Numerical Study of the Urban Heat Island in Sendai City with Potential Natural Vegetation and the 1850s and 2000s Land-Use Data

In this study, we investigated the impact of urbanization on surface air temperatures and the urban heat island (UHI) for Sendai City. We estimated the impact of urbanization during the 150-year period by comparing the 1850s to the 2000s case. We used the Weather Research and Forecasting (WRF) model with 1-km horizontal resolution and three land-use datasets: one for potential natural vegetation (PNV) data, and the other two for realistic land-use data (the 1850s and 2000s). First, the results from the control simulation (2000s land-use case) were verified against observations. The results show that the WRF model reasonably reproduced the diurnal variation of the observed surface air temperatures in the 2000s land-use case at six stations of the Miyagi prefecture. The model mean biases ranged from -0.29 to -1.18 degrees C in August (10-year average) and from -0.44 to -1.50 degrees C in February (10-year average). Second, the impact of urbanization on the surface air temperature distribution in and around Sendai City was evaluated. For the 1850s land-use case, the very small urban area of Sendai City led to a negligible UHI. Note that this case yields nearly the same surface air temperatures as experiments using PNV. If we compare the simulated monthly mean surface air temperatures in the central part of Sendai City between the 1850s and 2000s land-use cases, we find that the monthly mean temperature for February in the 2000s is 1.40 degrees C higher than that in the 1850s, whereas that for August is 1.30 degrees C. Similarly, we find a considerable nocturnal (1800 - 0500 JST) average surface air temperature increase of 2.20 degrees C in February and 2.00 degrees C in August.