DETACHED WARM FRONT - TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS

by ZAMG


Vertical cross sections of the Detached Warm Fronts do not differ from the classical band-type Warm Front.

As described before, the isentropes of the equivalent potential temperature across the Detached Warm Front show a crowding zone through the whole troposphere, which is inclined upwards from low to high levels. The colder air can be found in front of and below the crowding zone, the warmer air in front of and above it (see Meteorological physical background).

The field of humidity shows high values immediately behind and above the frontal surface of the Warm Front. Low values can be found below the crowding zone of the equivalent potential temperature.

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 Warm Front. The maximum of the WA can be found within the crowding zone where it often shows several maxima from low up to high levels. On the other hand CA can be found below and in front of the crowding zone. In actual cases the isentropes forming the lower boundary of the frontal surface do not represent the transition from WA to CA, but WA can also mostly be found far below the frontal surface while CA exists only at a larger distance from the surface front.

At the leading part above the frontal surface, at approximately 300 hPa, a pronounced isotach maximum can be observed.

Well developed fronts are accompanied by a zone of distinct convergence within and divergence above the frontal zone. Consequently, upward vertical motion can be found above the frontal zone, responsible for cloud development.

In the ideal case, the satellite signals across the Warm Front are characterized by typical distributions. While the IR image shows across the frontal area continuously 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). In contrast to this the WV image shows high pixel values within the frontal cloudiness and a pronounced minimum connected with the dry air in front. In reality, these variations of grey shades for Detached Warm Fronts are by far not as clear as in the ideal conceptual model.

22 March 1995/23.30 UTC - Meteosat IR image; position of vertical cross section indicated
23 March 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
23 March 1995/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
23 March 1995/00.00 UTC - Vertical cross section; magenta thin: divergence, magenta thick: convergence, cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values, orange thick: WV pixel values
23 March 1995/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), yellow: isotachs, orange thin: IR pixel values, orange thick: WV pixel values
The first cross section shows the crowding zones of the equivalent potential temperature of a pronounced surface Warm Front with intense WA, especially through the whole troposphere. The IR signals are characterized by high pixel values in the centre of the Warm Front cloudiness (pixel values between approximately 190 and 210 units). Lower pixel values, in connection with the dry (anticyclonic) side of the superimposed jet, can be found in the leading part of the Warm Front (pixel values between approximately 160 and 180 units). The field of relative humidity shows high values within the lower to mid-levels, also immediately behind the surface front (above 80%). The third cross section shows in the lower and mid-levels intense upward motion within the crowding zone which is caused by a zone of convergence in the lower and mid-levels of the troposphere situated within and below the frontal surface. Divergence can be found above the frontal surface. The fourth cross section is characterized by a pronounced isotach maximum within the leading part of the Detached Warm Front at approximately 200 hPa. This maximum is accompanied by a decrease of the WV pixel values.

SUB-MENU OF DETACHED WARM FRONT
KEY PARAMETERS
WEATHER EVENTS