Discrepancy between gauges and satellite estimates of rainfall in equatorial Africa

The Global Precipitation Climatology Project (GPCP) satellite estimates hav e approximately twice the magnitude of estimates produced from the rain gau ges used by the GPCP in central equatorial Africa. Different possible expla nations are identified and investigated. The first is that there may not be enough GPCP rain gauges in the area to provide accurate estimates of rainf all for comparisons with satellite estimates. A comparison of the time-aver aged GPCP rain gauge estimate with a long-term lover 40 yr climatology indi cates that the GPCP gauge estimates are similar to long-term rainfall avera ges, suggesting that the GPCP rain gauge analysis is not underestimating ra infall. Two other possible explanations related to the physical properties of the air masses in this region an studied. Evidence from the literature a nd from estimates of the effective radii of cloud droplets suggests that th ere may be an abundance of aerosols in central Africa, resulting in an abun dance of cloud condensation nuclei, small drops, and inefficient rain proce sses. The second explanation is that convective clouds forming under dry co nditions generally have cloud bases considerably higher than those of cloud s forming in moist environments. This leads to an increase in the evaporati on rate of the falling rain, resulting in less precipitation reaching the g round. Analysis of the moisture distributions from both the National Center s for Environmental Prediction numerical weather prediction model reanalysi s data and the National Aeronautics and Space Administration Water Vapor Pr oject global moisture dataset reveals that the lower troposphere in this re gion of Africa is relatively dry, which suggests that cloud bases are high.