Impact of WSR-88D MESO-SAILS Usage on MRMS-based Quantitative Precipitation Estimation Accuracy

The National Weather Service (NWS) Radar Operations Center (ROC) regularly adds new Volume Coverage Pattern (VCP) options to the operational Weather Surveillance Radar 1988-Doppler (WSR-88D) network. One of these additions was the supplemental adaptive intra-volume low-level scan (SAILS) capability starting in 2014, followed by the multiple elevation scan option for SAILS (MESO-SAILS). SAILS and MESO-SAILS allow NWS forecasters and radar operators to insert between one and three extra base scan(s) in the middle of certain VCPs. The Multi-Radar Multi-Sensor (MRMS) Quantitative Precipitation Estimation (QPE) system is arguably the gold standard QPE method used by the NWS. MRMS’s initial input is derived from WSR-88D Level-II estimates, and its algorithms primarily use the base scan for rainfall estimates. Since additional base scans at higher temporal resolution decreases the integration period for QPE, it would theoretically follow that WSR-88D-derived QPE products would have increased accuracy when SAILS and MESO-SAILS are in use, particularly during heavy rainfall periods and flash-flooding events.

In this study, every Automated Surface Observing System (ASOS) site in the CONUS during every rainfall event in the years of 2021 and 2022 is compared with the MRMS radar-only product. Over 500,000 events, summarized in hourly increments, are included in the database for comparison. The analysis shows that the MRMS radar-only product has decreased bias and increased accuracy with SAILS on versus SAILS off. Additionally, improvements in accuracy/bias are observed through at least the MESO-SAILSx2 level, indicating that faster base scans do indeed improve MRMS data quality. The analysis is also broken up by season and river forecast center, and several performance statistics are discussed. Finally, an analysis of bias by rainfall rate is presented, showing that SAILS and MESO-SAILS have an even higher impact on bias/accuracy as rain rate increases (e.g., in convection), making it a critical tool in flash-flooding scenarios. This information is useful for training forecasters that using MESO-SAILS in these situations can improve MRMS product accuracy and, hence, possibly warning performance as well.