Dynamics and Model Representation of Two Contrasting Extreme Precipitation Events in the Sahel

TL;DR

This study analyzes two extreme Sahel rainfall events (San 2012, Kenieba 2019) to diagnose dynamical forcings and evaluate ICON model performance under explicit convection (EXPLC) versus convective parameterization (PARAM). By integrating KASS-D rainfall gauges, IMERG satellite estimates, ERA5 reanalysis, and two ICON configurations with nested domains, the authors diagnose environmental drivers such as African easterly waves and equatorial wave activity. They introduce and apply spatial verification tools (FSS and SAL) within a moving vortex domain and perform spectral filtering to assess wave influences, revealing a strong case-dependency: EXPLC better captures convection in the dry San environment but struggles in the moister Kenieba context, where PARAM yields superior convective organization and rainfall representation. The findings highlight the nuanced role of environmental moisture, wave activity, and model configuration for forecasting extreme rainfall in the West Africa monsoon region, suggesting targeted verification and consideration of soil-moisture–convection coupling in future work.

Abstract

Two extreme flood-inducing precipitation events in two cities in Mali, on 08 August 2012 in San (127 mm) and on 25 August 2019 in Kenieba (126 mm), are investigated with respect to rainfall structures, dynamical forcings, and the ability of the ICOsahedral Nonhydrostatic (ICON) model to represent their evolution. Two sets of experiments with convective parameterization enabled (PARAM) and disabled (EXPLC), both at 6.5 km grid spacing, are conducted for each case. While the (thermo)dynamical fields of the simulations are compared with ERA5 reanalysis data, the rainfall fields are tested against the satellite-based precipitation dataset IMERG by applying the spatial verification methods Fractions Skill Score (FSS) and the Structure-Amplitude-Location (SAL) score. In addition, a spectral filtering of tropical waves is applied to investigate their impact on the extreme events. The most prominent results are: (1) Both cases were caused by organized convective systems associated with a westward propagating cyclonic vortex, but differ in their environmental setting. Although both cases featured an east African wave (AEW), the San case involved convective enhancement along dry Saharan airmasses, whereas the Kenieba case occurred within an unusual widespread wet environment extending deep into the Sahel. (2) Although EXPLC captures the rainfall distribution in the San case better than PARAM, it fails to organize convection in the moisture-laden Kenieba case, which PARAM is capable of simulating. (3) The FSS confirms the case-dependency of the ICON skill. The SAL method hints towards a systematic deficiency of EXPLC to represent the convective organization by producing too many scattered and weak rainfall systems, while PARAM is more effective in converting abundant moisture into excessive rainfall.

Figures (12)

• Figure 1: Map showing the terrain elevation (shading) for large parts of Africa, locations of the weather stations of the two Malian cities under study (Kenieba and San, black dots) and the spatial extent of the domains used for the ICON simulations (dark red rectangles). The first nest (R3B07 of the ICON nomenclature zangl2015icon) is the coarser domain with a model resolution of $\Delta x=13$ km and a model timestep $\Delta t=120$ s, whereas the second nest (R3B08) is the finer domain with resolutions of $\Delta x=6.5$ km and $\Delta t=60$ s.
• Figure 2: Evolution of daily rainfall (a) at the station of San in August 2012, and (b) at the station of Kenieba in August 2019 from rain gauge data (gray shaded), IMERG estimates (V6B and V7 in solid blue and dashed blue curves, respectively), and total ERA5 rainfall (red curves) and its partitions "grid-scale" (dotted green curves) and "convective" (dashed purple curves) rainfall.
• Figure 3: Evolution of 3-hourly IMERG rainfall at important timesteps between 06 and 08 August 2012. The star marker shows the location of San. The indicated time of day (in UTC) denote the end of the accumulation period, e.g., 03-06 UTC if indicated as 06 UTC. Two major westward propagating rainfall systems are identified and labelled as A and B.
• Figure 4: As Figure \ref{['fig:imerg_san']} but for the Kenieba case, showing selected timesteps between 22 and 25 August 2019.
• Figure 5: Evolution of weather systems at selected timesteps prior and during the San case from 05 to 08 August 2012 over West Africa. The red star and the purple circled dot show the positions of San and the center of the rain-bearing cyclonic vortex, respectively. Vectors denote 925–-600 hPa mass-weighted flow, shadings indicate anomalies of the 925–-600 hPa layer thickness (used as proxy for the location of the heat low) with respect to the 1981–-2010 long-term monthly mean (black dash contour), blue and red lines indicate the actual and climatological ITD position, respectively, defined by the 14°C isodrosotherm at 2m, and moisture flux convergence is represented by green contours at $-12\times10^{-3}$ to $-0.5\times10^{-3}$ kg m$^{-2}$ s$^{-1}$. In a), SHL, SHL*, AV, and CV denote the Saharan heat low, the climatological SHL in the month of August, the anticyclonic vortex over the Mediterranean and northern Africa, and the cyclonic vortex, respectively.