• Energy Research

Present-day and projected future changes in mean radiant temperature, T mrt in one northern, one mid-, and one southern European city (represented by Gothenburg, Frankfurt, and Porto), are presented, and the concept of hot spots is adopted. Air temperature, T a , increased in all cities by 2100, but changes in solar radiation due to changes in cloudiness counterbalanced or exacerbated the effects on T mrt. The number of days with high T mrt in Gothenburg was relatively unchanged at the end of the century (+1 day), whereas it more than doubled in Frankfurt and tripled in Porto. The use of street trees to reduce daytime radiant heat load was analyzed using hot spots to identify where trees could be most beneficial. Hot spots, although varying in intensity and frequency, were generally confined to near sunlit southeast-southwest facing walls, in northeast corner of courtyards, and in open spaces in all three cities. By adding trees in these spaces, the radiant heat load can be reduced, especially in spaces with no or few trees. A set of design principles for reducing the radiant heat load is outlined based on these findings and existing literature.

This dataset contains time-series of hourly climate constructed by merging records from various meteorological stations in and near Gothenburg, Sweden. The variables included are: - Central European Time (UTC+1) - Air temperature (degC). - Average wind speed (m/s) - Average wind direction (deg true) - Relative humidity (%) - Global radiation (short-wave radiation down on a horizontal surface, W/m/m). - Diffuse radiation (diffuse short-wave radiation down on a horizontal surface, W/m/m). - Mean sea-level pressure (hPa). - Direct-beam short wave radiation (W/m/m) incident on a plane normal to the solar angle. The dataset covers the period 1986-09-01 - 2020-12-31. Data files are provided in .csv and NetCDF format (https://www.unidata.ucar.edu/software/netcdf/) Further details are provided in the file README_utf8.txt. Detta dataset innehåller timvisa klimattidsserier konstruerade genom att kombinera data från meteorologiska stationer i och nära Göteborg, Sverige. De inkluderade variablerna är: - Centraleuropeisk tid (UTC+1) - Lufttemperatur (degC). - Genomsnittlig vindhastighet (m/s) - Genomsnittlig vindriktning (deg true) - Relativ luftfuktighet (%) - Global strålning (kortvågig strålning på en horisontell yta, W/m/m). - Diffus strålning (diffus kortvågsstrålning på en horisontell yta, W/m/m). - Havsnivåstryck (hPa). - Direktstrålande kortvågig strålning (W/m/m) på ett plan som är normalt till solvinkeln. Datasetet täcker perioden 1986-09-01 - 2020-12-31. Datafiler finns i .csv- och NetCDF-format (https://www.unidata.ucar.edu/software/netcdf/). Ytterligare information finns i filen README_utf8.txt.

Abstract A new model, Solar Energy on Building Envelopes (SEBE) for estimating shortwave irradiance on ground, roofs and building walls is presented. SEBE adopts a 2D raster modelling approach to derive 3D irradiance information, which makes it possible to compute extensive areas up to city scale. High resolution digital surface models (DSMs) are used to describe the urban geometry and additional DSMs including 3D vegetation structures can also be incorporated. Inclusion of vegetation is shown to be essential when modelling irradiances for wall surfaces within the urban environment. To obtain a detailed description of input forcing conditions, the model utilizes observed hourly data of shortwave radiation as meteorological input information. The model is evaluated for a tilted roof as well as for a wall location in Goteborg, Sweden. The overall performance of the model is high, both for the roof and wall evaluation point. Application of the model is exemplified by 3D visualisation and city scale model output presentation.

Future anthropogenic climate change is likely to increase the air temperature (T(a)) across Europe and increase the frequency, duration and magnitude of severe heat stress events. Heat stress events are generally associated with clear-sky conditions and high T(a), which give rise to high radiant heat load, i.e. mean radiant temperature (T(mrt)). In urban environments, T mrt is strongly influenced by urban geometry. The present study examines the effect of urban geometry on daytime heat stress in three European cities (Gothenburg in Sweden, Frankfurt in Germany and Porto in Portugal) under present and future climates, using T(mrt) as an indicator of heat stress. It is found that severe heat stress occurs in all three cities. Similar maximum daytime T(mrt) is found in open areas in all three cities despite of the latitudinal differences in average daytime T(mrt). In contrast, dense urban structures like narrow street canyons are able to mitigate heat stress in the summer, without causing substantial changes in T(mrt) in the winter. Although the T(mrt) averages are similar for the north-south and east-west street canyons in each city, the number of hours when T(mrt) exceeds the threshold values of 55.5 and 59.4 °C-used as indicators of moderate and severe heat stress-in the north-south canyons is much higher than that in the east-west canyons. Using statistically downscaled data from a regional climate model, it is found that the study sites were generally warmer in the future scenario, especially Porto, which would further exacerbate heat stress in urban areas. However, a decrease in solar radiation in Gothenburg and Frankfurt reduces T(mrt) in the spring, while the reduction in T(mrt) is somewhat offset by increasing T(a) in other seasons. It suggests that changes in the T(mrt) under the future scenario are dominated by variations in T(a). Nonetheless, the intra-urban differences remain relatively stable in the future. These findings suggest that dense urban structure can reduce daytime heat stress since it reduces the number of hours of high T(mrt) in the summer and does not cause substantial changes in average and minimum T(mrt) in the winter. In dense urban settings, a more diverse urban thermal environment is also preferred to compensate for reduced solar access in the winter. The extent to which the urban geometry can be optimized for the future climate is also influenced by local urban characteristics.

UMEP (Urban Multi-scale Environmental Predictor), a city-based climate service tool, combines models and tools essential for climate simulations. Applications are presented to illustrate UMEP's potential in the identification of heat waves and cold waves; the impact of green infrastructure on runoff; the effects of buildings on human thermal stress; solar energy production; and the impact of human activities on heat emissions. UMEP has broad utility for applications related to outdoor thermal comfort, wind, urban energy consumption and climate change mitigation. It includes tools to enable users to input atmospheric and surface data from multiple sources, to characterise the urban environment, to prepare meteorological data for use in cities, to undertake simulations and consider scenarios, and to compare and visualise different combinations of climate indicators. An open-source tool, UMEP is designed to be easily updated as new data and tools are developed, and to be accessible to researchers, decision-makers and practitioners.

AbstractOne important challenge facing the urbanization and global environmental change community is to understand the relation between urban form, energy use and carbon emissions. Missing from the current literature are scientific assessments that evaluate the impacts of different urban spatial units on energy fluxes; yet, this type of analysis is needed by urban planners, who recognize that local scale zoning affects energy consumption and local climate. Satellite-based estimation of urban energy fluxes at neighbourhood scale is still a challenge. Here we show the potential of the current satellite missions to retrieve urban energy budget fluxes, supported by meteorological observations and evaluated by direct flux measurements. We found an agreement within 5% between satellite and in-situ derived net all-wave radiation; and identified that wall facet fraction and urban materials type are the most important parameters for estimating heat storage of the urban canopy. The satellite approaches were found to underestimate measured turbulent heat fluxes, with sensible heat flux being most sensitive to surface temperature variation (−64.1, +69.3 W m−2 for ±2 K perturbation). They also underestimate anthropogenic heat fluxes. However, reasonable spatial patterns are obtained for the latter allowing hot-spots to be identified, therefore supporting both urban planning and urban climate modelling.

AbstractSite-specific meteorological forcing appropriate for applications such as urban outdoor thermal comfort simulations can be obtained using a newly coupled scheme that combines a simple slab convective boundary layer (CBL) model and urban land surface model (ULSM) (here two ULSMs are considered). The former simulates daytime CBL height, air temperature and humidity, and the latter estimates urban surface energy and water balance fluxes accounting for changes in land surface cover. The coupled models are tested at a suburban site and two rural sites, one irrigated and one unirrigated grass, in Sacramento, U.S.A. All the variables modelled compare well to measurements (e.g. coefficient of determination=0.97 and root mean square error=1.5°C for air temperature). The current version is applicable to daytime conditions and needs initial state conditions for the CBL model in the appropriate range to obtain the required performance. The coupled model allows routine observations from distant sites (e.g. rural, airport) to be used to predict air temperature and relative humidity in an urban area of interest. This simple model, which can be rapidly applied, could provide urban data for applications such as air quality forecasting and building energy modelling, in addition to outdoor thermal comfort.