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Detecting microclimatic biases in historical temperature records Runnalls, Kathryn Elizabeth

Abstract

The quality of historical climate data is a fundamental consideration in climate change research. This thesis considers the problem of microclimatic biases in observed surface-level air temperatures, and the effect of these biases on the homogeneity of historical temperature records. An extensive literature review describes causes of potential biases in air temperature records, and demonstrates that climate station siting biases, in the form of microclimatic change in the surrounding environment, are particularly difficult to detect by conventional homogeneity analysis techniques. Such biases are therefore potentially still present in many of the records used to monitor global climate change. The microclimatic processes responsible for thermal biases in temperature records are reviewed to demonstrate the relevant physical principles. A new technique to detect microclimatic inhomogeneities in temperature records is presented. The technique is based on the construction of time series of cooling ratios derived from nocturnal cooling at neighbouring climate stations calculated from daily maximum and minimum temperatures. The 'cooling ratio' is shown to be a particularly sensitive measure of the relative microclimatic differences between neighbouring climate stations, because larger-scale climatic influences common to both stations are removed by the use of a ratio and, because the ratio is invariant in the mean with weather variables such as wind and cloud. Discontinuities in the time series of cooling ratios indicate microclimatic change in one of the temperature records. Hurst rescaling (Hurst, 1951) is applied to several time series of cooling ratios, and is shown to be helpful in identifying discontinuities in the time series. The technique is tested on several Canadian historical temperature records, and proven to effectively identify subtle microclimatic changes such as minor station relocations, vegetation growth, and encroachment of buildings and parking lots. Results of this technique are compared to those from other homogeneity assessment techniques, and the cooling ratio approach is shown to be better suited to the detection of microclimatic biases. The technique is also shown to be useful in the assessment of the homogeneity of urban temperature records. Finally, since the research highlights the significance of having detailed station metadata, recommendations are made to improve the utility of such records, along with suggestions for future research.

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