Validation of Airs Moisture Products Using Three-way Intercomparisons With Radiosondes and GPS Sensors

Abstract

As technology advances, new ways of making measurements are developed and made cheaper, and it becomes feasible to make correlative measurements of the same atmospheric properties using multiple independent approaches. In the case of moisture, the availability of observations from conventional radiosondes, surface-based Global Positioning System (GPS) sensors, and the satellite-borne Atmospheric Infrared Sounder (AIRS) instrument suite make possible 3-way comparisons of precipitable water, which permit the AIRS moisture retrievals to be validated more rapidly and confidently than otherwise. For example, it has been noted that certain types of radiosondes exhibit a dry bias due to contamination by the packaging material in which they are stored. The dry tendency increases with time spent in the shipping container, and therefore varies from one radiosonde to another (Turner et al. 2003). Their experience with observations from the Atmospheric Radiation Measurement (ARM) sites has shown that this bias can be removed effectively by adjusting the radiosonde moisture profile to make the total precipitable water agree with an independent value obtained from an upward looking microwave water vapor radiometers (WVRs). Unfortunately, these sensors are few in number so this technique rarely can be applied in practice. On the other hand, the more widely available surface-based GPS Integrated Precipitable Water (IPW) measurements have many of the same desirable characteristics as WVRs, including accuracy, precision, and temporal resolution. In addition, they are available for areas experiencing heavy precipitation. Of particular interest for this study are a number of sites where the GPS instruments are deployed in close proximity to a conventional radiosonde launching facility. At these locations, 3-way matches between radiosonde, GPS, and AIRS IPW measurements are possible when a satellite overpass occurs. Also, the adjustment technique established using upward-looking microwave measurements can be adopted by substituting the GPS IPW as value used to constrain for correctively scaling the radiosonde profile. This paper summarizes three-way moisture intercomparisons performed as part of the AIRS validation project, which has been described by Fetzer et al. (2003). The three instruments, measurement principles, and retrieval procedures are outlined in Section 2. A description of the data set and the specific procedures for comparing corresponding IPW measurements and adjusting and comparing AIRS and adjusted radiosonde moisture profiles is presented in Section 3. The results are presented and discussed in Section 4, and conclusions and summary recommendations are in Section 5.