Day 4

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Paper title Revisiting the Seasonality of Surface Urban Heat Islands
  1. Panagiotis Sismanidis Ruhr Universität Bochum Speaker
  2. Benjamin Bechtel Ruhr Universität Bochum
  3. Mike Perry University of Leicester
  4. Darren Ghent University of Leicester
Form of presentation Poster
  • A5. Climate
    • A5.02 The role of Earth Observation in climate services
Abstract text Cities are warmer than their surroundings. This phenomenon is known as Urban Heat Island (UHI) and is one of the clearest examples of human-induced climate modification. Surface UHIs (SUHI) result from modifications of the surface energy balance at urban facets, canyons, and neighborhoods. The difference between urban and rural Land Surface Temperatures (LST)—known as SUHI Intensity (SUHII)—varies rapidly in space and time as the surface conditions, the weather, and the incoming radiation change, and is generally strongest during daytime and summertime. In this work we revisit the topic of SUHII seasonality and how it differs across climates. Our thesis is that aggregating global SUHII data without considering the biome (i.e., vegetation zone) of each city, can lead to erroneous conclusions and estimates that fail to reflect the actual SUHII characteristics. This is because SUHII is a function of both urban and rural features, and the phenology of the rural surroundings can differ considerably between cities even in the same climate zone. To test this hypothesis, we use 18 years (2000-2018) of global land cover and MODIS LST data from the European Space Agency’s Climate Change Initiative (ESA-CCI). Our analysis covers 1588 cities in 12 tropical, dry, temperate, and continental Köppen-Geiger sub-classes. This classification scheme empirically maps Earth in 5 main and 30 sub- classes by assuming that vegetation zones reflect climatic boundaries. To analyze our results, we calculate, for each climate class, the seasonal variation of SUHII and rural LST (at monthly resolution) by averaging the corresponding city data (we do this separately for daytime and nighttime). Our results reveal that the seasonality of tropical, dry, temperate, and continental SUHIs differs considerably during daytime and that it is more pronounced in temperate and continental climates. They also show that the seasonality of the dry and temperate sub-classes exhibits considerable intra-class variation. In particular, the month when the daytime SUHII is strongest can differ between temperate sub-classes by as much as 4 months (e.g., for the hot-Mediterranean sub-class it occurs in May and for the dry-winter subtropical highlands sub-class in September), while the corresponding SUHII magnitude by as much as 2.5 K. The strong intra-class variation of temperate climates is also evident in the corresponding hysteresis loops, where almost every sub-class exhibits a unique looping pattern. These finding support our thesis, and suggest that global SUHII investigations should consider, in addition to climate, and the distribution of biomes when aggregating their results. Our results provide the most complete typology of SUHII hysteresis loops to date and an in-depth description of how SUHIIs vary within the year across climates.