Clustering mesoscale convective systems with laser‐based water vapor δ 18 O monitoring in Niamey (Niger)

The isotopic composition of surface water vapor ( δ v ) has been measured continuously in Niamey along with the isotopic composition of event‐based precipitation ( δ p ) 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 δ v temporal evolution. We find that 51% of rain events (class A) exhibit a sharp decrease in δ 18 O v 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 δ 18 O v 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 δ 18 O v during rainfall (+0.8‰). We also examine the temporal evolution of water vapor deuterium excess ( d v ) which shows a sharp increase as δ 18 O v 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 δ 18 O v . We show that those two processes are dominant for class A, whereas other processes may exert a major role on δ 18 O v for classes B and C.