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An intra-urban nocturnal cooling rate model

Conference contribution
Authors Shiho Onomura
Björn Holmer
Fredrik Lindberg
Sofia Thorsson
Published in ICUC9 – 9 th International Conference on Urban Climate jointly with 12th Symposium on the Urban Environment. 20-24 July 2015, Toulouse, France
Publication year 2015
Published at Department of Earth Sciences
Language en
Links www.meteo.fr/cic/meetings/2015/ICUC...
Subject categories Climate Research, Meteorology and Atmospheric Sciences, Physical Geography

Abstract

Nocturnal urban heat island (UHI) and intra-urban heat island (IUHI) mainly develop through differences in cooling rates. The cooling process consists of two distinctive phases. In the first phase, around sunset, dense urban areas cool more slowly than more open sites, creating large intra-urban temperature differences that are preserved during the whole night. The intensity of this differential cooling is mainly determined by surface characteristics (geometry and material), prevailing weather conditions and season. On the other hand, the cooling during the rest of night, in the second phase, is independent of the surface characteristics. In this study, we investigated how intra-urban cooling rates in the two phases are statistically related to prevailing weather conditions, season, and sky view factor using observation data from Gothenburg, Sweden. Based on the results, a simple statistical intra-urban nocturnal cooling rate model was developed. The model requires only commonly-used meteorological variables and sky view factor. It was shown that the most intensive cooling rate at an open site, in the first phase, was chiefly dominated by the clearness of the sky and wind speed, i.e. the weather conditions. The cooling rate also had a linear relationship with maximum daily air temperature, which can be treated as the seasonal effect. Under clear sky condition, the magnitude of the cooling rate significantly decreased with lower sky view factor, but, under cloudy conditions, the cooling rate varied less or little. In the second phase, cooling rate seemed to linearly decrease as the night progressed and the slope of the decrease was determined by the clearness of the sky. The model was evaluated using three additional datasets, one from Gothenburg, one from London, UK and one from Ouagadougou, Burkina Faso. Gothenburg and London are classified to have a marine temperate climate (Cfb) and Ouagadougou has a tropical steppe climate (BSh) according to Köppen climate classification. The model simulated cooling rates along a smooth profile statistically determined, while observed cooling rates often fluctuated through night. Nevertheless, the model estimated well the total amount of cooling during the whole night. This resulted in the well-simulated nocturnal air temperature. Modeled cooling rates were deviated from the observation at the sites where the large effects of anthropogenic heat and evapotranspiration were present. The effects were not included in this model yet but were found to be significant. This model can be used for multiple applications such as nocturnal human thermal comfort estimation and climate-sensitive urban planning and design.

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