Uncertainties in TRMM-era Precipitation Measurements

The TRMM satellite ushered in an era of remarkable enrichment of global precipitation data products and data-producing techniques. Determining the uncertainties in the TRMM-based precipitation measurements is of fundamental importance to both data producers and end users, and to future missions including GPM. These uncertainties result mostly from the interplay of systematic errors and random errors. In this work, we will synthesize our recent results in quantifying the error characteristics in satellite-based precipitation estimates. Both systematic errors and total uncertainties have been analyzed for six different TRMM-era precipitation products (3B42, 3B42RT, CMORPH, PERSIANN, NRL and GSMaP). For systematic errors, we devised an error decomposition scheme to separate errors in precipitation estimates into three independent components, hit biases, missed precipitation and false precipitation. This decomposition scheme reveals more error features and provides a better link to the error sources than conventional analysis, because in the latter these error components tend to cancel one another when aggregated or averaged in space or time. For the random errors, we calculated the measurement spread from the ensemble of these six quasi-independent products, and thus produced a global map of measurement uncertainties. The map yields a global view of the error characteristics and their regional and seasonal variations, reveals many undocumented error features over areas with no validation data available, and provides better guidance to global assimilation of satellite-based precipitation data. Insights gained from these results and how they could help with GPM will be highlighted.