Urban warming and future air-conditioning use in an Asian megacity: importance of positive feedback

The impact of feedback between urban warming and air-conditioning (AC) use on temperatures in future urban climates is explored in this study. Pseudo global warming projections are dynamically downscaled to 1 km using a regional climate model (RCM) coupled to urban canopy and building energy models for current and six future global warming (ΔTGW) climates based on IPCC RCP8.5. Anthropogenic heat emissions from AC use is projected to increase almost linearly with ΔTGW, causing additional urban warming. This feedback on urban warming reaches 20% of ΔTGW in residential areas. This further uncertainty in future projections is comparable in size to that associated with: a selection of emission scenarios, RCMs and urban planning scenarios. Thus, this feedback should not be neglected in future urban climate projections, especially in hot cities with large AC use. The impact of the feedback during the July 2018 Japanese heat waves is calculated to be 0.11ºC

Asian megacity heat stress under future climate scenarios: impact of air-conditioning feedback

Future heat stress under six future global warming (ΔTGW) scenarios (IPCC RCP8.5) in an Asian megacity (Osaka) is estimated using a regional climate model with an urban canopy and air-conditioning (AC). An urban heat ‘stress’ island is projected in all six scenarios (ΔTGW = +0.5 to +3.0 ºC in 0.5 ºC steps). Under ΔTGW = +3.0 ºC conditions, people outdoors experience ‘extreme’ heat stress, which could result in dangerously high increases in human body core temperature. AC-induced feedback increases heat stress roughly linearly as ΔTGW increases, reaching 0.6 ºC (or 12% of the heat stress increase). As this increase is similar to current heat island mitigation techniques, this feedback needs to be considered in urban climate projections, especially where AC use is larg

Foehnlike Wind with a Traditional Foehn Effect plus Dry-Diabatic Heating from the Ground Surface Contributing to High Temperatures at the End of a Leeward Area

A foehn wind is an important factor in the occurrence of many extreme high-temperature events in geographically complex regions. In this study, the authors verified the hypothesis that a foehnlike wind contributes to high temperatures at the end of the leeward (eastward) area using three difference approaches: field experiments, numerical experiments, and statistical analyses. According to the hypothesis, a foehnlike wind has the features of the sum of a traditional foehn effect with adiabatic heating, plus dry-diabatic heating from the ground surface along the fetch of the wind. Field experiments conducted at seven observational points on Nobi Plain, Japan, where a mesoscale westerly wind blew, revealed that the westerly wind clearly had the features of a traditional foehn effect in the western part of the Nobi Plain. In addition to field experiments, a simplified estimate using a simple mixed-layer model demonstrated that the wind was further heated by dry-diabatic heating (sensible heat supply) from the ground surface along the fetch (especially in urbanized areas in the eastern region of the Nobi Plain) of the wind. This diabatic heating effect along the fetch of the wind on the high temperature at the end of the leeward area was also supported by both additional numerical experiments and a statistical analysis. These results proved that the hypothesis is correct and indicated that ground conditions and the land use and land cover in the windward area were strongly related to air temperature at the end of the leeward area, where an extremely high temperature was observed

Distribution of Air Temperature in Tajimi City in Summer

To research the distribution of air temperature in the city of Tajimi, Gifu prefecture, a total of 15 thermometers were placed at the schools and parks in the city of Tajimi, and the city of Kasugai, Aichi prefecture in August 2010. From the distribution of monthly average air temperature, air temperature at the center of Tajimi city was higher than the suburbs. Also, the days of daily minimum air temperature more than or equal to 25°C and daily maximum air temperature more than or equal to 35°C at the elementary school near the center of Tajimi city was more than those at other schools. This tendency appeared more clearly on the days of daily minimum air temperature more than or equal to 25°C. Also, the air temperature near the center of the city was higher than that ofthe suburbs in the early morning. Thus, it was indicated that the air temperature was hard to decrease as the bottom of the basin. From these results, the influence of urbanization to the formation of the daily minimum temperature in Tajimi city was indicated

Mechanism of Precipitation Increase with Urbanization in Tokyo as Revealed by Ensemble Climate Simulations

This study examines how urbanization affects the precipitation climatology in Tokyo, Japan. A unique aspect of this study is that an ensemble, regional climatological simulation approach is used with sensitivity experiments to reduce uncertainty arising from nonlinearity in the precipitation simulations. Another aspect is that the robustness of the precipitation response is tested with “stress response” simulations with increasing urban forcing. The results show that urbanization causes a robust increase in the amount of precipitation in the Tokyo metropolitan area and a reduction in the inland areas. These anomalies are statistically significant at the 95% and 99% levels in some parts. There is no measureable change in the surrounding rural and ocean areas. These precipitation responses are attributed to an increase of surface sensible heat flux in Tokyo, which destabilizes the atmosphere and induces an anomalous surface low pressure pattern and the convergence of grid-scale horizontal moisture flux. The anomalous convergence of grid-scale horizontal moisture flux is a consequence of urbanization modifying the sea breeze

Observational Study and Numerical Prediction Experiments on Wet-Bulb Globe Temperature in Tajimi, Gifu Prefecture: Consideration of Uncertainty with a Physics Parameterization Scheme and Horizontal Resolution of the Weather Research and Forecasting Model

As part of research on the actual conditions of the thermal environment surrounding the city of Tajimi, Gifu prefecture, and the city of Kasugai, Aichi prefecture, the surface air temperature and wet-bulb globe temperature (WBGT) were investigated by observation at these locations on clear-sky days in August 2010. Numerical prediction experiments on the WBGT were performed to confirm the utility of the Weather Research and Forecasting (WRF) model. Sensitivity experiments utilizing physics parameterization schemes and horizontal resolution of the WRF model were conducted to confirm the predicted WBGT. The resultsshowed that the maximum sensitivity with the parameterization scheme was 8.4°C for the daytime average, and especially, the simple thermal diffusion surface (SLAB) scheme caused an over/estimation of 6.8°C. On the other hand, the maximum sensitivity with horizontal resolution was 0.5°C, which is much less than that with the parameterization scheme

Factors causing climatologically high temperatures in a hottest city in Japan: a multi-scale analysis of Tajimi

In this study, multi-scale climatological features of extreme high temperature (EHT) events in Tajimi, the hottest cities in Japan, were investigated using observational data collected by the Japan Meteorological Agency over the past 23 years, and original data observed by the authors over the last 3 years. Results revealed the background factors that lead to climatologically high temperatures in Tajimi: the occurrence of a characteristic pressure pattern called ‘whale’: the synoptic-scale factors, and the urbanization of Tajimi: the meso-γ-scale factors. In addition, the high temperatures measured in Tajimi are affected by the foehn-like westerly airflow coming from the mountains located in the northwest/west towards the Nobi Plain where Tajimi is located at the east end: the meso-β-scale factors, and the location of the Tajimi observation site, which is within an urbanized area where the highest temperatures tend to be observed: the micro-scale factors. In contrast, statistical analysis demonstrated that the small-scale basin effects and soil dryness around Tajimi were of lesser importance than aforementioned factors, in the occurrence of EHT events in Tajimi