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

Abstract

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