Gérémy Panthou

Impact of model resolution and Mediterranean sea coupling on hydrometeorological extremes in RCMs in the frame of HyMeX and MED-CORDEX

In this study, we are interested in evaluating the potential improvement of: (i) coupled RCM simulations (with the Mediterranean sea) in comparison with atmosphere only (stand-alone) RCM simulations and (ii) RCM simulations at a finer resolution in comparison with coarser resolution. For that, three different RCMs (WRF, ALADIN, LMDZ4) were run, forced by ERA-Interim reanalyses, within the HyMeX/Med-CORDEX experiments. For each RCM, different versions (coupled/stand-alone, high/low resolution) were realized. This study focuses on extreme meteorological events (hot days, droughts and heavy precipitation) and evaluates the current RCM simulations in terms of return levels associated with these events. Additionally, a large set of indicators is proposed in order to better understand the performances of RCM simulations. These indicators were applied for three variables (daily precipitation amount, mean daily 2-m air temperature and dry spell length). Results show that the differences between coupled and stand-alone RCMs are localized very near the Mediterranean sea. For hot days and droughts statistics, high resolution runs display better performances than low resolution runs. The expected improvement for extreme precipitation with higher resolution runs was not observed in this study.

Runoff evolution due to land-use change in a small Sahelian catchment

Abstract Land-use changes have been significant these last decades in West Africa, particularly in the Sahel region; in this area, climatic and demographic factors have led to a rise in cropped areas in recent decades causing strong changes in the water cycle and in river regimes. This study compares the rainfall–runoff relationships for two periods (1991–1994 and 2004–2011) in two small and similar neighbouring Sahelian catchments (approx 0.1 km2 each). This allows identification of the different hydrological consequences of land-use/land-cover change, particularly the fallow shortening and the consequent degradation of topsoil. The main land surface change is a 75% increase in crusted soil area. Runoff increased by more than 20% on average between the two periods while flood duration decreased by 50% on average. However, runoff values remained largely constant in the lower part of the northern basin due to a strong increase in in-channel infiltration. Editor D. Koutsoyiannis; Associate editor T. Wagener

Trend in the Co‐Occurrence of Extreme Daily Rainfall in West Africa Since 1950

We propose in this paper a statistical framework to study the evolution of the co-occurrence of extreme daily rainfall in West Africa since 1950. We consider two regions subject to contrasted rainfall regimes: Senegal and the central Sahel. We study the likelihood of the 3% largest daily rainfall (considering all days) in each region to occur simultaneously and, in a 20 year moving window approach, how this likelihood has evolved with time. Our method uses an anisotropic max-stable process allowing us to properly represent the co-occurrence of daily extremes and including the possibility of a preferred direction of co-occurrence. In Senegal, a change is found in the 1980s, with preferred co-occurrence along the E-50-N direction (i.e., along azimuth 50∘) before the 1980s and weaker isotropic co-occurrence afterward. In central Sahel, a change is also found in the 1980s but surprisingly with contrasting results. Anisotropy along the E-W direction is found over the whole period, with greater extension after the 1980s. The paper discusses how the co-occurrence of extremes can provide a qualitative indicator on change in size and propagation of the strongest storms. This calls for further research to identify the atmospheric processes responsible for such contrasted changes in storm properties. Plain Language Summary We propose in this paper a statistical framework to study the evolution of the co-occurrence of extreme daily rainfall in West Africa since 1950. We consider two regions subject to contrasted rainfall regimes: Senegal and the central Sahel. In Senegal, a change is found in the 1980s, with preferred co-occurrence along the E-50-N direction (i.e., along azimuth 50∘) before the 1980s and weaker isotropic co-occurrence afterward. In the central Sahel, a change is also found in the 1980s but surprisingly with contrasting results. Anisotropy along the E-W direction is found over the whole period, with greater extension after the 1980s. The paper discusses how the co-occurrence of extremes can provide a qualitative indicator on change in size and propagation of the strongest storms. This calls for further research to identify the atmospheric processes responsible for such contrasted changes in storm properties.

Intensity–duration–frequency (IDF) rainfall curves in Senegal

Urbanization resulting from sharply increasing demographic pressure and infrastructure development has made the populations of many tropical areas more vulnerable to extreme rainfall hazards. Characterizing extreme rainfall distribution in a coherent way in space and time is thus becoming an overarching need that requires using appropriate models of intensity–duration–frequency (IDF) curves. Using a 14 series of 5 min rainfall records collected in Senegal, a comparison of two generalized extreme value (GEV) and scaling models is carried out, resulting in the selection of the more parsimonious one (four parameters), as the recommended model for use. A bootstrap approach is proposed to compute the uncertainty associated with the estimation of these four parameters and of the related rainfall return levels for durations ranging from 1 to 24 h. This study confirms previous works showing that simple scaling holds for characterizing the temporal scaling of extreme rainfall in tropical regions such as sub-Saharan Africa. It further provides confidence intervals for the parameter estimates and shows that the uncertainty linked to the estimation of the GEV parameters is 3 to 4 times larger than the uncertainty linked to the inference of the scaling parameter. From this model, maps of IDF parameters over Senegal are produced, providing a spatial vision of their organization over the country, with a north to south gradient for the location and scale parameters of the GEV. An influence of the distance from the ocean was found for the scaling parameter. It is acknowledged in conclusion that climate change renders the inference of IDF curves sensitive to increasing non-stationarity effects, which requires warning end-users that such tools should be used with care and discernment.