Temperature profiles of XBTs and ARGOs intercompared with ship based CTDs: some further considerations about their metrological comparability in the Mediterranean Sea
Creators: name: Raiteri, Giancarlo
name: Reseghetti, Franco
name: Cheng, Lijing
name: Fratianni, Claudia
name: Simoncelli, Simona
Publisher: name: Zenodo
Year: 2023
Subjects: XBT
Temperature profiles
Argo
Metrological comparability

description: Seawater temperature is the longest measured metric and currently more than half a million profiles arecollected each year using different instruments. Knowledge of both uncertainties about these measuresand their metrological comparability is of great importance in all studies of ocean warming and climatechange.The Expendable Bathythermographs (XBTs) provided a large part of the temperature profiles in theperiod 1970-2000, after 2000 the Argo Profiling Floats (PFLs) instead ensured a significant part of themeasurements. In any case, ship-based Conductivity-Temperature-Depth probes (CTDs) still maintain therole of reference transducers for measurements performed during field activities.The waters of the Mediterranean Sea (which has become a Hot Spot of planetary climate change in recentyears) have peculiar characteristics but there are few dedicated studies in the literature.The XBT and Argo temperature profiles recorded in the Mediterranean Sea and downloaded fromWOD18 (World Ocean Database, October 2022 release) are compared with CTD data (assumed asreference instrument) under strong space-time constraints and differences are estimated. For whatconcerns the profile matching conditions, following the indications for the first Rossby radius ofdeformation indicated in literature, XBT, PFL and CTD profiles were matched under the following 3Dspace-time conditions: DLat: ±0.10°, DLon: ±0.13°, Dtime: ±1 day, Ddepth: ±1 m. Rossby radius formode 1 has a mean value of (10.9±2.6) km, calculated by taking into consideration the variability both inspace and in time (along one typical year). The considered spatial window defines a square with a meanside of about ±11.4 km in the region of interest, therefore reasonably circumscribing the circumferencewhose radius is just the Rossby one.Comparison of XBTs vs PFLs and CTDs show good results, in particular for depths greater than 100 m,where the water column temperature is more stable and homogeneous: mean differences are in fact quitesimilar and equal to (0.02±0.11) °C and (0.03±0.12) °C, respectively.Furthermore, XBT profiles coming from the Ship Of Opportunity Programme (SOOP) monitoring in thewestern Mediterranean, and recorded in particularly accurate operating conditions, are compared withPFL data (using the previous space-time constraints) evaluating the effects of the application of thefollowing corrections on XBT depth and temperature values, respectively:1. new coefficients for the XBT Fall Rate Equation (FRE) specific for that basin2. temperature calculated by resistance measures3. temperature corrected for drift by tester measurements.By applying depth and temperature corrections on XBT profiles, a reduction of differences amongmatched profiles can be observed in comparison with values directly calculated by manufacturersoftware: mean differences, always for depths greater than 100m, change in fact from 0.033 °C to 0.015°C, with a distribution of values more similar to a Gaussian one.