Franco Reseghetti

XBT Science: Assessment of Instrumental Biases and Errors

AbstractExpendable bathythermograph (XBT) data were the major component of the ocean temperature profile observations from the late 1960s through the early 2000s, and XBTs still continue to provide critical data to monitor surface and subsurface currents, meridional heat transport, and ocean heat content. Systematic errors have been identified in the XBT data, some of which originate from computing the depth in the profile using a theoretically and experimentally derived fall-rate equation (FRE). After in-depth studies of these biases and discussions held in several workshops dedicated to discussing XBT biases, the XBT science community met at the Fourth XBT Science Workshop and concluded that XBT biases consist of 1) errors in depth values due to the inadequacy of the probe motion description done by standard FRE and 2) independent pure temperature biases. The depth error and temperature bias are temperature dependent and may depend on the data acquisition and recording system. In addition, the depth bia...

Drag Coefficients for Rotating Expendable Bathythermographs and the Impact of Launch Parameters on Depth Predictions

Computational fluid dynamics techniques have been applied to model fluid flow in the vicinity of oceanographic temperature probes. A major goal of the modeling effort is the determination of drag coefficients for probe descent into ocean water. These drag coefficients can be used, in conjunction with a dynamic model of the probe, to predict the depth of the probe during descent. Accurate depth information is essential for the proper measurement of ocean temperatures and, consequently, ocean heating associated with climate change. Until recently, probe depths were predicted with the use of experimental calibrations which relate time-of-flight and depth. Those calibrations are limited in their accuracy, they are confined to conditions that match the experiments from which the calibrations were determined, and they are unable to account for variations in quantities such as the drop height or initial probe mass. The dynamic model and drag coefficient calculations presented here are, to the best knowledge of the authors, the first to include the impact of probe rotation. It is hoped that this new technique can be applied to the archive of oceanographic probe measurements and that improvements to ocean temperature monitoring will result.

A New Method to Estimate the Systematical Biases of Expendable Bathythermograph

Abstract A new technique to estimate three major biases of XBT probes (improper fall rate, start-up transient, and pure temperature error) has been developed. Different from the well-known and standard “temperature error free” differential method, the new method analyses temperature profiles instead of vertical gradient temperature profiles. Consequently, it seems to be more noise resistant because it uses the integral property over the entire vertical profile instead of gradients. Its validity and robustness have been checked in two ways. In the first case, the new integral technique and the standard differential method have been applied to a set of simulated XBT profiles having a known fall-rate equation to which various combinations of pure temperature errors, random errors, and spikes have been added for the sake of this simulation. Results indicated that the single pure temperature error has little impact on the fall-rate coefficients for both methods, whereas with the added random error and spikes t...

Turbulent and transitional modeling of drag on oceanographic measurement devices

Computational fluid dynamic techniques have been applied to the determination of drag on oceanographic devices (expendable bathythermographs). Such devices, which are used to monitor changes in ocean heat content, provide information that is dependent on their drag coefficient. Inaccuracies in drag calculations can impact the estimation of ocean heating associated with global warming. Traditionally, ocean-heating information was based on experimental correlations which related the depth of the device to the fall time. The relation of time-depth is provided by a fall-rate equation (FRE). It is known that FRE depths are reasonably accurate for ocean environments that match the experiments from which the correlations were developed. For other situations, use of the FRE may lead to depth errors that preclude XBTs as accurate oceanographic devices. Here, a CFD approach has been taken which provides drag coefficients that are used to predict depths independent of an FRE.

Investigation of XBT and XCTD Biases in the Arabian Sea and the Bay of Bengal with Implications for Climate Studies

Long time series of XBT data in the Bay of Bengal and the Arabian Sea are valuable datasets for exploring and understanding climate variability. However, such studies of interannual and longer-scale variability of temperature requirean understanding, and, if possible,a correction of errors introduced by biases in the XBT and expendable conductivity‐temperature‐depth (XCTD) data. Two cruises in each basin were undertaken in 2008/09 on which series of tests of XBTs and XCTDs dropped simultaneously with CTD casts were performed. The XBT and XCTD depths were corrected by comparisonwith CTD data using a modification of an existing algorithm. Significant probe-to-probe fall-rate equation (FRE) velocity and deceleration coefficient variability was found within a cruise, as well as cruise-to-cruise variability. A small (;0.018C) temperature bias wasalso identifiedfor XBTson each cruise.No new FRE can be proposedfor either the Bayof Bengal or the Arabian Sea for XBTs. For the more consistent XCTD, basin-specific FREs are possible for the Bay of Bengal, but not for the Arabian Sea. The XCTD FRE velocity coefficients are significantly higher than the XCTD manufacturers’ FRE coefficient or those from previous tests, possibly resulting from the influence of temperature on XCTD FRE. Mean temperature anomalies versus a long-term mean climatology for XBT data using the present default FRE have a bias (which is positive for three cruises and negative for one cruise) compared to the mean temperature anomalies for CTD data. Some improvement is found when applying newly calculated cruise-specific FREs. This temperature error must be accounted for in any study of temperature change in the regions.

Experimental Verification of Drag Forces on Spherical Objects Entering Water

Experimental Verification of Drag Forces on Spherical Objects Entering Water Objects which pass from gas regions to liquid regions experience elevated impact forces associated with the acceleration of the surrounding liquid. In order to investigate these forces, complementary experiments and simulations were performed on a sphere that traveled from air to water with an impact velocity of 2 m/s. It was found that the two methods gave results that were in very good agreement. In particular, the depth vs. time trajectory of the sphere closely matched. A fitted polynomial allowed the entry region acceleration to be extracted.

Analysis of surface circulation structures along a frequently repeated XBT transect crossing the Ligurian and Tyrrhenian Seas

A dataset of XBT (eXpendable BathyThermograph) temperature profiles collected by ships of opportunity along the Genova–Palermo route, since September 1999, is analyzed, together with altimetric observations and model results, with the purpose of identifying and characterizing robust circulation features along the track and investigating their variability. An anticyclone is found in the Ligurian Sea, just north of the Corsica Channel, not present in previous descriptions of the Mediterranean Sea circulation. It appears to be a recurrent feature, better defined and stronger in summer and in the beginning of autumn. In the northern part of the Tyrrhenian Sea, the well-known Bonifacio dipole shows a similar seasonality, in agreement with previous observations. However, the Bonifacio anticyclone also displays a strong interannual variability, not previously recorded, with significant variations in position and shape. In fact, the data suggest the existence of two distinct summer circulation regimes related to the position and shape of the Ligurian anticyclone. When the latter is wider, filling the entire region north of the Corsica Channel, the circulation in the northern Tyrrhenian Sea is isolated from that in the Ligurian Sea, in agreement with the common picture. However, the altimeter maps show that there are several cases in the last two decades in which the Ligurian anticyclone is small and displaced to the west, allowing an inflow through the Corsica Channel into the Tyrrhenian Sea. The two regimes appear to result from a delicate balance between the forcings acting in the two sub-basins and the topographic constraints.

An algorithm for classifying unknown expendable bathythermograph (XBT) instruments based on existing meta data

AbstractTime-varying biases in expendable bathythermograph (XBT) instruments have emerged as a key uncertainty in estimates of historical ocean heat content variability and change. One of the chall...

Assessment of Quality and Reliability of Measurements with XBT Sippican T5 and T5/20

AbstractThe T5 expendable bathythermographs reach the greatest depth within the current XBT family. Since the early 1970s, in several areas they have been providing a significant part of available ...