Quality control and verification of Doppler spectra collected from a vertically pointing FMCW radar deployed during VORTEX-Southeast

The University of Massachusetts S-band frequency-modulated, continuous-wave radar (UMass FMCW) was deployed to monitor the growth of the convective boundary layer over northern Alabama during the Verification of the Origins of Rotation in Tornadoes Experiment-Southeast (VORTEX-SE). The Doppler spectra collected in 2016 from the vertically-pointing UMass FMCW contain “spurs”, or spurious spectral peaks, caused by high-voltage switching power supplies in the traveling wave tube amplifier. In the original data processing scheme for this radar, a median filtering method was used to eliminate most of the spurs, but the largest ones persisted, which significantly degraded the quality of derived radar moments (e.g., reflectivity, Doppler velocity, and spectrum width) and hindered further analysis of these data (e.g., boundary layer height tracking).

In this study, a novel “in-painting” image processing technique was applied to remove the spurs in the Doppler spectra. We hypothesized the in-painting method would exhibit superior performance to the median filter at removing large spectral peaks, and also improve downstream radar products derived from the spectra. First, a Laplacian filter identified and masked spikes in the spectra that were characteristic of the spurs in shape and amplitude. The in-painting method then filled in masked areas based on surrounding data. Via a histogram analysis, the in-painting method was found to be more effective than the median filter at removing the large spurs from the Doppler spectra. The radar moments were then recomputed using a coherent power (CP) technique, resulting in cleaner reflectivity, Doppler velocity, and spectrum width data. Improvement was also found downstream when a boundary layer height detection algorithm was applied to the moments generated from the in- painted spectra. Output from the boundary layer height detection algorithm was then used to verify forecast boundary layer height from the Advanced Regional Prediction System (ARPS) model for the 31 March 2016 VORTEX-SE tornadic case study.