Numerical Simulations of Outdoor Heat Stress Index and Heat Disorder Risk in the 23 Wards of Tokyo

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Abstract
In this study, the summertime outdoor heat stress hazard and heat disorder risks (HDR) were simulated numerically using a mesoscale meteorological model combined with an urban canopy model and a building energy model. Model grid maps including the 23 wards of Tokyo (23 Tokyo), Japan, were produced with a 1-km horizontal resolution for the period of July–September 2010. Model simulations of the daily maximum wet-bulb globe temperature (WBGTmax), which was adopted as a heat stress index, indicated the spatial heterogeneity of the heat stress hazard within 23 Tokyo. The heat stress hazard was greater in the inland western region, particularly for sunny conditions in July and August (based on the monthly mean; the maximum difference exceeded 2°C for both sunny and shaded conditions). This likely occurred as a result of greater spatial heterogeneity in the globe temperature than in the air temperature among model grid cells, with differences in the radiation environment induced by differences in urban geometric parameters. Gridded mapping of HDR simulations proceeded in two steps, using the incidence rate and the absolute number of heat disorder patients (HDP). These simulations were achieved by combining the exponential relationships between the actual WBGTmax and the number of HDP with the daytime grid population. Eventually, the resulting HDR maps incorporated the effects of the spatial heterogeneities of both the outdoor heat stress hazard and the daytime grid population.

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