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Sunlit fractions on urban facets – Impact of spatial resolution and approach

Journal article
Authors Fredrik Lindberg
C. S. B. Grimmond
Alberto Martilli
Published in Urban Climate
Volume 12
Pages 65-84
ISSN 2212-0955
Publication year 2015
Published at Department of Earth Sciences
Pages 65-84
Language en
Keywords Urban geometry, Urban land surface model, LiDAR, Shadow fraction algorithm, London
Subject categories Physical Geography, Meteorology and Atmospheric Sciences


The extent of the surface area sunlit is critical for radiative energy exchanges and therefore for a wide range of applications that require urban land surface models (ULSM), ranging from human comfort to weather forecasting. Here a computationally demanding shadow casting algorithm is used to assess the capability of a simple single-layer urban canopy model, which assumes an infinitely long rotating canyon (ILC), to reproduce sunlit areas on roofs, walls and roads over central London. Results indicate that the sunlit road areas are well-represented but somewhat smaller using an ILC, while sunlit roofs areas are consistently larger, especially for dense urban areas. The largest deviations from real world sunlit areas are for roofs during mornings and evenings. Sunlit fractions on walls are overestimated using an ILC during mornings and evenings are found. The implications of these errors are dependent on the application targeted. For example, (independent of albedo) ULSMs used in numerical weather prediction applying ILC representation of the urban form will overestimate outgoing shortwave radiation from roofs due to the overestimation of sunlit fraction of the roofs. Complications of deriving height to width ratios from real world data are also discussed.

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