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Thermal and Geometric Controls on the Rate of Surface Air Temperature Changes in a Medium-Sized, Midlatitude City

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  • 1 Faculty of Science, Kumamoto University, Kumamoto, and Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan
  • | 2 Fluid Science Sector, Central Research Institute of Electric Power Industry, Abiko, Japan
  • | 3 Graduate School of Science and Technology, Kumamoto University, Kumamoto, Japan
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Abstract

Gradual cooling in the evening forms a wintertime nocturnal urban heat island. This work, with a mesoscale model involving urban canopy physics, is an examination of how four thermal and geometric controls—anthropogenic heat QF, heat capacity C, thermal conductivity k, and sky-view factor ψs—modify the rate of surface air temperature changes ΔTt. In particular, the time dependence is diagnosed through numerical experiments. The controls QF and k are major agents in the evening, when QF changes the evening ΔTt linearly and k is logarithmic. The effects of C and ψs are large in the morning and in the afternoon with those of k. The impact of QF is, however, substantial only in the evening. Because the time dependence of C and k is different, the thermal inertia used as a parameter in the urban climate studies should be divided into two parameters: C and k. To improve the thermal environment in urban areas, the modification of QF and k could be effective.

* Current affiliation: Center for Computational Sciences, University of Tsukuba, Tsukuba, Japan

+ Current affiliation: Applied Technology Co., Ltd., Tokyo, Japan

Corresponding author address: Tomohiko Tomita, Faculty of Science, Kumamoto University, Kumamoto, Japan. Email: tomita@sci.kumamoto-u.ac.jp

Abstract

Gradual cooling in the evening forms a wintertime nocturnal urban heat island. This work, with a mesoscale model involving urban canopy physics, is an examination of how four thermal and geometric controls—anthropogenic heat QF, heat capacity C, thermal conductivity k, and sky-view factor ψs—modify the rate of surface air temperature changes ΔTt. In particular, the time dependence is diagnosed through numerical experiments. The controls QF and k are major agents in the evening, when QF changes the evening ΔTt linearly and k is logarithmic. The effects of C and ψs are large in the morning and in the afternoon with those of k. The impact of QF is, however, substantial only in the evening. Because the time dependence of C and k is different, the thermal inertia used as a parameter in the urban climate studies should be divided into two parameters: C and k. To improve the thermal environment in urban areas, the modification of QF and k could be effective.

* Current affiliation: Center for Computational Sciences, University of Tsukuba, Tsukuba, Japan

+ Current affiliation: Applied Technology Co., Ltd., Tokyo, Japan

Corresponding author address: Tomohiko Tomita, Faculty of Science, Kumamoto University, Kumamoto, Japan. Email: tomita@sci.kumamoto-u.ac.jp

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