Retrieval of precipitation extinction using ground-based sun-tracking millimeter-wave radiometry

Sun-tracking millimeter-wave radiometry exploits the Sun as a beacon source by tracking it along its diurnal ecliptic. The atmospheric brightness temperature is measured by alternately pointing toward-the-Sun and off-the-Sun according to ad hoc switching strategy. By properly developing a retrieval algorithm, we can estimate the atmospheric path attenuation in all-weather conditions. The Langley method, based on elevation-scanning, is proposed to estimate the equivalent brightness temperature of the Sun, which is a critical step for precipitation extinction estimation. An application to available Sun-tracking radiometric measurements at V and W band in Rome (NY, USA) is shown, discussed and compared with the conventional technique using the clear-air approximation of the mean radiative temperature. Results show an appealing potential of Sun-tracking technique in order to exploit millimeter-wave radiometry for atmospheric retrievals even in a cloudy and rainy conditions.