Abstract. This paper presents the Semi-empirical URban canopY parametrization (SURY) v1.0, which bridges the gap between bulk urban land-surface schemes and explicit-canyon schemes. Based on detailed observational studies, modelling experiments and available parameter inventories, it offers a robust translation of urban canopy parameters – containing the three-dimensional information – into bulk parameters. As a result, it brings canopy-dependent urban physics to existing bulk urban land-surface schemes of atmospheric models. At the same time, SURY preserves a low computational cost of bulk schemes for efficient numerical weather prediction and climate modelling at the convection-permitting scales. It offers versatility and consistency for employing both urban canopy parameters from bottom-up inventories and bulk parameters from top-down estimates. SURY is tested for Belgium at 2.8 km resolution with the COSMO-CLM model (v5.0_clm6) that is extended with the bulk urban land-surface scheme TERRA_URB (v2.0). The model reproduces very well the urban heat islands observed from in situ urban-climate observations, satellite imagery and tower observations, which is in contrast to the original COSMO-CLM model without an urban land-surface scheme. As an application of SURY, the sensitivity of atmospheric modelling with the COSMO-CLM model is addressed for the urban canopy parameter ranges from the local climate zones of http://WUDAPT.org. City-scale effects are found in modelling the land-surface temperatures, air temperatures and associated urban heat islands. Recommendations are formulated for more precise urban atmospheric modelling at the convection-permitting scales. It is concluded that urban canopy parametrizations including SURY, combined with the deployment of the WUDAPT urban database platform and advancements in atmospheric modelling systems, are essential.
Urban areas are usually warmer than their surrounding natural areas, an effect known as the urban heat island effect. As such, they are particularly vulnerable to global warming and associated increases in extreme temperatures. Yet ensemble climate‐model projections are generally performed on a scale that is too coarse to represent the evolution of temperatures in cities. Here, for the first time, we combine unprecedented long‐term (35 years) urban climate model integrations at the convection‐permitting scale (2.8 km resolution) with information from an ensemble of general circulation models to assess temperature‐based heat stress for Belgium, a densely populated midlatitude maritime region. We discover that the heat stress increase toward the mid‐21st century is twice as large in cities compared to their surrounding rural areas. The exacerbation is driven by the urban heat island itself, its concurrence with heat waves, and urban expansion. Cities experience a heat stress multiplication by a factor 1.4 and 15 depending on the scenario. Remarkably, the future heat stress surpasses everywhere the urban hot spots of today. Our results demonstrate the need to combine information from climate models, acting on different scales, for climate change risk assessment in heterogeneous regions. Moreover, these results highlight the necessity for adaptation to increasing heat stress, especially in urban areas.
a b s t r a c tIn order to improve the representation of the water balance in urban land-surface models, we present a new impervious waterstorage parametrization that assumes a distribution of water reservoirs. It has been implemented in TERRA-URB, a new urban parametrization for COSMO-CLM's standard land-surface module TERRA-ML. The water-storage capacity and the maximal wet surface fraction of the urban impervious land cover consisting of streets and buildings are estimated for Toulouse centre by matching the modelled and observed evapotranspiration (ET) rates. They amount to 1.31 ± 0.20 kg m À2 and 12 AE 4%, respectively. The model successfully reproduces the timespan and magnitude of increased ET for both urban observations campaigns CAPITOUL and BUBBLE. Our sensitivity study reveals that water-storage parametrization largely affects the performance of modelled ET rates. Hereby, the simulation employing the new water-storage parametrization is improved compared to arbitrary or existing water-storage parametrizations. The ET, surface sensible heat exchange and upwelling infra-red radiation are all affected until 12 day-time hours after rainfall on average. The modelled annual-mean ET during the CAP-ITOUL campaign from the urban land in Toulouse is an order of magnitude lower than that observed for the natural surroundings.
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