Numerical prediction of heat island mitigation effect on decrease in air temperature in Tokyo

In Tokyo metropolitan area, urban heat island phenomenon is feared as a factor of increase in heatstroke patients and raising cooling energy consumption in summer. Therefore, Tokyo Metropolitan Government promotes some heat island mitigation measures concerning with urban greening, water retentive pavement, high light-reflective paint, and reducing anthropogenic heat. In this study, a three-dimensional numerical simulation model developed by authors is applied to Tokyo 23 wards (hereinafter referred to as “Tokyo”) in order to estimate the effect of heat island mitigation measures mentioned above on decrease in air temperature.

In the numerical simulation model mentioned above, atmospheric temperature, humidity, and wind velocities are calculated unsteadily by finite-difference-methods with hydrostatic and Bousinesq approximations. At land surface, heat budget is calculated under the influence of solar radiation, atmospheric radiation, and anthropogenic heat. The horizontal resolution of this model is one or a few kilometers in atmosphere. For land surface, on the other hand, it is possible to calculate heat budget by finer horizontal resolution about 500m. For urban surface, a simplified “urban canopy model” is applied in order to consider the effect of buildings on radiation and wind velocity. In this urban canopy model, temperature around buildings is predicted such as the surface temperature at roof, wall, and road, and the air temperature in a ravine of buildings. The numerical parameters for a geometric pattern of buildings such as height, width, and interval of buildings are averaged in each horizontal numerical grid. Predicted parameters are also area-averaged values in a horizontal grid. In non-urban area, on the other hand, roughness model is applied.

In order to evaluate the numerical model, we use field observation data with METROS (Metropolitan Environmental Temperature and Rainfall Observation System) set up by Tokyo Metropolitan Research Institute for Environmental Protection and Tokyo Metropolitan University. The strong point of METROS is to be having high-density continuous observation for air temperature, relative humidity, wind velocity, wind direction, air pressure, and precipitation in Tokyo. Especially for air temperature and relative humidity just above ground level, METROS has 100 observation points over Tokyo (61500 ha). The METROS data with such high resolution can grasp the horizontal distribution of temperature over Tokyo. In this study, numerical simulation of the model is compared with METROS data in a typical summer day of August 29, 2002. We confirmed that the numerical results are in good agreement with the observation of horizontal distribution of air temperature, wind velocity, and wind direction near the surface.

In order to estimate the effect of heat island mitigation measures by numerical simulation, it is needed to determine the surface physical parameters for roof greening and water retentive pavement. The evaporation efficiency, which is one of the important surface parameters, is estimated from the surface heat budget field data of roof greening and water retentive pavement observed by Tokyo Metropolitan Research Institute for Environmental Protection and Institute of Civil Engineering of Tokyo Metropolitan Government, respectively. The parameter values of the evaporation efficiency for roof greening and water retentive pavement are estimated by trial and error comparing with the column model and the above observation heat budgets. As a result, the evaporation efficiencies of roof greening and water retentive pavement are presumed about 0.2 and 0.05, respectively.

Finally, the effect of heat island mitigation measures on decrease in temperature is estimated by numerical simulation. Here, a mitigation scenario is supposed by introducing five kinds of mitigation measures mentioned below, simultaneously.

1) Introducing roof greening to 45.0% of buildings over 1000 square meters in area and greening to 4.4% of urban ground surface in Tokyo

2) Introducing water retentive pavement to 29% of road in central area of Tokyo (1600ha)

3) Introducing high light-reflective paint to 20% of building roofs in Tokyo

4) Decrease in 41.5% of anthropoge nic heat from automobiles in Tokyo by improvement of fuel cost and increase in travel speed of cars

5) Decrease in 21.5% of anthropogenic heat of houses and 26.5% of buildings for business use in Tokyo by improvement of adiabatic efficiency of buildings and improvement of system for energy conservation

The numerical values for the above mitigation measures are settled as expected values for Tokyo in 2030. Another numerical simulation in a typical summer day condition (August 29, 2002) is carried out, but with the future condition after mitigation measures in 2030. The difference between two numerical results with and without the five measures mentioned above reveals the effect of these measures. As a result, decrease in area-averaged temperature over Tokyo (61500ha) is predicted about 0.5K in daytime, and 0.2K in nighttime, respectively. In central area of Tokyo (1600ha), on the other hand, temperature decrease is predicted about 0.8K in daytime.