Microwave radiometry: the impact on observed brightness temperature due to water droplets on the feed system
The water vapour radiometer Astrid at the Onsala Space Observatory measures the downward radiation from atmospheric water vapour. During and after a rainfall droplets stay on the reflector and on the transmission window and cause an increase in the measured antenna brightness temperature, TA. Four different cases were investigated in this report. Case 1, when the reflector was sprayed with droplets resulted in a 4 K increase in TA. In Case 2, where the transmission window was sprayed with droplets, a 34 K increase in TA was found. In Case 3 both of them was sprayed with droplets which resulted in a 74 K increase in TA. In Case 4 droplets were placed in a grid on the reflector which resulted in a 13 K increase in TA. As a reference the sky brightness temperature during the measurements was independently monitored by a second radiometer at the site. It shows stable results in all four cases. The weather conditions during the first experiment day (Case 1–3), July 26, were sunny and clear with a ground temperature that ranged from 17 C to 19 C, a relative humidity of 74 % and an air pressure of 1015 hPa. On the second day (Case 4), August 31, the weather was partly cloudy with a ground temperature at 17 C, relative humidity of 76 % and air pressure at 1011 hPa.