Abstract
Abstract. The Town Energy Balance (TEB) model has been refined and improved in order to explicitly represent street trees and their impacts on radiative transfer: a new vegetated stratum on the vertical plane, which can shade the road, the walls, and the low vegetation has been added. This modification led to more complex radiative calculations, but has been done with a concern to preserve a certain level of simplicity and to limit the number of new input parameters for TEB to the cover fraction of trees, the mean height of trunks and trees, their specific leaf area index, and albedo. Indeed, the model is designed to be run over whole cities, for which it can simulate the local climatic variability related to urban landscape heterogeneity at the neighborhood scale. This means that computing times must be acceptable, and that input urban data must be available or easy to define. This simplified characterization of high vegetation necessarily induces some uncertainties in terms of the solar radiative exchanges, as quantified by comparison of TEB with a high-spatial-resolution solar enlightenment model (SOLENE). On the basis of an idealized geometry of an urban canyon with various vegetation layouts, TEB is evaluated regarding the total shortwave radiation flux absorbed by the elements that compose the canyon. TEB simulations in summer gathered best scores for all configurations and surfaces considered, which is precisely the most relevant season to assess the cooling effect of deciduous trees under temperate climate. Mean absolute differences and biases of 6.03 and +3.50 W m−2 for road, respectively, and of 3.38 and +2.80 W m−2 for walls have been recorded in vegetationless canyons. In view of the important incident radiation flux, exceeding 1000 W m−2 at solar noon, the mean absolute percentage differences of 3 % for both surfaces remain moderate. Concerning the vegetated canyons, we noted a high variability of statistical scores depending on the vegetation layout. The greater uncertainties are found for the solar radiation fluxes received and absorbed by the high vegetation. The mean absolute differences averaged over the vegetation configurations during summertime are 21.12 ± 13.39 W m−2 or 20.92 ± 10.87 % of mean absolute percentage differences for the total shortwave absorption, but these scores are associated with acceptable biases: −15.96 ± 15.93 W m−2.
Highlights
For counteracting the adverse environmental effects that can result from continuous process of urban expansion, numerous projects of local urban planning or design support and favor the preservation and reintroduction of vegetation in the city
To take into account the tree canopy in Town Energy Balance (TEB), it is required to add a new vegetated stratum on the vertical plane, which can shade the road, the walls, and the low vegetation. This modification led to more complex radiative calculations, but is done with a concern to preserve a certain level of simplicity and to limit the number of new input parameters for TEB
The evaluation is focused on effects of deciduous trees which are typical and widely present in cities under temperate climate. Such trees are leafless during winter, so that they have a negligible impact on thermal comfort and energy demand in this season
Summary
For counteracting the adverse environmental effects that can result from continuous process of urban expansion, numerous projects of local urban planning or design support and favor the preservation and reintroduction of vegetation in the city. From an environmental point of view, the natural soils and vegetation play a important role and bring significant benefits in different sectors (Nowak and Dwyer, 2007; Mullaney et al, 2015). They act at a microclimate level through the processes of soil water evaporation and of transpiration of plants (Qiu et al, 2013). These processes cool the ambient air (Zhang et al, 2013) and mitigate the effect of the urban heat island (Feyisa et al, 2014; Önder and Akay, 2014; Alavipanah et al, 2015). Redon et al.: Implementation of street trees within the solar radiative exchange parameterization
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