Quantification of the Intraseasonal to Interannual Variation of Total Precipitable Water Derived from Microwave Radiometers using COSMIC Radio Occultation from 2006 to 2016

Water vapor is a primary variable that affects cloud radiative effects and hydrological feedbacks. Trends in global and regional vertically integrated total atmospheric water vapor, or Total Precipitable Water (TPW), are important indicators of climate warming because of the strong positive feedback between temperature and water vapor increases. Accurate observations of TPW are therefore important in identifying climate change and in verifying climate models, which estimate a wide range of TPW trends. Global TPW can be derived from satellite visible, infrared, and microwave sensors. However, no single remote sensing technique is capable of completely fulfilling the needs for climate studies in terms of spatial and temporal coverage and accuracy. Passive microwave (MW) radiometers are among the very few satellite missions that are able to provide long-term (close to 30 years) all-weather time series of water vapor measurements using a similar satellite sensors and retrieval techniques. Because MW radiation is significantly affected (absorbed or scattered) by heavy rain, the derived TPW is only retrieved under conditions of no or light-to-moderate rain, the uncertainty of TPW under all weather conditions may be large.

In this study, we quantify the uncertainty of the intra-seasonal to inter-annual variation of TPW derived from multiple MW radiometers under different meteorology (i.e., clear, cloudy, non-precipitation/cloudy and precipitation/cloudy) conditions using collocated TPW estimates derived from COSMIC (Constellation System for Meteorology, Ionosphere and Climate) radio occultation (RO). The Global Positioning System (GPS) Radio Occultation (RO) is an active remote sensing technique, which is complementary with the passive microwave and infrared sounders and microwave imagers. Because GPS RO data are not sensitive to clouds and precipitation, GPS RO derived water vapour products are very useful to identify the possible TWP biases retrieved from measurements of passive microwave sounders and imagers under different meteorology conditions. Results show that the mean microwave radiometer - COSMIC TPW differences range from 0.06-0.18 mm for clear skies, 0.79-0.96 mm for cloudy skies, 0.46-0.49 mm for cloudy but non-precipitation conditions, and 1.64-1.88 mm for precipitation conditions. Because RO measurements are not significantly affected by clouds and precipitation, the biases mainly result from MW retrieval uncertainties under cloudy and precipitating conditions. All COSMIC and MW radiometers detect a positive TPW trend over these ten years. 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). These two trends from independent observations are larger than previous estimates and are a strong indication of the positive water vapor-temperature feedback in a warming planet.