Quantitative characterisation of sky conditions on Paranal with the microwave radiometer LHATPRO – five years and learning

"A Low Humidity and Temperature Profiling (LHATPRO) microwave radiometer, optimized for measuring small amounts of atmospheric precipitable water vapour (PWV), has now been in use for more than five years to monitor sky conditions over ESO\u2019s Paranal observatory (median PWV ~2.5 mm). We\u2019ll summarise the performance characteristics of the unit and the current applications of its data in scheduling observations in Service Mode to take advantage of favourable conditions for infrared observations. We\u2019ll elaborate on our improved understanding of PWV over Paranal, including an analysis of PWV homogeneity addressing an important calibration issue. In addition we\u2019ll describe how the capabilities of the LHATPRO can be used in the future to further strengthen science operations and calibration by also offering line-of-sight support for individual VLT observations. Using its IR data we developed a method for an automated classification of photometric observing conditions in a quantitative way, supporting high precision photometry. Its highly precise PWV measurements enable new low PWV science during episodes of extremely low water vapour that result in a strongly increased transmission also outside the standard atmospheric windows. A goal for the future is to combine various diagnostics measurements (altitude resolved profiles) by LHATPRO and other instruments and sophisticated atmospheric modeling to better characterize relevant properties of the atmosphere and to thus enable more precise, local short-term forecasting for optimised science operations."