✍ Scribed by TAKAKIYO NAKAZAWA; MISA ISHIZAWA; KAZ HIGUCHI; NEIL B. A. TRIVETT
No coin nor oath required. For personal study only.
Digital ®ltering and harmonic regression curve ®tting techniques are applied to CO 2 ¯ask data from four stations in North America (Pt. Barrow, Alert, Sable Island and Cape St. James) to evaluate these two dierent methodologies in terms of growth rate and seasonal cycle in the atmospheric CO 2 concentration. Both methods agree relatively well in producing long-term atmospheric CO 2 trend at each of the monitoring stations, as well as in capturing relatively large interannual variations in the annual growth rate. Furthermore, they both agree in indicating the dependency of the variation in the seasonal amplitude on the seasonal minimum concentration. The digital ®ltering technique is able to capture the local temporal variation in CO 2 measurements much better than the harmonic regression method, although in some cases this variability is exaggerated in the digital ®ltering approach. The harmonic regression approach tends to smooth out the data, with much of the power in the very long period oscillations. Also, the timing of the occurrence of the seasonal minimum calculated by the digital ®ltering method tends to be earlier than that calculated by the harmonic regression method, although both methods do not indicate any major secular change in the timing. The overall assessment of the two methods applied to the CO 2 ¯ask data underscores the importance of using more than one curve ®tting method before any conclusions can be drawn from the ¯ask data about the interannual variability in the trend and seasonal cycle of the atmospheric CO 2 concentration.
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