COSMO-BEP-Tree v1.0: a coupled urban climate model with explicit representation of street trees

Abstract

Abstract. Street trees are more and more regarded as an effective measure to reduce excessive heat in urban areas. However, the vast majority of mesoscale urban climate models do not represent street trees in an explicit manner and for example do not take the important effect of shading by trees into account. In addition, urban canopy models that take interactions of trees and urban fabrics directly into account are usually limited to the street or neighbourhood scale and, hence, cannot be used to analyse the citywide effect of urban greening. In order to represent the interactions between street trees, urban elements and the atmosphere in realistic regional weather and climate simulations, we coupled the vegetated urban canopy model BEP-Tree and the mesoscale weather and climate model COSMO. The performance and applicability of the coupled model, named COSMO-BEP-Tree, are demonstrated over the urban area of Basel, Switzerland, during the heatwave event of June–July 2015. Overall, the model compared well with measurements of individual components of the surface energy balance and with air and surface temperatures obtained from a flux tower, surface stations and satellites. Deficiencies were identified for night-time air temperature and humidity, which can mainly be traced back to limitations in the simulation of the night-time stable boundary layer in COSMO. The representation of street trees in the coupled model generally improved the agreement with observations. Street trees produced large changes in simulated sensible and latent heat flux, and wind speed. Within the canopy layer, the presence of street trees resulted in a slight reduction in daytime air temperature and a very minor increase in night-time air temperature. The model was found to realistically respond to changes in the parameters defining the street trees: leaf area density and stomatal conductance. Overall, COSMO-BEP-Tree demonstrated the potential of (a) enabling city-wide studies on the cooling potential of street trees and (b) further enhancing the modelling capabilities and performance in urban climate modelling studies.

COSMO-BEP-Tree v1.0: a coupled urban climate model with explicit representation of street trees

“…G. Mussetti et al: Coupling an urban canopy model with trees with a regional climate model Appendix A: UCPgenerator v1.0 UCPgenerator v1.0 is a Python programme that generates the additional input fields required by COSMO-BEP-Tree (Mussetti, 2019b). The list of the additional input field required by COSMO-BEP-Tree is provided in Table A1.…”

“…The code of COSMO-BEP-Tree is made freely available to any user with a valid CLM community membership. All research data and scripts to produce figures and analysis are archived on Zenodo (https://doi.org/10.5281/zenodo.3377392, Mussetti, 2019a). UCPgenerator v1.0 can be downloaded from the GitHub repository (https://doi.org/10.5281/zenodo.3352214, Mussetti, 2019b).…”

“…All research data and scripts to produce figures and analysis are archived on Zenodo (https://doi.org/10.5281/zenodo.3377392, Mussetti, 2019a). UCPgenerator v1.0 can be downloaded from the GitHub repository (https://doi.org/10.5281/zenodo.3352214, Mussetti, 2019b). The LUCY model, used to calculate the anthropogenic heat flux, can be downloaded from https://doi.org/10.…”

The role of urban trees in reducing land surface temperatures in European cities

“…However, while our analysis based on remote-sensing data shows clear spatial patterns in the LST reduction provided by different vegetation types, it is essential to note that the benefits of trees and green spaces are manifold. For example, pedestrian thermal comfort can substantially vary if the effects of trees on shading, wind speed and humidity are taken into account 12,20,29 . Our results show that the average LST differences between vegetated and urban land can diverge from LST differences during hot extremes.…”

“…The potential of trees to reduce temperatures via transpiration is influenced by local- and micro-scale climatic conditions and may differ for trees in a city and trees or forests in rural areas 19 , 20 . The environmental conditions in an urban context could either increase or decrease the temperature reduction caused by trees 8 .…”