Advanced Weather Forecasts for Ethiopia by Optimized Initialization using a 3DVAR hybrid approach

Ethiopia is frequently affected by extreme rainfall events with devastating consequences for its society and economy. For example, they caused a deadly flash flood in Dire Dawa town in April 2020, and in Addis Ababa in August 2021. The frequency and severity of extreme rainfall events varies by region and season. Their variability depends mainly on the advection of moist air and the location and intensity of precipitation-bearing systems approaching Ethiopia. The westward propagation of low-pressure systems developing over the Indian Ocean and Arabian Sea as well as southerly moisture flow are widely known precipitation-producing features in this region.

Due to the vulnerability to extreme precipitation, especially over the orographically complex regions of Ethiopia, a very high resolution of Numerical Weather prediction (NWP) models with high quality initial conditions are required to enhance the advance warning time. The accuracy of initial states of the NWP models can be enhanced through advanced variational Data Assimilation (DA) techniques. With this aim, we simulated an extreme precipitation event using the Weather Research and Forecasting (WRF) model over the Horn of Africa centering the target region Ethiopia. The model is set up at approx. 2 km horizontal resolution and 100 vertical levels. The impact of DA is also evaluated using single observation tests and 3DVar simulations at three cycles. The 3DVar simulations are compared with the control run (without DA) and observations from Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS). This shows that the DA improves the results in comparison with the control run. Finally, we combined the Ensemble Transform Kalman Filter (ETKF) with the 3-Dimensional Variational DA.

The hybrid ETKF-3DVar (3DEnsVar) simulations are performed within a Rapid Update Cycle (RUC) with 6 hourly updates and 15 ensemble members. The 3DEnsVar simulations are compared with observations from Global Precipitation Measurement Mission (GPM), Climate Prediction Center morphing method (CMORPH), and European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) products. Results indicate that the 3DEnsVar method is an appropriate DA method to advance extreme rainfall prediction over Ethiopia.