Assessing two different commercial aircraft-based sensing systems

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 TAMDAR program, which uses a system of two capacitor sensors.

A test of the WVSS-I capacitor system 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 demonstrated 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 were conducted in 2005. Approximately 30 B757 aircraft participated in the WVSS-II test, while 60 Mesaba Saab 340 aircraft were involved in the TAMDAR evaluation. Radiosondes and other ground-based systems again served as the comparison standard for most of the tests.

However, in order to gauge whether the aircraft data are fully compatible with NWP data assimilation systems, additional assessments were conducted in which the aircraft data are compared with model analysis background fields. These types of assessments are essential to assure the optimal use of the data in operational assimilation systems.

Results were obtained for all three observation types - humidity, temperature and wind. Preliminary results show distinct differences in some of the basic characteristics of the different observing systems. These include such things as differences in wind speed biases between the smaller and larger aircraft and changes in the bias of humidity data as a function of the humidity itself. Additional results will be presented at the meeting, including further subdivision of the data by phase of flight (ascent / descent / en-route), elevation and season.