Simulating the surface energy balance over two contrasting urban environments using the Community Land Model Urban

Abstract

A single-layer urban canopy model Community Land Model Urban (CLMU) is evaluated over two contrasting urban environments of Toulouse (France) and Melbourne (Australia). For the latter, three measurement sites are available characterized by a varying amount of vegetation, which supports a detailed assessment of the representation of urban vegetation in CLMU. For Toulouse, observed roof, wall and road surface temperatures allow for a detailed evaluation of the anthropogenic heat parameterization. Overall, CLMU performs well in simulating the canyon and urban surface temperatures, anthropogenic heat flux and urban energy balance, with an overall better performance for the dense old city centre of Toulouse in comparison to the more vegetated sites in Melbourne. Results for the latter sites reveal that the pervious road fraction provides a reasonable approximation of vegetation in the urban canyon while the tile approach often results in an underestimation of latent heat fluxes. A detailed analysis of the radiative, turbulent and anthropogenic heat fluxes as well as surface temperatures for Toulouse point to a complex interaction between urban surfaces and canyon properties. Decoupling the roof from the urban canyon to the atmosphere aloft is shown to be important. Our findings suggest that more evaluation is necessary for contrasting urban geometries in order to obtain a better understanding of the interaction between the roof surface on the one hand and canyon air and air aloft on the other hand. The results simultaneously reveal a trade-off in errors between surface temperatures, radiative and turbulent fluxes and anthropogenic heat which again stresses the importance of the intended model application. Also, our results suggest that model complexity should, perhaps, relate to the site complexity. These results provide a robust basis for the construction of additional sensitivity experiments, tailored towards the intended application for urban climate mitigation studies.

The impact of impervious water-storage parametrization on urban climate modelling

“…3 and 4). This agrees with the observations for which only small evaporation rates (ie., lower than 25 Wm À2 ) are found most of the nighttime in Toulouse centre Demuzere et al, 2013, see their Figure 4). In contrast, the evaporation is much larger at daytime in both model results and observations.…”

“…As a result, the representation of the urban water balance in existing urban land-surface parametrizations is very rudimentary: For instance, various urban models do not consider evaporation from the impervious surfaces as a first approximation to the reduction of ET in cities (Hénon et al, 2012;Martilli et al, 2002;Schubert et al, 2012). Up to now, the accuracy, validity and applicability of water-storage parametrizations employed in existing land-surface models (Masson, 2000;Kanda et al, 2005;Oleson et al, 2008;Demuzere et al, 2013Demuzere et al, , 2014a have not been investigated in detail. The consequential deficiencies in urban water-storage parametrizations lead to very poor model performances in terms of ET: Grimmond et al (2011) showed that the evapotranspiration is the least well-modelled flux of the SEB.…”

“…The DRY run assumes no evaporation from the impervious land cover, which means that ET only originates from the natural fraction. The PTEB run refers to a waterstorage 'P'arametrization based on that of the 'T'own 'E'nergy 'B'udget (TEB) from Masson (2000), which is similar to that used in CLM-U from Oleson et al (2008), Demuzere et al (2013Demuzere et al ( , 2014a. The configuration of PTEB is explained in Appendix B, and results of PTEB will be discussed later on in Section 4.5.…”

“…Pearlmutter et al, 2009;Nakayoshi et al, 2009) are limited, especially during rainfall periods. Therefore, the investigation and evaluation of modelling the water balance for cities is limited as well (eg., Grimmond and Oke, 1991;Kanda et al, 2005;Nakayoshi et al, 2009;Demuzere et al, 2013). As a result, the representation of the urban water balance in existing urban land-surface parametrizations is very rudimentary: For instance, various urban models do not consider evaporation from the impervious surfaces as a first approximation to the reduction of ET in cities (Hénon et al, 2012;Martilli et al, 2002;Schubert et al, 2012).…”

Simulating the impact of urban development pathways on the local climate: A scenario-based analysis in the greater Dublin region, Ireland

“…In a study by Demuzere, Oleson, Coutts, Pigeon, and van Lipzig (2013) which simulated the impact of different urban environments on the surface energy balance, a similar energetic-hierarchy was found between compact and openset residential thermal climate zones, which are an earlier iteration of the LCZ 2 and LCZ 6 classes respectively. The reduction in vegetation fraction leads to a minor decrease in evapotranspiration though the larger impact is found in terms of heat storage as the heat capacity of these areas significantly increases due to the additional buildings and pavements.…”