Validation studies of WVSS-II moisture observations

Knowing the accuracy and representativeness of new observing systems is critical for their optimal use in data assimilation and other applications. One such new system is the laser-diode based Water Vapor Sensing System (WVSS-II) currently being deployed and tested on commercial aircraft in the US and, to a more limited degree, in Europe. The paper discusses the results of four WVSS-II-to-Rawinsonde intercomparison studies, not only in terms of assessing the accuracy of the humidity data, but also for determining how best to use these data as a supplement to other upper-air moisture measurements and options for using the data as a source of asynoptic observations to validate/calibrate a variety of satellite moisture products.

At previous meetings, we presented the results of three earlier, multi-seasonal validation experiments which were collected using the UW mobile observing system at airports where ~80% of the UPS B757 WVSS-II equipped aircraft land and/or take-off daily. Results of early tests indicated that ascent data showed generally excellent agreement, especially in the lower troposphere, with RMS fits < 0.5 g/kg for mixing ratio (<10% for relative humidity). Data from aircraft descents and from later times in the test periods, however, showed substantially worse results, including biases and instrument drift. As a result, a number of substantial engineering changes were made to the instrumentation during 2008. Chamber tests of the revised instrument have been successful and installation of the final systems began in late 2009.

Results of rawinsonde intercomparison tests conducted during the fall of 2009 and spring/summer of 2010 will be presented. Although the fall test lasted for only a few days, the initial results indicate that the problems identified with the earlier engineering versions of the systems have been addressed. The spring/summer test is expected to be long enough to provide statistically significant results over a wide range of humidity environments, as well as to show whether the instruments degrade over time. During each of the co-location tests, validation data are collected from the rawinsondes launched 3 times nightly – immediately before, between and after periods of multiple aircraft arrivals/departures. At least 5 co-locations (within 1 hour and 50 km) are obtained daily, including Temperature, Wind, and Humidity. The predominance of data collection during night-time hours also eliminated many of the radiation errors typical of day-time rawinsonde reports.

Results that will be shown include: 1) Similarity of reports from the different observing systems and different aircraft, 2) Biases/Standard Deviations between ascent and descent reports from individual aircraft with the rawinsonde data, 3) Variability between different aircraft (to assess instrument calibration and effects of aging), 4) Capabilities of the WVSS-II systems to capture sharp moisture gradients accurately, including when aircraft encounter clouds, 5) The performance of the instruments in very dry/cold and very warm/moist environments, and 6) Possible methods of using temporal and spatial variability between the combined rawinsonde/WVSS-II data sets to assess background atmospheric moisture variability.