On 23 February 1972 from 0650 LSI to 0726 LST, with the LEE hove to and 

 drifting and the thermistor chain hanging vertical in the water, seven XBT profiles were 

 attempted. Of these seven attempts, one was a catastrophic failure, one was a partial success 

 that yielded measurements to 90 m, and five were successes. Figure 32 summarizes the 

 results of a comparison of these profiles with single thermistor chain scans, beginning at the 

 same time the XBT probe was released. The XBT probe was at a depth of about 50 m by 

 the time the thermistor chain completed its 10-s scan. The left-hand part of figure 32 is a 

 plot of the six thermistor chain scans. In the near-surface layer, surface to 85 m, the 

 observed variation in the thermistor chain temperature measurements was small, while in 

 the thermocline, variations up to 0.80°C at 158 m were observed. The right-hand figure is 

 a plot of the differences associated with the six pairs of measurements. Also shown is the 

 XBT drop time in seconds. The average difference at each depth is shown by the symbol (x). 

 Of a total of 237 differences, the largest positive difference was 0.1 2°C, and the largest nega- 

 tive difference was -0.52°C. All the larger negative differences were in the thermocline and 

 could have resulted from small depth differences. Of major interest is that these six XBT 

 probes, on the average, measured temperatures that were lower than those measured by the 

 thermistor chain at all 44 sensor depths. The average of 237 differences was -0.07°C, with a 

 standard deviation of 0.09°C. Out of the total of 237 differences, only 54 (22.8%) were 

 positive, and 1 75 (73.8%) were negative. In the surface layer (surface to 85 m) and below the 

 thermochne (220 to 242 m), layers where vertical temperature gradients are small, the largest 

 positive difference was 0.1 2°C, and the largest negative difference was 0.2 1°C. The average 

 difference and standard deviation for the in-layer differences was -0.063°C and 0.08°C; for the 

 below-thermocline differences they were -0.046°C and 0.09°C; and for all 121 in-layer and 

 below-thermocline differences, they were -0.060°C and 0.08°C. 



Statistics for the individual XBT probes are summarized in table 37 for the in-layer, 

 below-thermocline, and for the combined set of differences. In the layer all, or nearly all, of 

 the differences were negative for four of the six probes, with the average differences varying 

 from -0.04rc, XBT 278L, to -0.162°C, XBT 277L. Only XBT 275L measured an average 

 positive difference. Note that this probe failed at 90 m. XBT 276L showed the best agree- 

 ment with the thermistor chain with seven positive, seven negative, and two zero differences. 

 Five of the six probes measured temperatures below the thermocline. The average differ- 

 ences for four of the five probes were negative, and for one probe it was positive. Thus the 

 six probes used in this comparison show a marked propensity to measure temperatures that 

 are less than those measured simultaneously by the thermistor chain. Only one of the six 

 probes, XBT 276L, showed excellent agreement in the statistical sense. These measure- 

 ments were made serially over a 26-min period on 23 February 1972. The biasing of the 

 differences may be a consequence related to a "run" of consecutive XBT probes. Addi- 

 tional examples of biased results related to "runs" of probes will be discussed later. 



During the station 3 and 4 experiments, 73 XBT profiles were attempted while the 

 LEE towed the thermistor chain at a speed of 3 knots. Out of the 73 attempts, 62 (84.9%) 

 were successes, two (2.7%) were partial successes, and nine (12.3%) were catastrophic 

 failures. This study compares the 64 visually acceptable profiles with the thermistor chain 

 scans that started at the same time the XBT probes were released. 



An inspection of the differences for the individual profiles showed that at all depths 

 greater than 34 m, the largest differences were associated with XBT 1 88L. The differences 

 were all negative, varying from -0.43°C at 39 m to a very large difference of -3.08°C at 



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