Validation of operational seasonal rainfall forecast in Ethiopia

Abstract

[1] Operational rainfall forecasts using the analog method have been issued in Ethiopia since 1987. We evaluate the performance of the forecast system for February-May and June-September rainy seasons over the period 1999-2011. Verification is performed using rainfall data obtained from Ethiopian meteorological stations covering eight homogeneous rainfall regions used in the forecasts. The results reveal that forecasts issued by the National Meteorological Agency (NMA) of Ethiopia, for the past 12 years have a weak positive skill for all eight regions compared with climatology. In terms of ranked probability skill scores, the values are all lower than 10% indicating that the forecast skill is modest. The results further suggest that the forecasting system has bias toward forecasting near-normal conditions and has problems in capturing below average events. In contrast, the forecast has some positive skills in ranking the wet years of February-May season, particularly over the regions where there is high seasonal rainfall variability with significantly positive rank correlations for the above average years. For the main season, however, the forecast is not able to rank wet years or dry years. The extreme low and high rainfall events are mostly missed by the forecast scheme. The results indicate rather low forecast skill for extreme rainfall events in both seasons. Generally, the results indicate that NMA's forecasts have low but positive skill as it is common with results from other forecasting systems for the Greater Horn of Africa region. The underforecasting of dry events is the most serious shortcoming of the system.

Seasonal-to-Interannual Variability of Ethiopia/Horn of Africa Monsoon. Part II: Statistical Multimodel Ensemble Rainfall Predictions

“…(Camberlin 1997;Vizy and Cook 2003). Also investigated have been the hydrological responses to rainfall (Conway 2000), temporal trends and decadal shifts of the rain (Seleshi and Zanke 2004;Bowden and Semazzi 2007;Cheung et al 2008;Williams et al 2012;Viste et al 2013;Berhane et al 2014), and seasonal spring (Belg) and Kiremt rainfall predictability (Gissila et al 2004;Korecha and Barnston 2007;Block and Rajagopalan 2007;Diro et al 2008;Korecha and Sorteberg 2013;Jury 2013;Nicholson 2014). Segele et al (2009b, hereinafter Part I) used wavelet analysis to identify temporal and spectral characteristics of the seasonal-to-interannual variability of 5-day average Ethiopian JJAS rainfall, and to present a timefrequency quantification of teleconnections between the rainfall and large-scale atmospheric circulation and global sea surface temperature (SST) patterns.…”

Assessing the Contributions of Local and East Pacific Warming to the 2015 Droughts in Ethiopia and Southern Africa

“…The impact of ENSO on Ethiopian rainfall is well documented (Fig. S15.1;Camberlin 1997;Degefu 1987;Diro et al 2011;Gissila et al 2004;Korecha and Barnston 2007;Korecha and Sorteberg 2013;Segele and Lamb 2005): the warm phase of ENSO is associated with suppressed rains during the main wet season (JJAS) over north and central Ethiopia. There have also been numerous papers documenting a negative teleconnection between El Niño and SA rainfall (Supplemental Fig.…”

Does objective cluster analysis serve as a useful precursor to seasonal precipitation prediction at local scale? Application to western Ethiopia

“…Skillful and credible predictions are produced for some regions in western Ethiopia, particularly under a clustered indirect statistical approach. At the regional scale, the approach shows promise for northwestern Ethiopia (Clusters 1, 3, 5, and 7), particularly compared to current NMA operational forecasts, which are only moderately more skillful than climatology (Korecha and Sorteberg, 2013). The regional average RPSS in this study under the clustered case ranges from 10 to 33 % for northwestern Ethiopia, as opposed to values under 6 % for NMA operational forecast (Korecha and Sorteberg, 2013).…”

“…At the regional scale, the approach shows promise for northwestern Ethiopia (Clusters 1, 3, 5, and 7), particularly compared to current NMA operational forecasts, which are only moderately more skillful than climatology (Korecha and Sorteberg, 2013). The regional average RPSS in this study under the clustered case ranges from 10 to 33 % for northwestern Ethiopia, as opposed to values under 6 % for NMA operational forecast (Korecha and Sorteberg, 2013). The approach adopted here also advances on previous studies (Gissila et al, 2004;Block and Rajagopalan, 2007;Korecha and Barnston, 2007;Diro et al, 2011b;Segele et al, 2015) by first applying an objective cluster analysis and then conditionally constructing high-resolution predictions.…”

“…The majority of agriculture and infrastructure are in western Ethiopia, where water resources are relatively rich compared to other parts of the country (Awulachew et al, 2007). Operational precipitation predic- tions in Ethiopia have been issued by its National Meteorological Agency (NMA) since 1987 using an analog methodology (i.e., locating a similar climate condition in the past -an analog -to predict future conditions); however, this approach has produced only marginally skillful outcomes (Korecha and Sorteberg, 2013). For NMA's prediction, the country is divided into eight homogeneous regions for which NMA produces independent predictions.…”