Impacts of Urban Dynamical Effect on Precipitation in Tokyo Metropolitan Area

  • Sanae OTAKA, 2014: Impacts of Urban Dynamical Effect on Precipitation in Tokyo Metropolitan Area .

Abstract

In this study, urban drag force effect on precipitation is investigated by numerical experiments with the WRF coupled to the Multi-Layer Urban Canopy Model (MLUCM). MLUCM enables to represent vertical structure of wind explicitly by dividing urban canopy into multi layers, and considering building drag effects in the equation of motion. However, MLUCM has not been used in previous research for urban precipitation because of its complexity and high computational cost.

Therefore, this research examines MLUCM’s performance and determines appropriate urban parameters. As a result of experiments conducted by tuning building height, building width, urban ratio, thermal inertia and anthropogenic heat to metropolitan area, simulation results were well corresponded with observation.

From sensitive experiments with different building’s resistance, it was found that the bigger the building resistance becomes, the more urban wind speed is reduced, and on the windward side of the city convergence occurred. This effects cause weaken wind speed of sea breeze in the urban canopy layer, and temperature and precipitable water vapor decreased slightly. Updraft caused by convergence in windward in the city modified convergence area of sea breeze, but the range of it was only within canopy. In the experiment with four times higher buildings, turbulence and sensible heat increased because of building resistance, and temperature and precipitable water vapor also increased slightly. Updrafts in windward in the city modified with convergence area of sea breeze in the only urban canopy upper layer. However, results did not show clear correlation between urban dynamical effects and precipitation in Tokyo metropolitan area.

From the above results, impact of urban dynamical effects on the precipitation is not remarkable. However, in the case of bigger urban scale, the possibility which makes an effect was suggested.

Key Words: urban dynamical effect, multi-layer urban canopy model, building energy model, precipitation