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Numerical study of the daytime planetary boundary layer over an idealized urban area. Influence of surface properties, anthropogenic heat flux, and geostrophic wind intensity
No full textLarge-eddy simulations of an idealized diurnal urban heat island are performed using the Weather Research
and Forecasting Model. The surface energy balance over an inhomogeneous terrain is solved considering
the anthropogenic heat contribution and the differences of thermal and mechanical properties
between urban and rural surfaces. Several cases are simulated together with a reference case, considering
different values of the control parameters: albedo, thermal inertia, roughness length, anthropogenic heat
emission, and geostrophic wind intensity. Spatial distributions of second-moment statistics, including the
turbulent kinetic energy (TKE) budget, are analyzed to characterize the structure of the planetary boundary
layer (PBL). The effect of each control parameter value on the turbulent properties of the PBL is investigated
with respect to the reference case. For all of the analyzed cases, the primary source of TKE is the buoyancy in
the lower half of the PBL, the shear in the upper half, and the turbulent transport term at the top. The vertical
advection of TKE is significant in the upper half of the PBL. The control parameters significantly influence the
shape of the profiles of the transport and shear terms in the TKE budget. Bulk properties of the PBL via
proper scaling are compared with literature data. A log-linear relationship between the aspect ratio of the
heat island and the Froude number is confirmed. For the first time, the effect of relevant surface control
parameters and the geostrophic wind intensity on the bulk and turbulent properties of the PBL is systematically
investigated at high resolution
Laboratory simulations of an urban heat island in a stratified atmospheric boundary layer
No full textIn 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
Impact of Highly Reflective Materials on Meteorology, PM10 and Ozone in Urban Areas: A Modeling Study with WRF-CHIMERE at High Resolution over Milan (Italy)
Full text linkThe Urban Heat Island (UHI) is a well-known phenomenon concerning an increasing percentage of the world’s population due to the growth rates of metropolitan areas. Given the health and economic implications of UHIs, several mitigation techniques are being evaluated and tested. In this study, we consider the use of highly reflective materials for urban surfaces, and we carried out numerical experiments using the Weather Research and Forecasting model coupled with the CHIMERE model in order to investigate the effects of these materials on the meteorology and air quality in the urban area of Milan (Italy). Results show that an increase in albedo from 0.2 to 0.7 for urban roofs, walls and streets leads to a decrease in UHI intensity by up to 2–3 °C and of the planetary boundary layer (PBL) height of about 500 m. However, the difference of PM10 and ozone between urban and surrounding areas increases by a factor of about 2, attributable to the reduction of PBL height and wind speed and to the increased reflected solar radiation that may enhance photochemical production during the daytime. Therefore, if anthropogenic emissions are held at the same levels, the potential benefit to the UHI in terms of thermal discomfort may have negative repercussions on air quality
High resolution numerical and experimental modeling of the Urban Heat Island circulation
No full textThe three-dimensional non-hydrostatic meteorological model WRF (Weather Research and Forecast) is used in Large-Eddy Simulation (LES) mode to reproduce the coherent structures associated to the Urban Heat Island (UHI) circulation. Horizontal resolution of 50 m and a vertically stretched grid with a finer resolution close to the surface is employed in the numerical model in order to compute the mean variables and the turbulence statistics. Experiments are conducted in a thermally controlled test section filled with distilled water. The velocity field is determined through image analysis techniques. Temperatures are measures by means of thermocouples arrays. Different UHI intensities and background initial stratifications are investigated with both the numerical code and the experimental apparatus and results compared with literature data. The turbulent structures of the UHI circulation are correctly reproduced by the LES model while the standard deviations are underestimated in the laboratory experiments
Decarbonization of Summer Cooling Energy Demands of Buildings Employing Absorption Systems in the Framework of Climate Change in Italy
Full text linkTemperatures in the Mediterranean area have gradually risen in the last decades due to climate change, especially in the Italian Peninsula. This phenomenon has increased the cooling needs to ensure thermal comfort in buildings and, consequently, the use of refrigeration machines. Summer air conditioning is carried out mainly using compression machines powered by electricity supplied by the national network. All this contributes to the emission of climate-changing gases. To avoid this disadvantageous chain, compression machines could be replaced by absorption cooling systems powered by solar energy. The energy needs of the buildings in a time are directly proportional to the sum of positive differences between the outdoor air temperature and the indoor set point of the systems (equal to 26 C). The annual sum of hourly temperature differences defined above can be computed for each grid cell thanks to a numerical weather prediction model, namely the Weather Research and Forecasting model, that simulates the hourly temperatures on high-resolution computation grids and over fairly large extents. Maps of cooling consumption for buildings are thus produced. Choosing absorption solar energy-powered systems instead of vapor compression refrigeration systems leads to a drop in electrical energy consumption and therefore in emissions of greenhouse gases. In this work, different hypothetical scenarios of penetration of this technology have been considered. And the subsequent consumption of electricity withdrawn from the national grid has been estimated together with the reduction of greenhouse gas emissions
High-resolution air quality modeling: Sensitivity tests to horizontal resolution and urban canopy with WRF-CHIMERE
No full textThe European Directive (2008/50/EC) encourages the use of models in the assessment and forecasting of air quality, and assigns them a supporting or replacing role with respect to fixed ground-based measurements. A thorough knowledge of performance of the modeling tools over urban areas is therefore required. In this study, we analyze sensitivity tests with the WRF-CHIMERE modeling system in order to investigate the effect of (1) the horizontal model grid size, (2) the resolution of the anthropogenic emission inventory, and (3) the introduction of urban canopy models. The work focuses on L'Aquila and Milan, two Italian cities widely differing for the number of inhabitants, the extension and the geographical location. We found a clear advantage in increasing the model resolution up to ∼4 km, but a further increase at ∼1 km resolution does not seem to be justified. Moreover, we found that the ozone simulation is generally degraded at higher resolution. The introduction of a more detailed treatment of the urban canopy and of the anthropogenic emissions suggests the potential for further improvement, but this requires a fine tuning on the area of application. For example, the Building Environment Parameterization corrects the surface wind speed daily cycle, but it also increases the planetary boundary layer height, resulting in excessive dilution of primary pollutants. The anthropogenic emissions should be refined proportionally to the increase in dynamic model resolution, possibly through new bottom-up inventories, rather than through a downscaling of a coarser inventory. We suggest that future work should primarily focus on intensive campaign periods, where a comprehensive observational characterization of the three dimensional structure and evolution of the planetary boundary layer is available
Future energy demands of European buildings in the framework of climate change: A scoping study
Full text linkClimate change is considered an important global threat, with a significant impact on the energy performance, since buildings will be subjected to higher average outdoor temperatures. This paper explores the relative impact of global warming across the different regional climates of Europe comparing present and estimated future energy needs of a hypothetical residential house located in 19 cities characterized by different latitude and Köppen-Geiger class. Building performance simulations with EnergyPlus are performed in order to simulate the heating and cooling needs of the building and the associated CO2 emissions in the present and in the future. The progressive increase in average temperatures in 2050 and 2080 leads to a general decrease of thermal energy request for heating and to an increase in the demand for electricity for cooling especially in the southern Europe, where high carbon intensity coefficients cause large CO2 emissions. The resulting vicious circle can be interrupted by increasing the energy efficiency of buildings and properly converting thermoelectric power plants
Effect of heatwaves on urban sea breeze, heat island intensity, and outdoor thermo-hygrometric comfort in Rome (Italy)
Full text linkThe purpose of this study is to investigate the impact of heatwaves on sea breeze, urban heat island intensity, and outdoor thermo-hygrometric comfort analysing ground-based remote sensing and in-situ meteorological measurements collected in a Mediterranean coastal metropolitan area (Rome, Italy) during the summer of 2022. The dataset (01 May - 31 August 2022) has been divided into heatwaves and not-heatwaves days to highlight the distinctive features of extreme heat episodes. Furthermore, the year 2022 was compared to 2020, the latter having been characterised by temperatures in line with the last two-decades.The heatwaves do not significantly affect the onset time and the duration of sea breeze, while its cooling effect intensifies during the heatwaves. The urban heat island intensity (UHII) is evaluated by computing the hourly-averaged air temperature difference between two stations, one in the urban centre and the other in rural surroundings. Although the daily-averaged UHII does not show significant differences among the years under investigation and the subsets, marked differences can be identified in the urban heat island daily cycle. In fact, during daytime the highest temperatures are detected in rural inland areas while, at night, the city experiences higher temperatures, especially during the heatwaves. Finally, the Mediterranean outdoor comfort index suggests that a heatwave seems to be a not sufficient condition for human thermohygrometric stress, even if stronger discomfort is experienced during heatwaves
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