Absolute Calibration of 94/95-GHz Radars Using Rain

Abstract

Absolute calibration of cloud radars is very difficult. A new method is proposed for 94/95-GHz radars that exploits the fact that at this frequency, the radar reflectivity factor of rain measured at a range of 250 m is approximately constant at 19 dBZ for rain rates between 3 and 10 mm h?1, due to the combined effects of extinction and non-Rayleigh scattering. The standard deviation of around 1.5 dB is due to natural variations in the number concentration of drops and is consistent with the variation predicted from theory, but averaging over a number of different rain events over a month or more should be sufficient to reduce the calibration error to less than 1 dB. A thin layer of rainwater on the radomes of the 94-GHz radar at Chilbolton, in southern England, was found to cause a two-way attenuation of between 9 and 14 dB, but it is shown here that the technique may be successfully implemented by operating the radar at a low elevation angle and employing a shelter to keep it dry. Most 94-GHz cloud radars worldwide use the same amplifier, and monitoring the calibration of this radar over a 2-yr period of continuous use reveals a loss of power of around 1 dB in the first year and 10 dB in the second. Frequent calibration is therefore recommended.