The Availability and Reliability of Precipitation and Zonal Wind Estimates over Africa

Two of the most important indicators for studying African climate are rainfall and zonal winds. Unfortunately, obtaining information on these parameters for Africa has become increasingly difficult during the last two decades. This paper describes the history of gauge information in the region and the current paucity of observational data, as well as the changing availability of upper-air data. Three alternatives to basic gauge data exist: gridded data sets based on the statistical manipulation of the available gauge data, satellite estimates of precipitation, and reanalysis estimates of precipitation. This paper describes all three as applied to equatorial Africa. It also considers the ability of reanalysis estimates to accurately depict winds over Africa.

Throughout Africa gauge data was plentiful in the mid-twentieth century. The gauge networks generally declined when countries became independent and generally began to experience economic hardships. In most countries extensive gauge networks continued to operating into the 1980s or so. In the Democratic Republic of the Congo, the decline began much earlier, in the 1960s. However, a large number of stations were still available throughout most of the 1980s. In recent decades only a handful have been operative.

Numerous satellite data sets are available but their performance over Africa varies markedly and differs from one region to the next. Several studies have validated various estimates. Overall, it appears that TRMM, CHIRPS, and PERSIANN provide the best estimates. In general, the best performing products are those that combine satellite assessments with gauge data and the reliability of the estimates is strongly based on the density of gauge data going into the estimates.

Several reanalysis data sets also provide gridded estimates of precipitation. These are generally much less reliable than satellite estimates of the NIC131 data set. The validation of zonal winds from several reanalyses has been carried out for the Sahel. The most reliable appear to be MERRA2, NCEP 1, and the ERA products.