FOEHN - TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS
by ZAMG
The typical appearance in vertical cross sections is somewhat false, because of the fact that the effect of mountains is not treated realistically within cross sections, neither from radiosoundings nor from numerical models. Radisonde stations have too big a spatial resolution while numerical models usually do not contain the fine structure and real height of mountainous areas. Nevertheless some interesting features can be derived.
- Equivalent potential temperature:
Moist adiabatic isentropes show a zone of high gradients on the windward side crossing the mountains and a different air mass below this zone
on the lee side; this can be seen much better in vertical cross sections from radiosounding stations.
- Relative humidity:
Reflects the differing cloud situation on both sides of the mountains: a maximum of relative humidity is located on the windward side from the
surface up to the mountain tops; a secondary zone of high humidity can be found at high levels on the lee side representing the high Lee Cloud;
below this zone it is much drier or even very dry.
- Convergence:
There is convergence on the windward side from the surface up to middle levels and convergence on the lee side from middle to high levels;
divergence occurs above the mountains as well as in the low layer on the lee side.
- Vertical motion:
Upward motion is associated with the convergence areas and, therefore, strong upward motion in a thick layer can be found on the windward side
and also on the lee side but at middle to high levels. The lowest layers on the lee side show sinking.
The example below, used to demonstrate general Foehn dynamics, is from the Alpine region. Therefore the following relation between vertical cross section, frontal cloud and mountains exists.
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06 November 2000/12.00 UTC - Meteosat IR image
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Equivalent potential temperature: vertical cross sections from radiosonde stations compared with the output from numerical models
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06 November 2000/12.00 UTC - Vertical cross section; blue: moist adiabatic isentropes - radiosounding, black: moist adiabatic
isentropes: ECMWF, orange thin: IR pixel values, orange thick: WV pixel values
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Relative humidity: vertical cross section from radiosounding stations
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06 November 2000/12.00 UTC - Vertical cross section; blue: relative humidity, black: isentropes (ThetaE), orange thin: IR pixel values,
orange thick: WV pixel values
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Convergence: vertical cross sections from numerical model
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06 November 2000/12.00 UTC - Vertical cross section; magenta: divergence, black: isentropes (ThetaE), orange thin: IR pixel values,
orange thick: WV pixel values
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Vertical motion: vertical cross sections from numerical model
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06 November 2000/12.00 UTC - Vertical cross section; cyan: vertical motion, black: isentropes (ThetaE), orange thin: IR pixel values,
orange thick: WV pixel values
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