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

Gianluca Mussetti,Stephan Henne,Roland Vogt,Dominik Brunner,Jan Carmeliet,Jonas Allegrini,E Scott Krayenhoff,Andreas Wicki,Sebastian Schubert,Christian Feigenwinter

doi:10.5194/gmd-13-1685-2020

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 Building Effect Parameterisation with Trees (BEP-Tree) vegetated urban canopy model and the Consortium for Small-scale Modeling (COSMO) mesoscale weather and climate model. 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 nighttime air temperature and humidity, which can mainly be traced back to limitations in the simulation of the nighttime 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 nighttime 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.

Highlights

Street trees – trees located in street canyons – and urban vegetation in general are more and more regarded as an effective measure to reduce excessive heat conditions in urban areas (e.g. Shashua-Bar et al, 2009; Armson et al, 2012; Coutts et al, 2016; Tan et al, 2016; Gunawardena et al, 2017; Manickathan et al, 2018)
The results corresponding to the reference simulation (STD) and to the simulation without street trees (LA0) are shown
An urban climate model with explicit representation of street trees was developed featuring a two-way coupling between the vegetated urban canopy model BEP-Tree (Krayenhoff et al, 2020) and the mesoscale weather and climate model Consortium for Small-scale Modeling (COSMO)-Community Land Model (CLM) (COSMO, 2019)

Summary

Introduction

Street trees – trees located in street canyons – and urban vegetation in general are more and more regarded as an effective measure to reduce excessive heat conditions in urban areas (e.g. Shashua-Bar et al, 2009; Armson et al, 2012; Coutts et al, 2016; Tan et al, 2016; Gunawardena et al, 2017; Manickathan et al, 2018). Excessive heat conditions are typically caused by the combination of regional-scale hot weather (heatwave) situations and the urban heat island (UHI) effect. G. Mussetti et al.: Coupling an urban canopy model with trees with a regional climate model is caused by the alteration of the surface energy balance due to the presence of man-made structures and activities (Oke et al, 2017). Climate change is expected to further accentuate the magnitude and frequency of excessive heat conditions in cities with potentially severe impacts on human health, energy consumption, air pollution and urban ecology (Rosenzweig et al, 2018)

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