The National Weather Service (NWS) began installation of about 160 WSR-88D radars covering the United States over ten years ago. These radars host a number of scientific algorithms that produce various value-added, derived products for our customers. One of these algorithms is the Precipitation Processing System (PPS) that generates rainfall accumulation products of various durations in graphical and digital formats. Recent detailed data analyses of these rainfall products have uncovered a minor software deficiency in the algorithmic processing that has been latent in the software since the radars were first deployed. This software error, called the PPS "truncation error," was relatively simple to correct and has since been implemented on all WSR-88D radars in Software Build 1 of the Open Systems Radar Product Generator.

The impact of the truncation error on the integrity of the products was non-trivial, particularly for stratiform rainfall events, and may likely explain the long-known tendency for the PPS to underestimate rainfall for these events. For stratiform rainfall cases, the truncation resulted in a 20-90% loss of hourly rainfall accumulation depending on the rainfall intensity and its duration. When summed over many rainfall events, the reduction in point maximum rainfall totals ranged from 15% for convective events to 60% for stratiform events.

Users of historical PPS rainfall products such as the hourly Digital Precipitation Array (DPA), and other NWS products derived from them such as the so-called Stage III precipitation products, that are resident in the NWS archive should be aware of these errors if they use them for historical case study analyses or other quantitative purposes. This paper will describe the truncation error, what caused it, when it was fixed, and a quantitative evaluation of the impacts on the computed radar rainfall amounts.