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

The ballast facet of a goods station with its railway track leading towards the rural surroundings of the city of Osnabruck, 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 (SF6) 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 Osnabruck 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.