Application of corrections to original XBT data: Cheng corrections
Application of corrections to original XBT data based on Cowley, R., S. Wijffels, L. Cheng, T. Boyer, S. Kizu: Biases in Expendable BathyThermograph data: a new view based on historical side-by-side comparisons, accepted by Journal of Atmospheric and Oceanic Technology.
Step 1: Identify appropriate correction
Corrections are provided for T4/T6 (Sippican), T7/Deep Blue (Sippican), TSK T6 and TSK T7/Deep Blue.
If manufacturer is not given the deploying country and year of deployment and maximum depth should be used to identify probe type.
Deploying country:
TSK types are applied to: Japan, Taiwan, Korea, Thailand, China. All other countries are designated Sippican.
Table 1 contains the information on earliest to market and depth cutoffs for each probe type.
| Probe Type | Earliest date to market (dd/mm/yyyy) |
Nominal depth (m) |
Depth cutoff (m) |
Approximate percentage in WOD09 (of total XBTs) |
|---|---|---|---|---|
| Sippican T4 | 6/14/1965 | 460 | 550 | 49.00% |
| Sippican T6 | 4/14/1968 | 460 | 550 | |
| Sippican T7 | 6/20/1967 | 760 | 950 | 29.70% |
| Sippican Deep Blue | 4/20/1981 | 760 | 950 | |
| TSK T6 | 7/1/1972 | 460 | 550 | 12.40% |
| TSK T7 | 4/1/1978 | 760 | 950 | 1.10% |
| TSK Deep Blue | 8/1/1997 | 760 | 950 | |
| Other types (no corrections supplied): | ||||
| Sippican T5 | 6/3/1971 | 1830 | 2500 | 0.80% |
| Sippican Fast Deep | 9/25/1991 | 1000 | 2500 | 0.25% |
| Sippican T10 | 3/24/1972 | 200 | 350 | 5.50% |
| TSK T10 | 1/1/1979 | 200 | 350 | |
| TSK T5 | 8/1/1971 | 1000 | 2500 | 0.07% |
Probes designated TSK T4 use TSK T6 (TSK does not make T4s). All Sparton XBTs use Sippican corrections.
In the paper, the following additional corrections were applied to other probe types for the Global Ocean Heat Content (GOHC) calculation:
Where a correction was not available for a particular year, T4/T6 corrections were used for T7/DB probes and vice versa for Sippican types. The equivalent Sippican correction was used for TSK types. Sippican T4/T6 corrections were applied to all T10, T11, and unknown types with terminal depth < 550 m. Sippican T7/DB corrections were applied to Sippican Fast Deeps and unknown types with terminal depth = 550 m and < than 1005m. Corrections were not applied to T5s or to probes with depths = 1005m. XBT data from 1996 to the present with no depth equation information were not included in the GOHC calculation.
Step 2: Convert to Hanawa (1995) fall rates if required.
Step 3: Apply the corrections:
Cowley et al. Cheng Corrections
Back calculate time from depth, t = q - (q2 - z/bh)0.5
Where q = ah/(2 * bh) and time t is in seconds.
And ah,bh are the fall rate coefficients used (Hanawa : ah = 6.691, bh = 2.25x10-3)
Z(corrected) = a*t - b*t2 - c
Where a,b are provided in Table 2,
Table 3,
and c which is the depth offset term is provided in
Table 4.
T(corrected) = T(original) - Thermal_bias
Thermal _bias for Cheng corrections is provided in Table 5.
