Assessing Two Different Commercial AIRCRAFT-BASED Sensing Systems

Abstract

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 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 capacitive relative humidity sensor (WVSS-I) into 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 capacitive sensors. A test of the WVSS-I system was conducted in 1999 by comparing aircraft data taken in ascent and descent with nearly simultaneous measurements from co-located radiosonde launches and other ground-based observing systems. These tests demonstrated the importance of the observations at non-synoptic times 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 was conducted in 2005. Approximately 30 B757aircraft participated in the WVSS-II test, while 63 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 were compared with model analysis background fields. These types of assessments are essential to assure the optimal use of the data in operational assimilation systems. Sample results of these aircraft-model comparisons are presented below.