Using a network of 2D video disdrometers for external radar calibration of NASA's S-band polarimetric radar

A fundamental requirement of weather radars is accurate calibration of the absolute reflectivity, the oft quoted figure being within ± 1dB but for quantitative application (rain rate estimation) a smaller uncertainty is often desired (±0.5dB). It is generally accepted that the “standard” methods such as engineering calibration, solar calibration or metallic sphere calibration cannot achieve uncertainty levels of ±0.5 dB (through error budget considerations). Other methods such as self-consistency use polarimetric data from rain [Zh, Zdr, Kdp] to calibrate the system offsets in Zh but uncertainties in the assumed mean drop axis ratio versus D relation as well as filtering/thresholding of the polarimetric data precludes achieving the elusive figure of ±0.5 dB. Here we use a network of 2D-video disdrometers to calibrate the absolute Zh measured by NASA's NPOL radar. The use of a 6-disdrometer network greatly increases the sample size. Rapid PPI scan updates (cycle time of 40 s) allows for estimating the spatial correlation of Zh at high spatial resolution permitting the “point-to-area” variance to be calculated and used in determining the accuracy of the reflectivity calibration. On two days during the MC3E campaign in northern Oklahoma, NASA's S-band polarimetric radar (NPOL) performed repeated PPI scans over a network of six 2D video disdrometer (2DVD) sites, located 20 to 30 km from the radar. The scans were repeated approximately every 40 seconds. We consider here the two cases, one a rapidly evolving multi-cell rain event (with large drops) on 24 April 2011 and the second a somewhat more uniform rain event on 11 May 2011.

For both events, the calibration offsets for radar reflectivity and differential reflectivity were determined by comparing the radar data extracted over the disdrometer sites with those determined from scattering simulations using the 2DVD data. Time series comparisons show excellent agreement for all six sites, and a technique has been developed to determine the reflectivity offsets for the NPOL data from the comparisons.

The radar data were then used to determine the rain rates over the six sites and compared with those derived from the 2DVD measurements. Once again, excellent agreement was obtained for all six sites, both for rain rates and for rain accumulations. Good consistency was also seen in terms of the DSD parameters.