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 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.

Impact of Urban Canopy Parameters on a Megacity’s Modelled Thermal Environment

“…This problem is typical not only for TERRA_URB, but also for more comprehensive UCMs such as DCEP in COSMO [95] and various schemes in WRF [96,97]. On the other hand, several studies have already demonstrated the noticeable impacts of explicitly accounting for urban vegetation in UCMs [98,99]. Thus, our study highlights the importance of the accurate ISA definition taking into account urban vegetation, and calls for more attention to urban vegetation in further UCM developments.…”

Radiative Transfer Model 3.0 integrated into the PALM model system 6.0

“…Calculation of the plant canopy transpiration rate is based on the Jarvis-Stewart model in the form described in Daudet et al (1999) and Ngao et al (2017). Namely, the evaporation rate from the leaf surface E r is computed as…”

Recognizing the Role of Forests in Urban Climate Mitigation and Adaptation: State of the Art, Lessons Learned, and the Way Forward

“…Urban forest governance under climate change has been mostly conceptualised within broader developments in urban sustainability and climate governance. However, there have also been efforts for creating specific frameworks regarding the climate-city-forest nexus [4], [33], [45], tailored decision-support software [25], [40], scenario modelling and simulations [27], [35]. Thus, it is possible to see the emergence of a more systematic understanding of urban forests and their peculiarities under climate change.…”