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XBT Science: assessment of instrumental biases and errors


A new study funded by CPO’s Climate Observation Division was recently published in the Bulletin of the American Meteorological Society. The study by Cheng et al.examines in-depth studies and offers recommendations for correcting biases in expendable bathythermograph (XBT) data.
XBT data were the major component of ocean temperature profile observations from the late 1960s to the early 200s. Even today, XBTs continue to provide critical data to monitor surface and subsurface currents, meridional heat transport, and ocean heat content. However, according to Cheng et al., scientists have identified systematic errors in this XBT data, some of which originate from computing the depth in the profile using a theoretically- and experimentally-derived fall rate equation (FRE). 
After competing in-depth studies of these biases and holding discussions at several workshops dedicated to XBT biases, the XBT science community met at the Fouth XBT Science Workshop and concluded that XBT biases consist of: errors in depth values to due to the inadequacy of the probe motion description done by standard FRE as well as independent pure temperature biases. 
According to this study, the depth error and temperature bias are temperature dependent and therefore may depend on the data acquisition and recording system. The depth bias also includes and offset term. “Some biases affecting the XBT-derived temperature profiles vary with manufacturer/probe type and have been shown to have a time dependence,” said the authors.
While the authors offered best practices for historical XBT data corrections and recommendations for future collection of metadata to accompany XBT data in the study, their analysis found that, despite the existence of these biases, historical XBT data without bias corrections are still suitable for many scientific applications. Bias corrected data can be used for future climate research.
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