Updated uncertainties on XBT measurements in Central Western Mediterranean Sea from in situ comparison (2001-2017)
Creators: name: Reseghetti, Franco
name: Cheng, Lijing
name: Raiteri, Giancarlo
name: Bordone, Andrea
name: Borghini, Mireno
name: Fratianni, Claudia
name: Simoncelli, Simona
Publisher: name: Zenodo
Year: 2023
Subjects: XBT
Temperature Profiles
Standard Uncertainty
Accuracy

description: The quality of seawater temperature measurements and a good understanding of theassociated uncertainties are of great importance in any study of ocean warming andclimate change. Among the various instruments used, the XBT (eXpendableBathyThermograph) probes provided the majority of temperature profiles collected inthe period 1970-2000 but uncertainties in their recordings have often createddifficulties in oceanographic studies. The main problems are due to the absence of asensor that measures pressure/depth so that the depth values are only estimated froma fall rate equation with phenomenological coefficients valid for the world ocean, and tothe less than excellent performance of the thermistor NTC used.The waters of the Mediterranean Sea (which in recent years has become a Hot Spot ofplanetary climate change) have peculiar characteristics in terms of temperature values,profile shape and salinity content but there are few studies in the literature thatanalyze the uncertainties on instrument measurements used for monitoring in thebasin. The accuracies indicated by the XBT manufacturer for the measurements carriedout by the XBT probes are rather high for both temperature and depth and do not makethem suitable for accurate analyzes of climate change.This presentation illustrates the preliminary results of in-situ tests carried out between2001 and 2017 in the western Mediterranean in which nearly simultaneoustemperature measurements were recorded from the same vessel with two instruments(XBT and CTD) to try to measure the same measurand. The CTD (e.g. an SBE 911plus) isthe standard reference instrument for field measurements of EOVs and has accuracies,sensitivities and uncertainties that are at least one order of magnitude better thanthose of the XBT probes.Various XBT models have been launched, in particular the DB version, currently themost used, and of which the sample for the analyzes is robust from a statistical point ofview, verifying which was the best fall rate equation describing the movement of theXBT probes , the possible presence of temperature offsets, and the possible influence ofthe launch height and the calibration of the acquisition system.New more effective fall rate equations have been determined for each XBT type, butthose used so far have been found to work fairly well. A small thermal offset wasidentified, depending both on the type of probe and, to a lesser extent, on the acquisition system used.

Finally, a depth-dependent correction was calculated(probably linked to a pressure effect on the thermistor) but, unexpectedly, variablewith the XBT type.Analyzes are underway applying these results to the OHC calculations but it can bestated that the new uncertainties calculated for the measurements with XBT probes inthe western Mediterranean (which are in good agreement with those shown by G.Raiteri in his presentation) lead us to consider how the evidence of warming of thewaters of the Tyrrhenian Sea provided by the latest measurements with XBT probes isstatistically robust over the entire measured water column.