Urban Microclimate Monitoring in Seoul, Korea: Fine Scale Summer Heating along the Cheonggye Stream Renewal Project

The city of Seoul is the 6^th most densely populated city in the world, and has a growing metropolitan area. The increasing density has enhanced the urban heat island (UHI) effect over the last four decades. This UHI along with projected warming over the coming century will cause heightened energy use and human heat stress.  Changes to the built environment can provide heat adaptation strategies and improve urban sustainability, helping human health and the economy of the city.  The highest surface and air temperatures are commonly found at very fine scales (from the touch scale of 1cm, to a human scale of 1m).  Quantifying the complex urban environment at human scales of exposures with mobile and place-based measurements is needed to generate a new evidence-base to guide climate adaptation in large, growing cities.  Seoul’s restoration of the Cheonggye stream in the city center provides such an opportunity using natural surfaces, water, and vegetation.  Fine-scale intra-urban surface and air temperatures (T_a), radiant heat load, wind speed, and relative humidity were analyzed along transect routes both directly adjacent to the lowered stream and on the upper streets, as well as extending into the city, during ‘tropical’ synoptic weather type days in the summer of 2016 in Seoul, South Korea.  Five bioclimatically distinct microclimates were also analyzed in a stationary fashion along the stream.  Human thermal comfort (TC) was modeled using the COMFA energy budget (EB) model and the Wet Bulb Globe Thermometer (WBGT) Index.  Results demonstrate an average increase of 0.9°C in T_a when moving away from the stream to the inner-city, with human EBs and overall heat stress being more closely correlated to incoming solar radiation than to T_a.  An average EB and WBGT difference of 16.9 W m^-2 and 0.9°C, respectively, is found between the lower and upper transect routes.  Additionally, of the five distinct environments, the environment in full sun with vegetation had the highest average T_a (33.1°C) and was the least thermally comfortable (EB = 68.2 W m^-2, WBGT = 29.0°C), with the full shade environment providing the greatest comfort.  Shade also played a large role in decreasing the surface temperatures and WBGT.  A growing number of studies have found that manmade and natural open water bodies influence the microclimate of urban areas through both evaporative cooling and the support of urban green space.  However, the Cheonggye stream, characterized as a small stream with low-lying vegetation, had only a small effect in urban heat mitigation, with shade (whether along the street or stream) playing a larger role in comfort on hot, humid days.  This study provides evidence for bioclimatic design recommendations for landscape architects and urban planners, which can be applied in densely populated cities for human-scale heat stress mitigation as a form of climate change adaptation.