CPC/FEWS Rainfall Estimates over West Africa

This paper presents an overview of the Climate Prediction Center (CPC)/Famine Early Warning System (FEWS) technique for estimating rainfall accumulation over Africa, and provides the results of preliminary comparisons of CPC/FEWS estimates with other satellite/rain gauge-based and model-based precipitation estimates. The period of study covers the 1998 West African rainy season.

The CPC/FEWS technique uses a combination of Meteosat data, GDAS model analyses and rain gauge observations. Initial estimates are based on Meteosat cold-cloud duration using a technique similar to the GOES Precipitation Index (GPI) method. This is coupled with a method to calculate the warm-cloud precipitation, which uses a combination of Meteosat cloud information, orography, and GDAS model data analyses of relative humidity and wind. The cold-cloud and warm-cloud rainfall estimates are merged with optimally-interpolated rain gauge data from the Global Telecommunications System (GTS) to produce the final product. Currently, the CPC/FEWS operational estimates cover 10-day spans. In addition, experimental rainfall estimates are being computed at the 1-day resolution.

Comparisons between the CPC/FEWS rainfall estimates and monthly Tropical Rainfall Measuring Mission (TRMM)-based satellite/rain gauge merged analyses are being performed. Preliminary results for August and September 1998 indicate that the monthly-averaged CPC/FEWS estimates are 50-150 mm lower on average than the corresponding TRMM-based estimates. The sources of these differences are currently under investigation.