Global Validation of Atmospheric Total Precipitable Water Derived COSMIC Radio Occultation Using Microwave Radiometers over Oceans under Clear and Cloudy Skies

Water vapor (WV) is one of the most important greenhouse gases in the atmosphere. Accurate observations of long-term water vapor under both clear and cloudy skies are important for understanding the role of water vapor on climate, which is still one of the largest uncertainties in understanding climate change mechanisms. The Global Navigation Satellite System (GNSS) radio occultation (RO) is the first space-based measurement technique that can provide estimates of global total precipitable water (TPW) in all weather. Launched in June 2006, COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) GPS RO data to study atmospheric temperature and refractivity trends in the lower stratosphere, and modes of variability above, within, and below clouds. Passive microwave (MW) radiometers are among the very few satellite missions that are able to provide long-term (more than 25 years) all-weather time series of water vapor measurements using a similar satellite sensors and retrieval techniques. Recently a new version of daily ocean products mapped to 0.25 degree grid from SSM/I and similar satellite-borne microwave radiometers including SSMIS, AMSR, AMSR-E, WindSat, and TMI are released by Remote Sensing System (RSS). In this study, we compare atmospheric TPW derived from SSM/I (Special Sensor Microwave Imager) and SSMIS (Special Sensor Microwave Imager Sounder) radiometers and WindSat to collocated TPW estimates derived from COSMIC under clear and cloudy conditions over the oceans from June 2006 to December 2013.

The causes of the global TWP differences under different meteorological conditions (i.e., clear, cloudy, non-precipitation/cloudy and precipitation/cloudy) are specified. The trend using all COSMIC observations collocated with MW pixels is 1.79 mm/decade, with a 95% confidence interval of (0.96, 2.63), which is in close agreement with the trend estimated by all MW observations (1.78 mm/decade with a 95% confidence interval of 0.94, 2.62).