lmd_Risi2008_abstracts.html

2008 .

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, 35:24808, 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.

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), 113:19306, 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), 113:19305, 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.