Formation of the Urban Air Mass and the Associated Local Circulation

[摘要] A two dimensional numerical model is designed to investigate phenomena of urban heat islands under a steady condition. The model describes temperature and wind velocity distributions in the Ekman layer under a certain combination of factors. These factors are anthropogenic heat generation, evaporability, roughness and prevailing flow. The surface temperature is decided from the energy balance equation at the air-earth interface. The anthropogenic heat is given to the air at 50 m level instead of at the interface. Many facts pointed out in past studies are successfully simulated.The characteristics of this study and the results are following: 1) The diurnal condition including insolation is considered and the importance of the non-uniform evaporability is denoted. 2) The difference of influences between the anthropogenic heat and the evaporation whose each effect acts on the different level, becomes clear in the temperature distributions at the 50 m and the ground. (These two effects are referred to as the thermal effect.) 3) The effect of variation of the prevailing flow intensity on the wind pattern is investigated in detail within the extent from 0 to 10 m/sec in the flow intensity. 4) When the prevailing flow intensity increases, the non-uniform roughness becomes a main controlling factor instead of the thermal effect. In these two regimes, the direction of the perturbed wind is reverse with each other.

[学科分类] 大气科学