Complexity of surface temperatures in cities – let’s talk about what we don’t know

By Simone Kotthaus

With summer upon us we want to spend more time outdoors. However, as temperatures rise, conditions in cities may become uncomfortable. The urban heat island (UHI) effect, whereby cities are warmer than their surroundings, may exacerbate the higher temperatures. This UHI is amongst the best known phenomena of climate conditions in cities.

A number of processes contribute to the distinct climates in cities and many environmental variables provide useful insights. One such variable is the surface (skin) temperature of the urban canopy (the temperatures of all the surface of all the different facets – roads, buildings, gardens, etc). It responds to the amount of solar radiation absorbed by the surface and the anthropogenic heat emitted (from buildings, vehicles, people), and serves as an indicator for the amount of energy being stored in the urban fabric which in turn is used to drive turbulent surface exchanges (heating the air and evaporating water). Given the significance of surface temperature to the heating of the lowermost atmosphere, where people live and work, it is a critical variable in many model parameterisations of energy exchange processes.

For larger areas, satellite remote sensing techniques have proven extremely useful in observing thermal contrasts, e.g. urban vs rural differences for large cities at a spatial resolution common to thermal infrared satellite images (~ 1 km). However, thermal patterns are increasingly being investigated at more detailed scales, either using remotely sensed imagery from airborne platforms, ground based imagers or by decomposing mixed-pixel information in satellite imagery. Despite great advances in thermal remote sensing methods over the last three decades, many challenges remain when studying the thermal response of the urban surface in detail given the huge heterogeneity of the urban canopy layer (i.e. the three-dimensional urban surface composed of buildings, roads, vegetation and potentially open water surfaces).

Given the 3D-structure of the urban surface the observed surface temperature depends on the viewing geometry and the solar position. This is referred to as thermal anisotropy. Shaded areas are usually much cooler than the average temperatures, while sunlit areas are warmer. The average surface temperature in a pixel (the smallest area a satellite ‘sees’), is a function of the shadows ‘seen’ by the remote sensor but also the type of facets sampled (Figure 1). Another challenge is the complexity of construction materials used in urban areas. These have a wide range of radiative and conductive properties. Hence they not only absorb and store the solar energy in different ways, but also require different corrections to be applied when processing the remotely sensed imagery. Significant amounts of research currently are focused on this topic.

This week, a range of experts in the fields of thermal remote sensing and urban climate will come together at the University of Reading to discuss this hot topic. This is particularly important given current rates of urbanization world-wide, climate change and predicted increases in the frequency and duration of heatwaves. The Department of Meteorology hosts the Fourth International Workshop of the EarthTEMP Network, an initiative to stimulate new international collaboration in measuring and understanding the surface temperatures of Earth across all domains and methods. Previous meetings discussed the characterisation of surface temperatures in key land regions, extreme regions or specifically data sparse regions.

This year’s meeting aims to addresses the ‘Complexity of Urban Surface Temperatures’. International scientists with various backgrounds have been invited to identify the key challenges in quantifying surface temperatures in urban areas. The workshop provides a platform for establishing new collaborations with the objective to advance thermal research in urban areas. The programme starts today, Monday 8 June, with a practical exercise taking place at University of Reading’s Whiteknights campus. This is followed by two days with a range of high-profile keynote presentations that will provide the background for fruitful discussions.

Figure 1. (Upper) Brightness temperature image and (lower) visible image of an urban surface in central London with a westward view, taken in November 2010 around midday. The roof surfaces (~ 20 °C) are clearly hotter than the east-facing walls (~ 5 °C) and windows appear warmer (~ 9 °C) due to their differing radiative properties.

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