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| Titel |
Spatial and temporal variability of turbulent vertical fluxes in Helsinki, Finland |
| VerfasserIn |
L. Järvi, A. Nordbo, S. Haapanala, J. Moilanen, T. Vesala |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061984
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| Zusammenfassung |
| The eddy-covariance technique has been widely used above vegetated surfaces to measure the
turbulent exchange of momentum, heat and gases between the surface and the atmosphere.
Above an urban surface, however, observations are scarce and complex measurement
surroundings bring challenges to the measurements and the representativeness of the fluxes in
a city scale.
The fluxes of sensible (QH) and latent heat (QE), and CO2 (Fc) have been measured at
three sites in Helsinki, Finland. At the SMEAR III station the measurements have been
ongoing since December 2005 and the site is located next to a busy road about 4 km from
downtown Helsinki. Two of the sites, Erottaja Fire Station (EFS) and Hotel Torni (HT), are
located in downtown within a distance of 400 meters from each other. In EFS, the
measurements have been carried out in June 2010 – January 2011, while in HT, the
measurements have been ongoing since September 2010. The present dataset allows the
studying of the inter-site variability of the exchange processes. Simultaneous measurements
from all three sites cover four months in autumn/winter time. The high-latitude location
allows a detailed examination of the effect of seasonal variation to the exchange
processes.
QH tends to be higher in city centre than in SMEAR III and a difference of 50 W m-2 is
observed in winter. During the simultaneous measurements, stable atmospheric stratification
is observed half of the time at SMEAR III whereas the occurrence in the city centre is less
than 5%. This is a result of the urban heat island effect which is stronger in downtown than in
the outside region. On the other hand, higher QE is measured in SMEAR III than in
downtown particularly during spring and summer months when a difference of
100 W m-2 is observed. In downtown the low fraction of green areas limits the
evaporation. Despite the short distance there are also differences between the two
downtown sites. Both the median QH and QE are 7 W m-2 smaller in EFS than in
HT. This difference arises likely from variations in storage and anthropogenic heat
emissions. Fc reaches higher values in downtown than at SMEAR III, where the
anthropogenic emissions are weaker. In summer, downtown acts as a source for CO2
through the day while in SMEAR III downward median fluxes are observed. In
SMEAR III however, the fluxes are highly dependent on the wind direction and
downwind from the road Fc is as high as in downtown. Annually, the surroundings of
SMEAR III act as a net source for CO2 with 1760 g C m-2, with a biased error of
6.1 g C m-2caused by the gap filling method. Differences between the years are
small and are mainly related to changes in the prevailing wind direction. Seasonal
differences in Fc downwind from the road are mainly caused by reduced traffic
rates in summer, while in other directions road traffic is the most important factor
in winter but in summer the exchange is mainly determined by solar radiation. |
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