COLD FRONT - TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS

by ZAMG


The isentropes of the equivalent potential temperature show a downward inclined crowding zone, which extends through the whole troposphere, with the warmer air found in front and above, the colder air at the rear and below the crowding zone (see Meteorological physical background). Besides the surface front, often an upper level front, which is situated ahead of the surface front, can be observed.

The relative humidity has high values in front of and low values behind the crowding zone. With the help of the models of Ana Cold Front and Kata Cold Front, the field of humidity can be interpreted with the warm ascending air in front of and cold descending air behind the frontal surface. Furthermore, in the case of an Ana Cold Front the field of humidity is characterized by high values (approximately 80%) extending on the warmer side of the crowding zone with a backward inclination from the lower to the higher levels of the troposphere. In contrast to this, the distribution of the relative humidity in the case of a Kata Cold Front shows high values (approximately 80%) in front of the warmer side of the crowding zone from the lower levels with a forward inclination into higher layers of the troposphere, thereby moving away from the highest surface of the crowding zone. The dry cold air (with minimum values of about 20%) extends in the case of an Ana Cold Front from the highest layer of the frontal surface downward below the frontal surface. In contrast to this, in the case of a Kata Cold Front the dry air extends downward below as well as within the frontal surface, thereby leading to cloud dissolution (see Meteorological physical background).

The field of temperature advection shows WA in front (ahead) of the crowding zone, indicating warm ascending air of the warm conveyor belt and/or the upper relative stream from behind the front. CA can be found behind (below) the frontal surface. In general, the CA is much more pronounced than the WA.

The whole crowding zone from the lower to the upper levels of the troposphere is characterized by positive vorticity advection. In the ideal case the values of the vorticity advection are continously increasing. Therefore its maximum (often situated at 300 hPa) can be found in the upper levels of the troposphere and behind the surface front line.

The field of divergence shows a pronounced zone of convergence within and a zone of divergence above the frontal surface.

As a consequence of this distribution the field of omega is characterized by negative values (upward motion) within and above the crowding zone. As convergence has its highest values in the lower troposphere the greatest negative values of omega can be found in the mid-levels of the troposphere. Weak positive values (downward motion) can be found within the cold air at the rear side below the crowding zone.

The pixel values of the satellite image show in the VIS image high values for both the Ana Cold Front and Kata Cold Front cases. In contrast to this the distribution of the pixel values of the IR and WV images are dependant on the existing cold front type. In the case of an Ana Cold Front the pixel values of the IR and WV images are increasing backwards i.e. behind the TFP, but in the case of a Kata Cold Front the pixel values mostly are increasing forewards i.e. ahead of the TFP (see Cloud structure in satellite image).

26 September 1995/06.00 UTC - Meteosat IR image; Ana Cold Front; position of vertical cross section indicated
29 February 1996/06.00 UTC - Meteosat IR image; Kata Cold Front; position of vertical cross section indicated
The first two figures show the main differences in the humidity distribution between Ana and Kata Cold Fronts. The other parameters do not distinguish between the two types of Cold Front and are therefore chosen from the Kata Cold Front type example. All fields are very close to the schematics representing ideal situations with the exception of the isentropes in the Kata Cold Front case which unusually consists of two branches of the frontal zone, one reaching the ground (approximately at 57N/15E) and an upper level front slightly ahead of the surface front reaching down to about 750 hPa.
26 September 1995/06.00 UTC - Vertical cross section; Ana Cold Front; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
29 February 1996/06.00 UTC - Vertical cross section; Kata Cold Front; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
29 February 1996/06.00 UTC - vertical cross section; Ana and Kata Cold Fronts; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values
29 February 1996/06.00 UTC - vertical cross section; Ana and Kata Cold Fronts; black: isentropes (ThetaE), green thick: vorticity advection - PVA, green thin: vorticity advection - NVA, orange thin: IR pixel values, orange thick: WV pixel values
29 February 1996/06.00 UTC - Vertical cross section; Ana and Kata Cold Fronts; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values, orange thick: WV pixel values