Performance evaluation of a high‐resolution regional model over West Africa for operational use: A case study of August 2017

Abstract

The frequency of flash floods resulting from heavy rainfall over West Africa has increased in recent years with serious socio-economic consequences.Therefore, the need to utilize numerical weather prediction models to forecast heavy rainfall events reliably is also rising at many operational meteorological centres in West Africa. This paper evaluates the performance of the Consortium for Small-scale Modelling (COSMO) model of the German Meteorological Services (DWD) in predicting rainfall over West Africa for high-impact rainfall events that occurred between 19 and 26 August 2017. The paper aims to investigate the synoptic forcings modulating daily rainfall variability during that period. Results show that COSMO simulates adequately the spatio-temporal variability of rainfall distribution over West Africa, though with inherent biases. COSMO displays a decreasing skill in producing spatial rainfall distribution as rainfall amounts tend to 30 mm and above. Additionally, areas of heavy rainfall, mostly about 100-300 km southwest of the core of the Africa Easterly Jet (AEJ), often coincide with areas of decreasing mean sea level pressure of at least 0.6 hPa and areas of increasing convective available potential energy of at least 500 J/kg. Although not in all cases, the trough of the Africa Easterly Wave (AEW) is always located to the east of these areas. We show that not every storm, especially east of the prime meridian, is associated with an AEW trough. COSMO is able to reproduce the atmospheric dynamics modulating the daily rainfall variability, in addition to capturing the daily propagation of the AEW trough, and the core of the AEJ. However, the reproducibility skill of the model in predicting atmospheric dynamics may not transform into the predictive skill of the model in producing rainfall. Nevertheless, operational forecasters may be able to determine likely areas of heavy rainfall by estimating the position of the AEJ core based on the position of areas of the least falling pressure from COSMO. Finally, the incorporation of the fractions skill

Verification of multiresolution model forecasts of heavy rainfall events from 23 to 26 August 2017 over Nigeria

“…The period corresponds to the time when there was an east to west propagation of rainfall maximum across the domain. This convective activity is associated with the passage of an African Easterly Wave (AEW) over Nigeria (Olaniyan et al, 2022). At 21:00 UTC of August 24, 2017, GPM observed accumulated rainfall extending from the southern part of Nigeria to around latitude 10.5 o N (Figure 2).…”

“…Depending on the user's need and purpose, these methods give more detailed information than conventional metrics. Spatial verification methods have been used recently in Tropical Africa to assess the skill of convection‐permitting ensembles (East Africa; Cafaro et al, 2021) and an operational deterministic model (West Africa; Olaniyan et al, 2022).…”