Comparison of Column-Integrated Water Vapor Among Reanalysis Datasets

Atmospheric water vapor is a primary greenhouse gas and plays a major role in climate as a dominant feedback variable in association with moist dynamics and radiative effects. Fluxes of water vapor link the main components of the hydrological cycle and the release of energy into the atmosphere and, via the process of condensation in the upper atmosphere, provide a fundamental source of energy for driving global atmospheric circulation. However, unlike other greenhouse gases, water vapor is highly variable over space and time, both horizontally and vertically. In this study, we assess changes in global atmospheric water vapor, with a focus on the column-integrated amount, known as precipitable water. We quantitatively analyzed three leading reanalysis products from NOAA’s Climate Forecast System Reanalysis (CFSR), NASA’s Modern Era Retrospective analysis for Research and Applications (MERRA-2) and Twentieth Century Reanalysis (20CR) for seasonal and inter-annual variations in the water vapor. We used perceptible water derived from global positioning system (GPS) and sun-photometer based ground observations to validate reanalysis estimates. The main advantage of the performed validation is the independence of the GPS water estimates compared to studies using water datasets from radiosondes that are already assimilated into the NWP models. We also used satellite estimates of column integrated water vapor from special sensor microwave imager (SSM/I) over ocean, Moderate Resolution Imaging Spectroradiometer (MODIS), Atmospheric Infrared Sounder (AIRS) globally to compare against reanalysis. Result of this study will benefit suite of tests that are currently under development at NOAA/NCEP to evaluate the performance of new versions of the coupled model (UGCS) assimilation/reanalysis.