The ground-based microwave radiometer RPG-HATPRO-G5, designed by Radiometer Physics GmbH (RPG), has been operating at CIAO since December 2022 and replaced the old microwave profiler MP3014 (designed by Radiometrics Corporation) not operational anymore. It measures the sky brightness temperature (Tb) at 14 frequencies: 7 in the K-band, distributed from the centre onto the wing of the 22 GHz water vapour resonance absorption line, the rest are in the V-band, distributed on one shoulder of the 60 GHz oxygen spin-rotation band. The radiometer is equipped with a scanning infrared radiometer, able to measure cloud base brightness temperature (Tb) within the spectral range of 9.6 to 11.5 µm, and with an automatic weather station for measuring surface temperature, humidity, pressure, rain rate and wind (speed and direction). A mitigation system consisting of a blower, a heater and the hydrophobic coating of radome minimize errors resulting from the accumulation of precipitations and dew on the radiometer’s radome. The radiometer is also equipped with an azimuth positioner for full sky scanning.

Temperature, humidity and cloud liquid water profiles up to 10 km above the ground (level 2 products) are retrieved from Tb at K and V-band frequencies, using a neural network algorithm trained by forward modelling several years of historical radiosonde dataset. The above profiles have vertical resolution that increases with the altitude range with values less than 100 m up to 2.5 km, between 120 and 200 m up to 6 km and 300 m above. Finally, the inversion algorithm provides also an estimation of the Integrated Precipitable Water Vapour (IPWV) and the cloud Liquid Water Path (LWP).