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

Mekonnen Gebremichael,Haowen Yue,Vahid Nourani

doi:10.3390/rs14040937

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).

Highlights

Academic Editor: Christopher KiddMedium-range precipitation forecasts are needed for a number of applications, such as early warning systems for floods and droughts, reservoir management and operations, and decisions in agriculture [1,2,3,4,5,6]
In a Global Forecast System (GFS) forecast sensitivity experiment, it was found that the impact of the inclusion of field data on the root-mean-squared-error (RMSE) of the forecasts was large at short lead times, whereas the impact of the type of Kalman Filter data assimilation method considered lasted throughout the 7-day period [14]
This study evaluated the accuracy of medium-range (1-day to 15-day lead time) forecasts available from the Global Forecast System (GFS), for the watershed of the Akosombo dam in the Volta basin, West Africa, using satellite-gauge merged IMERG Final and Climate Hazard’s group Infrared Precipitation with Stations (CHIRPS)

Summary

Introduction

Academic Editor: Christopher KiddMedium-range precipitation forecasts are needed for a number of applications, such as early warning systems for floods and droughts, reservoir management and operations, and decisions in agriculture [1,2,3,4,5,6]. Medium-range precipitation forecasts are becoming increasingly available from global weather forecasting models, such as the Global Forecast. System (GFS) [7], the NCEP climate forecast system (NSF CFS) [8], the European Centre for Medium-Range Weather Forecasts (ECMWF) [9], and the Global Spectral Model (GSM) [10]. In India, the GFS forecast showed some skills at 1-day and 2-day lead times, but lower skills from 3 days and over [12]. In a GFS forecast sensitivity experiment, it was found that the impact of the inclusion of field data on the root-mean-squared-error (RMSE) of the forecasts was large at short lead times (about 1 day), whereas the impact of the type of Kalman Filter data assimilation method considered lasted throughout the 7-day period [14]. Lien et al [15] compared the global statistical properties of Received: 13 January 2022

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