOI | ADS link ]
E. Guilyardi, P. Braconnot, F.-F. Jin, S. T. Kim, M. Kolasinski, T. Li, and I. Musat. Atmosphere Feedbacks during ENSO in a Coupled GCM with a Modified Atmospheric Convection Scheme. Journal of Climate, 22:5698, 2009. [ bib | DOI | ADS link ]
G. Gastineau, L. Li, and H. Le Treut. The Hadley and Walker Circulation Changes in Global Warming Conditions Described by Idealized Atmospheric Simulations. Journal of Climate, 22:3993, 2009. [ bib | DOI | ADS link ]
J.-I. Yano and M. Bonazzola. Scale Analysis for Large-Scale Tropical Atmospheric Dynamics. Journal of Atmospheric Sciences, 66:159, 2009. [ bib | DOI | ADS link ]
A. Plana-Fattori, G. Brogniez, P. Chervet, M. Haeffelin, O. Lado-Bordowsky, Y. Morille, F. Parol, J. Pelon, A. Roblin, G. Sèze, and C. Stubenrauch. Comparison of High-Cloud Characteristics as Estimated by Selected Spaceborne Observations and Ground-Based Lidar Datasets. Journal of Applied Meteorology and Climatology, 48:1142, 2009. [ bib | DOI | ADS link ]
J. P. Duvel, C. Basdevant, H. Bellenger, G. Reverdin, A. Vargas, and J. Vialard. The Aeroclipper: A New Device to Explore Convective Systems and Cyclones. Bulletin of the American Meteorological Society, 90:63, 2009. [ bib | DOI | ADS link ]