Takahashi et al. (2025) Evaluation of the flagGraupelHail Product from Dual-Frequency Precipitation Radar Onboard the Global Precipitation Measurement Core Observatory Using Multi-Parameter Phased Array Weather Radar

Identification

• Journal: Remote Sensing
• Year: 2025
• Date: 2025-11-17
• Authors: Nobuhiro Takahashi, Takayuki Kosaka
• DOI: 10.3390/rs17223741

Research Groups

• Institute for Space–Earth Environmental Research, Nagoya University, Nagoya, Japan
• Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan

Short Summary

This study evaluates the GPM/DPR flagGraupelHail product in a humid convective environment over Tokyo, Japan, using high-resolution Multi-Parameter Phased Array Weather Radar (MP-PAWR) data and a novel volume-matching method. It demonstrates that incorporating storm-top height information significantly improves the accuracy of DPR's graupel/hail detection.

Objective

• To evaluate the GPM/DPR flagGraupelHail product in a humid convective environment.
• To develop an improved volume-matching approach between DPR and MP-PAWR.
• To propose an additional index to enhance graupel and hail detection performance by DPR.

Study Configuration

• Spatial Scale: Tokyo region of Japan (humid environment); MP-PAWR observation radius of 60 km, covering altitudes up to 15 km; XRAIN radar network with 80 km range per radar; DPR footprint diameter of approximately 5 km.
• Temporal Scale: Case study on 12 July 2022, with GPM Core Observatory overpass at 21:49 JST (Japan Standard Time); MP-PAWR volumetric scans every 30 seconds; XRAIN PPI scans every 5 minutes.

Methodology and Data

• Models used:
  • X-band fuzzy-logic-based hydrometeor classification algorithm (Kouketsu et al., [18]) for MP-PAWR.
  • Bias correction method for radar reflectivity (Z) and differential reflectivity (ZDR) (Gourley et al., [16]).
  • Parameterized Textural Index (PTI) algorithm for DPR graupel/hail detection (Le and Chandrasekar, [7,8]).
  • T-matrix method (Mishchenko et al., [21]) for simulating Z values.
  • Python-based pyDDA (ver. 1.5) software (Jackson et al., [23]) for dual-Doppler wind field retrieval.
• Data sources:
  • Spaceborne: Global Precipitation Measurement (GPM) Core Observatory Dual-Frequency Precipitation Radar (DPR) (Ku-band and Ka-band), specifically the flagGraupelHail product (Version 07).
  • Ground-based: Multi-Parameter Phased Array Weather Radar (MP-PAWR) (X-band, dual-polarization) located in Saitama City, Japan.
  • Ground-based network: XRAIN (X-band dual-polarization radar network) including KANTOU, SHINYOKO, FUNABASHI, and YATTAJIMA radars.
  • Surface observations: Japan Meteorological Agency’s AMeDAS (Automatic Meteorological Data Acquisition System).

Main Results

• MP-PAWR reflectivity bias correction using XRAIN data improved agreement with KuPR observations, reducing the bias from approximately -5 dB to 0.91 dB with a root mean square error (RMSE) of 3.18 dB.
• A new grid-matching method, convolving high-resolution MP-PAWR data (0.25 km resolution) with the DPR antenna pattern, enabled comparison of 3293 points, significantly more than conventional methods (339 points), with a bias of 0.91 dB and RMSE of 3.18 dB.
• DPR flagGraupelHail detections generally corresponded to regions of high MP-PAWR Graupel-Hail ratio (GHratio), but DPR tended to detect fewer events and showed spatial discreteness.
• A moderate negative correlation (r = -0.409) was observed between DPR PTI values and the MP-PAWR GHratio.
• The height difference between the 35 dBZ echo top and the 0 °C level (STH35-FH) showed the highest correlation (r = 0.735) with the MP-PAWR GHratio among tested reflectivity thresholds (25–40 dBZ).
• Incorporating STH35-FH into the DPR algorithm, with a relaxed PTI threshold of 10, improved consistency with MP-PAWR detections, reducing false positives and enhancing overall detection accuracy for graupel/hail in humid environments.
• MP-PAWR successfully captured fine-scale vertical structures and echo towers exceeding 10 km, while KuPR provided broader horizontal coverage.
• High GHratio regions from MP-PAWR coincided with strong updrafts (exceeding 10 m s⁻¹) identified by XRAIN dual-Doppler analysis.

Contributions

• Provides the first evaluation of the GPM/DPR flagGraupelHail product in a humid convective environment (Tokyo, Japan), addressing a gap in previous studies focused on continental or semi-arid regions.
• Introduces and validates an improved, high-resolution volume-matching method between spaceborne (DPR) and ground-based (MP-PAWR) radar, particularly suitable for localized hydrometeor type comparisons.
• Proposes an additional index, STH35-FH (height difference between the 35 dBZ echo top and the 0 °C level), to enhance DPR's graupel and hail detection performance, especially in humid environments.
• Demonstrates the value of combining high-temporal-resolution (30 s) and high-spatial-resolution dual-polarization ground-based radar (MP-PAWR) with spaceborne radar for refining global precipitation products and understanding microphysics.

Funding

• 4th Research Announcement on the Earth Observations of the Japan Aerospace Exploration Agency (JAXA) JX-PSPC-576316.

Citation

@article{Takahashi2025Evaluation,
  author = {Takahashi, Nobuhiro and Kosaka, Takayuki},
  title = {Evaluation of the flagGraupelHail Product from Dual-Frequency Precipitation Radar Onboard the Global Precipitation Measurement Core Observatory Using Multi-Parameter Phased Array Weather Radar},
  journal = {Remote Sensing},
  year = {2025},
  doi = {10.3390/rs17223741},
  url = {https://doi.org/10.3390/rs17223741}
}