The Accuracy of Precipitation Forecasts at Timescales of 1–15 Days in the Volta River Basin

Abstract

Medium-range (1–15 day) precipitation forecasts are increasingly available from global weather models. This study presents evaluation of the Global Forecast System (GFS) for the Volta river basin in West Africa. The evaluation was performed using two satellite-gauge merged products: NASA’s Integrated Multi-satellitE Retrievals (IMERG) “Final Run” satellite-gauge merged rainfall observations, and the University of California Santa Barbara’s Climate Hazard’s group Infrared Precipitation with Stations (CHIRPS). The performance of GFS depends on the climate zone, with underestimation bias in the dry Sahel climate, overestimation bias in the wet Guinea Coastal climate, and relatively no bias in the moderately wet Savannah climate. Averaging rainfall over the watershed of the Akosombo dam (i.e., averaging across all three climate zones), the GFS forecast indicates low skill (Kling-Gupta Efficiency KGE = 0.42 to 0.48) for the daily, 1-day, lead GFS forecast, which deteriorates further as the lead time increases. A sharp decrease in KGE occurred between 6 to 10 days. Aggregating the forecasts over long timescales improves the accuracy of the GFS forecasts. On a 15-day accumulation timescale, GFS shows higher skills (KGE = 0.74 to 0.88).

Verification of operational numerical weather prediction model forecasts of precipitation using satellite rainfall estimates over Africa

“…In many countries, global NWP centers are well connected with their National Meteorological Agencies or Forecasting Offices, where forecasters are well trained and make best use of NWP products. The variations in skill of NWM across African geographies and climatologies are well studied (Gebremichael et al, 2022; Gebrechorkos et al, 2018; Kniffka et al, 2020; Linden et al, 2020; Maidment et al, 2013; Ogutu et al, 2017; Taraphdar et al, 2016; Tompkins & Feudale, 2010 among others). However, a large proportion of research efforts have focused on seasonal or climate timescales, or restricted analysis to specific regions of Africa, or individual weather phenomena or events.…”