Various studies over the past decade have shown that additional detailed measurements of the vertical, horizontal and temporal atmospheric moisture structure are necessary to improve forecasts of precipitation location, intensity and timing, as well as the onset and strength of severe convective storms. To meet this need, several projects have been established to provide low cost moisture sensors that are appropriate for use on commercial aircraft. These instruments have the potential for filling in the space and time gaps left by all other existing observations by providing 10 or more high-resolution tropospheric moisture profiles (along with wind and temperature needed to determine moisture flux) at different locations throughout the day. One of these system, the Water Vapor Sensing System has evolved from using a radiosonde-like thin-film capacitor relative humidity sensor (WVSS-I) to a more precise laser diode mixing ratio measurement system (WVSS-II). A second development has occurred through the Tropospheric Airborne Meteorological DAta Reports (TAMDAR) program, which uses a system of two capacitor sensors.

A test of the WVSS-I was conducted in 1999 by comparing aircraft data taken in ascent and descent with nearly simultaneous measurements from collocated radiosonde launches and other ground-based observing systems. These tests demonstrate the importance of the off-time observations but also pointed to several areas of concern, including different biases in ascent versus descent reports and potential instrument aging effects.

A series of two separate tests of both the newly installed WVSS-II and the TAMDAR systems is planned for spring of 2005. Approximately 30-40 B757 aircraft will participate in the WVSS-II test, while 64 Masaba Saab 340 aircraft will be equipped by the time of the TAMDAR evaluation. Radiosondes and other ground-based systems will again be used as the comparison standard. In order to avoid the logistical complications of launching radiosonde in areas of congested air traffic near major airports, the tests will be conducted at smaller airports that serve as hubs for the major participating airlines, Louisville for UPS and Memphis for Masaba. Approximately 2 weeks of collocated radiosonde and aircraft data will be collected are each site; with radiosonde launches scheduled immediately before and after periods of multiple aircraft arrivals/departures. Evaluations will be made of: 1) the overall similarity of the reports, 2) biases that exist between ascent and descent reports, 3) variability between different aircraft (including instrument aging), and 4) the ability of the systems to capture sharp moisture gradients accurately, including after an aircraft emerges from clouds. All of these factors will be important for the optimal objective use of the aircraft data in combination with other data sources. The results are planned to be available in real time.