Projection of Change in Urban Heat Island Intensity of Tokyo Metropolitan Area during Winter

During the recent 100 years, mean surface air temperature (SAT) increased about 3 ºC in Tokyo, while mean SAT of the world increased only 0.66 ºC. The major reason of the difference is the effect of urban heat island (UHI). The intensity of UHI is often large during winter. Although a large number of case studies have been carried out, climatological features of UHI are still unknown. In the urbanizing areas, increase of SAT may be more serious in the future. This study investigates change in UHI intensity of Tokyo metropolitan area related with the global climate change. First, two high-resolution simulations including urban canopy model, i.e., hindcast (CTL) and hindcast without urbanization (CTLNU) were performed to validate the result of CTL as well as to estimate SAT change by the urbanization. Spatial distribution and frequency of appearance of the SAT were simulated well in CTL run comparing with AMeDAS observation data. In CTLNU run, the grid points categorized as urban were replaced by that of grassland. This run indicates that the maximum SAT decreases by about 2.5 ºC over the urban area. The other two simulations, future projection (PGW) and future projection without urbanization (PGWNU) were performed to investigate the impact of the future climate change. Pseudo-Global-Warming method (Sato et al., 2008; Kimura and Kitoh, 2008) is adopted for the PGW and PGWNU runs. Boundary conditions during 2070s and 1990s are given by a CGCM, MIROC 3.2 CGCM assumed SRES A2 scenario. We calculate UHI intensity (UHII) using the results of four experiments as follows, UHII_CTL = T_CTL - T_CTLNU, UHII_PGW = T_PGW - T_PGWNU. The difference between UHII_PGW and UHII_CTL (UHII_PGW-UHII_CTL) shows UHII change due to the future climate change. The UHII is 2 to 3 ºC during night, while it is almost zero at noon in the CTL and PGW. Change in UHII due to the global climate change reaches more than 20% of the UHII (about 0.5 ºC) during night. Surface heat capacity of the urban area is larger than that of the rural area because of taller buildings. SAT is difficult to rapidly increase in the urban area during daytime. On the other hand, the buildings release heat more than rural area during nighttime, when the released heat tends to be restricted in the lower atmosphere because of weak turbulence at night. Since these processes are sensitive on cloud fraction and the atmospheric stability in the lower boundary layer, the UHII may be also affected by these factors.