92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Wednesday, 25 January 2012
Relationship Between Urban Warming and Surface Wind Speed–A Numerical Study on Climatic Changes Induced by Land Surface Modifications
Hall E (New Orleans Convention Center )
Toshinori Aoyagi, MRI, Tsukuba, Japan; and N. Seino

Poster PDF (1.6 MB)

The aim of this study is to figure out the relationships between the urban warming rate and the increase and decrease rate of the wind speed, which are induced by land surface modifications (mainly by the urbanizations). We prepared datasets of the land use area fraction at the year of 1976 and 2006, the horizontal distributions of anthropogenic heats and the shapes of the urban buildings (i.e. height – width ratio) in the Tokyo metropolitan area, Japan. Then we operated some numerical simulations on the effects of the bottom boundary modifications to the lower atmosphere under summer atmospheric conditions. The atmospheric model used in this study is the Japan Meteorological Agency - Nonhydrostatic Model (JMA-NHM). A single-layered Square Prism Urban Canopy scheme (SPUC) was coupled to the land surface scheme of the JMA-NHM in order to be able to estimate the verticalization effects of the urban buildings.

From the simulation results on the effects of land use changes (from the vegetations to the urban surfaces) between 1976 and 2006, we could see the temperature rises almost of all the grids in the simulation domain. The simulations showed the decreases of wind speeds especially at the grids with larger land surface modifications. The temperature rises were seemed to be the result of the Bowen ratio increases induced by vegetation decreases. The decreases of wind speed were mainly caused by the increase of roughness lengths along with the urban settings. The temperature increases were about 0.1°C to the maximum at around the center of Tokyo. About 17% decreases of wind velocities could be seen at the grids with the larger change to urban surfaces. The decreases of wind speed were smaller in daytime and larger in nighttime. Even some grids having wind increases in nighttime could be seen. Such grids seemed to have larger temperature rises in nighttime. It implied that the increases of local winds might be larger than the wind decreases by the roughness increase.

The addition of the anthropogenic heat caused the temperature rise and the increase of wind speed occurred centering around the warming grids. The temperature rises caused by the verticalization of urban buildings showed also the increasing effects on wind speed.

From the simulations, there seemed to be mainly positive correlations between the temperature rises and the wind speed changes as total effects of urbanization. On the other hands, it is shown that there are mainly negative correlations between the urban warming trends and the wind speed trends in recent several tens of years from the quite careful analyses of observational datasets. Further discussions are needed when we compare the simulation results in this study and the previous observational researches.

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