Rapid cooling of urban surfaces during rainfall: physical basis, dominant energy fluxes, and sensitivity to pavement and rainfall properties

Using model for the heat transfer between pavements and runoff during rainfall, we investigate the importance of different pavement and rainfall properties, as well as crucial energy budget terms that drive the cooling processes. The results indicate that the pavement and runoff temperature and energy fluxes are very sensitive to the rain temperature. In addition, pavement albedo has a significant effect on the simulated temperature since it modifies the initial pavement temperature before the rain starts. The results also indicated that among the different energy budget terms, evaporation and long wave radiation are the main cooling terms, while the shortwave radiation dominates energy input into the runoff-pavement system

Greening rooftops to reduce heat islands: How large is large enough?

Green roofs, with adequate water supply, have been proven as effective measures to reduce urban environmental temperature. The benefits of large-scale deployment of green roofs have been studied mainly through numerical simulations with unrealistic high penetration scenarios, where all rooftops across the entire metropolis is assumed to be retrofitted. In this study, the scale dependence of the cooling effect of green roofs is investigated with a coverage of 25% over buildings at local, city, or regional scales. We compared results at 6 major U.S. cities to assess the response of the scale dependence to geoclimatic conditions. High-resolution weather simulations reveal that the cooling of near-surface air temperature by green roofs increases non-linearly with the scale of deployment. The shape and geoclimatic setting (geographic and climatic characteristics) of metropolitan areas control the scaling that some city centers are not able to achieve a significant cooling by greening their own rooftops. Uniform deployment of green roofs at the regional scale, on the other hand, provides a substantial temperature reduction with a very low cooling efficiency per intervention area. Cities should carefully revisit the scale dependences of cooling benefit and efficiency of green roofs to develop resilient plans meeting their expectations

Using advanced Urban Canopy Models to investigate the potential of thermochromic materials as urban heat island mitigation strategies

Recent trends in urbanization processes are causing serious threats at both local and global environmental scale. Greenhouse gas emissions, heat waves, and the heat island effect are constantly growing in intensity and produce increasing discomfort and health impacts in urban populations. In this context, the building sector is currently developing advanced and adaptive materials for building envelope and paving surface applications characterized by high energy performance and low embodied energy. Most of these innovative materials are firstly analysed at the component scale by means of laboratory investigations, while their effect on the built environment is generally assessed at a later stage, by means of advanced computer simulations in buildings and urban microclimate monitoring or modelling. In this context, this work focuses on the evaluation of the UHI modulation potential of materials with advanced dynamic optical properties, i.e. variable surface albedo, for surface urban canyon applications. Specifically, the Princeton Urban Canopy Model (PUCM) is applied with the aim of investigating the potential of advanced urban roofing material to modulate the urban heat island. The aim is to minimize the heat island in the summer but to let it develop in the winter, using roofing applications characterized by a dynamic temperature-dependent optical behavior. In particular, the effect of thermochromic materials on local energy transport phenomena is assessed and benchmarked against more common cool roof solutions. Results show that the modified UCM can effectively be implemented to represent temperaturedependent albedo variations. Additionally, this study demonstrates that using thermochromic materials produces a smart optical response to local environmental stimuli and allows enhanced short wave solar reflection in summer conditions, reduced reflected solar fraction in winter, and adaptive properties during transition periods

Improving the representation of convective heat transfer in an urban canopy model

The urban street canyon has been widely recognized as a basic surface unit in urban micrometeorological studies. Urban canopy models (UCMs), which quantify the exchange of energy and momentum between the urban surface and the overlying atmosphere, often adopt this type of street canyon representation as the fundamental surface element. Since UCMs can be coupled to regional-scale weather and climate models such as the Weather Forecast and Research Model (WRF), parametrizations of the surface momentum and scalar fluxes in UCM are of paramount importance. However, many current single-layer UCMs rely on empirical relations that were obtained over 80 years ago and often invoke the exponential wind profile derived from the existing literature for vegetation canopy. In this study, we conducted wallmodeled large-eddy simulations (LES) to study the forced (very weak buoyancy) convective heat transfer over idealized two-dimensional street canyons. It shows that the transfer efficiency computed following commonly applied resistance formulations can be one order of magnitude lower than LES results. The main reasons for the deviation include inaccurate wind speed parameterization and the use of a log-law based formulation for turbulent heat exchange between canyon air and the flow above

Scale dependence of subgrid-scale model coefficients: An a priori study

Dynamic subgrid-scale models require an a priori assumption about the variation in the model coefficients with filter scale. The standard dynamic model assumes independence of scale while the scale dependent model assumes power-law dependence. In this paper, we use field experimental data to investigate the dependence of model coefficients on filter scale for the Smagorinsky and the nonlinear models. The results indicate that the assumption of a power-law dependence, which is often used in scale dependent dynamic models, holds very well for the Smagorinsky model. For the nonlinear model, the power-law assumption seems less robust but still adequat