The first WetNet precipitation intercomparison project (PIP‐1): interpretation of results

The first WetNet Precipitation Intercomparison Project (PIP‐1) was the first major intercomparative study of global monthly precipitation retrieval techniques based on satellite imagery. As primary products, monthly estimates of rainfall were submitted to the project based on SSM/I satellite passive microwave image data, passive microwave sounding data and infrared data. Rainfall totals were also produced from numerical weather prediction model estimates and surface rain gauges. Instantaneous rain rate retrievals were also analysed in the study along selected latitudes and longitudes. Secondary products were generated from these results in order to more fully display, intercompare and validate various techniques of rainfall retrieval. Examples of both primary and secondary products are presented in this paper. Among the results submitted there was a large variation in coverage. In some data sets, rainfall retrievals were restricted to over ocean surfaces; in others coastal zones, high latitudes, ice and snow surfaces were flagged, and no estimation of precipitation made in some or all of these regions. The large number of products presented in this paper provide examples of the PIP‐1 analyses and allow the comparisons of data sets, withstanding the differences in coverage. This paper describes the process of generating results for this intercomparison project and some of the problems encountered in this exercise.