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Föhn as a response to changing upstream and downstream air masses
Author(s) -
Mayr Georg J.,
Armi Laurence
Publication year - 2008
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.295
Subject(s) - advection , crest , mesoscale meteorology , ridge , warm front , geology , radiosonde , upstream and downstream (dna) , cold front , upstream (networking) , air mass (solar energy) , airflow , potential temperature , meteorology , climatology , atmospheric sciences , geography , oceanography , boundary layer , mechanical engineering , paleontology , computer network , physics , quantum mechanics , computer science , engineering , thermodynamics
Abstract Observations of föhn from the field phase of the Mesoscale Alpine Programme (MAP) are used to study how differences between the air masses upstream and downstream of the central Alpine crest determine whether the flow can descend to the lee as either shallow föhn, when it passes through passes in the mountains, or deep föhn, when it overflows the Alpine crest. First, the föhn case of 30 October 1999 is examined using ECMWF analyses and radiosonde data at various upstream and downstream locations. Additional measurements from aircraft, dropsondes, an instrumented car and automatic weather stations are then used for a detailed study of the föhn flow across the Brenner Pass. Advection of cold air around the eastern edges of the Alps and warm air around the western edge of the Alps ahead of a synoptic ridge set up a reservoir of colder air on the south side of the Alps and a reservoir of warmer air to the north. The depth to where the air was colder on the southern side was sufficient for a shallow föhn to flow through the pass. After the passage of the ridge axis, synoptic cold air advection provided another source of colder air, this time from the southwest, growing deeper with time and having a synoptically imposed cross‐barrier flow component. The maximum depth to where the air upstream was colder than downstream extended just above the peaks of the highest mountains. An analysis of the detailed measurements across the Brenner Pass showed that this depth was also the top of the layer that descended and accelerated down the lee slopes of the Wipp Valley. Upstream, air above the föhn layer had an even stronger cross‐barrier component yet did not descend because it did not have lower potential temperatures than the downstream side at that level. Deep föhn never developed. An examination of other well‐documented MAP föhn cases confirmed the conclusion from the 30 October event that shallow and deep föhns – at least for the central Alps – are mostly a response to differences in air masses between the upstream and downstream side. A cross‐barrier component of the flow was only a modification but in itself not sufficient to cause the flow to both descend and accelerate down the lee slope, unless potential temperatures on the upstream side were lower in this layer than on the downstream side. Copyright © 2008 Royal Meteorological Society