An investigation of 19.5 GHz sky noise temperature over Taiwan area

A number of ground-based instruments, including 19.5 GHz radiometer, optica l raingauge, portable weather station, and high resolution disdrometer, wer e set up to conduct the Ka band propagation experiment of the Experimental Communication Payload (ECP) for ROCSAT-1. In this article, 19.5 GHz backgro und sky noise temperatures measured at Chung-Li and Tainan sites are presen ted and investigated. Long-term statistics of the 19.5 GHz background sky n oise temperature observed by a vertically pointed radiometer in precipitati on-free condition over the Taiwan area shows that the percentages of time t hat the sky noise temperature exceeds 20 K, 30 K, 40 K and 50 K are, respec tively, 98%, 85%, 53%, and 27%. However, in precipitating environments, sta tistics shows that the percentage of time that the sky noise temperature ex ceeds 55 K, 100 K, 150 K, and 200 K are, respectively, 22%, 13%, 4.5%, and 2%. The statistics of sky noise temperatures observed at different zenith a ngles under environments without precipitation is also made. The results sh ow that 80% of the observed sky noise temperatures at zenith angles of 100, 300, 500 and 70 degrees are, respectively, in the ranges of 92 - 180 K, 39 - 52 K, 26 - 33 K, and 21 - 27 K. In addition, a comparison between surfac e rainfall rate recorded by the optical raingauge and sky noise temperature measured by 19.5 GHz radiometer shows that the former lags behind the latt er by about 5 minutes, implying non-uniform and inhomogeneous distribution of precipitation in the air. In order to measure the precipitation aloft, t he Chung-Li VHF radar was operated simultaneously. Champaign observation sh ows that there is no latency between sky noise temperature and VHF backscat ter from precipitation. This result implies that the VHF backscatter from p recipitation can be employed to validate the observed sky noise temperature . In addition, we also find that the sky noise temperature may be as high a s 155 K (corresponding to 3.6 dB attenuation) under an environment without surface precipitation. This feature is attributed to the dense water vapor and heavy cloud.