Retrieval of sun brightness temperature and precipitating cloud extinction using ground-based sun-tracking microwave radiometry

Sun-tracking (ST) microwave radiometry is a technique where the Sun is used as a microwave signal source and it is here rigorously summarized. The antenna noise temperature of a ground-based microwave radiometer is measured by alternately pointing toward-the-Sun and off-the-Sun while tracking it along its diurnal ecliptic. During clear sky the brightness temperature of the Sun disk emission at K and Ka band and in the unexplored millimeter-wave frequency region at V and W band can be estimated by adopting different techniques. Using a unique dataset collected during 2015 through a ST multifrequency radiometer, the Sun brightness temperature shows a decreasing behavior with frequency with values from about 9000 K at K band down to about 6600 K at W band. In the presence of precipitating clouds the ST technique can also provide an accurate estimate of the atmospheric extinction up to about 32 dB at W band with the current radiometric system. Parametric prediction models for retrieving all-weather atmospheric extinction from ground-based microwave radiometers are then tested and their accuracy evaluated.