WARM FRONT SHIELD - TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS

by ZAMG


The isentropes of the equivalent potential temperature across the Warm Front Shield show two crowding zones: a surface Warm Front zone and an upper level Warm Front zone. Both crowding zones are inclined from low to high levels and merge within the upper level of the troposphere. In both cases the colder air can be found in front of, and below, the warmer air behind and above the crowding zones (see Meteorological physical background).

The field of humidity shows high values immediately behind and within both frontal surfaces of the Warm Front. Lower values can be found below the crowding zones. Of special interest in this connection are the lower humidity values between the two frontal zones, indicating air masses of different origin.

Like the distribution of the humidity, the field of temperature advection can also be separated into two parts. Therefore WA exists above and within the crowding zone of the surface warm front. CA can be found below and ahead of the surface front. In contrast to this, the upper level front is completely within WA, with higher values of WA above and within, and lower values below, the crowding zone. The maximum of the WA can be found within the crowding zones of both frontal surfaces. In the case of the surface warm front the WA maximum is situated at approximately 800 hPa while in the case of the upper level front it can be found at approximately 600 hPa. These two maxima correspond to the two WA maxima in the isobaric fields, the main one in front of and the second one behind the TFP (see Key parameters).

All three channels of satellite images across the Warm Front Shield show a broad area of high values. In the ideal case typical distributions can be observed. While the IR image shows across the Warm Front Shield only weakly increasing values of grey shades from the rear to the leading edge, the distribution of the grey shades in the VIS image is inverse (see Cloud structure in satellite image). The most pronounced feature in the WV image is the minimum connected with the dry air at the leading edge of the shield.

The sharp decrease of IR and WV pixel values at the leading edge of the Warm Front Shield is connected to a jet streak, which can be found at approximately 300 hPa.

27 January 1995/00.30 UTC - Meteosat IR image; position of vertical cross section indicated
The first cross section shows the two inclined crowding zones of isentropes (black) connected with typical temperature discontinuities. The IR and WV grey shades show high values nearly across the whole cross section. The sharp decrease in height above the station 22845 marks the leading edge of the cloud shield. The main precipitation events are connected with the surface Warm Front zone. The second figure shows temperature advection in green. The main maximum is connected with the surface Warm Front zone, the second weaker one can be found in the upper level frontal zone. According to the position of the surface front line, or the TFP, the main maximum is in front of it, the second one behind. The third figure shows relative humidity (green). A zone of lower values discriminates the two zones of high humidity on top of the two frontal surfaces. The fourth figure finally shows a good coincidence between the jet maximum at 300 hPa and the sharp decrease of WV and IR pixel values, already mentioned before.
27 January 1995/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), red: temperature, orange thin: IR pixel values, orange thick: WV pixel values
27 January 1995/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values
27 January 1995/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
27 January 1995/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), yellow: isotachs, orange thin: IR pixel values, orange thick: WV pixel values

SUB-MENU OF WARM FRONT SHIELD
KEY PARAMETERS
WEATHER EVENTS