Principles of Urban Meteorology (re atmospheric CO2)

(by Andrew)

The generalization of fundamental meteorological principles over urban environments is more problematic than to other land surfaces. This is for three reasons. First, the heterogeneity of roughness elements within the complex 3-D geometry of the urban “canopy” creates spatial variability in turbulence patterns. Second, the multiple sources and sinks of momentum, heat, moisture, and emissions creates spatial variability in fluxes and concentrations. Third, the impact of human activities to continually re-shape and alter, in new and distinct ways, the urban environment itself limits the long-term validity of any findings. In short, there is considerable uncertainty concerning the dynamics of the unique microclimates found within previously understudied and increasingly complex urban areas.

Both across cities and between cities, significant spatial and temporal variability in CO2 concentrations and fluxes can be expected as a consequence of the distribution of anthropogenic sources (mobile and fixed), processes of urban vegetation (including irrigation, and patterns of atmospheric convection and advection. Previous studies have shown that there is a marked and distinct diurnal cycle in the concentration of CO₂ with a morning peak attributable to anthropogenic (largely traffic), biogenic (nocturnal respiration), and meteorological (atmospheric stability) factors. In contrast, a mid-afternoon minimum can be attributed to vegetative photosynthesis and strong convective turbulence; concentrations then begin to rise again during the evening “rush-hour” traffic-flow.

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The main roughness elements in urban environments are trees and buildings. Other than being large, trees and buildings share few characteristics of meteorological significance. Aerodynamically, buildings are true “bluff” bodies because of their impermeability, inflexibility, and sharp edges. When exposed to airflow they create strong positive and negative pressure differences over their surface, leading to flow separation and vortex shedding. Trees are also good generators of mechanical turbulence but buildings have to be judged as more effective roughness elements. The effects of smaller roughness elements, such as cars or paved surfaces, are minimum in comparison.

Three spatial scales are commonly utilized for studying urban environments:

-       the micro-scale (10¹–10²m) involves spatial differences in response to individual roughness elements (variability in building/canyon dimensions, trees) and proximity to localized emissions sources (e.g. roads, vegetation);

-       the local-scale (10²–10⁴ m) represents the integrated response of an array of roughness elements with spatial variability reflecting the unique characteristics of different neighborhoods/land-uses;

-       and the meso-scale (10⁴–10⁵ m) considers the city in its entirely, and differentiated from its surroundings, areas of forest, agriculture, etc.

The urban canopy layer (UCL) is defined as being from the ground to the mean height of the roughness elements, usually just below roof –level, where micro-scale effects of the site characteristics are dominated. The UCL is most clearly delineated in areas of high building density; it may be discontinuous or absent in less densely developed suburban areas.

The layer extending from the top of the UCL, to a height where urban surface influences are no longer perceptible, is defined the urban boundary layer (UBL). It includes the roughness sub-layer immediately affected by the individual roughness elements, the turbulent surface layer (local-scale), and the outer mixed layer (meso-scale).

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Of those studies employing atmospheric-based measurement methods to study CO2 concentrations in urban environments to date, virtually all, with a few exceptions, have focused on the micro-scale, considering processes and patterns within the UCL. Inadequate attention has as yet focused on how micro-scale results can be extrapolated to larger scales and on how to accurately study the local-scale using atmospheric-based measurement methods. 

In regards to the latter, current debates focus on determining the height (or “depth”) of the roughness sub-layer, in which the perturbations caused by individual roughness elements are “blended” together due to atmospheric turbulence. It is as this height that instruments are to be placed in order to study at the local-scale that is spatially representative of a distinct urban neighbourhood/land-use. To be sure, placing instruments are greater heights than this leads to increased risk of incurring errors due to advection from dissimilar upwind surfaces and storage changes below the measurement level due to vertical flux divergence.

It is known that the height of the roughness sub-layer is a function of both the length/height of roughness elements (z_H) and their horizontal spacing. More recent research suggests that the latter factor may in fact be the primary determinant. It has been estimated that, as a general “rule-of-thumb”, instruments must be mounted at a height at least twice the mean height of the roughness elements (approximately 20-90m) to ensure that they are above the influence of individual roughness elements and, therefore, that the measurements represent an integrated response at the local-scale.

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References

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