Downscaling of seasonal ensemble forecasts to the convection-permitting scale over the Horn of Africa using the WRF model

The new SEAS5 global ensemble forecast system was dynamically downscaled over the Horn of Africa for summer (June-July-August) 2018. For this purpose, a multi-physics ensemble was designed with a grid increment of 3 km and without any intermediate nest based on the Weather Research and Forecasting model (WRF). The WRF and the SEAS5 model output were compared with each other and reference datasets to assess the biases in 4 different regions of Ethiopia. Also, the WRF ensemble variability was investigated in relation to model parameterization and lateral boundary conditions. Over the summer, the SEAS5 has a positive temperature bias of 0.17°C compared to ECMWF analysis average for the study domain, while the WRF bias is +1.14°C. Concerning precipitation, the WRF model had average accumulated values of 264 mm, compared to 248 mm for SEAS5 and 236 mm for the observations. Over south Ethiopia, however, the downscaling produced over 50% more precipitation than the other datasets. The maximum northward extension of the tropical rain belt was reduced by about 2° in both models when compared to observations. Downscaling increased reliability for precipitation, correcting the SEAS5 underdispersion: ensemble spread for precipitation was increased by about 70% in the WRF ensemble in three of the four Ethiopian sub-regions, whereas the very dry Somali region remained unaffected. The WRF ensemble analysis revealed that the ensemble spread is mainly caused by the perturbed boundary conditions, as their effect is often 50% larger than the physics-induced variability in the mountainous part of Ethiopia for precipitation and temperature

Projected effects of 1.5 °C and 2 °C global warming levels on the intra-seasonal rainfall characteristics over the Greater Horn of Africa

This study examines the effects of 1.5 °C and 2 °C global warming levels (GWLs) on intra-seasonal rainfall characteristics over the Greater Horn of Africa. The impacts are analysed based on the outputs of a 25-member regional multi-model ensemble from the Coordinated Regional Climate Downscaling Experiment project. The regional climate models were driven by Coupled Model Intercomparison Project Phase 5 Global Climate Models for historical and future (RCP8.5) periods. We analyse the three major seasons over the region, namely March–May, June–September, and October–December. Results indicate widespread robust changes in the mean intra-seasonal rainfall characteristics at 1.5 °C and 2 °C GWLs especially for the June–September and October–December seasons. The March–May season is projected to shift for both GWL scenarios with the season starting and ending early. During the June–September season, there is a robust indication of delayed onset, reduction in consecutive wet days and shortening of the length of rainy season over parts of the northern sector under 2 °C GWL. During the October–December season, the region is projected to have late-onset, delayed cessation, reduced consecutive wet days and a longer season over most of the equatorial region under the 2 °C GWL. These results indicate that it is crucial to limit the GWL to below 1.5 °C as the differences between the 1.5 °C and 2 °C GWLs in some cases exacerbates changes in the intra-seasonal rainfall characteristics over the Greater Horn of Africa