Climate variability and change, scale-interactions and precipitation processes in West Africa. Limitations and challenges of modeling efforts

There has been a downturn in precipitation rates during the growing season (JJA) in much of West Africa over the last three decades. The reduction precipitation has been linked to sea surface temperature anomalies (SSTAs), land-use change and quite possibly climatic change. A number of studies have examined the causes for reduced West African rainfall with global climate models (GCMs). While many GCMs can capture the pattern and total amounts of precipitation to a first order approximation, the processes associated with simulated precipitation remains unresolved. We use the 1998 and 1999 JJA data from the TRMM satellite to examine the precipitation processes over West Africa in the GENESIS and CCM3 climate models. The largest discrepancies between the GCMs and the TRMM data is the lack of nonconvective (stratiform) precipitation over West Africa. This is discrepancy is caused by the convective parameterizations and horizontal resolution of the GCM. Convective and stratiform precipitation in the tropics are dependent on each other while in the GCM they are independent of each other. Further, meso-scale convective systems such as squall lines which are often associated with easterly waves but which cannot be resolved in GCMs are responsible for producing convective and stratiform precipitation. If we are to understand interannual variability or climate change in West Africa, then it will be necessary to use meso-scale models that incorporate explicit physics of water and ice processes in the convective schemes. This will increase the computational cost of model simulations, but may provide insight on both the long term downward trends in rainfall and future changes in rainfall associated with elevated greenhouse gases.