Die Wirkung von Vegetation in randstädtischen Luftleitbahnen : Studien zur Kaltluft in der Stadt Aachen

• The effect of vegetation on suburban ventilation paths : studies on cold air drainage in the city of Aachen

Sachsen, Timo Gerhard; Schneider, Christoph (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2013, 2014)
Dissertation / PhD Thesis

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.

Institutions

• Division of Earth Sciences and Geography [530000]
• Physical Geography and Climatology Teaching and Research Area [551520]