Influence of urban green vegetation fraction on the urban heat island effect across Europe Arjan Droste Supervised by: Reinder Ronda and Natalie Theeuwes Meteorology and Air Quality Group, Wageningen University June 2015 i Contents Abstract .................................................................................................................................................... 1 Introduction .............................................................................................................................................. 1 Data and methodology ............................................................................................................................ 3 General data and methodology ............................................................................................................ 3 ECMWF data ..................................................................................................................................... 4 Urban Heat Island calculation ........................................................................................................... 5 Green fraction and Local Climate Zones ........................................................................................... 5 Statistics ............................................................................................................................................ 6 Rotterdam ............................................................................................................................................. 6 Temperature data and analyses ........................................................................................................ 6 Green fraction and Local Climate Zones ........................................................................................... 7 Madrid ................................................................................................................................................... 8 Urban and rural data ......................................................................................................................... 9 Green cover ....................................................................................................................................... 9 Oslo .................................................................................................................................................... 10 Urban and rural data ....................................................................................................................... 10 Green cover ..................................................................................................................................... 11 Results ................................................................................................................................................... 11 Data validation .................................................................................................................................... 11 Validation of the Wunderground data .............................................................................................. 11 Validation of the ECMWF data ........................................................................................................ 12 UHI vs vegetation ............................................................................................................................... 16 NDVI and GVF................................................................................................................................. 16 Green cover estimate ...................................................................................................................... 19 Seasonality of the Urban Heat Island ................................................................................................. 22 Rotterdam ........................................................................................................................................ 22 Madrid .............................................................................................................................................. 24 Oslo ................................................................................................................................................. 26 Discussion ............................................................................................................................................. 27 Rotterdam ........................................................................................................................................... 28 Madrid ................................................................................................................................................. 29 Oslo .................................................................................................................................................... 30 Conclusions ........................................................................................................................................... 31 Outlook and recommendations.............................................................................................................. 31 Acknowledgments ................................................................................................................................. 32 References ............................................................................................................................................ 33 Appendices ............................................................................................................................................ 36 ii Appendix A: table and map of the Rotterdam stations ....................................................................... 36 Appendix B: table and map of the Madrid stations ............................................................................. 38 Appendix C: table and map of the Oslo stations ................................................................................ 40 Appendix D: table of measurement accuracy of weather stations ..................................................... 42 Appendix E: Histogram of the difference between Wunderground and CPC data, Rotterdam ......... 43 Appendix F: hysteresis of Tryvannshøgda ......................................................................................... 45 iii Abstract The difference in temperature between urban areas and their rural surroundings is known as the Urban Heat Island (UHI). In the light of climate change and the ongoing world-wide urbanisation, it is important to take measures to reduce the UHI. Studies have shown that increased vegetation fraction in cities correlates with a reduction in mean UHI values. Though this relation has been researched for single cities and countries, it is yet unknown whether this relation is universal across different climate zones. This study researches the applicability of measurements from hobby meteorologists combined with ECMWF model data to calculate the UHI in an innovative new approach. We research the influence of the vegetation fraction of neighbourhoods on the maximum UHI, in order to establish a statistical relationship between vegetation and UHI reduction. This analysis is performed for several European cities in varying climates (Rotterdam, the Netherlands; Oslo, Norway; Madrid, Spain) to determine the role of climate zones on this relationship. Our results establish that using the ECMWF operational model data as means of rural background temperature is a valid new way of calculating UHI. For Rotterdam we find a significant (R2 of 72%) relationship between vegetation and maximum UHI reduction, but we are unable to reproduce a significant relation for the cities of Madrid and Oslo. Water availability for the rural surroundings, the anthropogenic water flux and the regional climate appear to play a large role in the way vegetation affects the UHI. Introduction From 2008 onwards, over half of the world’s population has been living in cities, and projections show urbanisation to increase even further (United Nations, 2011). This rapid urbanisation, combined with the issue of climate change, can bring various health issues related to the Urban Heat Island (UHI) effect. This is the phenomenon of temperatures being several degrees higher in cities than in the rural surroundings. The elevated urban temperature can lead to additional heat stress or heat-related illness during hot days. The urban heat island is caused by enhanced radiative energy storage by the built-up urban area during the day, which is subsequently released during the night. The resulting elevated nocturnal temperatures can lead to thermal discomfort, when people cannot cool down at night which can lead to sleep deprivation and other health issues (Bell, 1982). During the 2003 summer heat wave in Western Europe over 70,000 people lost their lives due to heat-related stress (Poumadère et al., 2005). Groups at risk include those suffering from cardiovascular disease, pregnant women, children and the elderly (Kovats and Hajat, 2008; Reid et al., 2009). In the light of climate change which will increase extreme temperatures further, it is important to mitigate the urban heat island. Investigating potential measures to mitigate the UHI is therefore of great interest to society, especially in strongly urbanised areas such as Europe, North America and parts of East Asia. One way of reducing urban temperatures is by using