lmd_Risi2010_abstracts.html

2010 .

(7 publications)

C. Risi, S. Bony, F. Vimeux, and J. Jouzel. Correction to ”Water-stable isotopes in the LMDZ4 general circulation model: Model evaluation for present-day and past climates and applications to climatic interpretations of tropical isotopic records”. Journal of Geophysical Research (Atmospheres), 115:24123, December 2010. [ bib | DOI | ADS link ]

C. Risi, S. Bony, F. Vimeux, C. Frankenberg, D. Noone, and J. Worden. Understanding the Sahelian water budget through the isotopic composition of water vapor and precipitation. Journal of Geophysical Research (Atmospheres), 115:24110, December 2010. [ bib | DOI | ADS link ]

The goal of this paper is to investigate the added value of water isotopic measurements to estimate the relative influence of large-scale dynamics, convection, and land surface recycling on the Sahelian water budget. To this aim, we use isotope data in the lower tropospheric water vapor measured by the SCIAMACHY and TES satellite instruments and in situ precipitation data from the Global Network for Isotopes in Precipitation and collected during the African Monsoon Multidisciplinary Analysis field campaign, together with water-tagging experiments with the Laboratoire de Météorologie Dynamique general circulation model (LMDZ) fitted with isotopes. We show that some isotopic biases in LMDZ reveal the misrepresentation of dehydrating processes that would be undetected without isotopic measurements. In dry regions, the vapor isotopic composition is primarily controlled by the intensity of the air dehydration. In addition, it may also keep some memory of dehydration pathways that is erased in the humidity distribution, namely the relative contribution of dehydration in the tropical upper troposphere versus midlatitudes. In wet regions, vapor and rain isotope compositions are primarily controlled by changes in convection, through rain reevaporation and through the progressive depletion of the vapor by convective mixing along air mass trajectories. Gradients in vapor isotope composition along air mass trajectories may help estimate continental recycling intensity, provided that we could quantify the effect of convection on the isotopic composition of water vapor.

A. Landais, C. Risi, S. Bony, F. Vimeux, L. Descroix, S. Falourd, and A. Bouygues. Combined measurements of ¹⁷O _excess and d-excess in African monsoon precipitation: Implications for evaluating convective parameterizations. Earth and Planetary Science Letters, 298:104-112, September 2010. [ bib | DOI | ADS link ]

Water stable isotopes (δ ¹⁸O, δD) are useful tools to depict and to understand the atmospheric water cycle. In tropical regions, they record the variations of convective activity and their implementation in convection schemes brings constraints on our understanding and parameterization of this phenomena. Here, we present for the first time measurements of a new isotopic marker of the hydrological cycle ( ¹⁷O _excess resulting from the combination of δ ¹⁷O and δ ¹⁸O of water) in convective regions on two different time scales: (i) during the African monsoon onset and intra-seasonal variability (Banizoumbou, 2006) and (ii) during the squall line of the 11th of August 2006 (Niamey). ¹⁷O _excess responds to the monsoon onset by a ˜ 30 per meg increase as well as to different convective processes in squall lines by ˜ 20 per meg variations. These variations parallel those of d-excess at first order and display significant correlation with relative humidity in the lower troposphere. Still, higher correlation coefficients are observed between d-excess and relative humidity than between ¹⁷O _excess and relative humidity, suggesting a higher influence of relative humidity on d-excess than on ¹⁷O _excess. Using a simple reevaporation model and a more sophisticated 2D model of a squall line, we show that reevaporation is the process explaining the increase of d-excess and ¹⁷O _excess with relative humidity for these two studies. We also show that the combination of ¹⁷O _excess and d-excess is a powerful tool to constrain the representation of isotopic processes during rain reevaporation. In turn, a good representation of such processes enables to use water isotopes to evaluate convective parameterization in atmospheric models.

C. Risi, S. Bony, F. Vimeux, and J. Jouzel. Water-stable isotopes in the LMDZ4 general circulation model: Model evaluation for present-day and past climates and applications to climatic interpretations of tropical isotopic records. Journal of Geophysical Research (Atmospheres), 115:12118, June 2010. [ bib | DOI | ADS link ]

We present simulations of water-stable isotopes from the LMDZ general circulation model (the LMDZ-iso GCM) and evaluate them at different time scales (synoptic to interannual). LMDZ-iso reproduces reasonably well the spatial and seasonal variations of both δ¹⁸O and deuterium excess. When nudged with reanalyses, LMDZ-iso is able to capture the synoptic variability of isotopes in winter at a midlatitude station, and the interannual variability in mid and high latitudes is strongly improved. The degree of equilibration between the vapor and the precipitation is strongly sensitive to kinetic effects during rain reevaporation, calling for more synchronous vapor and precipitation measurements. We then evaluate the simulations of two past climates: Last Glacial Maximum (21 ka) and Mid-Holocene (6 ka). A particularity of LMDZ-iso compared to other isotopic GCMs is that it simulates a lower d excess during the LGM over most high-latitude regions, consistent with observations. Finally, we use LMDZ-iso to explore the relationship between precipitation and δ¹⁸O in the tropics, and we discuss its paleoclimatic implications. We show that the imprint of uniform temperature changes on tropical δ¹⁸O is weak. Large regional changes in δ¹⁸O can, however, be associated with dynamical changes of precipitation. Using LMDZ as a test bed for reconstructing past precipitation changes through local δ¹⁸O records, we show that past tropical precipitation changes can be well reconstructed qualitatively but not quantitatively. Over continents, nonlocal effects make the local reconstruction even less accurate.

C. Risi, A. Landais, S. Bony, J. Jouzel, V. Masson-Delmotte, and F. Vimeux. Understanding the ¹⁷O excess glacial-interglacial variations in Vostok precipitation. Journal of Geophysical Research (Atmospheres), 115:10112, May 2010. [ bib | DOI | ADS link ]

Combined measurements of δ¹⁸O, δ¹⁷O, and δD in ice cores, leading to d excess and ¹⁷O excess, are expected to provide new constraints on the water cycle and past climates. We explore different processes, both in the source regions and during the poleward transport, that could explain the ¹⁷O excess increase by 20 per meg observed from the Last Glacial Maximum (LGM) to Early Holocene (EH) at the Vostok station. Using a single-column model over tropical and subtropical oceans, we show that the relative humidity at the surface is the main factor controlling ¹⁷O excess in source regions. Then, using a Rayleigh-type model, we show that the ¹⁷O excess signal from the source region is preserved in the polar snowfall, contrary to d excess. Evaporative recharge over mid and high latitudes and δ¹⁸O seasonality in polar regions can also affect the Vostok ¹⁷O excess but cannot account for most of the 20 per meg deglacial increase from LGM to EH. On the other hand, a decrease of the relative humidity at the surface (rh_s) by 8 to 22% would explain the observed change in ¹⁷O excess. Such a change would not necessarily be incompatible with a nearly unchanged boundary layer relative humidity, if the surface thermodynamic disequilibrium decreased by 4degC. Such a change in rh_s would affect source and polar temperatures reconstructions from δ¹⁸O and d excess measurements, strengthening the interest of ¹⁷O excess measurements to better constrain such changes.

C. Risi, S. Bony, F. Vimeux, M. Chong, and L. Descroix. Evolution of the stable water isotopic composition of the rain sampled along Sahelian squall lines. Quarterly Journal of the Royal Meteorological Society, 136:227-242, January 2010. [ bib | DOI | ADS link ]