COLD FRONT - METEOROLOGICAL PHYSICAL BACKGROUND

by ZAMG


If cold and warm air are situated next to each other an inclined boundary (oriented downward from high to low layers) can be found between the two air masses. The main physical process for the development of Cold Fronts is the movement of the colder against the warmer air. As a consequence of this movement, and relative to it, the warm air tends to ascend this air mass boundary while the cold air tends to sink below it. This upward motion often leads to condensation and subsequently the development of clouds and precipitation.
Many conceptual models have been developed to depict fronts; the most classical ones are the for Ana Cold Front and the Kata Cold Front. From the viewpoint of satellite meteorology, the conveyor belt model presents new ideas. Therefore within the following paragraphs these cold front types are described.

Ana Cold Front

According to classical ideas cold air moves rapidly against warm air, thereby creating convergence at the surface line between the two airmasses. This convergence forces the warm, moist air to ascend on the frontal surface. The cloud band develops, inclined rearward from the surface cold front. Consequently, in this case the main zone of cloudiness and precipitation appear behind the surface front (indicated by the TFP). Satellite images confirm this structure (see Cloud structure in satellite image). Only in the case where there are high upper level winds, does the high cloud extend downstream ahead of the surface front, leading to a TFP maximum within the cloud band.

If one describes the situation using conveyor belt theory, the frontal cloud band and precipitation are related to an ascending warm conveyor belt. This conveyor belt has a rearward component relative to the movement of the front. This leads to the same result mentioned above, with the frontal cloud band and precipitation appearing behind the surface front. Parallel to the warm conveyor belt there is a dry stream (dry intrusion). The sharp rear cloud edge of frontal cloudiness marks the transition between the two relative streams and is accompanied by a limiting stream line.

As mentioned in literature, real examples of CF do not always show these model characteristics, but sometimes even show parallel or even forward inclined warm conveyor belts. While the rearward component can be explained by ageostrophic wind flow in the planetary boundary layer due to friction, the parallel or even foreward sloping warm conveyor within the middle and higher levels is in accordance with the geostrophic wind relation.

Kata Cold Front

According to classical ideas, in this type of front the warm air follows processes similar to the Ana Cold Front, however, the ascent of air is restricted by dry descending air originating from behind the front and, consequently, dissipating the higher clouds. In this case, the main zone of cloudiness and precipitation appear in front of the surface front. Satellite images show parts of this process of cloud decay (see Cloud structure in satellite image). It is now generally considered that a Kata Cold Front evolves from an Ana Cold Front.

In contrast to the Ana Cold Front, the ascending warm conveyor belt is overrun by dry air, which is transported within the relative stream of the dry intrusion. The air of this intrusion originates from upper levels of the troposphere or even the lower levels of the stratosphere and crosses the Cold Front from behind. Due to this process, the warm conveyor belt acquires a component which is forward inclined relative to the movement of the Cold Front. Therefore, frontal clouds and precipitation tend to lie ahead of the surface front. The cloud tops within the area of the dry sinking tropospheric and/or stratospheric air are lower (warmer) than in the case of an Ana Cold Front. At the leading edge of this dry air, an increase of the cloud tops can be observed (see Cloud structure in satellite image). This area indicates the so-called upper Cold Front. The air mass which is advected by the dry intrusion is colder than the air within the warm conveyor belt. The intrusion cools air above and, later, also ahead of the Cold Front. Furthermore, the air of the upper relative stream is indicated by lower values of equivalent potential temperature. The result of this situation is the development of a conditionally unstable layer close to the leading edge of the frontal cloud band. As a result of ascent, this area is suitable for the development of pronounced instability which is often observable by a change of cloud type from stratiform to cumuliform (see Weather events).

Discussion

As already mentioned, the literature identifies several uncertainties about,in particular, the structure of the Ana Cold Front. During its investigation of Cold Front cases, ZAMG has also become aware of some inconsistencies. The results are summarized as follows:

A typical warm conveyor belt from the south or south-east turning to more northely directions, can be observed in every case (see Warm Front Band , Warm Front Shield and Detached Warm Front ), but

Schematic Summary

Ana Cold Front Kata Cold Front
The schematics above summarise several aspects of conveyor belt theory and compare the different behaviour of Ana and Kata Fronts.
The most important feature is always the orientation of the jet streak relative to that of the cloud band, i.e. parallel to an Ana Front but across the cloud band of a Kata Front and descending.
04 October 1995/12 UTC - IR image (Ana Cold Front); relative streams on the isentropic surface of ThetaE = 310K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hPa; lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hPa but corrected with WV imagery, yellow: isobars, magenta: relative streams - system velocity: 267° 9 m/s, white: position of vertical cross section
04 October 1995/12 UTC - WV image (Ana Cold Front); relative streams on the isentropic surface of ThetaE = 318K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hPa; lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hPa but corrected with WV imagery, yellow: isobars, magenta: relative streams - system velocity: 267° 9 m/s
The above images show an example of an Ana Cold Front. On the lower isentropic surface (top) a warm conveyor belt can be observed crossing (rearwards) the frontal line approximately northward of the Bay of Biscay. Under its influence is the cloud band in front of the TFP as well as the relevant part behind it stretching from France across south-east England into the North Sea. The second relative stream is a broad one, from the north-western part of the trough behind the front. Frontal cloudiness to the rear of the TFP, from north-west Spain across the Bay of Biscay and Brittany into the English Channel is, increasingly, under its influence.

If this situation is compared with the vertical cross section below (upper cross section), it can be seen that the zone of high humidity ahead of the front, at the 310K surface around 800 hPa, represents the warm conveyor belt, while humidity values on this surface, between 800 and approximately 550 hPa, represent the relative stream approaching from the rear. This height also marks the position of the rearward edge of the cloud band, which coincides with a sharp decrease in IR and WV pixel values.

On the higher isentropic surface (lower vertical cross section) the relative stream from the rear can be divided into two parts. The one near the anticyclonic side of the jet axis originates from moist regions in the warm sector of the consecutive frontal system and is associated with Cold Front clouds to the rear of the TFP; the other drier relative stream can be found on the cyclonic side of the jet axis. Both are parallel to the frontal cloud band. On this higher surface the warm conveyor belt crosses the TFP much less than on the lower surface.

Looking again at the vertical cross section (upper cross section), the humidity maximum in front at the 318K surface between 500 and 400 hPa represents the warm conveyor belt (accompanied by peaks in IR and WV pixel values) while on this isentropic surface further upward, near 350 hPa, cloudiness is associated with the moist part of the relative stream from the rear side (accompanied by a second IR and WV peak in pixel values). The dry part of this stream shows up in the cross section as very low humidity values (around 300 hPa).

04 October 1995/12.00 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
The Ana Cold Front (top) shows a backward inclined zone of high humidity from low to high levels while the Kata Cold Front (bottom) shows a similar zone but forward inclined. The driest air, in the case of the Ana Cold Front, lies behind and below the frontal surface, but within and below the frontal surface in the case of the Kata Cold Front.
29 February 1996/06 UTC - IR image (Kata Cold Front); relative streams on the isentropic surface of ThetaE = 286K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hPa; lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hPa but corrected with WV imagery, yellow: isobars, magenta: relative streams - system velocity: 326° 10 m/s, white: position of vertical cross section
29 February 1996/06 UTC - WV image (Kata Cold Front); relative streams on the isentropic surface of ThetaE = 300K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hPa; lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hPa but corrected with WV imagery, yellow: isobares, magenta: relative streams - system velocity: 326° 10 m/s
In the case of thr Kata Cold Front, the zero line of shear vorticity, marking the jet axis, crosses the cloud band leading less bright (i.e. lower height) frontal clouds at the cyclonic side. This is the main difference from the situation of the Ana Cold Front described before, where the jet axis and frontal cloud band are parallel. On the lower isentropic surface (top image) a warm conveyor belt can be observed which is nearly completely restricted to the area of high cloudiness ahead of the TFP. Behind the TFP, relative stream lines are from the rear. Comparing with the vertical cross section (above, lower cross section) the moist zone ahead of the front, at the 286K surface from the ground up to about 800 hPa, is composed of two relative streams: the warm conveyor belt air mass only exists in the lowest layer (below about 900 hPa) and the air mass of the moist relative stream from behind, dominates above 900 hPa.

The second image above shows the situation on a higher isentropic surface with a crossing of the relative stream lines from the north-west over the TFP of the Cold Front. This belongs to the maximum of relative humidity in the vertical cross section (above, lower cross section) ahead of the front at the 300K surface (approximately 500 hPa). Consequently, the high clouds, indicated by the maximum of pixel values, is formed in the moist branch of the relative stream to the rear, while the warm conveyor belt is only associated with a layer of low level cloudiness.

The lower cloud tops on the cyclonic side of the jet axis should be the result of the dry part of the relative stream, which is confirmed in the vertical cross section; dry air above 300K associated with the IR peak at about 400 hPa.

In this case it is not easy to distinguish between the origins of the two branches of the relative stream from the north-west because the stream lines cross the jet axis to the rear of the trough. One possible explanation is that the system velocity is computed for the Cold Front and does not match that of the subsequent Warm Front system. The following image is the result of a system velocity computation based on propagation of the approaching Warm Front system. The differentiation between the dry and the moist branches in the upstream region is now much clearer.

29 February 1996/06 UTC - IR image (Kata Cold Front); relative streams on the isentropic surface at ThetaE = 300K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hPa;lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hPa but corrected with WV imagery, yellow: isobares, magenta: relative streams - system velocity: 278° 9 m/s, white: position of vertical cross section

SUB-MENU OF COLD FRONT
CLOUD STRUCTURE IN SATELLITE IMAGES
KEY PARAMETERS