Evaluation of the COSMO-CLM high-resolution climate simulations over West Africa

The evaluation of a high-resolution simulation at 0.11° (12 km) with the COnsortium for Small-scale MOdelling in CLimate Mode (CCLM) regional climate model, applied over West Africa, is presented. This simulation is nested in a CCLM run at resolution of 0.44°, driven with boundary forcing data from the ERA-Interim reanalysis, and covers the period from 1981 to 2010. The simulated temperature and precipitation are evaluated using three selected climate indices for temperature and eight indices for precipitation in five different regions against a new, daily precipitation climatology covering West Africa and against other state of the art climatologies. The obtained results indicate that CCLM is able to reproduce the observed major climate characteristics including the West African Monsoon within the range of comparable regional climate modeling evaluations studies, but substantial uncertainties remain, especially in the Sahel zone. The seasonal mean temperature bias for the rainy season from June to September ranges from -0.8°C to -1.1°C. The CCLM simulations also underestimate the observed precipitation with biases in precipitation reaching -10% in the high-resolution and -20% in the low-resolution model runs. CCLM extends the monsoon precipitation belt too far north, which results in an overestimation of precipitation in the Sahel zone of up to 60%. In the coastal zone, the precipitation is underestimated by up to -90%. These biases in precipitation amounts are associated with errors in the precipitation seasonality. The added value of the higher resolution of the nested run is reflected in a smaller bias in extreme precipitation statistics with respect to the reference data.