Observed and simulated interactions between electricity consumption and urban surface air temperatures in downtown Tokyo and Osaka

This study investigates interactional relationship between urban climate and energy demand. In urban areas, the consumed energy which turns into waste heat causes anthropogenic heating of urban surface atmosphere. And its consequent local climate change well known as urban heat island or urban warming will give the thermal environmental feedback effects to urban energy demands themselves, especially to the air-conditioning energy demands there.

In order to study the abovementioned interactions, summer field campaigns were conducted during August 2007 in two major Japanese cities, Tokyo and Osaka. The surface meteorological observations using the multi-site installed instruments were carried out in two couples of urban areas in the two cities. Each couple consists of an uptown low-rise residential area and a downtown high-rise business area within the same city. Those areas were contrastive in both terms of urban canopy structure and levels of energy demands and anthropogenic heating. The amounts of anthropogenic heat were estimated based on actual energy consumptions in each area, resulting in greater value up to 200W/land-m² in business areas and less value around 20W/land-m² in residential areas. Anthropogenic heat in business areas also indicated large decreases by around 50% at weekends compared to those on weekdays, whereas those in residential areas were very stable from weekends to weekdays. Then, differences in areal mean surface air temperatures between business area and residential area were analyzed for weekdays and weekends separately. As a result, observed differences in areal mean temperatures between the two contrastive urban areas were found to be decreased by up to 1 degree Celsius from weekday to weekend in both cities suggesting actual impacts of anthropogenic heat on air temperatures in downtown business areas. It was also indicated that those decreases were roughly proportional to decreases of anthropogenic heat in the office buildings areas on weekends.

To clarify the mechanism of the phenomena, Weather Research and Forecasting model (WRF) was applied to Tokyo and Osaka. Our original urban Canopy and Building energy Model (AIST-CBM) was incorporated into WRF for the consideration of dynamic and thermodynamic effects of urban canopy and anthropogenic heating. Those phenomena were qualitatively reproduced by numerical models, and the sensitivity of downtown air temperature to anthropogenic heat was estimated to be 0.7 to 0.9 degrees Celsius/100W/land-m² in Tokyo and Osaka. Additionally, based on those sensitivities and the dependence of the electric power consumption to the air temperature in the downtown area, the impacts on electricity demands due to thermal environmental feedback effect were estimated to be +2 ~ +2.5% for 2 p.m. ~ 4 p.m. in downtown Tokyo, resulting from increases in anthropogenic heat there from weekend to weekday, and suggesting vicious impact from the viewpoint of urban energy conservation.