Urban Heat Island
Heat Mitigation Strategies  

日本語

Urban Heat Island

The Urban heat Island is a phenomenon where cities become warmer than their surrounding suburbs. Many people think it appears most strongly during the daytime in the summer, but in fact it appears the strongest at night during fall and winter.

It is also often seen or heard that the explanation for the inland Kanto region being warmer (e.g. Kumagaya is hotter) is due to the heat from Tokyo carried by the sea breeze. Furthermore, the Kumagaya District Meteorological Observatory's previous website provided such an explanation, but this is not actually true. If this were the case, it would be caused by warm air advection, and Tokyo would theoretically be hotter than Kumagaya. The current explanation in the website of the Kumagaya District Meteorological Observatory has then been corrected.

The mechanism of the urban heat island has long been studied, but is still not fully understood. Kusaka and Kimura (2004) have provided a certain answer to the long debated question, "Does urban heat island intensity reach a maximum in the hours after sunset or early morning?".

The Kusaka Laboratory investigates the effects of past and future land use change and increased anthropogenic heat removal on urban heat islands and sea breezes. The research field has been expanding with the students' interests, starting from Tokyo, then Sendai, Ho Chi Minh City and Hanoi in Vietnam, and Sofia in Bulgaria.

Heat Mitigation Strategies

In recent years, the urban heat environment has been deteriorating due to global warming and the urban heat island. As the urban heat environment deteriorates, people are more likely to suffer from heat stroke, so the Kusaka Laboratory conducts research on heat mitigation strategies to improve such environments.

Now, what comes to mind with heat mitigation strategies? For example, Kusaka et al. (2022a) observed the heat index (i.e. wet bulb globe temperature; WBGT) under a wisteria trellises and a tent commonly used at sports day, and found that the wisteria trellises were able to reduce WBGT by about 2°C, which was about 1°C greater than the tent (Figure 1). Kusaka et al. (2022b) also observed WBGT under street trees, dry mist, and UV parasols, and found that street trees were the most effective of the three, reducing WBGT 1.9°C lower than under direct sunlight (Figure 2). Other heat mitigation strategies include greening of walls and rooftops, and the use of sea breezes.

The research on heat mitigation strategies includes observations and numerical simulations using City-LES and ENVI-met. Experiments are also conducted with subjects, and heat mitigation strategies are part of a research field that is closely related to the field of biometeorology.

Fig.1 Differences in the mean contribution of black-globe temperature (red), dry-bulb temperature (yellow), and wet-bulb temperature (blue) to WBGT between measurements under the wisteria trellis and in direct sunlight, between measurements under the wisteria trellis and a tent (Kusaka et al. 2022a).

Fig.2 The difference in WBGT under direct sunlight and other three locations. Each color represents the average contribution of wet bulb temperature (blue), black bulb temperature (orange), and dry bulb temperature (gray) to the WBGT reduction (Kusaka et al. 2022b).

(Written by: Satoka Morohashi; Edited by: Angela Magnaye and Chiho Numata)

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