Accuracy Requirements for Moisture Observations Using Space Geodetic Techniques

Most of our information about the structure and variability of moisture in the atmosphere comes from three operational sources: radiosondes, surface measurements of dew point temperature (convertible to relative humidity), and satellite measurements of brightness temperature in the infrared and microwave portions of the electromagnetic spectrum. These observing systems function synergistically, since no one system provides all of the information required for weather forecasting, climate monitoring, and scientific research. Notwithstanding the strengths of the operational composite moisture observing system, its shortcomings preclude significant advances in weather prediction, our understanding of atmospheric processes, and climate monitoring and prediction. In recent years, the use of space geodetic techniques for atmospheric remote sensing has been proposed, demonstrated and verified. Space geodetic techniques, especially techniques using the Global Positioning System, have certain attributes that strongly complement both operational (or soon to be operational) and research observing systems. NOAA's Forecast Systems Laboratory in Boulder, Colorado, has been evaluating techniques to retrieve integrated (total atmospheric column) precipitable water vapor (IPW) from zenith tropospheric signal delays since 1994, and use IPW in operational weather forecasting since 1997. As a result of these evaluations, the use of ground-based GPS-Met is expected to become operational within the National Weather Service over the next few years. However, space geodetic techniques will neither replace conventional observing systems, nor will they completely mitigate their deficiencies. In this talk, I will review the requirements for water vapor measurement accuracy, and show that this is application dependent. I will then discuss the strengths and weaknesses of the more common operational moisture sensing systems, and examine the attributes of some space geodetic techniques in this context.