Rainfall estimation in the Sahel: the EPSAT-NIGER experiment

Lebel, Thierry; Sauvageot, Henri; Hoepffner, Michel; Des̄bois, M.; Guillot, B.; Hubert, P.

doi:10.1080/02626669209492582

Cited by 104 publications

(41 citation statements)

References 12 publications

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“…However, these platforms are not geostationary, and their long return time means that they observe only a sample of the MCSs which actually occur. In a recent study, Mathon et al (2002) presented a comprehensive comparison of Meteosat cold-cloud imagery and precipitation recorded by the EPSAT * -Niger (Lebel et al 1992) rain-gauge network over a period of 9 years. From tracking algorithms at −40 and −60 • C thresholds, they defined a subset of cold-cloud structures called Organized Convective Systems (OCSs) which were extensive, long-lived and fast moving.…”

Section: Observing Land-atmosphere Coupling Using Satellite Data (A) mentioning

confidence: 99%

Observations of synoptic-scale land surface variability and its coupling with the atmosphere

Taylor

Parker

Lloyd

et al. 2005

Q. J. R. Meteorol. Soc.

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SUMMARYCloud-free brightness temperatures from satellite are used to interpret changes in the state of the land surface energy balance from day to day across the tropical semi-arid region of North Africa. The method provides an assessment of the spatial and temporal patterns in the surface characteristics. The variability in brightness temperature is closely linked to precipitation, as inferred from available rain-gauge data and satellite cold-cloud imagery. Temperatures drop sharply after rainfall, and increase gradually in subsequent days consistent with surface drying. The analysis shows that surface temperatures (and therefore fluxes) are well-organized at the synoptic scale, and can be linked to the westward propagation of mesoscale convective systems and African easterly waves (AEWs).The modulation of rainfall and cloud cover by weather systems is a key element in producing variability in surface fluxes of heat and moisture. To examine the relationships between surface variability and the atmosphere, a composite of anomalously warm (and dry) surface conditions with spatial coherence over a given longitude band is constructed from satellite data covering a single wet season. Operational analyses are used to examine atmospheric anomalies associated with these composited 'hotspots'. Surface moisture variations on the scale of several hundred km and larger can generate anomalous heat-lows during the day and, according to the operational analyses, generate cyclonic vorticity overnight. These vortices appear to be responsible for the observed modulation of cold cloud in the vicinity of the hotspot, and may influence the characteristics of AEWs in the Northern Sahel. Furthermore, theoretical ideas suggest that surface hotspots are likely to trigger a westward-propagating Rossbywave response; this process is demonstrated here in a simple numerical model.

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Section: Observing Land-atmosphere Coupling Using Satellite Data (A) mentioning

confidence: 99%

Observations of synoptic-scale land surface variability and its coupling with the atmosphere

Taylor

Parker

Lloyd

et al. 2005

Q. J. R. Meteorol. Soc.

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“…(c) Surface data Surface rain-gauge data from the region are used where available. In the Niamey area, the long-term EPSAT-Niger network (Estimation des Pluies par Satellite, expérience Niger; Lebel et al 1992) provided hourly observations at 34 sites. A number of synoptic stations in the region reported rainfall over the Global Telecommunication System, at varying time intervals.…”

Section: Flight Paths and Experimental Data (A) Flight Pathsmentioning

confidence: 99%

Linking boundary‐layer variability with convection: A case‐study from JET2000

Taylor

Ellis

Parker

et al. 2003

Quart J Royal Meteoro Soc

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SUMMARYThe properties of the planetary boundary layer (PBL) are crucial for understanding convection and dynamics in the tropical atmosphere. In semi-arid regions, there are strong links between the PBL and soil moisture. This study demonstrates the link between PBL characteristics and antecedent rainfall via soil moisture variability on length-scales from 10 to 100 km. Spatial variability of the PBL in the west African Sahel is explored using observations from an aircraft for one day in the wet season, as part of the JET2000 experiment. Meteosat and surface station data provide information about rainfall patterns during the previous days.The aircraft transect crossed several well-de ned bands of low surface temperature and albedo, as determined by the satellite imagery. These bands indicate high soil moisture beneath the paths of recent storms. Observed atmospheric properties are closely linked to the surface gradients. Above moist soils, the PBL tends to be shallower, cooler, moister and less turbulent than in adjacent dry areas. The study shows that the characteristic large-scale gradients of temperature and humidity across the Sahel are concentrated in zones de ned by recent rainfall.Evidence is presented which is consistent with a positive feedback at the storm-scale between deep convection and soil moisture. Atmospheric pro les indicate that soil moisture plays an important role in determining the stability of the atmosphere to subsequent convection. It was found that in a region of low surface moisture covering 80 000 km 2 , cold-cloud systems did not occur in the 48 hours following the ight. By contrast, mesoscale convective systems were observed in adjacent regions with wetter soils.

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“…The transition from the dry season to the wet season is controlled by the monsoon regime. In the Sahel severe drought-periods have been observed during the 1970's and 1980's with large meso-scale variability (Lebel et al, 1992(Lebel et al, , 2003Le Barbé and al., 2002;Messager andal., 2004, Balme et al, 2006) and specific hydrological processes (Goutorbe et al, 1994).…”

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Long term precipitation chemistry and wet deposition in a remote dry savanna site in Africa (Niger)

et al. 2009

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Abstract. Long-term precipitation chemistry have been recorded in the rural area of Banizoumbou (Niger), representative of a semi-arid savanna ecosystem. A total of 305 rainfall samples ∼90% of the total annual rainfall) were collected from June 1994 to September 2005. From ionic chromatography, pH major inorganic and organic ions were detected. Rainwater chemistry is controlled by soil/dust emissions associated with terrigeneous elements represented by SO concentrations of 11.6 and 18.1 µeq.l −1 , respectively. This is the signature of ammonia sources from animals and NO x emissions from savannas soil-particles rain-induced. The mean annual NH 3 and NO 2 air concentration are of 6 ppbv and 2.6 ppbv, respectively. The annual VWM precipitation concentration of sodium and chloride are both of 8.7 µeq.l −1 which reflects the marine signature of monsoonal and humid air masses. The median pH value is of 6.05. Acidity is neutralized by mineral dust, mainly carbonates, and/or dissolved gases such NH 3 . High level of organic acidity with 8µeq.l −1 and 5.2 µeq.l −1 of formate and acetate were also found. The analysis of monthly Black Carbon emissions and Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) values show that both biogenic emission from vegetation and biomass burning could explain the Correspondence to: C. Galy-Lacaux (lacc@aero.obs-mip.fr) rainfall organic acidity content. The interannual variability of the VWM concentrations around the mean (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) is between ±5% and ±30% and mainly due to variations of sources strength and rainfall spatio-temporal distribution. From 1994 to 2005, the total mean wet deposition flux in the Sahelian region is of 60.1 mmol.m −2 .yr −1 ±25%. Finally, Banizoumbou measurements are compared to other long-term measurements of precipitation chemistry in the wet savanna of Lamto (Côte d'Ivoire) and in the forested zone of Zoétélé (Cameroon). The total chemical loading presents a maximum in the dry savanna and a minimum in the forest (from 143.7, 100.2 to 86.6 µeq.l −1 ), associated with the gradient of terrigeneous sources. The wet deposition fluxes present an opposite trend, with 60.0 mmol.m −2 .yr −1 in Banizoumbou, 108.6 mmol.m −2 .yr −1 in Lamto and 162.9 mmol.m −2 .yr −1 in Zoétélé, controlled by rainfall gradient along the ecosystems transect.

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Rainfall estimation in the Sahel: the EPSAT-NIGER experiment

Cited by 104 publications

References 12 publications

Observations of synoptic-scale land surface variability and its coupling with the atmosphere

Observations of synoptic-scale land surface variability and its coupling with the atmosphere

Linking boundary‐layer variability with convection: A case‐study from JET2000

Long term precipitation chemistry and wet deposition in a remote dry savanna site in Africa (Niger)

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