Scientific contributions from GPS radio occultation sounding of the atmosphere: decadal-scale variability and vertical profiles of water vapor

Global Positioning System (GPS) radio occultation (GPSRO) was successfully demonstrated as a new remote sensing technique in 1995, through the GPS/METeorology (GPS/MET) proof–of–concept experiment. Observations continue to the present day and are planned for the future. GPSRO vertical profiles of atmospheric density, temperature, water vapor and geopotential height have low sensitivity to clouds or precipitation, making them highly complementary to other space–based observations. GPSRO measurement calibrations are traceable to atomic time standards, as long as potential biases are properly accounted for. The analysis of a long-term GPS RO time series of data and the improvement of its robustness has been a main focus since the development of the technique. Numerous studies have demonstrated the complementary nature and use of GPSRO observations in climate monitoring, numerical weather forecasting, and extreme weather research. Within the scope of expanding the applicability of GPSRO observations in climate research, we will present results on long–term atmospheric variability using data from the second and later generations of GPS instruments starting with the Blackjack on CHAMP (2001 launch), through to the present-day COSMIC constellation. Recent research suggests an expansion of the tropical belt as indicated by the tropical edge latitude of the lapse rate tropopause, although with asymmetric response in the northern and southern hemispheres. We will also present our studies of water vapor in the lower-mid troposphere (pressures above 350 hPa) using GPSRO. Differences from other observations (AIRS) and reanalyses suggest that unique new information is being obtained with these observations. We focus on the seasonal variability of the tropical tropospheric humidity climatology. We quantify humidity discrepancies with the European Center for Medium–Range Weather Forecasts (ECMWF) Re-Analysis Interim (ERA-Interim) and the Modern Era Retrospective Analysis for Research and Applications (MERRA) over specific regions such as, the Inter-Tropical Convergence Zone (ITCZ) and the edges of the Hadley Cells. There has been recent community interest in water vapor climatologies derived from GPSRO, particularly from the Water Vapor Assessment group (G-VAP) of the Global Energy and Water Cycle Experiment (GEWEX). We will present preliminary results that suggest the high vertical resolution and cloud-penetrating capability of GPSRO improves the fidelity of water vapor climatologies over the full range of atmospheric humidity, from very dry to very wet air. We conclude with remarks on expected improvements from the planned COSMIC-2 constellation of radio occultation satellites, which will be launched in late 2016 providing a factor of 20 increase over current sampling in the tropics.