Landais A., Risi C., Bony S., Vimeux Françoise, Descroix Luc, Falourd S., Bouygues A.
Source
Earth and Planetary Science Letters, 2010,
298 (1-2), p. 104-112 ISSN 0012-821X
Water stable isotopes (delta O-18, delta D) 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 (O-17(excess) resulting from the combination of delta O-17 and delta O-18 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). O-17(excess) responds to the monsoon onset by a similar to 30 per meg increase as well as to different convective processes in squall lines by similar to 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 O-17(excess) and relative humidity, suggesting a higher influence of relative humidity on d-excess than on O-17(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 O-17(excess) with relative humidity for these two studies. We also show that the combination of O-17(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.
