Objective tracking of African Easterly Waves in Met Office models

A simple and computationally efficient objective tracking technique is used to assess the representation of African Easterly Waves (AEWs) in the Met Office Unified Model (MetUM). The algorithm takes Hovmoller diagrams of curvature vorticity at 700 hPa and uses object-orientated image processing to identify the propagating waves. We use the tracking to evaluate how well AEWs are captured by the MetUM in comparison to ERA-Interim reanalysis and GPCP rainfall data. The waves are inspected from a dynamical perspective and also from a rainfall-coupling perspective. The results show that the MetUM is able to capture AEW features at weather forecasting and climate time-scales and has reasonable representation of the large-scale dynamics. Some rainfall coupling to waves is also captured in the forecasts. However, it is found that the rainfall enhancement due to wave passage is underestimated by the models, as is the strength and frequency of AEWs. Potential causes of forecast deficiency are investigated and both resolution and changes to convective parametrisation are found to have impacts on model performance. The implications for future model development over Africa are discussed and it is suggested that good convection representation within models is of key importance to realistic simulations of AEWs and their coupling to West African rainfall.