Experimental observation and numerical simulation of heat island plume in urban surface layer

A peculiar thermal plume is formed over urban areas under urban heat island conditions, which appear owing to huge energy consumption in megacities like Tokyo. In order to clarify the characteristics of this urban heat island phenomenon, the authors made a microscopic analysis for the thermal plume in the urban surface layer, where a lot of various structures including buildings, pavements and other facilities exist. In this article, a simplified experiment and 2-D numerical simulations were conducted by employing a new microscopic modeling for the urban surface layer. The purposes of this microscopic modeling are: (i) to find a similarity between urban building structures and porous media; and (ii) to apply the model of "flow in porous media" to heat island phenomena, which has been developed in the field of heat transfer. Results of the experimental observations agree well with that of the numerical simulations on the flow pattern of the thermal plume. It is indicated that micro-scale thermal plumes occur in the thermally stable stratified air if clusters of urban structures exist at the lower boundary layer. This means that real atmospheric condition is greatly influenced by this kind of micro-scale thermal plume in the urban surface layer.