Should Interpolation of Radar Reflectivity be Performed in Z or dBZ?

Interpolation of ground-based radar measurements is required when mapping data from their native spherical coordinates to a Cartesian grid. For reflectivity the question arises as to whether this processing should be performed in units of Z (mm 6 m −3 ) or dB Z . This study addresses this question using one year of data from three radars, operating in diverse climates across Australia. For each radar, a subset of 800 volume scans is processed to identify “triads”—groups of three consecutive gates with valid data—in each of the three coordinate directions: range, azimuth, and elevation. For every triad, the reflectivity at the central gate is estimated by linearly interpolating between the outer two gates in both Z and dB Z . The resulting values are then compared with the true reflectivity at the central gate to quantify the interpolation errors. For all three sites and in all three coordinate directions, we find that interpolation in Z is more accurate on average, especially in regions of high reflectivity and strong reflectivity gradient (i.e., convective cores). However, interpolation in dB Z is better in regions of low and monotonically increasing/decreasing reflectivity. It is therefore recommended that reflectivities be converted from dB Z to Z prior to interpolation except when identifying echo-top height or other low-reflectivity boundaries.