Characterizing the Tropospheric Water Vapor Variation using COSMIC Radio Occultation and ECMWF Reanalysis Data
Authors: Shao Xi , Ho Shu-Peng , Jing Xin , Zhou Xinjia , Chen Yong , Liu Tung-Chang , Zhang Bin , Dong Jun 
Published: February 2023

Abstract. Atmospheric water vapor plays an essential role in the global energy balance, hydrological cycle, and climate system. High-quality and consistent water vapor data from different sources are critical for numerical weather prediction and climate studies. This study evaluates the consistencies between Formosa Satellite Mission 3–Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) radio occultation (RO) and European Centre for Medium-Range Weather Forecasts (ECMWF) ReAnalysis Model 5 (ERA5) water vapor datasets. The COSMIC and ERA5 water vapor data at lower (850 hPa), mid- (500 hPa), and upper troposphere (300 hPa) from 2007 to 2018 are compared. These two water vapor datasets generally show good agreements in space and time. At 500 and 850 hPa, COSMIC water vapor retrieval is lower than water vapor from ERA5 globally, with asymmetric latitudinal variability between the southern and northern hemispheres. The water vapor increases around 2015–2016 due to the El Niño event are identifiable in both COSMIC and ERA5 water vapor time-series data. COSMIC global water vapor increasing trends are 3.47±0.24, 3.25±1.06, 2.03±2.93 %/Decade at 300, 500, and 850 hPa, respectively. COSMIC's increasing water vapor trends at 500 and 850 hPa are ~0.8 %/Decade lower than ERA5. Large regional water vapor trend variabilities with strong increasing and decreasing slopes are observed in the tropics and sub-tropics regions. At 500 and 850 hPa, strong water vapor increasing trends in the equatorial Pacific Ocean and the Laccadive Sea and decreasing trends in the Indo-Pacific Ocean region and the Arabian Sea are recognizable. This study also found that the increasing water vapor trends at 850 hPa estimated from COSMIC are significantly higher than ERA5 data for two low-height stratocumulus cloud-rich ocean regions to the west of Africa and the west of South America. Over land, significant water vapor increasing trends at 850 hPa are around the southern United States, and decreasing trends are observed at sites in the south of Africa and Australia. The differences between the water vapor trends of COSMIC and ERA5 are primarily negative in the tropical regions at 850 hPa. At 500 hPa, the negative differences between COSMIC and ERA5 trends are mainly distributed in the Indo-Pacific Ocean region. In contrast, the positive differences are in the northern Indian Ocean and its northern coast. These regions with notable water vapor trending differences between COSMIC and ERA are located in the Intertropical Convergence Zone (ITCZ) area with frequent occurrences of convection, such as deep clouds. The difference in characterizing water vapor distribution between RO and ERA5 in the presence of a deep cloud may cause such trending differences. Quantitative evaluation of the spatiotemporal variabilities of atmospheric water vapor data helps assure the qualities of RO-derived and reanalysis water vapor for climate studies.