Scale and Applications

Typically urban meteorological processes are considered to operate at three spatial scales; these relate to spatial units that are relevant for applications and decision making. The micro-scale, which in the vertical dimension relates to the urban canopy layer (from about the mean height of the roughness elements—buildings and trees to the ground), is what is experienced most directly by people. At this scale, there is significant spatial variability because of differences in radiation (e.g. shading/sunlit) and wind flow (blocked, channelled, open) that occur over very short distances. This spatial variability is not resolved in urban land surface parameterizations that are included in NWP. If they need to be resolved, some form of computational fluid dynamics (CFD) modelling is needed (e.g. large eddy simulation, LES).

The local or neighbourhood scale, where the vertical dimension extends from the blending height (minimum of about two times the roughness height) to a height dependent on the nature of the variability of the neighbourhood areas, is what urban land surface models parameterize in NWP models. The urban area is made of a series of such neighbourhoods which may have different characteristics related to urban form, vegetation cover etc. It is at this scale that understanding/modelling of meteorology is required for decision making related to response to flooding, assessing vulnerability to heat waves, response to hazardous dispersion, etc.

The meso- or city scale creates its own urban boundary layer, the height of which is a function of the different neighbourhoods in the city and the characteristics of the surrounding rural area. This city scale is normally an administrative unit for which decisions are made related to large scale meteorological and climatological processes; for example susceptibility to major storms or synoptic events (e.g., hurricanes, snow storms, regional heat waves) or climatological vulnerabilities (e.g., sea level rise).

Features of Urban Environment
from a Meteorological Perspective

The most well-known atmospheric feature of the urban environment is the urban heat island (UHI). The original definition of the UHI is related to the canopy layer (UCL)—the air temperature difference between an urban area and the surrounding rural area. However, when considering the UHI, a number of key issues need to be taken into account. The UHI is dynamic, and the size and location of the maximum varies with time of day and with season. This means that few people experience the maximum urban warming, rather an urban effect that is less than that. It is also important to keep



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