1.3 Calibration

Calibrating weather radars became routine soon after the discovery of its meteorological use. In 1951, the Weather Radar Group at the Massachusetts Institute of Technology discovered disparities between radar estimates and gauge measurements, which led them to research radar calibration (Atlas 2002). Traditional attempts at radar calibration made use of standard targets with known backscattering properties, such as BB gun pellets fired into radar beams; metalized ping pong balls dropped from light aircraft; or metalized spheres suspended from balloons or helicopters. While such physical methods work well for single-radar calibration and monitoring, they however pose challenges for networks of tens or hundreds of radars. Auxiliary instruments for calibration, such as radar profilers and disdrometers, measure drop size distribution at the same time as the radar. The corresponding reflectivities from the drop size distribution measured by the disdrometers and the reflectivity measured by the radar are then compared for consistency (Joss, Thams, and Waldvogel 1968; Ulbrich and Lee 1999). However, since radars measure precipitation aloft while disdrometers measure drop size distribution on the ground, the sample volumes between those two instruments can differ by as much as eight orders (Droegemeier et al. 2000). The height difference between these sample volumes mean that external factors such as wind and temperature can change the microphysical characteristics of the droplets that reach the disdrometer, e.g. drop size change through fusion/breakup, change of state through melting.

Relative calibration (defined as the assessment of reflectivity bias between two radars) has been gaining popularity, in particular the comparison with spaceborne precipitation radars (SR) (such as the precipitation radar on-board the Tropical Rainfall Measuring Mission (TRMM; 1007-2014; Kummerow et al. (1998)) and Global Precipitation Measurement (GPM; 2014-present; Hou et al. (2013)). The precipitation radars on-board these satellite platforms are calibrated to within 1 dBZ (Kawanishi et al. 2000; Takahashi, Kuroiwa, and Kawanishi 2003; Furukawa et al. 2015; Toyoshima, Masunaga, and Furuzawa 2015), and hence they are accurate enough to serve as a reference for relative calibration. The measured reflectivities from the on-board spaceborne precipitation radars are matched with the ground radar measurements, where the reflectivities (the primary measured quantity) are compared (Warren et al. 2018) or the estimated rainfall from both instruments (Kirstetter et al. 2012; Speirs, Gabella, and Berne 2017; Joss et al. 2006; Amitai, Llort, and Sempere-Torres 2009; Gabella et al. 2017; Petracca et al. 2018) for the same event in areas of overlap for calibration. In addition, a major advantage of relative calibration in contrast to absolute calibration (i.e. minimizing the bias in measured power between an external reference noise source and the radar at hand) is that they can be carried out a posteriori, and this be applied to historical data. The large spatial coverage of spaceborne radars enables the calibration of multiple radars in a large network against a single, stable reference (Hong and Gourley 2015), making them particularly helpful for countries like the Philippines with a sparse rain-gauge network.

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