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 À1.2 C and RSME of 2.7 C. For Tokyo, Nagoya, and Osaka, these biases are À0.6, À0.1, and À0.4 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 di¤er 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.

Reconsidering the Quality and Utility of Downscaling

“…Kusaka et al (2012) dynamically downscaled local climate around three major metropolitan areas in Japan and succeeded in separating the heat island effect from the general climate warming around these areas. In this case, temperature within the urban canopy was controlled mainly at the scale of the urban area, and the local model could successfully represent dynamic effects arising at that scale.…”

Benefits of explicit urban parameterization in regional climate modeling to study climate and city interactions

“…As an example, Yang et al (2016) performed RCM simulations at 4-km resolution with the WRF atmospheric model over Phoenix area (AZ, US) to study impact of urbanization on precipitation. Using the same set-up over Tokyo (Japan), Kusaka et al (2012) investigated evolution of heat stress for population, by comparing occurrences of August warm nights between 2080 and the present day. The computational time of these numerical studies that focus on specific cities with high spatial resolution significantly constrains the simulation duration.…”

Detailed Urban Heat Island Projections for Cities Worldwide: Dynamical Downscaling CMIP5 Global Climate Models

“…McCarthy et al [8] applied a 25-km resolution to study the UHIs of several cities in the U.K. under an A1B scenario. Kusuka et al [9] used an even higher resolution of 3 km, but limited their simulations to one month in a study over the largest urban areas in Japan. In another study on both urban expansion and climate change with a 2-km resolution mesoscale model, Argüeso et al [10] found a strong effect on nocturnal temperatures due to urbanization, which enhanced the climate change signal at local scales in Sydney (Australia).…”