Laboratory simulations of an urban heat island in a stratified atmospheric boundary layer
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R E G UL A R P A P E R
Serena Falasca • Monica Moroni • Antonio Cenedese
Laboratory simulations of an urban heat island in a stratified atmospheric boundary layer
Received: 25 June 2012 / Revised: 6 September 2012 / Accepted: 17 October 2012 / Published online: 2 December 2012 Ó The Visualization Society of Japan 2012
Abstract In the atmospheric boundary layer (ABL), under high pressure conditions and negligible geostrophic winds, problems associated with pollution are the most critical. In this situation, the urban heat island plays a major role in the close-to-the-ground atmospheric dynamics and in dispersion processes at scales in the order of tens of meters (small scales). This article presents water tank laboratory simulations of an urban heat island in a stably stratified ABL, neglecting geostrophic winds and the effects of Coriolis force. The phenomenon is studied in the framework of a similarity theory developed for a nocturnal and low-aspect ratio urban heat island extended to the diurnal case. Image analysis techniques appear suitable to fully describe the phenomenon. The high resolution data provides a detailed fluid dynamic characterization of the urban heat island circulation. Present laboratory results, normalized by similarity theory scaling parameters, compare well with literature data. Keywords Urban heat island Water tank model Local climate Planetary boundary layer turbulence
1 Introduction The urban heat island (UHI) is the characteristic warmth associated with the urban (or industrial) areas with respect to the surrounding rural zones. Water tank simulations of the UHI have been performed in the absence of synoptic winds and neglecting the effects of the Coriolis force. Such meteorological conditions are the most critical for problems associated with pollutant dispersion. Water tanks have been widely used to investigate the time evolution of the convective boundary layer. The thermal properties of water allow both a large heating rate and sufficient time to take measurements of the evolving thermal structures (Yuan et al. 2011). The most significant limitation of water tank laboratory measurements is the inability to achieve all similarity conditions (i.e., Reynolds, Froude, Prandtl numbers), but the parameters that govern the phenomenon (i.e., vertical temperature gradient and heat fluxes) can be easily controlled. Furthermore, for sufficiently large values of Re and for Pr larger than unity, it can be assumed that the phenomenon does not depend on those nondimensional numbers themselves (Snyder 1981; Lu et al. 1997a). Wind tunnels represent a valid alternative for simulating properly scaled atmospheric boundary layer phenomenon (Kozmar 2010; Varshney and Poddar 2011; Varshney and Poddar 2012). The advantages of wind tunnels reside in their ability to model the effects of local topography as well as the combined effects of wind shear and convective turbulence at high wind speeds, thus complementing the zero to light wind regime that is best simulated in water tank models. On the other
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