Abstract: Based on the land use data in 1993 and 2005,the effects of urban expansion on the near-surface wind and thermal environment and boundary layer were investigated using the UCM+AH scheme in the WRF model under the conditions of two clear days in August 2005.The results show that UCM+AH scheme is able to properly and accurately simulate the diurnal variations of 2 m temperature and 10 m wind speed,and the accuracy of simulated 2 m temperature is better than that of 10 m wind speed.The scheme overestimates the diurnal range of the simulated 2m temperature and the wind speed systematically by 0.5-1.0 m·s^-1.Urban expansion can lead to a general warming in the urban area especially in nighttime.The maximal increasing amplitudes of 7 ℃ and 3 ℃ at nighttime are distributed in the expanded and old urban area,respectively.The increasing amplitude is larger in the upwind direction than in other directions.The warming with a range between 0-1 ℃ at daytime is not obvious in the urban area.In addition,after urban expansion,the wind speed decreases generally with a magnitude smaller than 1.0 m·s^-1 in the old urban area and close to 1.0 m·s^-1 in the expanded urban area.At the same time,a high-temperature convergent zone in the urban area accelerating the near ground wind speed in the surrounding areas is probably caused by urban expansion.Besides,the urban expansion affects significantly the boundary layer in the afternoon and results in Turbulent Kinematic Energy with an increase of 0.2-0.3 m²·s^-2 expanding its range of influence.Moreover,the boundary layer height is risen up by 100-200 m at 14:00 and the local thermal circulation and vertical upward motion are strengthened over the expanded area.

Key words: Urban expansion, Urban canopy model, Land use category, WRF model