Detection of Global Water Vapor Variation in the Troposphere from 2007 to 2018 using COSMIC Radio Occultation Data

Atmospheric water vapor is essential to Earth's climate system, regulating the global energy balance and hydrological cycle. Accurate and consistent water vapor data products are crucial for advancing weather prediction and Earth's climate studies. This study demonstrates the accurate detection of the global water vapor variation in the troposphere from 2007 to 2018 using Formosa Satellite Mission 3–Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC) Global Navigation Satellite System (GNSS) radio occultation (RO) data. European Centre for Medium-Range Weather Forecasts (ECMWF) ReAnalysis Model 5 (ERA5) was used as the reference to correct the biases due to the limited time and location coverage in COSMIC RO data by using the sampling error removal method. We further validated the COSMIC RO data through comparison with ERA5 water vapor data in terms of biases and trend differences at different atmospheric pressure levels (300 hPa, 500 hPa, and 850 hPa) from 2007 to 2018. In general, the COSMIC and ERA5 water vapor data show good spatial and temporal agreements with the regional differences in areas abundant with stratocumulus clouds, as well as within the Intertropical Convergence Zone (ITCZ) characterized by frequent deep cloud cover. Such differences can stem from the distinctive cloud-penetration capability of the RO signal, whereas the water vapor from the reanalysis data is assumed from the cloud-free scenes. The long-term time series of global, latitudinal, and regional COSMIC RO water vapor variations are further analyzed to characterize water vapor trends at different spatial scales. Increases in water vapor around 2010-2011 and 2015-2016 due to El Niño events are discernible in COSMIC time-series data. The global water vapor trends derived from COSMIC RO data are 3.47±0.24%, 3.25±1.06%, and 2.03±2.93% per decade at 300, 500, and 850 hPa, respectively. The latitude-mean water vapor trends are mostly positive (increasing) except in the southern -80^o to -60^o latitude zone and have substantial latitudinal variabilities. Large regional water vapor trend variabilities with strong increasing and decreasing slopes are observed in the tropical and subtropical regions. At 500 and 850 hPa, strong water vapor growing trends are noted in the equatorial Pacific Ocean and the Laccadive Sea, while decreasing trends are evident in the Indo-Pacific Ocean region and the Arabian Sea. Over land, substantial upward trends at 850 hPa are observed in the southern United States, contrasting with downward trends in South Africa and Australia. Evaluating the spatiotemporal variability in atmospheric water vapor data derived from RO data is crucial to ensuring its reliability in climate research.