Geoscience Reference
In-Depth Information
96], provide useful data to understand the climate at the macroclimatic scale, rather
than at the mesoclimatic or local scales. Furthermore, in most cases data are not
accompanied by metadata (data about data). This means that often researchers only
have access to long lists of figures and no information is provided as to exact
location (latitude and longitude in degrees and minutes are insufficient), site
characteristics and changes over time, instrumentation used, observation procedures
and time of measurement. Moreover, as Grimmond [GRI 06b] pointed out stations
from different institutions have different standards for instruments exposure, time of
observation, and quality control of data. Furthermore, lack of information in urban
stations about the instruments' location in relation to the street and building
orientation, height of the instruments above the ground, percentage of impervious
and green areas around the devices prevent firm conclusions [OKE 06b].
In some countries, the meteorological services have installed networks of urban
meteorological stations. An “urban” station has been installed in Portugal for each
100,000 inhabitant of urban areas (http//www.meteo.pt). However, progress in the
study of urban atmospheric processes has led to a greater awareness of the
importance of correctly selecting the location of meteorological instrumentation in
towns; often groups of scholars install their own networks in order to choose both
location and device characteristics suitable for their research and expected results.
Furthermore, improved instrumentation is now available to researchers due to the
advances in technology, and it is possible to purchase reliable and accurate sensors
and data loggers at affordable prices.
The “Initial guidance to obtain representative meteorological observation at
urban sites” [OKE 06b, p. 2] has contributed to provide instructions for monitoring
atmospheric parameters within the urban perimeter that are used in research at
different scales, in order to understand the complex processes that occur in the urban
atmosphere. As stated by T.R. Oke [OKE 06b] in urban areas “rigid rules have little
utility”, as “it is sometimes necessary to accept exposure over non-standard surfaces,
at non-standard heights [….] or to be closer than usual to buildings or waste heat
exhausts”.
If the objective of establishing a meteorological network is clear, then reliable
data can be obtained. Studies both at the mesoclimatic (settlement) and at the
microclimatic scales have been carried out in Lisbon where two different networks
were installed for the CLIMLIS project “Prescription of climatic principles for urban
planning in Lisbon”. Relying on Tim Oke's advice, consultant to this project, a
mesoclimatic network (Figure 5.4b) was installed in order to detect air temperature
variations due to the position of the measuring points in town, as independently as
possible of the microclimate environment.
The first permanent data loggers, placed in meteorological shelters (Figure 5.3d-
e), were installed in October 2004 following a first trial over shorter periods in
which 12 measuring points were used. Seven are still active, recording data every 15
minutes and constitute the CEG (Centre for Geographical Studies of the University
of Lisbon) “mesoscale measurement network”. The immediate urban influence has
been avoided by selecting measuring points with high sky-view factors (SVFs
