Greater Toronto Area Urban Heat Island: Analysis of Temperature and Extremes
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GREATER TORONTO AREA URBAN HEAT ISLAND: ANALYSIS OF TEMPERATURE AND EXTREMES by Tanzina Mohsin A thesis submitted in conformity with the requirements for the Degree of Doctor of Philosophy Graduate Department of Geography University of Toronto ©Copyright by Tanzina Mohsin (2009) Greater Toronto Area Urban Heat Island: Analysis of Temperature and Extremes Doctor of Philosophy (2009) Tanzina Mohsin Department of Geography University of Toronto Abstract 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 (∆Tu-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 ∆Tu-r suggest that the choice of the rural station is crucial in the estimation of ∆Tu-r, and thus can overestimate or underestimate its prediction depending on the location and topographical characteristics of a rural station ii 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. iii Acknowledgements I wish to acknowledge the financial support from the Government of Ontario Graduate Scholarship (OGS), Ontario Graduate Scholarship in Science and Technology, and the George Tatham/Geography Alumni Graduate Scholarship. I also wish to thank Maria Petrou, Climate Services in Environment Canada for data and information without which some part of this research project would have been incomplete. Special acknowledgements to Vincent Cheng, GIS specialist, Environment Canada, for his suggestions on the GIS figures. A special thanks to my husband, Ehsan-ul Quayyum and my two children, for their support throughout the completion of this dissertation. Finally, many thanks to my supervisor, William A. Gough for his constant guidance and motivation without which this research project would have not been possible. Also, many thanks to my supervisory committee: David Etkin, Scott Munro and Sarah Finkelstein for their suggestions towards the completion of this dissertation. iv TABLE OF CONTENTS CHAPTER 1: INTRODUCTION 1 1.1: Background and rationale 1 1.2 Objectives 9 CHAPTER 2: TREND ANALYSIS OF LONG TERM TEMPERATURE TIME SERIES IN THE GREATER TORONTO AREA (GTA) 12 2.1 Introduction 12 2.2 Data and Analysis 15 2.3 Results and Discussion 22 2.4. Conclusions 44 CHAPTER 3: CHARACTERIZATION OF TORONTO’S URBAN HEAT ISLAND: IMPACT OF SELECTION OF RURAL SITES 47 3.1. Introduction 47 3.2. Data and Methodology 50 3.3. Results and discussion 55 3.4 Summary and Conclusions 78 v CHAPTER 4: ANALYSIS OF TRENDS FOR EXTREMES INDICES OF DAILY OBSERVED TEMPERATURE IN THE GREATER TORONTO AREA (GTA) 80 4.1. Introduction 80 4.2. Data and methodologies 83 4.3. Results and discussion 88 4.4 Summary and conclusions 118 CHAPTER 5: DISCUSSION AND CONCLUSIONS 121 REFERENCES 128 APPENDIX 144 LIST OF ACRONYMS 146 vi List of Figures Figure 1.1 The growth of the build up areas in Toronto, 1861-1941. 3 Figure 1.2 Population change in Toronto census metropolitan area for the period of 1986- 2006. 4 Figure 1.3 Comparison of population density at Toronto and its surrounding areas between 1971 and 2001. 5 Figure 1.4 Idealized illustrations of a typical lake breeze circulation and its associated front. Common features are labeled. The dashed line represents the outer boundary of the inflow layer (Sills, 1998). 7 Figure 2. 1 Map showing the locations of the meteorological stations in the GTA. 17 Figure 2.2 Statistical time series of annual maximum, minimum and mean temperatures. 23 Figure 2.3 Abrupt changes in climate for annual mean temperature for the stations in the GTA. (the test statistics from sequential Mann-Kendall test for forward and backward time series are plotted to detect the abrupt changes). 33 Figure 2.4 Comparison of trends for the statistical time series of annual temperatures for Toronto and Beatrice as derived from the sequential Mann-Kendall test. (Test sample values are shown corresponding to the temperature time series for each year). 36 Figure 2.5 Comparison of the changes in annual maximum, minimum and mean temperature between two periods, 1970-2000 and 1989-2000, at the stations in the GTA with increasing distance from Toronto downtown. 41 Figure 2.6 Spatial patterns of annual temperature changes for the period of 1989-2000 and 1970-2000. Figures on the left are based on 12 years annual average and figures on the vii right are based on 31 years annual average for the annual maximum, minimum and mean temperatures respectively (top to bottom). 43 Figure 3.1 Locations of the meteorological stations used in the analysis. 52 Figure 3.2 Year-to-year variation of UHI intensity at Toronto and Pearson with three rural stations for the period of 1970-2000. 58 Figure 3.3 Monthly mean changes of urban-rural temperature difference at Toronto and Pearson with three different rural stations for the period of 1970-2000. 63 Figure 3.4 Seasonal changes in ∆Tu-r at Toronto and at Pearson for three different rural stations for the period of 1970-2000. 66 Figure 3.5 Frequency of intensity of UHI at Toronto and at Pearson for three different rural stations for the period of 1970-2000. 68 Figure 3.6 Change in the amplitude of UHI intensity at Toronto and Pearson in relation to wind speed for July and February, 2006. 73 Figure 3.7 Comparison of the diurnal variation of UHI intensity in July and February between Toronto and Pearson. 74 Figure 3.8 Diurnal changes in UHI intensity at Toronto and Pearson on July (02-03) and February (09-10), 2006;(the bars are proportional to the magnitude of UHI intensity). 77 Figure 4. 1 Map showing the stations used for extreme temperature analysis in the GTA. 84 Figure 4.2 Spatial distribution of hot days, summer days, ice days and frost days in the GTA. The circles are proportional to the magnitude of the average number of days for the viii period of 1971-2000. The average number of days per year is indicated inside the circles. 93 Figure 4. 3 (a) Trends in indices based of daily maximum temperature (hot days, summer days, warmest days, warm days, cold days and ice days) for the period of 1971-2000. 99 Figure 4.3 (b) Same as 4.3a, only for indices based on daily minimum temperature. 102 Figure 4.3 (c) Same as 4.3a, only for the indices based on both maximum and minimum temperatures (diurnal temperature range and extreme temperature range). 103 Figure 4.4 Extreme temperature indices for seasonal temperature time series for the GTA. The linear trend is shown by the bold line. A) Warm Days, B) Warm Nights, C) Cold Days and D) Cold Nights. 110 Figure 4.5 Comparison of the changes in heating degree days (HDD) and cooling degree days (CDD) for two different periods, 1971-2000 and 1991-2000. 113 Figure 4. 6 (a) Spatial comparison of the changes in HDD between 1971-2000 and 1991- 2000 (positive sign indicates increasing trend and negative sign indicates negative trends). 116 Figure 4.6 (b) Spatial comparison of the changes in CDD between 1971-2000 and 1991-2000 (positive sign indicates increasing trend and negative sign indicates negative trends). 117 ix List of Tables Table 2.1 Meteorological stations in the GTA. 16 Table 2.2 The autocorrelation coefficients for lags 1, 2 and 3 for the period of 1970 - 2000 20 Table 2.3 The u(t) test statistic and p-values from the sequential Mann-Kendall test and the statistical significance of the trend in terms of p-values. 24 Table 2.4 The magnitude of the trend computed using the Kendall’s tau and TSA (1970- 2000).