Combined measurements of 17O excess and d-excess in African monsoon precipitation: Implications for evaluating convective parameterizations

Abstract

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.