Impact of Radar-Constrained Effective Drop-Shape Relations on Polarimetric Radar Quantitative Precipitation Estimation in Typhoons

Accurate typhoon quantitative precipitation estimation (QPE) with polarimetric radar depends on the assumed drop-shape relation (DSR), but typhoon DSRs derived from surface 2D video disdrometer (2DVD) observations may not represent elevated radar sampling volumes. To address this issue, we apply the established polarimetric self-consistency framework to estimate radar-constrained effective linear DSRs from quality-controlled horizontal reflectivity factor (ZH), differential reflectivity (ZDR), and specific differential phase (KDP) observations using multi-sample optimization. The method estimates βeff, the slope parameter of the effective linear DSR, and is first evaluated using a separate non-typhoon event, for which the radar-constrained relation agrees well with the median axis ratios measured by a nearby 2DVD. It is then applied to six landfalling typhoons observed by the Guangzhou S-band polarimetric radar. The resulting βeff values range from 0.046 to 0.049 mm−1, indicating stable event-to-event behavior and greater effective oblateness than two published surface 2DVD-derived typhoon DSRs. Scattering simulations show that DSR choice has little effect on ZH but substantially affects ZDR and KDP, causing overly spherical DSRs to underestimate R(ZH, ZDR) and overestimate R(KDP). Gauge validation with more than 400 gauges shows that the radar-constrained effective and Brandes et al. DSRs provide more consistent rainfall estimates than the two surface-derived typhoon DSRs. Application without recalibration to Typhoon Lekima, observed by the independent Wenzhou S-band radar, reproduces the same overall performance grouping. These results identify a transferability limitation of surface-derived DSRs and demonstrate the value of radar-volume constraints for typhoon polarimetric QPE.