Abstract
Abstract. While water vapor is the most important tropospheric greenhouse gas, it is also highly variable in both space and time, and water vapor concentrations range over 3 orders of magnitude in the troposphere. These properties challenge all observing systems to accurately measure and resolve the vertical structure and variability of tropospheric humidity. In this study we characterize the humidity measurements of various observing techniques, including four separate Global Positioning System (GPS) radio occultation (RO) humidity retrievals (University Corporation for Atmospheric Research (UCAR) direct, UCAR one-dimensional variational retrieval (1D-Var), Wegener Center for Climate and Global Change (WEGC) 1D-Var, Jet Propulsion Laboratory (JPL) direct), radiosonde, and Atmospheric Infrared Sounder (AIRS) data. Furthermore, we evaluate how well the ERA-Interim reanalysis and NCEP Global Forecast System (GFS) model perform in analyzing water vapor at different levels. To investigate detailed vertical structure, we analyzed time–height cross sections over four radiosonde stations in the tropical and subtropical western Pacific for the year 2007. We found that the accuracy of RO humidity is comparable to or better than both radiosonde and AIRS humidity over 800 to 400 hPa, as well as below 800 hPa if super-refraction is absent. The various RO retrievals of specific humidity agree within 20 % in the 1000–400 hPa layer, and differences are most pronounced above 600 hPa.
Highlights
Tropospheric humidity is one of the key parameters driving weather and climate, and it plays an important role in the development of many extreme events
In this study we focus on the water vapor variability in both a temporal and spatial sense by comparing data from multiple observing techniques (RO, RS, Atmospheric Infrared Sounder (AIRS)) and modelanalyses (ERA-Interim, Global Forecast System (GFS)) at particular locations in the tropics and sub-tropics over an entire year
The mean and standard deviation values of the differences for each pressure layer are depicted in each panel
Summary
Tropospheric humidity is one of the key parameters driving weather and climate, and it plays an important role in the development of many extreme events. To accurately model current and future climate, it is crucial to understand the distribution, transport, and vertical structure of tropospheric water vapor. Measuring water vapor accurately is a great challenge, as it is highly variable on both spatial and temporal scales, and its tropospheric concentration varies over 3 orders of magnitude between the tropical planetary boundary layer (PBL) and the tropopause. No single observing system can provide accurate tropospheric humidity data on a global scale with high vertical resolution. Passive (microwave and infrared) nadir-sounding systems provide data globally, but with relatively low vertical resolution. Weighting functions are used to quantify vertically resolved humidity information, and these vertical scales are large (2 to 3 km) compared to the variability of water vapor in the vertical. Infrared-based systems cannot provide data within or below clouds
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