C. Risi, S. Bony, F. Vimeux, L. Descroix, B. Ibrahim, E. Lebreton, I. Mamadou, and B. Sultan. What controls the isotopic composition of the African monsoon precipitation? Insights from event-based precipitation collected during the 2006 AMMA field campaign. Geophysical Research Letters, 35:L24808, December 2008. [ bib | DOI | ADS link ]
The stable isotopic composition of the tropical precipitation constitutes a useful tool for paleoclimate reconstructions and to better constrain the water cycle. To better understand what controls the isotopic composition of tropical precipitation, we analyze the δ 18O and deuterium-excess of the precipitation of individual events collected in the Niamey area (Niger) during the monsoon season, as part of the 2006 AMMA field campaign. During the monsoon onset, the abrupt increase of convective activity over the Sahel is associated with an abrupt change in the isotopic composition. Before the onset, when convective activity is scarce, the rain composition records the intensity and the organization of individual convective systems. After the onset, on the contrary, it records a regional-scale intra-seasonal variability over the Sahel, by integrating convective activity both spatially and temporally over the previous days.
S. Verma, C. Venkataraman, and O. Boucher. Origin of surface and columnar Indian Ocean Experiment (INDOEX) aerosols using source- and region-tagged emissions transport in a general circulation model. Journal of Geophysical Research (Atmospheres), 113:D24211, December 2008. [ bib | DOI | ADS link ]
We study the relative influence of aerosols emitted from different sectors and geographical regions on aerosol loading in south Asia. Sectors contributing aerosol emissions include biofuel and fossil fuel combustion, open biomass burning, and natural sources. Geographical regions include India (the Indo-Gangetic plain, central India, south India, and northwest India), southeast Asia, east Asia, Africa-west Asia, and the rest of the world. Simulations of the Indian Ocean Experiment (INDOEX), from January to March 1999, are made in the general circulation model of Laboratoire de Météorologie Dynamique (LMD-ZT GCM) with emissions tagged by sector and geographical region. Anthropogenic emissions dominate (54-88%) the predicted aerosol optical depth (AOD) over all the receptor regions. Among the anthropogenic sectors, fossil fuel combustion has the largest overall influence on aerosol loading, primarily sulfate, with emissions from India (50-80%) and rest of the world significantly influencing surface concentrations and AOD. Biofuel combustion has a significant influence on both the surface and columnar black carbon (BC) in particular over the Indian subcontinent and Bay of Bengal with emissions largely from the Indian region (60-80%). Open biomass burning emissions influence organic matter (OM) significantly, and arise largely from Africa-west Asia. The emissions from Africa-west Asia affect the carbonaceous aerosols AOD in all receptor regions, with their largest influence (AOD-BC: 60%; and AOD-OM: 70%) over the Arabian Sea. Among Indian regions, the Indo-Gangetic Plain is the largest contributor to anthropogenic surface mass concentrations and AOD over the Bay of Bengal and India. Dust aerosols are contributed mainly through the long-range transport from Africa-west Asia over the receptor regions. Overall, the model estimates significant intercontinental incursion of aerosol, for example, BC, OM, and dust from Africa-west Asia and sulfate from distant regions (rest of the world) into the INDOEX domain.
J.-J. Morcrette, A. Beljaars, A. Benedetti, L. Jones, and O. Boucher. Sea-salt and dust aerosols in the ECMWF IFS model. Geophysical Research Letters, 35:L24813, December 2008. [ bib | DOI | ADS link ]
The ECMWF IFS model has recently been modified to include prognostic aerosols in its analysis and forecast modules. For the sea salt and dust components, comparisons of three versions of the model are presented: (i) a forecast only model started from conventional analysis with free-running aerosols, (ii) a full analysis including aerosols, and (iii) as in (i) but with sea salt and dust sources revised to account for the 10-m wind including gustiness and calibrated on the aerosol analysis results. It is shown that this new formulation of the sources of the main natural aerosols gives an improved agreement with AERONET surface observations where sea salt and dust aerosols are dominant. It also shows how the information brought by the aerosol analysis can be used to improve the representation of aerosols in numerical weather prediction and climate-type general circulation models.
C. Risi, S. Bony, and F. Vimeux. Influence of convective processes on the isotopic composition (δ18O and δD) of precipitation and water vapor in the tropics: 2. Physical interpretation of the amount effect. Journal of Geophysical Research (Atmospheres), 113:D19306, October 2008. [ bib | DOI | ADS link ]
In the tropics, the proportion of heavier water isotopes in precipitation is anticorrelated with the precipitation amount. The physical processes underlying this so-called amount effect are still poorly understood and quantified. In the present study, stable water isotopes (H218O and HDO) have been introduced in a single column model including the Emanuel convection parameterization. We investigate the physical processes underlying the amount effect and propose a methodology to quantify their relative contributions. We focus on convective processes, since the idealized framework of the single column models does not allow us to consider the effects of large-scale horizontal advections of air masses of different isotopic signatures. We show that two kinds of processes predominantly explain the amount effect: first, the reevaporation of the falling rain and the diffusive exchanges with the surrounding vapor; and second, the recycling of the subcloud layer vapor feeding the convective system by convective fluxes. This highlights the importance of a detailed representation of rain evaporation processes to simulate accurately the isotopic composition of precipitation in the tropics. The variability of the isotopic composition on different timescales (from days to months) is also studied using a unidimensional simulation of the Tropical Ocean-Global Atmosphere-Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) campaign. The amount effect is best observable at intraseasonal or longer timescales. The period of time over which convective activity significantly affects the isotopic composition of precipitation is related to the residence time of water within atmospheric reservoirs.
S. Bony, C. Risi, and F. Vimeux. Influence of convective processes on the isotopic composition (δ18O and δD) of precipitation and water vapor in the tropics: 1. Radiative-convective equilibrium and Tropical Ocean-Global Atmosphere-Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) simulations. Journal of Geophysical Research (Atmospheres), 113:D19305, October 2008. [ bib | DOI | ADS link ]
Cumulus convection constitutes a key process in the control of tropical precipitation and the vertical transport of atmospheric water. To better understand the influence of convective processes on the isotopic composition of precipitation and water vapor, water stable isotopes (H218O and HDO) are introduced into a single column model including the Emanuel convective parameterization. This paper analyzes unidimensional simulations of the tropical atmosphere in a state of radiative-convective equilibrium, and simulations forced by data from the Tropical Ocean-Global Atmosphere-Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). This study shows that deep convective atmospheres are associated with robust isotopic features such as an isotopic composition of the air below the tropical tropopause layer (around 12-13 km) close to the typical values observed in the lower tropical stratosphere, and an isotopic enrichment of the upper tropospheric water that starts well below the tropopause. It highlights the critical role of condensate lofting and convective detrainment in these features, and the role of convective unsaturated downdrafts in the control of the isotopic composition of precipitation. Finally, it shows that the so-called “amount effect” primarily reveals the influence of large-scale atmospheric circulation changes on the isotopic composition of the precipitation, and that temperature changes not associated with circulation changes lead to an “anti-amount effect”. The detailed analysis of the physical processes underlying the “amount effect” is presented in a companion paper.
G. Gastineau, H. Le Treut, and L. Li. Hadley circulation changes under global warming conditions indicated by coupled climate models. Tellus Series A, 60:863-884, October 2008. [ bib | DOI | ADS link ]
We use the mean meridional tropical circulation of the Atmospheric Ocean Coupled General Circulation Models (AOGCM) to diagnose and quantify the modifications of the mean meridional circulation of the atmosphere under global warming conditions. The AOGCMs generally show a weakening of the Hadley circulation for the winter cell in both hemispheres, accompanied by a poleward extension of the Hadley circulation area. The conditions explaining these modifications are analysed using detailed outputs from IPSL-CM4. The AOGCM IPSL-CM4 shows changes, under CO2 doubling, that are in accordance with the other models, for austral winter. On the other hand, for boreal winter, the winter cell shows little change in intensity and in extension. The poleward shift of the Southern Hemisphere winter Hadley cell corresponds to changes in the transient eddies, whereas the increase of the dry static stability is mainly responsible for the mean weakening of the cell. For boreal winter, a strong shrinking of the ascending branch area, and an increase of the latent heating, is found to cancel the weakening of the circulation due to dry static stability increase.
A. Crespin, S. Lebonnois, S. Vinatier, B. Bézard, A. Coustenis, N. A. Teanby, R. K. Achterberg, P. Rannou, and F. Hourdin. Diagnostics of Titan's stratospheric dynamics using Cassini/CIRS data and the 2-dimensional IPSL circulation model. Icarus, 197:556-571, October 2008. [ bib | DOI | ADS link ]
The dynamics of Titan's stratosphere is discussed in this study, based on a comparison between observations by the CIRS instrument on board the Cassini spacecraft, and results of the 2-dimensional circulation model developed at the Institute Pierre-Simon Laplace, available at http://www.lmd.jussieu.fr/titanDbase [Rannou, P., Lebonnois, S., Hourdin, F., Luz, D., 2005. Adv. Space Res. 36, 2194-2198]. The comparison aims at both evaluating the model's capabilities and interpreting the observations concerning: (1) dynamical and thermal structure using temperature retrievals from Cassini/CIRS and the vertical profile of zonal wind at the Huygens landing site obtained by Huygens/DWE; and (2) vertical and latitudinal profiles of stratospheric gases deduced from Cassini/CIRS data. The modeled thermal structure is similar to that inferred from observations (Cassini/CIRS and Earth-based observations). However, the upper stratosphere (above 0.05 mbar) is systematically too hot in the 2D-CM, and therefore the stratopause region is not well represented. This bias may be related to the haze structure and to misrepresented radiative effects in this region, such as the cooling effect of hydrogen cyanide (HCN). The 2D-CM produces a strong atmospheric superrotation, with zonal winds reaching 200 m s -1 at high winter latitudes between 200 and 300 km altitude (0.1-1 mbar). The modeled zonal winds are in good agreement with retrieved wind fields from occultation observations, Cassini/CIRS and Huygens/DWE. Changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension, and therefore affect chemical distributions, especially in winter polar regions. When a higher altitude haze production source is used, the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude. There is an overall good agreement between modeled chemical distributions and observations in equatorial regions. The difference in observed vertical gradients of C 2H 2 and HCN may be an indicator of the relative strength of circulation and chemical loss of HCN. The negative vertical gradient of ethylene in the low stratosphere at 15deg S, cannot be modeled with simple 1-dimensional models, where a strong photochemical sink in the middle stratosphere would be necessary. It is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense. Near the winter pole (80deg N), some compounds (C 4H 2, C 3H 4) exhibit an (interior) minimum in the observed abundance vertical profiles, whereas 2D-CM profiles are well mixed all along the atmospheric column. This minimum can be a diagnostic of the strength of the meridional circulation, and of the spatial extension of the winter polar vortex where strong descending motions are present. In the summer hemisphere, observed stratospheric abundances are uniform in latitude, whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar, at latitudes south of 40-50deg S. The strength, as well as spatial and temporal extensions of this structure are a difficulty, that may be linked to possible misrepresentation of horizontally mixing processes, due to the restricted 2-dimensional nature of the model. This restriction should also be kept in mind as a possible source of other discrepancies.
R. James, M. Bonazzola, B. Legras, K. Surbled, and S. Fueglistaler. Water vapor transport and dehydration above convective outflow during Asian monsoon. Geophysical Research Letters, 35:L20810, October 2008. [ bib | DOI | ADS link ]
We investigate the respective roles of large-scale transport and convection in determining the water vapor maximum at 100 hPa in the Asian monsoon region. The study uses backward trajectories with ECMWF ERA-Interim heating rates. It includes simple microphysics with supersaturation and takes into account convective sources based on CLAUS data with a simple parameterization of overshoots. A good agreement between reconstructed water vapor and observations is obtained over Asia. It is found that parcels belonging to the water vapor maximum have been first lifted by convection over the Bay of Bengal and the Sea of China and then transported through the tropical tropopause layer (TTL) via the monsoon anticyclonic circulation towards North-West India, where they are eventually dehydrated, avoiding the coldest temperatures of the TTL. Convective moistening accounts for about 0.3 ppmv in the Asian monsoon region and overshoots do not have a significant impact on the water vapor budget.
H. Le Treut, G. Gastineau, and L. Li. Uncertainties attached to global or local climate changes. Comptes Rendus Geoscience, 340:584-590, September 2008. [ bib | DOI | ADS link ]
The successive reports of the Intergovernmental Panel on Climate Change (IPCC) illustrate an apparent contradiction. On the one hand, the large scale climatic change in response to the increase of greenhouse gases is structured following patterns which have remained surprisingly stable throughout the development of climate models. Already in the 1980s model simulations of climate change were characterized by a larger warming in polar areas and over the continents, and a tendency for precipitations to accentuate existing contrasts, with a drier climate in semiarid regions and more precipitations at mid-latitudes or near the Equator. On the other hand, models have made little progress in predicting more unanimously and more reliably the global amplitude of climate changes and their geographical distributions. This lack of progress is certainly linked with the role of the atmospheric dynamics in shaping up certain aspects of climate response, either small scales which affect atmospheric stratification, or synoptic scales, whose inherent complexity and nonlinear interactions also limit the possibility of more accurate regional predictions.
H. Chepfer, S. Bony, D. Winker, M. Chiriaco, J.-L. Dufresne, and G. Sèze. Use of CALIPSO lidar observations to evaluate the cloudiness simulated by a climate model. Geophysical Research Letters, 35:L15704, August 2008. [ bib | DOI | ADS link ]
New space-borne active sensors make it possible to observe the three-dimensional structure of clouds. Here we use CALIPSO lidar observations together with a lidar simulator to evaluate the cloudiness simulated by a climate model: modeled atmospheric profiles are converted to an ensemble of subgrid-scale attenuated backscatter lidar signals from which a cloud fraction is derived. Except in regions of persistent thick upper-level clouds, the cloud fraction diagnosed through this procedure is close to that actually predicted by the model. A fractional cloudiness is diagnosed consistently from CALIPSO data at a spatio-temporal resolution comparable to that of the model. The comparison of the model's cloudiness with CALIPSO data reveals discrepancies more pronounced than in previous model evaluations based on passive observations. This suggests that space-borne lidar observations constitute a powerful tool for the evaluation of clouds in large-scale models, including marine boundary-layer clouds
J.-A. Wartho, Z.-X. Li, S. A. Occhipinti, and S. Reddy. 40Ar/39Ar UV laser dating, EBSD and EMP analysis of 1040-940 Ma metamorphic/deformation/cooling events recorded in Sibao Orogen white micas, South China. Geochimica et Cosmochimica Acta Supplement, 72:A1006, July 2008. [ bib | ADS link ]
P. Yiou, K. Goubanova, Z. X. Li, and M. Nogaj. Weather regime dependence of extreme value statistics for summer temperature and precipitation. Nonlinear Processes in Geophysics, 15:365-378, May 2008. [ bib | ADS link ]
Extreme Value Theory (EVT) is a useful tool to describe the statistical properties of extreme events. Its underlying assumptions include some form of temporal stationarity in the data. Previous studies have been able to treat long-term trends in datasets, to obtain the time dependence of EVT parameters in a parametric form. Since there is also a dependence of surface temperature and precipitation to weather patterns obtained from pressure data, we determine the EVT parameters of those meteorological variables over France conditional to the occurrence of North Atlantic weather patterns in the summer. We use a clustering algorithm on geopotential height data over the North Atlantic to obtain those patterns. This approach refines the straightforward application of EVT on climate data by allowing us to assess the role of atmospheric variability on temperature and precipitation extreme parameters. This study also investigates the statistical robustness of this relation. Our results show how weather regimes can modulate the different behavior of mean climate variables and their extremes. Such a modulation can be very different for the mean and extreme precipitation.
J. Quaas, O. Boucher, N. Bellouin, and S. Kinne. Satellite-based estimate of the direct and indirect aerosol climate forcing. Journal of Geophysical Research (Atmospheres), 113:D05204, March 2008. [ bib | DOI | ADS link ]
The main uncertainty in anthropogenic forcing of the Earth's climate stems from pollution aerosols, particularly their “indirect effect” whereby aerosols modify cloud properties. We develop a new methodology to derive a measurement-based estimate using almost exclusively information from an Earth radiation budget instrument (CERES) and a radiometer (MODIS). We derive a statistical relationship between planetary albedo and cloud properties, and, further, between the cloud properties and column aerosol concentration. Combining these relationships with a data set of satellite-derived anthropogenic aerosol fraction, we estimate an anthropogenic radiative forcing of -0.9 +/- 0.4 Wm-2 for the aerosol direct effect and of -0.2 +/- 0.1 Wm-2 for the cloud albedo effect. Because of uncertainties in both satellite data and the method, the uncertainty of this result is likely larger than the values given here which correspond only to the quantifiable error estimates. The results nevertheless indicate that current global climate models may overestimate the cloud albedo effect.
C. Rio and F. Hourdin. A Thermal Plume Model for the Convective Boundary Layer: Representation of Cumulus Clouds. Journal of Atmospheric Sciences, 65:407-425, February 2008. [ bib | DOI | ADS link ]
J.-L. Dufresne and S. Bony. An Assessment of the Primary Sources of Spread of Global Warming Estimates from Coupled Atmosphere Ocean Models. Journal of Climate, 21:5135, 2008. [ bib | DOI | ADS link ]
O. Arzel, T. Fichefet, H. Goosse, and J.-L. Dufresne. Causes and impacts of changes in the Arctic freshwater budget during the twentieth and twenty-first centuries in an AOGCM. Climate Dynamics, 30:37-58, January 2008. [ bib | DOI | ADS link ]
The fourth version of the atmosphere-ocean general circulation (AOGCM) model developed at the Institut Pierre-Simon Laplace (IPSL-CM4) is used to investigate the mechanisms influencing the Arctic freshwater balance in response to anthropogenic greenhouse gas forcing. The freshwater influence on the interannual variability of deep winter oceanic convection in the Nordic Seas is also studied on the basis of correlation and regression analyses of detrended variables. The model shows that the Fram Strait outflow, which is an important source of freshwater for the northern North Atlantic, experiences a rapid and strong transition from a weak state toward a relatively strong state during 1990-2010. The authors propose that this climate shift is triggered by the retreat of sea ice in the Barents Sea during the late twentieth century. This sea ice reduction initiates a positive feedback in the atmosphere-sea ice-ocean system that alters both the atmospheric and oceanic circulations in the Greenland-Iceland-Norwegian (GIN)-Barents Seas sector. Around year 2080, the model predicts a second transition threshold beyond which the Fram Strait outflow is restored toward its original weak value. The long-term freshening of the GIN Seas is invoked to explain this rapid transition. It is further found that the mechanism of interannual changes in deep mixing differ fundamentally between the twentieth and twenty-first centuries. This difference is caused by the dominant influence of freshwater over the twenty-first century. In the GIN Seas, the interannual changes in the liquid freshwater export out of the Arctic Ocean through Fram Strait combined with the interannual changes in the liquid freshwater import from the North Atlantic are shown to have a major influence in driving the interannual variability of the deep convection during the twenty-first century. South of Iceland, the other region of deep water renewal in the model, changes in freshwater import from the North Atlantic constitute the dominant forcing of deep convection on interannual time scales over the twenty-first century.
A. Hollingsworth, R. J. Engelen, C. Textor, A. Benedetti, O. Boucher, F. Chevallier, A. Dethof, H. Elbern, H. Eskes, J. Flemming, C. Granier, J. W. Kaiser, J.-J. Morcrette, P. Rayner, V.-H. Peuch, L. Rouil, M. G. Schultz, and A. J. Simmons. Toward a Monitoring and Forecasting System For Atmospheric Composition: The GEMS Project. Bulletin of the American Meteorological Society, 89:1147, 2008. [ bib | DOI | ADS link ]
M. T. Woodhouse, G. W. Mann, K. S. Carslaw, and O. Boucher. New Directions: The impact of oceanic iron fertilisation on cloud condensation nuclei. Atmospheric Environment, 42:5728-5730, 2008. [ bib | DOI | ADS link ]