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Mountain torque events at the Tibetan Plateau

Egger, Joseph and Hoinka, Klaus-Peter (2008) Mountain torque events at the Tibetan Plateau. Monthly Weather Review, 136, pp. 389-404. DOI: 10.1175/2007MWR2126.1.

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Official URL: http://ams.allenpress.com/perlserv/?request=get-archive&issn=1520-0493

Abstract

The interaction of large-scale wave systems with the Tibetan Plateau (TP) is investigated by regressing pressure, potential temperature, winds, precipitation, and selected fluxes in winter onto the three components Toi of this massif’s mountain torque on the basis of the 40-yr ECMWF reanalysis (ERA-40) data. Events with respect to the equatorial “Greenwich” axis of the global angular momentum exhibit by far the largest torques (To1,), which essentially represent north–south pressure differences across the TP. The axial torque To3 peaks when the surface pressure is high at the eastern slope of the TP. The torque To2 with respect to the 90°E axis is closely related to To3 with To2 ∼ −To3. The maximum (minimum) of To1 tends to occur about 1 day earlier than the minimum (maximum) of To2. All torque events are initiated by equivalent barotropic perturbations moving eastward along the northern rim of the TP. In general, the initial depression, for example, forms a southward-protruding extension at the eastern slope of the TP and a new high grows near Japan. Later, the perturbation near Japan moves eastward in To2 events but extends northward in To1 events. These flow developments cannot be explained by theories of topographic instability. The observed vertical motion at the lee slope is at best partly consistent with theories of linear quasigeostrophic wave motion along mountain slopes. These findings lead the authors to test the eventual usefulness of linear theories by fitting the linear terms of a novel statistical equation for the potential temperature θ to the observed changes of θ and the torque to the observations. This test indicates that the evolving regression patterns of θ can be explained by linear terms at least in specific domains. In turn, pressure tendency regressions at a selected level can be calculated on the basis of the linear θ tendencies above that level. The formation of the lee trough appears to be mainly caused by horizontal warm-air advection along the slopes, but changes of the potential temperature above the height of the TP also contribute significantly to the pressure changes in the lee. Cold-air advection aloft strengthens the Japan high. “Turbulent” transports appear to be mainly responsible for the decay of the perturbations but data accuracy problems impede the analysis. In particular, the noisiness of the vertical motion fields affects the skill of the linear calculations.

Document Type:Article
Title:Mountain torque events at the Tibetan Plateau
Authors:
AuthorsInstitution or Email of Authors
Egger, JosephUniv. München, München
Hoinka, Klaus-PeterUNSPECIFIED
Date:2008
Journal or Publication Title:Monthly Weather Review
Refereed publication:Yes
In Open Access:No
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:136
DOI:10.1175/2007MWR2126.1
Page Range:pp. 389-404
Status:Published
Keywords:Tibetian plateau, torques, Gebirgswiderstand
HGF - Research field:Aeronautics, Space and Transport (old)
HGF - Program:Space (old)
HGF - Program Themes:W EO - Erdbeobachtung
DLR - Research area:Space
DLR - Program:W EO - Erdbeobachtung
DLR - Research theme (Project):W - Vorhaben Atmosphären- und Klimaforschung (old)
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics > Cloud Physics and Traffic Meteorology
Deposited By: Jana Freund
Deposited On:27 Mar 2008
Last Modified:20 Oct 2014 14:31

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