Skill of global raw and postprocessed ensemble predictions of rainfall over northern tropical Africa

TL;DR

This study evaluates the performance of global ensemble rainfall forecasts over northern tropical Africa, demonstrating that raw ensembles are unreliable and that postprocessing improves calibration but often does not surpass climatology.

Contribution

It provides a comprehensive assessment of multiple ensemble prediction systems and applies advanced statistical postprocessing methods in a challenging tropical region.

Findings

Raw ensembles are uncalibrated and underperform climatology.

Postprocessed forecasts are calibrated and reliable but rarely outperform climatology.

Ensemble forecast skill remains limited, possibly due to convection parametrization issues.

Abstract

Accumulated precipitation forecasts are of high socioeconomic importance for agriculturally dominated societies in northern tropical Africa. In this study, we analyze the performance of nine operational global ensemble prediction systems (EPSs) relative to climatology-based forecasts for 1 to 5-day accumulated precipitation based on the monsoon seasons 2007-2014 for three regions within northern tropical Africa. To assess the full potential of raw ensemble forecasts across spatial scales, we apply state-of-the-art statistical postprocessing methods in form of Bayesian Model Averaging (BMA) and Ensemble Model Output Statistics (EMOS), and verify against station and spatially aggregated, satellite-based gridded observations. Raw ensemble forecasts are uncalibrated, unreliable, and underperform relative to climatology, independently of region, accumulation time, monsoon season, and ensemble. Differences between raw ensemble and climatological forecasts are large, and partly stem from poor prediction for low precipitation amounts. BMA and EMOS postprocessed forecasts are calibrated, reliable, and strongly improve on the raw ensembles, but - somewhat disappointingly - typically do not outperform climatology. Most EPSs exhibit slight improvements over the period 2007-2014, but overall have little added value compared to climatology. We suspect that the parametrization of convection is a potential cause for the sobering lack of ensemble forecast skill in a region dominated by mesoscale convective systems.