Clustering mesoscale convective systems with laser-based water vapor δ18O monitoring in Niamey (Niger)

International audienceThe 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

A 1-year long delta O-18 record of water vapor in Niamey (Niger) reveals insightful atmospheric processes at different timescales

International audienceWe present a 1-year long representative delta O-18 record of water vapor (delta O-18(v)) in Niamey (Niger) using the Wavelength Scanned-Cavity Ring Down Spectroscopy (WS-CRDS). We explore how local and regional atmospheric processes influence delta O-18(v) variability from seasonal to diurnal scale. At seasonal scale, delta O-18(v) exhibits a W-shape, associated with the increase of regional convective activity during the monsoon and the intensification of large scale subsidence North of Niamey during the dry season. During the monsoon season, delta O-18(v) records a broad range of intra-seasonal modes in the 25-40-day and 15-25-day bands that could be related to the well-known modes of the West African Monsoon (WAM). Strong delta O-18(v) modulations are also seen at the synoptic scale (5-9 days) during winter, driven by tropical-extra-tropical teleconnections through the propagation of a baroclinic wave train-like structure and intrusion of air originating from higher altitude and latitude. delta O-18(v) also reveals a significant diurnal cycle, which reflects mixing process between the boundary layer and the free atmosphere during the dry season, and records the propagation of density currents associated with meso-scale convective systems during the monsoon season. Citation: Tremoy, G., F. Vimeux, S. Mayaki, I. Souley, O. Cattani, C. Risi, G. Favreau, and M. Oi (2012), A 1-year long delta O-18 record of water vapor in Niamey (Niger) reveals insightful atmospheric processes at different timescales, Geophys. Res. Lett., 39, L08805, doi:10.1029/2012GL051298