SPLIT FRONT - TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS

by ZAMG


In the ideal case the isentropes of the equivalent potential temperature show two frontal crowding zones, an upper level front and a surface front. Both crowding zones have Cold Front - like inclinations. While the upper level front is connectied to the high leading cloud band, the surface front represents the low-level cloudiness at the back side of the frontal system. Whereas the crowding zone of the surface front is strongly pronounced, the crowding zone of the upper level front is mostly weak. Therefore the upper level front is not characterized as a thermal boundary but rather as a moisture boundary (see Meteorological physical background).

The field of temperature advection shows often pronounced WA in front of and above the upper level frontal zone, which is connected with the upper level cloudiness. CA, typical for Cold Fronts, is situated below the surface front.

The most characteristic feature of the humidity distribution is a dry area in higher levels between the two frontal zones. High values of humidity can be found in front of the frontal zones.

In the case of superimposed EC cloudiness a distinct isotach and PVA maximum can be found above the surface front at approximately 300 hPan (see Front Intensification By Jet Crossing - Typical appearance in vertical cross section ).

According to the distribution of humidity the satellite signals in IR and WV images show the highest pixel values in front of the upper level front and, if existing, within the EC - like cloud part. At the backside of the upper level front the VIS signals are usually higher while IR and WV signals are much lower than in front of the upper level front (see Cloud structure in satellite image).

Compare also the chapter Cloud structure in satellite image, where the cross section is indicated in the image. In contrast to the ideal case the surface cold front has a superadiabatic layer in the lower levels of the troposphere. The upper level Cold Front is well developed. The distribution of humidity shows the described insertion of drier air between the two frontal zones. The IR and WV images show the pronounced decrease of temperature from the low to the high cloud part.

02 September 1995/12.00 UTC - Meteosat IR image; position of vertical cross section indicated
02 September 1995/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
02 September 1995/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), green thick: vorticity advection - PVA, green thin: vorticity advection - NVA, orange thin: IR pixel values, orange thick: WV pixel values
02 September 1995/12.00 UTC - Vertical cross section; black: isentropes (ThetaE),yellow: isotachs , orange thin: IR pixel values, orange thick: WV pixel values

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