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

Climate, Not Conflict, Explains Extreme Middle East Dust Storm

The recent dust storm in the Middle East (September 2015) was publicized in the media as a sign of an impending ‘Dust Bowl.’ Its severity, demonstrated by extreme aerosol optical depth in the atmosphere in the 99th percentile compared to historical data, was attributed to the ongoing regional conflict. However, surface meteorological and remote sensing data, as well as regional climate model simulations, support an alternative hypothesis: the historically unprecedented aridity played a more prominent role, as evidenced by unusual climatic and meteorological conditions prior to and during the storm. Remotely sensed normalized difference vegetation index demonstrates that vegetation cover was high in 2015 relative to the prior drought and conflict periods, suggesting that agricultural activity was not diminished during that year, thus negating the media narrative. Instead, meteorological simulations using the Weather Research and Forecasting (WRF) model show that the storm was associated with a cyclone and ‘Shamal’ winds, typical for dust storm generation in this region, that were immediately followed by an unusual wind reversal at low levels that spread dust west to the Mediterranean Coast. These unusual meteorological conditions were aided by a significant reduction in the critical shear stress due to extreme dry and hot conditions, thereby enhancing dust availability for erosion during this storm. Concluding, unusual aridity, combined with unique synoptic weather patterns, enhanced dust emission and westward long-range transport across the region, thus generating the extreme storm

The Effects of Building Representation and Clustering in Large-Eddy Simulations of Flows in Urban Canopies

We perform large-eddy simulations of neutral atmospheric boundary-layer flow over a cluster of buildings surrounded by relatively flat terrain. The first investigated question is the effect of the level of building detail that can be included in the numerical model, a topic not yet addressed by any previous study. The simplest representation is found to give similar results to more refined representations for the mean flow, but not for turbulence. The wind direction on the other hand is found to be important for both mean and turbulent parameters. As many suburban areas are characterised by the clustering of buildings and homes into small areas separated by surfaces of lower roughness, we look at the adjustment of the atmospheric surface layer as it flows from the smoother terrain to the built-up area. This transition has unexpected impacts on the flow; mainly, a zone of global backscatter (energy transfer from the turbulent eddies to the mean flow) is found at the upstream edge of the built-up are

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

Field study of the dynamics and modelling of subgrid-scale turbulence in a stable atmospheric surface layer over a glacier

A field experiment - the Snow Horizontal Array Turbulence Study (SnoHATS) - has been performed over an extensive glacier in Switzerland in order to study small-scale turbulence in the stable atmospheric surface layer, and to investigate the role, dynamics and modelling of the subgrid scales (SGSs) in the context of large-eddy simulations. The a priori data analysis aims at comparing the role and behaviour of the SGSs under stable conditions with previous studies under neutral or unstable conditions. It is found that the SGSs in a stable surface layer remain an important sink of temperature variance and turbulent kinetic energy from the resolved scales and carry a significant portion of the fluxes when the filter scale is larger than the distance to the wall. The fraction of SGS fluxes (out of the total fluxes) is found to be independent of stability. In addition, the stress-strain alignment is similar to the alignment under neutral and unstable conditions. The model coefficients vary considerably with stability but in a manner consistent with previous findings, which also showed that scale-dependent dynamic models can capture this variation. Furthermore, the variation of the coefficients for both momentum and heat SGS fluxes can be shown to be better explained by stability parameters based on vertical gradients, rather than vertical fluxes. These findings suggest that small-scale turbulence dynamics and SGS modelling under stable conditions share many important properties with neutral and convective conditions, and that a unified approach is thus possible. This paper concludes with a discussion of some other challenges for stable boundary-layer simulations that are not encountered in the neutral or unstable case