Quantitative Precipitation Estimation with NSSL’s Advanced Technology Demonstrator Phased Array Radar

The Weather Surveillance Radar–1988 Dopplers (WSR-88Ds) are an operational network of S-band (10-cm wavelength) radars with dual-polarization capabilities. Typically, WSR-88Ds have a volumetric update time of about 4-6 minutes, where the update time of the lowest elevation can be reduced to about 2 minutes with the implementation of the Supplemental Adaptive Intra-Volume Low Level Scan (SAILS). SAILS scans provide some improvement in temporal sampling, but at the expense of radar volume time. Currently, the National Oceanic and Atmospheric Administration (NOAA) is considering replacement options for its next generation of weather radars. One option is phased array radar (PAR), which employs electronic scanning and therefore has a faster volumetric update time (~1 minute) and an even faster update time for the lowest elevation. The Advanced Technology Demonstrator (ATD), operated by the National Severe Storms Laboratory (NSSL), is an example of a research PAR that is being used to evaluate the feasibility of dual-polarization PAR technology.

In this study, PAR quantitative precipitation estimation (QPE) is of particular interest. Questions have been raised regarding the impact an upgrade to PARs would have on QPE accuracy. Presumably, the improved temporal and vertical resolution of the PARs will lead to more accurate radar-based QPE. However, polarimetric PARs require more complex calibration compared to parabolic dish radars, and their measurement accuracy is not as robustly quantified compared to parabolic dish radars. To test the hypothesis that PARs can improve QPE accuracy, the ATD and KOUN, a WSR-88D also operated by the NSSL and collocated with the ATD, will be used. This work will explore two cases, 2 May 2024 and 4 June 2024, which exhibited heavy rainfall that led to the issuance of several flash flood warnings in central Oklahoma. After interpolating each radar to the MRMS grid via a two-pass Barnes method, instantaneous rainfall rates are calculated based on the methodology for the NSSL Multi-Radar Multi-Sensor (MRMS) Surface Precipitation Rate (SPR) product. KOUN-derived and ATD-derived QPE is then compared to Oklahoma Mesonet observations that have been interpolated to the common grid via isotropic ordinary kriging. The QPE-ground truth comparison will be used to evaluate rapid-update PAR-based QPE according to the MRMS methodology. It will also present the opportunity to test alternate methodology (e.g., updated polarimetric relationships) based on differing error characteristics between parabolic dish radars and PARs. Additionally, QPE error between the radars as a function of PAR-specific characteristics (i.e., changes in beam width due to electronic beam steering) will be explored.