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Cold-air ventilation and the nocturnal boundary layer structure above an urban ballast facet

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The ballast facet of a goods station with its railway track leading towards the rural surroundings of the city of Osnabrück, Germany, was studied in terms of its thermal behaviour and its significance as an urban ventilation path for nocturnal cold-air transport from the rural surroundings. The investigations are based on sulfurhexafluoride (SF) tracer experiments, tethersonde soundings and energy-balance measurements. Although the ballast facet absorbs much of the incoming solar radiation during the day it is able to cool significantly throughout the night. The dispersion of nocturnal cold-air about the urban centre of Osnabrück is shown to be spatially and temporally variable and affected by the vertical structure of the nocturnal boundary layer. Even in moderate topography the near-surface flow is able to partly decouple from the flow aloft resulting in a two-layered structure of the nocturnal boundary layer above the goods station area. The bottom part can be attributed to the cold-air flow from the eastern surroundings with a vertical extension of about 20–30 m while the upper part is influenced by the larger scale orography.

Die Schotterfläche mit der ins ¨ostliche Umland führenden Gleisanlage des Güuterbahnhofes der Stadt Osnabrück wurde hinsichtlich ihres thermischen Verhaltens sowie der Bedeutung als urbane Luftleitbahn füur nächtlichen Kaltlufttransport aus dem Umland untersucht. Als Untersuchungsmethoden kamen Schwefelhexafluorid (SF6)-Tracerexperimente, Vertikalsondierungen mit einem Fesselballon sowie Energiebilanzmessungen zum Einsatz. Trotz der starken Absorption der Einstrahlung im Tagesverlauf ist die Schotterfläche in der Lage, während der Nacht stark auszukühlen. Die Ausbreitung nächtlicher Kaltluft in das Stadtgebiet Osnabrücks zeigt sich als räumlich und zeitlich variabel und von der nächtlichen Grenzschichtstruktur beeinflusst. Die trotz moderater Topographie zeitweilig abgekoppelte bodennahe Strömung führt zu einer zweigeschichteten Struktur der nächtlichen Grenzschicht u¨ber dem Güterbahnhofbereich in Osnabrück. Der untere Teil wird von der Kaltluftströmung aus dem uöstlichen Umland mit einer vertikalen Mächtigkeit von 20–30 m charakterisiert, während der obere Teil von der großräumigeren Orographie beeinflusst wird.
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Document Type: Research Article

Publication date: October 1, 2004

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  • Meteorologische Zeitschrift (originally founded in 1866) is the joint periodical of the meteorological societies of Austria, Germany and Switzerland. It accepts high-quality peer-reviewed manuscripts on all aspects of observational, theoretical and computational research out of the entire field of meteorology, including climatology. Meteorologische Zeitschrift represents a natural forum for the meteorological community of Central Europe and worldwide.
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