With a large and growing share of the world population living in cities (United Nations, 2018), the impact weather-related risks magnified by climate change, such as heatwaves and flooding (Wilby, 2007), also increases. In cities, air temperatures are typically higher than in the rural surroundings due to the Urban Heat Island effect (UHI; Oke, 1982;Oke et al., 2017;Santamouris, 2014). The UHI originates from the difference between the rural and urban energy balances due to lower albedo, radiation trapping, less vegetation, higher heat storage capacity and anthropogenic heat release (Oke, 1982). Because of its positive effect on evaporative cooling that is complemented by shading, urban vegetation is often given a central role in attempts to improve thermal comfort (Ennos, 2010). Indeed, higher vegetation fractions are associated with lower urban air and canopy temperatures (e.g., Gallo et al., 1993;Theeuwes et al., 2017;Weng et al., 2004), although in specific situations vegetation can cause higher temperatures (Meili et al., 2021). Wei and Shu (2020) showed that expanding the vegetation fraction
<p>Urban Land Surface Models (ULSM) are developed to simulate the urban climate and vary in their complexity. The need for this complexity was assessed by two successive systematic intercomparison projects. Both projects focused on the energy balance and found the latent heat flux to be the most challenging flux to model. However, these projects did not address the closure of the water balance, although the energy balance is directly linked to the water balance. This study aims to assess the representation and dynamics of the water balance in 14 ULSMs from the Urban-PLUMBER project each ran for 20 sites. The water balance could not be evaluated by straightforwardly comparing the model results against measurements since most water balance fluxes are not measured. Therefore, the water storage dynamics were derived from the modelled water balance fluxes. We examined the inter-model variation in both the storage dynamics and the separate fluxes and developed seven indicators of a well-captured water balance. The variation in both the fluxes and the storage dynamics is in the same order of magnitude as the size of the fluxes themselves. The indicators show that no ULSM in this study can consistently reproduce a physically realistic water balance regardless of the model&#8217;s complexity. As the water balance is linked to the energy balance, the poor water balance representation may explain the poor performance for the latent heat flux. The linked balances illustrate model evaluations and comparisons should extend beyond the target variables of the model to all processes that directly influence these variables.</p>
<p>The amount and dynamics of urban water storage play an important role in mitigating urban flooding and heat. Assessment of the capacity of cities to store water remains challenging due to the extreme heterogeneity of the urban surface. Evapotranspiration (ET) recession after rainfall events during the period without precipitation, over which the amount of stored water gradually decreases, can provide insight on the water storage capacity of urban surfaces. Assuming ET is the only outgoing flux, the water storage capacity can be estimated based on the timescale and intercept of its recession. In this paper, we test the proposed approach to estimate the water storage capacity at neighborhood scale with latent heat flux data collected by eddy covariance flux towers in eleven contrasting urban sites with different local climate zones, vegetation cover and characteristics and background climates (Amsterdam, Arnhem, Basel, Berlin, Helsinki, &#321;&#243;d&#378;, Melbourne, Mexico City, Seoul, Singapore, Vancouver). Water storage capacities ranging between 1 and 12 mm were found. These values correspond to e-folding timescales lasting from 2 to 10 days, which translate to half-lives of 1.5 to 7 days. We find ET at the start of a drydown to be positively related to vegetation fraction, and long timescales and large storage capacities to be associated with higher vegetation fractions. According to our results, urban water storage capacity is at least one order of magnitude smaller than the known water storage capacity in natural forests and grassland.</p>
<p>The development of urban areas impacts the local climate and hydrology. Cities have been modelled with an array of models with different complexities. These models are called urban land surface models (ULSM) and focus on radiation, and turbulent sensible and latent heat fluxes. Grimmond et al. (2010) evaluated these models finding that the latent heat flux is the most challenging to simulate. This flux is part of both the energy balance and water balance, as the latent heat flux is the energy equivalent of the mass evapotranspiration. Thus, the hydrological circumstances may be crucial to correctly model the turbulent heat fluxes. However, the representation of the water balance in these models has not been the focus of a multi-model evaluation. As a part of the follow-up project to the work by Grimmond et al. and Urban-PLUMBER we evaluated the representation of the water balance in ULSMs with varying complexity and representation of the water balance. It is difficult to evaluate the water balance fluxes against observations, as not all terms are observed. For example, changes in water storage require knowledge of the state of all the individual stores (e.g. soil moisture, detention ponds). Analysis of 14 models shows a large spread in the magnitude of the individual water balance fluxes. The rate of reduction of the latent heat flux/evapotranspiration during periods without rainfall varies widely between models, consistent with literature (e.g. Jongen et al., 2022). Initial analysis suggests that models that simulate the water balance and conserve mass are more likely to accurately simulate turbulent heat fluxes. It is thus crucial that both the water and energy balance are accounted for in future urban model improvements.</p>
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