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lmd_Li2002_abstracts.html

2002 .

(4 publications)

R. Roca, M. Viollier, L. Picon, and M. Desbois. A multisatellite analysis of deep convection and its moist environment over the Indian Ocean during the winter monsoon. Journal of Geophysical Research (Atmospheres), 107:8012, August 2002. [ bib | DOI | ADS link ]

The aim of this paper is to characterize the deep convective systems over the Indian Ocean during Indian Ocean Experiment (INDOEX) and their relationship to cloudiness and to the Upper Tropospheric Humidity (UTH) of their environment together with the relevant longwave radiation fields. Multisatellite analyses are performed (Meteosat, Scanner for Radiation Budget (ScaRaB), and Special Sensor Microwave Imager (SSM/I)) to measure these environmental parameters. The use of Meteosat water vapor (WV) channel appears very efficient not only for estimating UTH but also for separating high level cloudiness, including thin cirrus, from clear sky and low clouds. The Meteosat infrared (IR) and WV channels are also used for correlating Meteosat and ScaRaB measurements, allowing to retrieve continuously the longwave radiative flux. The longwave flux is used to compute the cloud radiative forcing as well as the clear-sky greenhouse effect. Spatial relationships between upper level cloudiness and UTH are established. A strong positive linear relationship is found suggesting a local moistening of the upper troposphere by convection. The temporal analysis reveals that during the active phase of the intraseasonal oscillation, the longwave cloud radiative forcing reaches a mean value up to 40 W m-2 over a large region in the open ocean, while the average clear-sky greenhouse effect is in excess of 180 W m-2. These radiative parameters are strongly correlated with the upper level cloudiness and upper level moisture, respectively. The temporal variability of UTH explains up to 80% of the greenhouse effect variability. The structure of the convective cloud systems is then studied. The observed population of systems spans a wide spectrum of area from 100 to 1,000,000 km2. The contribution to the high level cloudiness of the systems with a strong vertical development is dominant. These systems, with at least one convective cell reaching the highest levels (below 210 K), present indices of overshooting tops and are the most horizontally extended. The largest system exhibits an average longwave radiative forcing of around 100 W m-2. Their contribution to the cloud forcing over the Indian Ocean is overwhelming. The spatial and temporal variability of the systems is finally related to the UTH and to the clear-sky greenhouse effect. Strong correlations are found indicating that these organized convective systems at mesoscale play a leading role in the Indian Ocean climate. The analysis suggests that deeper convection is associated with larger cloud desks with larger cloud radiative forcing. It is also associated with a moister upper troposphere and a larger clear-sky greenhouse effect. These two effects would provide a positive feedback on the surface conditions.

J.-L. Dufresne, L. Fairhead, H. Le Treut, M. Berthelot, L. Bopp, P. Ciais, P. Friedlingstein, and P. Monfray. On the magnitude of positive feedback between future climate change and the carbon cycle. Geophysical Research Letters, 29:1405, May 2002. [ bib | DOI | ADS link ]

We use an ocean-atmosphere general circulation model coupled to land and ocean carbon models to simulate the evolution of climate and atmospheric CO2 from 1860 to 2100. Our model reproduces the observed global mean temperature changes and the growth rate of atmospheric CO2 for the period 1860-2000. For the future, we simulate that the climate change due to CO2 increase will reduce the land carbon uptake, leaving a larger fraction of anthropogenic CO2 in the atmosphere. By 2100, we estimate that atmospheric CO2 will be 18% higher due to the climate change impact on the carbon cycle. Such a positive feedback has also been found by Cox et al. [2000]. However, the amplitude of our feedback is three times smaller than the one they simulated. We show that the partitioning between carbon stored in the living biomass or in the soil, and their respective sensitivity to increased CO2 and climate change largely explain this discrepancy.

T.-J. Zhou and Z.-X. Li. Simulation of the east asian summer monsoon using a variable resolution atmospheric GCM. Climate Dynamics, 19:167-180, February 2002. [ bib | DOI | ADS link ]

The East Asia summer monsoon (EASM) is simulated with a variable resolution global atmospheric general circulation model (GCM) developed at the Laboratoire de Météorologie Dynamique, France. The version used has a local zoom centered on China. This study validates the model's capability in reproducing the fundamental features of the EASM. The monsoon behaviors over East Asia revealed by the ECMWF reanalysis data are also addressed systematically, providing as observational evidence. The mean state of the EASM is generally portrayed well in the model, including the large-scale monsoon airflows, the monsoonal meridional circulation, the cross-equatorial low-level jets, the monsoon trough in the South China Sea, the surface cold high in Australia, and the upper-level northeasterly return flow. While the performance of simulating large-scale monsoonal climate is encouraging, the model's main deficiency lies in the rainfall. The marked rainbelt observed along the Yangtze River Valley is missed in the simulation. This is due to the weakly reproduced monsoonal components in essence and is directly related to the weak western Pacific subtropical high, which leads to a fragile subtropical southwest monsoon on its western flank and results in a weaker convergence of the southwest monsoon flow with the midlatitude westerlies. The excessively westward extension of the high, together with the distorted Indian low, also makes the contribution of the tropical southwest monsoon to the moisture convergence over the Yangtze River Valley too weak in the model. The insufficient plateau heating and the resulting weak land-sea thermal contrast are responsible for the weakly reproduced monsoon. It is the deficiency of the model in handling the low-level cloud cover over the plateau rather than the horizontal resolution and the associated depiction of plateau topography that results in the insufficient plateau heating. Comparison with the simulation employing regular coarser mesh model reveals that the local zoom technique improves, in a general manner, the EASM simulation.

F. M. Bréon, J. C. Buriez, P. Couvert, P. Y. Deschamps, J. L. Deuzé, M. Herman, P. Goloub, M. Leroy, A. Lifermann, C. Moulin, F. Parol, G. Sèze, D. Tanré, C. Vanbauce, and M. Vesperini. Scientific results from the Polarization and Directionality of the Earth's Reflectances (POLDER). Advances in Space Research, 30:2383-2386, 2002. [ bib | DOI | ADS link ]

The POLDER (POlarization and Directionality of the Earth's Reflectances) instrument, developed by the French Space Agency (CNES) has flown on board the ADEOS-1/NASDA platform from November 1996 until June 1997. The sensor has a wide field of view (2400km swath) for collecting global daily data and has multi-angle viewing capability. It measures the solar radiation reflected by the Earth in eight spectral bands. For three of these bands (0.443, 0.670 and 0.865 μm), measurements include the polarization ratio by the use of 3 polarizers. This measurement strategy provides unique information on aerosols, clouds and surfaces.

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