Numerical simulations of influence of heat island countermeasures on outdoor human heat stress in the 23 wards of Tokyo, Japan

• The summertime outdoor heat stress hazard and heat disorder risk were simulated.
• The changes by introducing urban heat island mitigation scenarios were evaluated.
• 1-km gridded maps including the 23 wards of Tokyo were produced with the WBGT index.
• Building greening, high-albedo coating, and roof-level waste heat were investigated.
• High-albedo coating on urban surfaces increased the heat stress and heat disorder risk.

In this study, the summertime outdoor heat stress hazard and heat disorder risk (HDR) were simulated, and their changes under several heat island mitigation scenarios were quantitatively evaluated. Gridded maps that included the 23 wards of Tokyo (23 Tokyo) were produced with 1-km horizontal resolution for consecutive summer months, facing extremely hot weathers, in 2010. The wet-bulb globe temperature (WBGT) was adopted as an outdoor heat stress indicator. Numerical simulations were applied to evaluate the effects of several heat island mitigation scenarios—including building greening (GR), high-albedo coating (ALB), and roof-level emissions of all waste heat by air conditioning (WH)—on the WBGT and HDR for 23 Tokyo. Results showed no cases that effectively lowered the outdoor WBGT and HDR for the studied region. Conversely, the ALB case increased the daily maximum WBGT by an average of 0.6 °C (maximum of 1.7 °C) over all the grids. This increase induced a 1.4 (3.3) times higher HDR in the area average (at the maximum grid) compared to a control case with no mitigation scenario. Hence, heat island mitigation methods reflecting solar radiation could potentially lead to an increase in the outdoor heat stress hazard and risk to a large extent in urban areas.