Comparison of TRMM and GPM Precipitation Radars to U.S. WSR-88D Radars in the Expanded GPM Validation Network

The Global Precipitation Measurement (GPM) Validation Network (VN) hosted by the NASA Goddard Space Flight Center consists of a suite of software and procedures to routinely acquire, quality control, process, analyze, and archive Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR), GPM Dual-Frequency Precipitation Radar (DPR), and coincident WSR-88D radar (GR) data products. A VN prototype based on TRMM PR has been in place since August, 2006. Following the launch of GPM in February, 2014 the VN prototype became operational, with DPR data availability starting in March, 2014. The VN had the advantage of overlapping GPM and TRMM observations between March, 2014 and the final shutdown of the TRMM PR on 1 April 2015.

An algorithm has been developed for the VN space/ground radar comparison that defines common volumes in terms of the geometric intersection of PR and GR rays on individual GR sweep surfaces, where smoothing of the PR and GR data are minimized and no interpolation is performed (Schwaller and Morris, 2011). The PR and GR volume-averaged reflectivity values of each sample volume are accompanied by descriptive metadata, for attributes including the variability and maximum of the reflectivity within the sample volume, and the fraction of range gates in the sample average having reflectivity values above an adjustable detection threshold (typically taken to be 18 dBZ for the PR). Sample volumes are further characterized by rain type (Stratiform or Convective), proximity to the melting layer, underlying surface (land/water/mixed), and the time difference between the PR and GR observations. This TRMM-based algorithm has been enhanced to support analysis of any and all of the GPM Level 2 Ka- and Ku-band DPR products and scan types. In addition, the near-global coverage of the GPM orbit has allowed the VN to expand to include WSR-88D sites at higher latitudes than TRMM, including northern CONUS and Alaskan locations. The volume-matched data and analysis capabilities now include WSR-88D Level II and derived dual-polarization parameters and DPR drop size distribution (DSD) parameters such as Dm and Nw, and derived rain rates.

In this paper, we will show results comparing the TRMM PR and GPM DPR to matching WSR-88D ground radar data, both for GR sites in common to TRMM and GPM, and at GR sites beyond the TRMM coverage. Mean reflectivity differences between the ground and space radars will be used to evaluate site-specific calibration differences between WSR-88D locations, and between the TRMM PR and GPM DPR. Preliminary results of comparison of DPR Ka-band to GR reflectivity, and of DPR to GR DSD parameters will be shown. Time series of site-specific PR/DPR and GR reflectivity differences will be shown as an indicator of the stability of the WSR-88D measurements and the expected case-by-case variability of the comparison technique. Use of unattenuated ground radar data to evaluate the PR and DPR attenuation correction algorithms will be discussed. Examples of current DPR Ku-band ground clutter issues will be shown, and conservative methods to mitigate these clutter issues will be described. Finally, procedures for outside users to obtain VN data and open source software and to participate in GPM VN activities will be described.

REFERENCES

Schwaller, M. R. and K. R. Morris, 2011: A Ground Validation Network for the Global Precipitation Measurement Mission. J. Atmos. Oceanic Technol., 28, 301–319.