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Observation and modelling of urban dew Richards, Katrina

Abstract

Despite its relevance to many aspects of urban climate and to several practical questions, urban dew has largely been ignored. Here, simple observations, an outof- doors scale model, and numerical simulation are used to investigate patterns of dewfall and surface moisture (dew + guttation) in urban environments. Observations and modelling were undertaken in Vancouver, B.C., primarily during the summers of 1993 and 1996. Surveys at several scales (0.02-25 km) show that the main controls on dew are weather, location and site configuration (geometry and surface materials). Weather effects are discussed using an empirical factor, Φw. Maximum dew accumulation (up to ~0.2 mm per night) is seen on nights with moist air and high Φw, i.e., cloudless conditions with light winds. Favoured sites are those with high Ψsky and surfaces which cool rapidly after sunset, e.g., grass and well insulated roofs. A 1/8-scale model is designed, constructed, and run at an out-of-doors site to study dew patterns in an urban residential landscape which consists of house lots, a street and an open grassed park. The Internal Thermal Mass (ITM) approach is used to scale the thermal inertia of buildings. The model is validated using data from fullscale sites in Vancouver. Patterns in the model agree with those seen at the fullscale, i.e., dew distribution is governed by weather, site geometry and substrate conditions. Correlation is shown between Ψsky and surface moisture accumulation. The feasibility of using a numerical model to simulate urban dew is investigated using a modified version of a rural dew model. Results for simple isolated surfaces—a deciduous tree leaf and an asphalt shingle roof—show promise, especially for built surfaces. [Scientific formulae used in this abstract could not be reproduced.]

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