A New Method to Estimate the Systematical Biases of Expendable Bathythermograph
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244 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 28 A New Method to Estimate the Systematical Biases of Expendable Bathythermograph LIJING CHENG AND JIANG ZHU Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China FRANCO RESEGHETTI Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Lerici, Italy QINGPING LIU China University of Mining and Technology, Beijing, China (Manuscript received 8 December 2009, in final form 24 July 2010) 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 the simulation leads to better results with the new integral technique than with the standard differential method. In the second case, two sets of profiles from actual XBT versus CTD comparisons, collected near Barbados in 1990 and in the western Mediterranean (2003–04 and 2008–09), have been used. The individual fall-rate coefficients and start-up transient for each XBT profile, along with the overall pure temperature correction, have been calculated for the XBT profiles. To standardize procedures and to improve the terms of comparison, the individual start-up transient estimated by the integral method was also assigned and included in calculations with the differential method. The new integral method sig- nificantly reduces both the temperature difference between XBT and CTD profiles and the standard de- viation. Finally, the validity of the mean fall-rate coefficients and the mean start-up transient, respectively, for DB and T7 probes as precalculated equations was verified. In this case, the temperature difference is reduced to less than 0.18C for both datasets, and it randomly distributes around the null value. In addition, the standard deviation on depth values is largely reduced, and the maximum depth error computed with the datasets near Barbados is within 1.1% of its real value. Results also indicate that the integral method has a good perfor- mance mainly when applied to profiles in regions with either a very large temperature gradient, at the thermocline or a very small one, toward the bottom. 1. Introduction of its low price and simple operation, XBTs have become an important part of ocean observation systems since the Since the 1960s expendable bathythermograph (XBT) 1970s. probes, originally invented for military applications, The XBT probe has a slim body with a sensor (a therm- have been widely used for collecting upper-ocean tem- istor) in the nose and twin wires dereeling from both the perature profiles mainly by ships of opportunity. Because probe tail and a canister on board, and it is usually deployed from moving ships. As the probe touches the seawater and Corresponding author address: Jiang Zhu, Institute of Atmospheric falls by a slightly decelerated motion, the water tempera- Physics, Chinese Academy of Sciences, Beijing 100029, China. ture is continuously sensed by the thermistor at a rate E-mail: [email protected] ranging from 10 to 20 Hz, which depends on the recording DOI: 10.1175/2010JTECHO759.1 Ó 2011 American Meteorological Society Unauthenticated | Downloaded 09/25/21 03:47 PM UTC FEBRUARY 2011 C H E N G E T A L . 245 system used. The acquisition system stops recording data efforts were focused on the fall-rate equation problem. according to two criteria, either based on a preselected Various techniques for computing the best fall-rate equa- depth or until the probe completely dereels the wire from tion for a specific dataset were developed until the early one of the two spools. Then, the wire breaks off and the 1990s, when, in collaboration with manufacturers, a task probe is discarded. Different XBT models are available; team sponsored by the Intergovernmental Oceanographic the most used versions have a nominal terminal depth of Commission (IOC) coordinated several XBT versus CTD 460 (T4/T6) and 760 m (T7/DB), but they can usually re- comparisons in different oceanic regions. As a final result, cord data down to about 500 and 850 m, respectively. Be- a comprehensive report and a paper mainly dedicated to cause an XBT carries no pressure sensor, XBT depth is the problem of calculating the correct depth were released estimated from a depth–time mapping originally provided (Hanawa et al. 1994, 1995, hereafter H95). Only the by the manufacturer (Lockheed Martin Sippican, hereafter structure of the manufacturer’s fall-rate equation was con- Sippican) based on oversimplified assumptions about the firmed, but new constant-temperature independent fall- probe motion. The elapsed time, beginning when the probe rate coefficients were calculated with a ‘‘temperature error hits the seawater and the recording starts and ending at the free’’ technique for the most used XBT models manufac- moment when the first wire breaks, is the actual recorded tured by Sippican and TSK (the Japanese manufacturer). parameter. The manufacturer’s fall-rate equation is z(t) 5 The use of these coefficients was strongly recommended At 2 Bt 2,wherez(t) is depth at the elapsed time t and the by the IOC and accepted by the United Nations Educa- coefficients are both positive and temperature independent. tional, Scientific and Cultural Organization (UNESCO). Since the 1970s, when XBT measurements started to As a consequence, it was recommended that the old pro- be compared with simultaneous temperature values re- files in the databases be converted to the new depth values. corded by other and usually more accurate and expensive Despite this improvement, discrepancies have still been instruments, such as salinity–temperature–depth (STD) or found both in the H95 method and the H95 equation. conductivity–temperature–depth (CTD), some discrepan- Some reports showed that the H95 equation underes- cies have become increasingly evident. For example, in a timated the actual fall-rate coefficients of the XBT report describing several intercomparisons involving a total probes (Boedecker 2001; Fang 2002). On the other hand, of about 2000 XBT profiles, Anderson (1980) pointed out Thadathil et al. (2002) found that the old manufacturer’s several problems in XBT measurements yielding a general fall-rate equation worked well in cold Antarctic wa- positive bias in measurements with XBT probes. Unfortu- ters, possibly because of viscosity changes, and they nately, that paper, with its important but widely ignored list suggested that the fall-rate coefficients should depend on of troubles and errors in the XBT system, remained un- latitude (they admitted that the probe motion depended known until recent years. on water temperature). This correlation was confirmed for In the 1980s and early 1990s some ‘‘cookbooks’’ and T5 probes manufactured by TSK (Kizu et al. 2005a), and for reports detailing the possible malfunctioning were pre- expendable conductivity–temperature–depth (XCTD) pared for the oceanographic community (see, e.g., Bailey probes (Kizu et al. 2008). Subsequently, Reseghetti et al. et al. 1994). Several researchers reported the inadequacy (2007) proposed a different method to deal with the XBT of the manufacturer’s fall-rate equation in the descrip- measurements in regions of the Mediterranean with high tion of the probe motion, both in the near-surface and temperature homogeneity because in those conditions the the deeper layers, which possibly correlates with the sea- H95 technique would not work. On the other hand, the ad water viscosity (e.g., Seaver and Kuleshov 1982; Heinmiller hoc method that was useful in the Mediterranean requires et al. 1983; Green 1984; Hanawa and Yoritaka 1987; Singer that some reference points in a profile must be visually 1990; Hanawa and Yoshikawa 1991; Hallock and Teague determined, and it is slightly complicated to extend this 1992; Kezele and Friesen 1993). International meetings procedure to all of the intercomparisons. These new co- involving the manufacturers were also dedicated to an- efficients also take into account the fall-rate differences alyzing problems in XBT measurements and eliminating between shallower (slower) and deeper (faster) probes. the biases (e.g., IOC 1992). These studies indicated that Table 1 shows a comparison between fall-rate coefficients XBT measurements contain systematic biases caused by suggested by manufacturers and reported publications. diverse factors, with significant probe-to-probe, cruise- In recent years, XBT measurements have attracted to-cruise, and time-to-time variability. In summary, the a renewed interest. Kizu and Hanawa (2002a,b) in- main part of the errors in the XBT measurements can be vestigated several types of recording systems looking for ascribed to inadequate fall-rate coefficients, pure tem- the start-up transient, which is the source of the main perature error, and start-up transient with an additional error in the upper layer. Their results suggested that the but hard-to-quantify component resulting from spikes and depth of the transient differed for different types of re- other random factors. Nevertheless, the most important corders. Reseghetti et al. (2007) suggested a possible Unauthenticated | Downloaded 09/25/21 03:47 PM UTC 246 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 28 TABLE 1.