Analysis on the solar influence to brightness temperatures observed with a ground-based microwave radiometer*

When the sun is located in the observation direction of the ground-based microwave radiometer, the brightness temperature data, especially for those deployed in low latitudes, will increase abnormally due to the influence of solar radiation. The paper collects the observation data of the MWP967KV-type ground-based microwave radiometer set up at Field Experiment Base on Lightning Science of China Meteorological Administration in Guangzhou, from June 1 to July 17, 2017, and May 21 to July 31, 2018. The observation data have been fitted and the fitting parameters have been quantitatively analyzed. The results show that the daily variation characteristics of brightness temperature increment (Delta Tb(0)) increase from the end of May or early June, reach the maximum on the summer solstice, and then decline. Higher maximum solar elevation angle (Hs max), greater atmospheric transmittance, and narrower beam width imply greater amplitudes of Delta Tb(0). The correlation coefficients between the amplitudes of Delta Tb(0) and Hs max for K-band and V-band in 2017 and 2018 are 0.96, 0.86, 0.98, and 0.94, respectively. And the maximum brightness temperature increment can reach up to 221.21 K. There's a linear relationship between the time of the amplitudes of Delta Tb(0) and the noontime. The correlation coefficients between them for K-band and V-band in 2017 and 2018 are 0.98, 0.90, 0.98, and 0.96, respectively. The longest duration of solar influence is 52.59min and decreases with the observation frequencies due to the beam width.