Meteorological field observations were conducted at a small urban area in the lakeshore basin environment in summer. During the daytime of a clear, calm day, about 2000m height of the mixing layer were developed by the thermally induced circulations over the basin. The maximum of air temperature around the lakeshore were recorded after 1500 LST, and at 1800 LST for the urban area. It is conjectured that the mechanism of a continuation of high temperature in urban area is mainly due to the complex factors such as the downward wind which brings high temperature of air mass by the circulations, the relatively high temperature of paved ground and roof surfaces, and the improved mass and heat exchange within and above the urban canopy layer. Numerical simulation was conducted to evaluate these convection and urban effects by a three-dimensional turbulent closure model. The 100x120km computational domain was discretized horizontally by a nested grid system consisting of overlapped primary 2x2km for the circulations, and secondary 1x1km, and tertialy 250x250m for then urban atmosphere, mesh resolutions. The results were in good agreement with observed value of air temperature, mixing ratio, and wind profiles. The large curculation also reproduced well, and downward wind over the lake brought warm and dry air mass to the ground. The air temperature in urban area rose and kept the highest values until around 18:00LST, even, the size of urbanized area has 3x3km and that is low-stories and low anthoropogenic heat release.