Should Interpolation of Reflectivity be Performed in Z or dBZ?

Spatial interpolation of radar reflectivities is commonly required when remapping data from its native spherical coordinates to a Cartesian or latitude–longitude grid or identifying features such as echo top height. It is often argued that this processing should be performed on the measured quantity Z (units of mm⁶ m^-3), rather than its decibel equivalent, dBZ. However, for linear interpolation the better quantity to operate on is whichever varies more linearly in space. Using a simple analysis method applied to reflectivity data from a single WSR-88D site, Lakshmanan (2012) showed that interpolation in dBZ produces consistently smaller errors. Here, we extend his analysis to several radars around Australia and show that, in fact, interpolating in dBZ is only better for low reflectivities (less than about 30 dBZ). Since these occur more frequently than higher values, dBZ gives better results overall; however, for severe weather applications, where high reflectivities are most relevant, interpolation in Z is recommended. In general, interpolating in Z leads to overestimation due to positive curvature of the reflectivity field while interpolating in dBZ leads to underestimation due to negative curvature of the reflectivity field. These results are consistent across all three coordinate directions (range, azimuth, and elevation) and a range of precipitation regimes.

References:

Lakshmanan, V., 2012: Image processing of weather radar reflectivity data: Should it be done in Z or dBZ? Electronic J. Severe Storms Meteor., 7, 1–8.