Climatology-Focused Evaluation of CMORPH and TMPA Satellite Rainfall Products over the Nile Basin

Abstract

ith many operational satellite rainfall products being available for long periods, it is now possible to examine whether these products can reproduce climatologically known rainfall characteristics over large river basins that suffer from poor surface monitoring resources. Such assessment is a prerequisite for any further hydrologic applications that rely on these products. The current study evaluates two satellite rainfall products, the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis research-grade product (TMPA-3B42) and the Climate Prediction Center (CPC) morphing technique (CMORPH), over the domain of the Nile basin in eastern Africa. The large latitudinal extent of the basin, its complex topography, and its diverse land use result in widely contrasting regimes and distributions of annual and seasonal rainfall. The results suggest that the two products are fairly successful in reproducing some of the region-specific rainfall patterns across climatologically different parts of the basin. However, significant overestimation and underestimation by CMORPH and TMPA-3B42, respectively, are clearly evident over the majority of the basin and can exceed 100% of the mean annual rainfall. The biases are also evident in the seasonal rainfall cycle. The bias shows a complex dependency, in terms of magnitude and sign, on topography and latitude, especially in the central parts of the basin and over the Ethiopian Highlands region. The performance of both products is better over the equatorial region of the basin. The significant underestimation by the gauge-adjusted TMPA-3B42 product relative to CMORPH is attributed to the sparsity in operational gauges, which can adversely affect bias adjustment procedures in the TMPA algorithm. Current and future algorithmic developments are expected to bring much-needed improvements for satellite rainfall products to see full operational utility in these regions of the world.