caa a22 4500 467938105 CHVBK 20180406153003.0 cr unu---uuuuu 170328e20060101xx s 000 0 eng 10.1007/s00703-004-0107-6 doi (NATIONALLICENCE)springer-10.1007/s00703-004-0107-6 Local foehn effects in the upper Isar Valley, Part 2: numerical simulations [Elektronische Daten] [M. Hornsteiner, G. Zängl] Summary: The local wind system in the upper Isar Valley (Bavarian Alps) near Mittenwald has the peculiarity that regularly strong foehn-like nocturnal flows occur, mainly during clear nights in autumn and winter. We will refer to this phenomenon as "Minifoehn”, as its properties are similar to the classical deep foehn in the sense that its breakthrough into the Isar Valley usually brings a striking increase in temperature and a concomitant decrease in relative humidity. Numerical simulations with the MM5 model reveal that this phenomenon is related to a nocturnal drainage flow originating from a plateau south of Mittenwald. This flow is driven by the temperature difference between this plateau (1180 m) and the free atmosphere above Mittenwald (920 m, 15 km north of the plateau) at the same level. The air masses flow through two different valleys that merge again further downstream. The upper part of one of the two drainage currents goes over a small mountain ridge (1180 m) south-west of Mittenwald and then descends into the Isar Valley, leading to an advection of potentially warm air towards Mittenwald. This branch of the drainage current constitutes the Minifoehn. The remaining part of the drainage current flows through a narrow gap towards the Isar Valley and then joins the drainage flow of this valley. As these air masses are significantly cooler than the Minifoehn branch, large horizontal temperature gradients can be found around Mittenwald. The dynamical behaviour of the cold air flow turns out to be qualitatively consistent with shallow-water theory only in the absence of a forcing by large-scale winds. Otherwise, gravity-wave induced pressure perturbations interact with the drainage flow and modify the low-level flow field. The simulations show that the gravity waves are excited by the mountain range that separates the two valleys mentioned above. Moreover, the simulations indicate that the structure of this nocturnal wind system is not very sensitive to the direction of synoptic-scale winds as long as they come from the southern sector. On the other hand, ambient northerly winds are able to prevent the drainage flow and therefore the local foehn effects in the Isar Valley provided that synoptic winds are strong enough. The results of the MM5 simulations are in good agreement with the measurements and observations described in part 1 of this study. Springer-Verlag/Wien, 2005 Hornsteiner M. Meteorological Institute, University of Munich, Germany aut Zängl G. Meteorological Institute, University of Munich, Germany aut Meteorology and Atmospheric Physics Springer-Verlag 91/1-4(2006-01-01), 63-83 0177-7971 91:1-4<63 2006 91 703 https://doi.org/10.1007/s00703-004-0107-6 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s00703-004-0107-6 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Hornsteiner M. Meteorological Institute, University of Munich, Germany aut NATIONALLICENCE 700 1- Zängl G. Meteorological Institute, University of Munich, Germany aut NATIONALLICENCE 773 0- Meteorology and Atmospheric Physics Springer-Verlag 91/1-4(2006-01-01), 63-83 0177-7971 91:1-4<63 2006 91 703 Metadata rights reserved Springer special CC-BY-NC licence nationallicence SWISSBIB 467937621 BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer