Towards Improved Agreement Between Measurements of UTLS Water Vapor from Balloons, Aircraft and Satellites

During the last several decades a variety of instruments aboard balloons, aircraft and satellites have been used to measure water vapor in the upper troposphere and lower stratosphere (UTLS). Measurement discrepancies between some instruments, manifested as biases and/or drifts, have hindered our ability to quantitatively understand dehydration processes and accurately determine long-term water vapor trends. Contemporary biases and drifts are, for the most part, smaller than their predecessors, but the associated uncertainties are still large enough to limit our confidence in the results of UTLS water vapor studies. For example, a decade ago, disparities >50% between some aircraft- and balloon-based instruments made it difficult to ascertain if air masses in the tropical tropopause layer (TTL) were frequently or rarely super-saturated. More recent comparisons of balloon-, aircraft- and satellite-based water vapor instruments have exposed measurement biases of <20%, demonstrating conformity that is improved but often still inadequate to differentiate between saturated and super-saturated air masses. In this presentation we will show important biases and drifts in UTLS water vapor measurements during the last decade as well as the results of a very recent comparison between balloon- and aircraft-based measurements in the TTL.