A Comparison of Radar Rainfall Estimators During The South China Sea Monsoon Experiment (SCSMEX)

The South China Sea Monsoon Experiment was conducted over the South China Sea (SCS) during the period May 5-June 25 1998. An Intensive Flux Array was established in the northern portion of the SCS and included a dual-Doppler radar network located near Dongsha Is (20.7N 116.7E). Included was the NASA C-band Doppler radar (USA) located on the research vessel Shi Yan #3 (20.4N 116.8E) and the BMRC CPOL polarimetric radar (Australia) on Dongsha Is. The radars sampled a variety of convective systems occurring during the onset of the 1998 monsoon.

The remote measurement of rainfall over this oceanic environment using polarimetric rainfall estimators is described. Rain gauges and disdrometers were located on Dongsha Is and the Shi Yan #3 (optical gauges) and used for validation but primary validation was undertaken using acoustic raingauge (ARG) attached to an Atlas Mooring located 20.36N 116.52 E.

Calibration of the CPOL was undertaken using a variety of techniques. Electronic and solar calibrations; vertical pointing data, bias inferences deduced from microphysical phase and inter comparison of the polarimetric variables provided time series of ZH and ZDR offsets. Attenuation correction procedures were applied to the CPOL reflectivity and differential reflectivity radar data using the self consistent method described by Bringi et al. (2001). Here differential phase measurements are employed as a constraint. Then rainfall was estimated using a variety of estimators including R(KDP), R(AH), R(KDP, ZDR), R(AH,ZDR) and R(ZH_CORR). Radar data were averaged over a square of 4 km 2 and gauge data over about 15 minutes for the comparison. The primary rainfall intercomparisons were over the period 15-20 May 1998 during the monsoon onset.

The polarimetric rainfall estimators showed consistent results especially with the ARG data. Mean square errors in rain rate ranged from 3.6-4.1 mm h-1 and correlation coefficients in the range 0.70-0.76 with R (AH) showing best overall agreement (marginally).