Etude de la composition isotopique (deutérium et oxygène 18) de la vapeur d'eau à Niamey (Niger) (vers une meilleure compréhension des processus atmosphériques en Afrique de l'Ouest)

Les isotopes stables de l'eau constituent un outil intéressant pour étudier les variations passées et actuelles du cycle de l'eau. Sous les tropiques, la convection est un facteur important agissant sur la composition isotopique ( 18O, D) de l'eau. Néanmoins, plusieurs questions subsistent sur la compréhension du signal isotopique, notamment en raison d'un manque de données dans la phase vapeur. Le but de cette thèse est d'une part de mesurer en continu pour la première fois au Sahel la composition isotopique de la vapeur d'eau en surface, puis de déterminer quelle est la valeur ajoutée de telles mesures pour l'étude du cycle de l'eau atmosphérique de la mousson africaine. Dans un premier temps, nous présentons notre protocole de mesure, à travers plusieurs tests réalisés en laboratoire avec une technologie laser récemment commercialisée. L'installation d'un instrument laser à Niamey (Niger) nous a permis d'acquérir des mesures pendant plus de deux ans. Nous discutons ces données à différentes échelles de variabilité. A l'échelle saisonnière, nous soulignons le rôle prépondérant de l'activité convective pendant la mousson et de la dynamique de grande échelle. Nous mettons en évidence de forts modes de variabilité intra-saisonniers évoluant au cours de l'année, illustrant le caractère intégrateur de la composition isotopique de la vapeur vis-à-vis de la convection pendant la mousson, et le rôle des interactions entre la circulation atmosphérique et les téléconnexions tropiques/extra-tropiques pendant la saison sèche. Nous discutons aussi de la variabilité diurne et des informations que peuvent nous apporter ces mesures sur les processus convectifs.Water stable isotopes are a useful tool to investigate past and present-day atmospheric water cycle. If the isotopic composition ( 18O, D) of tropical precipitation is strongly affected by convective processes at different timescales, large uncertainties remain in the understanding of its variabilty, since a variety of factors can control , from local to large-scale processes. One way to better understand the atmospheric controls on the isotopic composition of precipitation is to monitor that of the water vapor in parallel.The goal of this thesis is to monitor continuously for the first time the near-surface water vapor isotopic composition in the Sahel region, and second to study what is the added value of such measurements for investigating the atmospheric water cycle of the West African Monsoon.First, we present our measurement protocol, through several experimental results conducted in laboratory. We have set up a commercially available laser instrument in Niamey (Niger) and monitored the isotopic composition of water vapor during more than two years. Then, we present and discuss the data, through several temporal scales of variability. We show that the water vapor isotopic composition is strongly impacted by convection during the monsoon period, and large-scale dynamics during the dry season. It also records strong intraseasonal fluctuations that could be linked to convection modes of variability during the monsoon, and interactions between atmospheric circulation and tropical/extratropical teleconnections during the dry season. We also discuss diurnal variations, and investigate the variability at the scale of the rain events.VERSAILLES-BU Sciences et IUT (786462101) / SudocSudocFranceF

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

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

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