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| Titel |
The energy budget of the urban surface: Two locations in Dublin |
| VerfasserIn |
Stephanie Keogh, Gerald Mills, Rowan Fealy |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055725
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| Zusammenfassung |
| The energy budget is a statement of the principle of the conservation of energy. Applied to a
surface it describes the magnitude and types of surface-air energy exchanges that give rise to
distinctive near-surface climates. Traditionally observation programmes to measure its terms
were conducted over ideal surfaces that could be described as flat and extensively
homogenous. Hence, a measurement system located above the vegetative canopy at sufficient
distance from the edge of the surface could be assured of a suitable fetch. Thus the
energy fluxes acquired were substantially invariant with wind direction and could be
clearly linked to the underlying surface. Conducting energy budget observations
in an urban setting is more complex owing to the nature of the urban canopy, the
heterogeneous nature of the urban surface and the presence of additional terms.
Consequently, there are relatively few observations of urban energy budgets in
different urban environments. However, this information is needed to improve our
understanding of urban climate processes and to aid in the development of urban climate
models.
A statement of the urban energy budget is as follows:
Q* + QF = QH + QE + ΔQS + ΔQA
Which states that net radiation (Q*) plus anthropogenic heat (QF) is partitioned into the
vertical exchange of sensible and latent heat (QH+QE) via turbulence, the storage of heat in
the system under study (ΔQS) and the horizontal transport of energy through the system
(ΔQA) by advection. Each term is expressed as a flux density, the flow of energy (Watts, W)
per unit surface area (m2). Observing each of these terms is difficult in an urban setting and
experimental designs have employed both measurements and estimates to partition
energy exchanges. To acquire information on distinctive urban surface types requires
careful placement of the instruments both vertically and horizontally. The vertical
placement of instruments should be within the constant flux layer located above
the urban ‘surface’. The available evidence suggests that this layer is located a
considerable distance above the urban roughness elements (H) at a height -¥1.5H. The
horizontal placement within the settlement should be such as to ensure a suitable fetch
over a ‘homogenous’ urban surface (so that ΔQA can be ignored). This requires
careful site selection to ensure that instruments are located within a neighbourhood
that has consistent properties (street widths, building heights, vegetative cover,
etc.).
This paper presents the results of an energy budget measurement programme within
Dublin. Two sites were selected to represent two urban subtypes:
A sub-urban location consisting of extensive semi-detached, two storey houses
with private gardens and large, mature trees.
A densely built city centre site with little vegetation.
At both sites instrumented masts have been observing energy exchanges for approximately one
year. These exchanges include net radiation (and the individual short- and long-wave terms)
and turbulent sensible and latent heat fluxes. The nature of the urban surface cover at each site
has been captured and is used to interpret the results. The paper will present the results from
each site and compare these with comparable measurement programmes in other cities. |
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