Modeling the Partitioning of Turbulent Fluxes at Urban Sites with Varying Vegetation Cover

Abstract

21 36 37 Key Words: Bowen ratio, irrigation, JULES, land surface model, spin-up strategy, urban vegetation 38 39 40National University urban canopy model (Ryu et al., 2011). Typically these models are designed to 47 represent the energy balance of the various facets that make up an idealized urban canopy. Often 48 this idealized urban canopy is treated as a symmetric street canyon geometry with varying degrees 49 of complexity, ranging from a bulk canyon (e.g., Best, 2005), separate roof, walls and road, with 50 single (e.g., Masson, 2000) or multiple (e.g., Krayenhoff and Voogt, 2007) energy balances, and 51 even intersections separate from street canyons (e.g., Kawai et al., 2009). Whilst this may be a good 52 representation of the central downtown areas of major cities, this design alone (i.e., without a 53 representation of vegetation) does not capture the influence of vegetation present in many street 54 canyons and abundant in the suburbs of many cities. 55 56The implications are that vegetation also needs to be modelled for urban areas. Indeed, the first 57 international urban model comparison experiment (PILPS-Urban) concluded that for the two urban 58 sites considered in the study (Vancouver and Melbourne), models that included a representation of 59 vegetation performed much better in simulating the sensible (QH) and latent heat (QE) densities than 60 models that neglected it (Grimmond et al., 2010, 2011, Best and Grimmond, 2013, 2015a. Urban also concluded that the way in which the vegetation was modelled, i.e., as a separate 62 independent surface (e.g., Dupont and Mestayer, 2006) or integrated within the urban street canyon 63 (e.g., Lee and Park, 2008) was not as important. However, the main focus of PILPS-Urban results 64 was a suburban site, so it is not clear how robust these conclusions are for other sites with varying 65 fractions of vegetation within the footprint of the observations. 66 67 Observational data have quantified directly QH and QE and hence how the Bowen Ratio β (i.e., β = 68 QH/QE) varies with the vegetation fraction across a range of values (Grimmond and Oke, 2002, 69 Loridan and Grimmond, 2012a). Here we investigate if an urban model that includes a 70 representation of vegetation can reproduce this observed behaviour. 71 72

A surface modelling approach for attribution and disentanglement of the effects of global warming from urbanization in temperature extremes: application to Lisbon

“…The improvement was physically grounded in the strong sensitivity of summer temperature variability to local urban effects and land-use properties, which were better represented in the high-resolution SURFEX-TEB simulations when compared with the coarseresolution E20C. This sensitivity was systematically investigated, and urban built-fraction (fraction of surface covered by buildings or roads), surface albedo and surface emissivity emerged as key parameters, in agreement with previous findings [7,15,21].…”

“…Urban canopy numerical models are a useful tool for studying and predicting temperature extremes over urban regions. This has been demonstrated by several previous studies focusing on different cities across the world, under different global warming scenarios, urban density growth scenarios, urban vegetation fractions, and human energy consumption scenarios [15][16][17][18][19]. However, accurate simulation of local urban effects requires very high-resolution (on order of a few kilometers or less) [20,21].…”

Quantifying Water and Energy Fluxes Over Different Urban Land Covers in Phoenix, Arizona

“…Intraurban studies have been conducted in European cities (Christen & Voogt, 2004;Offerle et al, 2006) to explore energy partitioning and the surface energy balance (SEB), with an emphasis on comparing across different urban land covers and to nearby rural areas. Nevertheless, few studies have observed the effects of different types of urban land covers on the SEB in arid and semiarid environments and the partitioning of turbulent fluxes in a comparative manner such that the effects of precipitation and outdoor water use can be discerned (e.g., Best & Grimmond, 2016;Coutts et al, 2007;Liang et al, 2017). grass) and xeric (drip irrigated trees with gravel cover) landscaping (e.g., Song & Wang, 2015;Volo et al, 2014;Yang & Wang, 2015).…”

Sensitivity of Surface Urban Energy and Water Balance Scheme (SUEWS) to downscaling of reanalysis forcing data

“…To date, reanalysis products have been little used in the modelling of urban water and energy exchanges. As the WATCH WFDEI reanalysis data (Weedon et al 2011(Weedon et al , 2014 are specifically derived for hydrological modelling from ERA-Interim, and have been used to model surface energy fluxes in 20 different urban areas (Best and Grimmond 2016), they are chosen for this study.…”