Résumé
The isotopic composition of surface water vapor (δv) has been measured continuously in Niamey along with the isotopic composition of event-based precipitation (δp) since 2010. We investigate the evolution of water vapor and precipitation isotope ratios during rain events of the 2010, 2011, and 2012 monsoon periods. We establish a classification of rain systems into three types based on the δv temporal evolution. We find that 51% of rain events (class A) exhibit a sharp decrease in δ18Ov in phase with the surface air temperature drop, leading to a depletion of water vapor by −1.9‰ on average during rainfall. Twenty-nine percent of rain events (class B) show a similar decrease in δ18Ov in phase with the temperature drop but are characterized by a progressive enrichment of the vapor in the stratiform region, resulting in a depletion of water vapor by −1.2‰ on average during rainfall. The last 20% of the rain events (class C) are associated with a progressive increase in δ18Ov during rainfall (+0.8‰). We also examine the temporal evolution of water vapor deuterium excess (dv) which shows a sharp increase as δ18Ov decreases, followed by a progressive decrease in the stratiform part for classes A and B. Using a basic box model, we examine for each class the respective roles that mesoscale subsidence and rain evaporation play on the evolution of δ18Ov. We show that those two processes are dominant for class A, whereas other processes may exert a major role on δ18Ov for classes B and C.