53
• In urban districts (such as LCZ 4 and LCZ 5) with scattered tall buildings, the
recommendations are as in previous point, but with special attention toward
avoiding the wake zone of the tall structures.
According to Harman I.N., 2003, setting up an urban climate station in a street
canyon requires consideration of different airflows, depending on canyon sizes.
Horizontal air currents above the roof level create eddies, caused by flow blockage
from buildings. If the street canyon is wide enough, the current system within the
street canyon consists of two parts: one part that has closed circulation, and another
part which is blown through. The ratio of these two, easily discernible parts depends
on the buildings and the width of the street.
If the width of the street is at least three times the height of the buildings (Fig.
2.9a), then the eddying air currents have no effect whatsoever on the neighboring
building, and the street canyon has free airflow.
If the width of the street is gradually decreased within the model, then the closed
eddy will dominate the entire width of the street, resulting in the characteristic feature that the direction of the prevailing wind blowing at street level is the opposite
of the direction of the currents at roof level (Fig. 2.9b).
If the width of the street is narrowed further, so that the ratio of the height of the
buildings and the width of the street exceeds 2/3 (Fig. 2.9c), then the entire street
canyon will be characterized by a closed eddy that prevents airing through, causing
hot air to become trapped and allowing air pollutants produced by motor vehicles to
accumulate.
2.4.4 Precipitation
In urban areas, instruments and methods for the measurement of precipitation are
the same as an open site. The measurement of precipitation (such as rain or snow)
is very susceptible to changes in airflow in the vicinity of the measurement.
In urban environments, measurement errors are associated with the following
main causes:
• the interception of precipitation during its trajectory to the ground by nearby collecting surfaces, such as tress and buildings,
• hard surfaces near the gauge which may cause splash-in into the gauge, and overhanging objects which may drip precipitation into the gauge,
• the spatial complexity of the wind field around obstacles in the LCZ, causing
significant localized concentration or absence of rain- or snow-bearing airflow,
• the gustiness of the wind in combination with the physical presence of the gauge
itself, causing anomalous turbulence around it and leading to under- or
over-catch.
In open country, standard exposure requires that obstacles should be no closer
than two times their height. In some ways, this guideline is less restrictive than for
2 Urban Heat Island Gold Standard and Urban Heat Island Atlas
• In urban districts (such as LCZ 4 and LCZ 5) with scattered tall buildings, the
recommendations are as in previous point, but with special attention toward
avoiding the wake zone of the tall structures.
According to Harman I.N., 2003, setting up an urban climate station in a street
canyon requires consideration of different airflows, depending on canyon sizes.
Horizontal air currents above the roof level create eddies, caused by flow blockage
from buildings. If the street canyon is wide enough, the current system within the
street canyon consists of two parts: one part that has closed circulation, and another
part which is blown through. The ratio of these two, easily discernible parts depends
on the buildings and the width of the street.
If the width of the street is at least three times the height of the buildings (Fig.
2.9a), then the eddying air currents have no effect whatsoever on the neighboring
building, and the street canyon has free airflow.
If the width of the street is gradually decreased within the model, then the closed
eddy will dominate the entire width of the street, resulting in the characteristic feature that the direction of the prevailing wind blowing at street level is the opposite
of the direction of the currents at roof level (Fig. 2.9b).
If the width of the street is narrowed further, so that the ratio of the height of the
buildings and the width of the street exceeds 2/3 (Fig. 2.9c), then the entire street
canyon will be characterized by a closed eddy that prevents airing through, causing
hot air to become trapped and allowing air pollutants produced by motor vehicles to
accumulate.
2.4.4 Precipitation
In urban areas, instruments and methods for the measurement of precipitation are
the same as an open site. The measurement of precipitation (such as rain or snow)
is very susceptible to changes in airflow in the vicinity of the measurement.
In urban environments, measurement errors are associated with the following
main causes:
• the interception of precipitation during its trajectory to the ground by nearby collecting surfaces, such as tress and buildings,
• hard surfaces near the gauge which may cause splash-in into the gauge, and overhanging objects which may drip precipitation into the gauge,
• the spatial complexity of the wind field around obstacles in the LCZ, causing
significant localized concentration or absence of rain- or snow-bearing airflow,
• the gustiness of the wind in combination with the physical presence of the gauge
itself, causing anomalous turbulence around it and leading to under- or
over-catch.
In open country, standard exposure requires that obstacles should be no closer
than two times their height. In some ways, this guideline is less restrictive than for
2 Urban Heat Island Gold Standard and Urban Heat Island Atlas