Urban climate projection by the WRF model at 3-km horizontal grid increment: Dynamical downscaling and predicting heat stress in the 2070's August for Tokyo, Osaka, and Nagoya metropolises

Abstract

This study presents the projected urban climate for the 2070s' August in the three largest urban areas, Tokyo, Osaka, and Nagoya in Japan. To accurately evaluate the urban climate, the simulations use the Weather Research and Forecast (WRF) model with 3-km grid increment coupled to an urban canopy model (UCM). To project future urban climate, the simulations apply dynamical downscaling to three GCMs (MIROC3.2-medres, MRI-CGCM2.3.2a, CSIRO-Mk3.0) and use the ensemble average for results. The results provide estimates of the heat stress to future residents of Tokyo, Osaka, and Nagoya. The WRF-UCM model reproduces the observed spatial distribution of the surface air temperature in the 2000s' August, giving an all-domain mean bias of -1.2°C and RSME of 2.7°C. For Tokyo, Nagoya, and Osaka, these biases are -0.6, -0.1, and -0.4°C. Moreover, the diurnal temperature variations at these urban stations are well reproduced. The projected monthly average August temperatures in the 2070s are about 2.3°C higher than those in the 2000s at the three urban areas and comparable to those in the record-breaking hot summer of 2010. (Predictions by individual ensemble members differ by 0.8-1.2°C.) As a result, urban areas will experience uncomfortable sleeping nights nearly every day in August, with roughly as many heat-induced sleeping-discomfort nights as those in 2010. Moreover, application of the wet-bulb globe temperature (WBGT) shows that people in Tokyo will be warned not to strenuously exercise outdoors for 62% of the daytime hours in the 2070s, a sharp increase from the 30% of the 2000s. (Predictions by individual ensemble members range from 54-67%). Osaka and Nagoya will have even more restrictions on outdoor exercise. Finally, the urban heat island intensity is 1.5°C in Tokyo of the 2070s, comparable to the background climate warming of 2.3°C. © 2012, Meteorological Society of Japan.