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@PHDTHESIS{Sachsen:229472,
author = {Sachsen, Timo Gerhard},
othercontributors = {Schneider, Christoph},
title = {{D}ie {W}irkung von {V}egetation in randstädtischen
{L}uftleitbahnen : {S}tudien zur {K}altluft in der {S}tadt
{A}achen},
address = {Aachen},
publisher = {Publikationsserver der RWTH Aachen University},
reportid = {RWTH-CONV-144442},
pages = {XVI, 229 S. : Ill., graph. Darst., Kt.},
year = {2013},
note = {Zsfassung in dt. und engl. Sprache. - Prüfungsjahr: 2013.
- Publikationsjahr: 2014; Aachen, Techn. Hochsch., Diss.,
2013},
abstract = {The aspects of nocturnal cold air drainage flow were
investigated in two suburban valleys of the city of Aachen,
Germany, during a two year measurement period – from
February 2009 to February 2011. The investigation intended
to answer the following questions:1. What are the
characteristics of cold air drainage flow in the suburban
valleys of the city of Aachen?2. How does vegetation –
especially trees and hedges – affect the cold air drainage
flow?3. Can these effects be reproduced by a model,
calibrated with the collected data?In 2009 four weather
stations were set up, each of them representing different
locations within the valleys. One of them collected data of
cold air drainage without being influenced by vegetation.
The data of this station was used as a reference for a
comparative analysis. The other three stations were placed
within vegetation stands, one at the lee side of a hedge and
two at the lee side of a grove of alders with a height of up
to 15 m. Halfway through the measurement period a section of
the hedge and several alders were cut down, in order to get
two data sets for comparison. This data was complemented by
mobile cross valley section and tethered balloon
measurements, as well as by air quality measurements and
measurements taken from busses carrying GPS equipment and
temperature sensors. Data evaluation took account of annual
and seasonal variations in regard to changing vegetation
structures. In order to improve the input detail for
modeling and model calibration further, the vegetation in
two larger areas within the valleys was mapped. Modeling was
done by the numerical cold air drainage flow model $KLAM_21$
from the DWD (Deutscher Wetterdienst/German Weather
Service). The results show that vegetation has a
considerable effect on cold air drainage, differing for each
location and climatic element measured: The wind speed of
cold air is reduced by vegetation structures and influenced
by the density and diameter of the vegetation and the
vegetated surface. Therefore wind can vary over short
distances. A large expanse of vegetation has a large effect
on wind speed. Increased surface roughness, e.g. caused by
hedges or trees, reduces wind speed. Wind speed can be
directly impacted by changing surface roughness, for
instance by reducing tree coverage. Additionally wind speed
shows annual and seasonal variations. Wind direction as well
as wind speed is influenced by the vegetation within the
valley. Thus different wind directions were measured at
different heights but in the same location. Large expanses
of dense vegetation can block and deviate cold air, which
can even result in an overflow into surrounding valleys.
Vegetation shades the valley surface, influencing surface
and soil temperatures and consequently air temperature.
Therefore cold air production rates change. This was proved
by calculation, mobile measurements and modeling. Air
quality has to be considered carefully, taking account of
the origin of cold air. In case of clean air, e.g.
originated from non-industrialized areas with less traffic,
air quality can be improved by cold air drainage. In case of
noxious substances polluting the cold air, air quality can
be decreased. The area of investigation shows increased
amounts of (presumably biogenic) particulate matter within
the drainage flow, derived from the valley vegetation. A
good match between modeled and measured data was achieved by
calibrating the cold air drain-age flow model with
high-resolution land use data. This allows the results to be
transferred to sur-rounding valleys. Modeled and measured
results are presented in a diagram showing cold air
drain-age behavior related to obstacles. The investigated
area shows, although a severe data filtering process has
been carried out, a high frequency of nocturnal cold air
drainage flow with cold air at high altitudes. The presence
of cold air within urban areas was proved, by setting up,
amongst others, a measurement system carried by public
transport busses. At a small scale it was proved, that even
vegetation with the highest density is permeable to cold air
drainage. Vegetation is not able to cause effects like
buildings. In order to increase the amount of cold air
drainage considerably, major changes in vegetation
structures have to be carried out. A critical review on
height and density within suburban valleys can be helpful to
city planners, especially in light of rising urban
temperatures due to climate change. Nevertheless, suburban
valleys cannot be characterized solely by climatological
factors, as this single-disciplinary view on their functions
would be inadequate. An interdisciplinary assessment in due
consideration of all functions, like biodiversity, local
recreation, is advisable.},
keywords = {Kaltluft (SWD) / Stadtklima (SWD) / Vegetation (SWD) /
Modellierung (SWD) / Luftströmung (SWD) / Katabatischer
Wind (SWD)},
cin = {530000 / 551520},
ddc = {550},
cid = {$I:(DE-82)530000_20140620$ / $I:(DE-82)551520_20140620$},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-opus-48841},
url = {https://publications.rwth-aachen.de/record/229472},
}
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