Greater Toronto area urban heat island: Analysis of temperature and extremes.

摘要

This study analyzes the trends in temperature, and their extremes, in the Greater Toronto Area (GTA) in the context of urban heat island. The trends in annual and seasonal temperature changes were investigated in the GTA over the past century and a half with special focus on 1970-2000. The Mann-Kendall test is used to assess the significance of the trends and the Theil-Sen slope estimator is used to identify their magnitude. Statistically significant increasing trends for mean and minimum temperatures are observed mainly at the urban and suburban stations. The sequential Mann-Kendall test is used to identify any abrupt change in the time series of temperature (31 -161 years), and the results indicate that increasing trend for annual mean temperature has started after 1920 at Toronto downtown, after the 1960s at the suburban stations, and has increased significantly during the 1980s at all stations, which is consistent with the pace of urbanization during these periods in the GTA. The observed urban heat island (UHI) in Toronto is quantified and characterized by considering three different rural stations. The UHI intensity (DeltaT u-r) in Toronto is categorized as winter dominating or summer dominating depending on the choice of a rural station. The results from the trend analysis of annual and seasonal DeltaTu-r suggest that the choice of the rural station is crucial in the estimation of DeltaTu-r, and thus can overestimate or underestimate its prediction depending on the location and topographical characteristics of a rural station relative to the urban station. The trends in extreme temperature indices are also investigated and the results indicate that indices based on daily maximum temperature are more pronounced at the urban and suburban stations compared to that at the rural stations. The changes in the trends for extreme indices based on daily minimum temperature are consistent at all stations for the period of 1971-2000. With the decrease in the percentage of cold nights and the increase in the percentage of warm nights, the diurnal temperature range has decreased throughout the GTA region. The analysis of heating degree days and cooling degree days revealed that the former is associated with decreasing trends and the latter exhibited increasing trends at almost all stations in the GTA. Finally, it is evident from the results that urban heat island phenomenon exerts warmer influence on the climate in cities, and with the current pace of urbanization in the GTA, it is imperative to understand the potential impact of the emerging UHI on humans and society.