Comprehensive Analysis of an Unusual Winter Graupel Shower Event Recorded by an S-Band Polarimetric Radar and Two Optical Imaging Surface Instruments

This paper presents comprehensive analysis of one unusual graupel shower event which was recorded during a Colorado winter campaign project called MASCRAD. The event is unusual because it recorded negative differential reflectivity, Zdr, values along a vertical column. The event was captured by the S-band polarimetric CSU-CHILL radar. The edge of the negative Zdr column went over a well-instrumented site, which includes a multi-angle snowflake camera (MASC) and a two-dimensional video disdrometer (2DVD).

It is impossible, in general, to explain all of the polarimetric radar measurables in winter precipitation using spheroidal shape. Even for Rayleigh scattering, where the spherical or spheroidal shape assumption is reasonable for reflectivity computation, it is not sufficient for computing the full scattering matrix and related radar measurables, required for radar-based particle classification. So, even at the S-band (all WSR-88D radars), some radar observables significantly depend on the shape of particles, and sophisticated computational electromagnetic methods are needed for scattering computations. In addition, with the advent of optical imaging disdrometers, we can reconstruct more realistic three-dimensional (3D) shapes of hydrometeors when compared to spheroidal approximations.

The 2DVD provides 2D contours of a graupel particle or another hydrometeor such as a rain drop, a snowflake, or a hailstone in two mutually orthogonal cross sections. The reconstructed shapes – after a suitable generation of the surface mesh composed of generalized quadrilateral patches – are used as input to a scattering analysis technique based on a higher order method of moments (MoM) in the surface integral equation (SIE) formulation and the frequency domain. The MoM-SIE modeling technique is used to compute the scattering matrices and polarimetric radar observables on a “particle-by-particle” basis. We use a “stacked ellipses” interpolation method for 3D shape reconstruction of particles from the two orthogonal contour images provided by the 2DVD. In particular, after obtaining scan line data from the 2DVD, contours are made for the particles, and the contours in two orthogonal planes are then reconstructed into a 3D shape with ellipses being created for each horizontal scan line received from the 2DVD. The MASC captures high-resolution photographs of snow and ice particles in freefall from multiple views. We use the visual hull image processing method to reconstruct 3D shapes of particles from the MASC photographs, and convert these shapes into models for MoM-SIE numerical electromagnetic scattering analysis.

This paper focuses on the graupel shower event of 16 Feb 2015 and in particular the time period ~19:00–21:00 UTC at the MASCRAD Field Site. The negative Zdr is demonstrated by CSU-CHILL Radar Zdr data, as well as by the KFTG (Denver) NEXRAD Zdr images, for the MASCRAD site area. The 2DVD contour images and MASC photographs show vertically ‘elongated' hydrometeors, mixed in with more ‘round' shaped hydrometeors with some fine structural features. Also, the radar measured negative Zdr areas seemed to correlate with the stronger reflectivity columns. The CHILL Radar showed higher LDR values than what was seen with snow aggregates outside of the considered time period. The 3D shapes of hydrometeors are reconstructed using the “stacked ellipses” method and the visual hull method, and then re-meshed by ANSYS ICEM CFD software. Scattering simulations are performed using the higher order MoM-SIE method, and they show that such particles indeed give rise to negative Zdr. We also compute other polarimetric radar observables (Zh, LDR, Kdp, and ρhv) and correlate the results with measurements from the CHILL Radar.