Rainfall estimation from C-band polarimetric radar in Okinawa, Japan : Comparisons with 2D-video disdrometer and 400 MHz Wind Profiler

The Okinawa Subtropical Environment Remote-Sensing Center of NICT operates an advanced C-band polarimetric radar (referred to as COBRA) with 2 Klystron transmitters and 2 digital receivers. A 443 MHz Wind Profiler and a 2D-video disdrometer are located about 15 km from the COBRA radar site. In this paper we utilize the co-polar measurements of Zh, Zdr and Kdp; the radiated polarization state was slant 45 deg with simultaneous reception of the H and V signals in 2 separate receivers. The COBRA Zdr calibration had to be carefully adjusted because of the dependence of the differential gain between the H and V receivers on the input signal level.

A long duration ‘Baiu' frontal event with mixed stratiform and embedded convective rain was used for the inter-comparison. The 2D-video disdrometer data from this long event were used to measure the drop size distributions (DSD) and the mean axis ratios which were in turn used in developing the polarimetric-based radar retrieval algorithms (Do from Zdr, and R from Kdp) as well as the attenuation-correction methods. The wind profiler data were processed using an iterative retrieval method (involving repeated convolutions) for arbitrary shaped raindrop size distributions.

Results of the inter-comparison showed excellent agreement amongst all three instruments for the initial stratiform period, both in terms of Do and R. During the convective rain events, the COBRA-retrieved Do showed reasonable agreement with the 2DVD derived Do considering the highly fluctuating DSD. The estimated rainfall rate from the three instruments compared very well for the entire selected period of 12 hours despite considerable differences in the sampling volumes. The Kdp-based rain rate estimator derived in this environment was very close to previously published studies at C-band in a tropical environment.