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2012 .

(7 publications)

A. Arakelian and F. Codron. Southern Hemisphere Jet Variability in the IPSL GCM at Varying Resolutions. Journal of Atmospheric Sciences, 69:3788-3799, December 2012. [ bib | DOI | ADS link ]

A. A. Mrowiec, C. Rio, A. M. Fridlind, A. S. Ackerman, A. D. Del Genio, O. M. Pauluis, A. C. Varble, and J. Fan. Analysis of cloud-resolving simulations of a tropical mesoscale convective system observed during TWP-ICE: Vertical fluxes and draft properties in convective and stratiform regions. Journal of Geophysical Research (Atmospheres), 117:19201, October 2012. [ bib | DOI | ADS link ]

We analyze three cloud-resolving model simulations of a strong convective event observed during the TWP-ICE campaign, differing in dynamical core, microphysical scheme or both. Based on simulated and observed radar reflectivity, simulations roughly reproduce observed convective and stratiform precipitating areas. To identify the characteristics of convective and stratiform drafts that are difficult to observe but relevant to climate model parameterization, independent vertical wind speed thresholds are calculated to capture 90% of total convective and stratiform updraft and downdraft mass fluxes. Convective updrafts are fairly consistent across simulations (likely owing to fixed large-scale forcings and surface conditions), except that hydrometeor loadings differ substantially. Convective downdraft and stratiform updraft and downdraft mass fluxes vary notably below the melting level, but share similar vertically uniform draft velocities despite differing hydrometeor loadings. All identified convective and stratiform downdrafts contain precipitation below 10 km and nearly all updrafts are cloudy above the melting level. Cold pool properties diverge substantially in a manner that is consistent with convective downdraft mass flux differences below the melting level. Despite differences in hydrometeor loadings and cold pool properties, convective updraft and downdraft mass fluxes are linearly correlated with convective area, the ratio of ice in downdrafts to that in updrafts is 0.5 independent of species, and the ratio of downdraft to updraft mass flux is 0.5-0.6, which may represent a minimum evaporation efficiency under moist conditions. Hydrometeor loading in stratiform regions is found to be a fraction of hydrometeor loading in convective regions that ranges from 10% (graupel) to 90% (cloud ice). These findings may lead to improved convection parameterizations.

J.-E. Lee, C. Risi, I. Fung, J. Worden, R. A. Scheepmaker, B. Lintner, and C. Frankenberg. Asian monsoon hydrometeorology from TES and SCIAMACHY water vapor isotope measurements and LMDZ simulations: Implications for speleothem climate record interpretation. Journal of Geophysical Research (Atmospheres), 117:15112, August 2012. [ bib | DOI | ADS link ]

Observations show that heavy oxygen isotope composition in precipitation (δ18Op) increases from coastal southeastern (SE) China to interior northwestern (NW) China during the wet season, contradicting expectations from simple Rayleigh distillation theory. Here we employ stable isotopes of precipitation and vapor from satellite measurements and climate model simulations to characterize the moisture processes that control Asian monsoon precipitation and relate these processes to speleothem paleoclimate records. We find that δ18Op is low over SE China as a result of local and upstream condensation and that δ18Op is high over NW China because of evaporative enrichment of 18O as raindrops fall through dry air. We show that δ18Op at cave sites over southern China is weakly correlated with upstream precipitation in the core of the Indian monsoon region rather than local precipitation, but it is well-correlated with the δ18Op over large areas of southern and central China, consistent with coherent speleothem δ18Op variations over different parts of China. Previous studies have documented high correlations between speleothem δ18Op and millennial timescale climate forcings, and we suggest that the high correlation between insolation and speleothem δ18Op in southern China reflects the variations of hydrologic processes over the Indian monsoon region on millennial and orbital timescales. The δ18Op in the drier part (north of 30degN) of China, on the other hand, has consistently negative correlations with local precipitation and may capture local hydrologic processes related to changes in the extent of the Hadley circulation.

N. Huneeus, F. Chevallier, and O. Boucher. Estimating aerosol emissions by assimilating observed aerosol optical depth in a global aerosol model. Atmospheric Chemistry & Physics, 12:4585-4606, May 2012. [ bib | DOI | ADS link ]

This study estimates the emission fluxes of a range of aerosol species and one aerosol precursor at the global scale. These fluxes are estimated by assimilating daily total and fine mode aerosol optical depth (AOD) at 550 nm from the Moderate Resolution Imaging Spectroradiometer (MODIS) into a global aerosol model of intermediate complexity. Monthly emissions are fitted homogenously for each species over a set of predefined regions. The performance of the assimilation is evaluated by comparing the AOD after assimilation against the MODIS observations and against independent observations. The system is effective in forcing the model towards the observations, for both total and fine mode AOD. Significant improvements for the root mean square error and correlation coefficient against both the assimilated and independent datasets are observed as well as a significant decrease in the mean bias against the assimilated observations. These improvements are larger over land than over ocean. The impact of the assimilation of fine mode AOD over ocean demonstrates potential for further improvement by including fine mode AOD observations over continents. The Angström exponent is also improved in African, European and dusty stations. The estimated emission flux for black carbon is 15 Tg yr-1, 119 Tg yr-1 for particulate organic matter, 17 Pg yr-1 for sea salt, 83 TgS yr-1 for SO2 and 1383 Tg yr-1 for desert dust. They represent a difference of +45 %, +40 %, +26 %, +13 % and -39 % respectively, with respect to the a priori values. The initial errors attributed to the emission fluxes are reduced for all estimated species.

H. Zhang, Z. Wang, Z. Wang, Q. Liu, S. Gong, X. Zhang, Z. Shen, P. Lu, X. Wei, H. Che, and L. Li. Simulation of direct radiative forcing of aerosols and their effects on East Asian climate using an interactive AGCM-aerosol coupled system. Climate Dynamics, 38:1675-1693, April 2012. [ bib | DOI | ADS link ]

An interactive system coupling the Beijing Climate Center atmospheric general circulation model (BCC_AGCM2.0.1) and the Canadian Aerosol Module (CAM) with updated aerosol emission sources was developed to investigate the global distributions of optical properties and direct radiative forcing (DRF) of typical aerosols and their impacts on East Asian climate. The simulated total aerosol optical depth (AOD), single scattering albedo, and asymmetry parameter were generally consistent with the ground-based measurements. Under all-sky conditions, the simulated global annual mean DRF at the top of the atmosphere was -2.03 W m-2 for all aerosols including sulfate, organic carbon (OC), black carbon (BC), dust, and sea salt; the global annual mean DRF was -0.23 W m-2 for sulfate, BC, and OC aerosols. The sulfate, BC, and OC aerosols led to decreases of 0.58deg and 0.14 mm day-1 in the JJA means of surface temperature and precipitation rate in East Asia. The differences of land-sea surface temperature and surface pressure were reduced in East Asian monsoon region due to these aerosols, thus leading to the weakening of East Asian summer monsoon. Atmospheric dynamic and thermodynamic were affected due to the three types of aerosol, and the southward motion between 15degN and 30degN in lower troposphere was increased, which slowed down the northward transport of moist air carried by the East Asian summer monsoon, and moreover decreased the summer monsoon precipitation in south and east China.

S. Brachet, F. Codron, Y. Feliks, M. Ghil, H. Le Treut, and E. Simonnet. Atmospheric Circulations Induced by a Midlatitude SST Front: A GCM Study. Journal of Climate, 25:1847-1853, March 2012. [ bib | DOI | ADS link ]

C. Shi, V. Daux, Q.-B. Zhang, C. Risi, S.-G. Hou, M. Stievenard, M. Pierre, Z. Li, and V. Masson-Delmotte. Reconstruction of southeast Tibetan Plateau summer climate using tree ring δ18O: moisture variability over the past two centuries. Climate of the Past, 8:205-213, February 2012. [ bib | DOI | ADS link ]

A tree-ring δ18O chronology of Linzhi spruce, spanning from AD 1781 to 2005, was developed in Bomi, Southeast Tibetan Plateau (TP). During the period with instrumental data (AD 1961-2005), this record is strongly correlated with regional CRU (Climate Research Unit) summer cloud data, which is supported by a precipitation δ18O simulation conducted with the isotope-enabled atmospheric general circulation model LMDZiso. A reconstruction of a regional summer cloud index, based upon the empirical relationship between cloud and diurnal temperature range, was therefore achieved. This index reflects regional moisture variability in the past 225 yr. The climate appears drier and more stable in the 20th century than previously. The drying trend in late 19th century of our reconstruction is consistent with a decrease in the TP glacier accumulation recorded in ice cores. An exceptional dry decade is documented in the 1810s, possibly related to the impact of repeated volcanic eruptions on monsoon flow.

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