Cold Front crossing with remarkable "Lee clearing"

The typical synoptic situation can be summarized as:

• A Cold Front crosses the Alpine region from north to south accompanied by a distinct thickness gradient
• WA changes to CA during the passage of the front over the Alps
• PVA exists but impact on precipitation not clear
• Main cloud features: Cold Front band, Stau features, "Occlusion - formed Stau", Lee clearing

For the complete sequence of satellite images during this phase see Overview - Phase 4 and Overview - Phase 7 .

The typical example is from 09/12.00 UTC (where the frontal cloud band can be identified exactly over and north of the Alps) to 09/18.00 UTC (where the frontal cloud band is already south of the Alps extending from N. Italy to SE Austria). There are several typical cloud features that result from the propagation of a Cold Front band across the Alps from north to south:

• Cold Front cloud band south of Alps extending from SW to NE;
• Clearing to the lee of the Alps is especially marked in NW Italy;
• Cloud band immediately north of and within the Alpine mountain range, resembling an Occlusion - like cloud spiral; however, this band feature is the consequence of remnants of frontal cloudiness and Stau Cloud (partly formed by deformation).

A second example from 20/18.00 UTC shows similar cloud features.

Height contours for the first case show a surface low which becomes cut off while moving across the Alps, and an upper level trough over France which approaches the W. Alps with little change during this 6 hours period. Frontal conditions are well marked. A high thickness gradient with a well developed TFP at its southern boundary moves southward across the W. Alps. CA propagates south and eastward across the Alps. The wind fields at 850 and 500 hPa in this case seem to strongly support the development of the Occlusion - like Stau cloud and the lee clearing. Over the W. Alps winds at 850 hPa change from westerly to northerly components during the 6 hours while there are westerly components at high levels at both points in time. Stau cloud at the W. Alps and descent in the lee over NW Italy behind the front at 18.00 UTC are to be expected.

Over the area of Central and E. Alps the situation is somewhat different. At 850 hPa, there are small northerly components north of the Alps, especially over Bavaria, and rather strong southerly components south of the Alps accompanying the frontal cloud band. The latter might be responsible for the lower cloudiness over NE Italy and S. Austria. At 500 hPa, strong westerly components prevail. The form of the Occlusion - like Stau cloud is therefore a combination of several effects, namely Stau, descent and deformation at lower levels.

For 09/12.00 UTC, a lobe of PVA can be found to the rear of the Cold Front band over France which is approaching the W. Alps. It accopanies the Occlusion - like cloudiness at 09/18.00 UTC. Precipitation is wide-spread. Whilst there is snow in the W. Alps (especially over Switzerland and Tyrol), throughout the whole period, a precipitation area moves from Bavaria southward into Austria. Although the PVA lobe is associated with the precipitation, no clear relationship between the PVA maximum and intensified cloud and precipitation areas can be found.

As already mentioned in the chapter of the Comma and Comma crossing there are some reservations when using vertical cross sections in the region of the Alps, because of a lack of orography in the numerical model data as well as in the vertical cross section algorithm. Even so, remarkable results can be achieved by using this material.

The cross section lines have been chosen so that they are perpendicular to the W. Alps in Switzerland and to the frontal cloud band and the area of lee clearing in NW. Italy. The vertical cross sections for both points in time reveal a very pronounced downward inclined frontal surface which reaches the surface south of the Alps. A southward movement of about 1/2 degree in latitude and longitude takes place during the 6 hours. But there is also another interesting phenomenon: There is a second more northern frontal zone, which moves from a position north of the Alps (48.3N/6.8E) at 09/12.00 UTC to a position exactly over the Alps (46N/08E) at 18.00 UTC. This is a new feature associated with the Occlusion - like band structure over the Alps at 18.00 UTC and means that there is a double frontal structure already inherent in the frontal system which might support the development of this distinct band feature over the Alps. It is interesting to see whether other parameters also show this double structure. Several interesting phenomena can be noted:

1. There is a rather classical distribution of TA around the frontal surface with WA in front and above, and CA below for both points in time; but:
2. The CA propagates faster at higher levels and, therefore, lies above the WA layer at 18.00 UTC (750 - 500 hPa)
3. The northern, weaker frontal zone (double structure in isentropes) is within CA and represents (if it is analysed separately at all) a Cold Front in Cold Advection
4. There is a CA area above the upper level frontal zone at 400 hPa which indicates colder air already in advance of the frontal zone which intensifies between 12.00 and 18.00 UTC. This preceding CA maximum at high levels is a typical feature associated with a Cold Front or cold air systems crossing the Alps.

The model is not able to separate the two frontal surfaces, but there is a broad zone of convergence including both surfaces at 09/12.00 UTC and some indication of different systems at low levels at 18.00 UTC. For both points in time, strong upward motion is associated with the frontal surfaces. The inherent double structure is separated quite clearly at 18.00 UTC, when this cloud band lies over the Alps.

Another very pronounced effect, the lee clearing, is poorly represented in these cross sections: the location 45N/09E is quite representative of the area of the lee clearing, but vertical motion only shows smaller upward components and a very small area of positive values at the surface. But as the area of the "lee clearing" is quite small and the cross section F is more to the eastern part, close to the area of frontal cloudiness, a more western cross section G has been analysed.

The vertical motion field in this cross section clearly shows an area with descent between 400 and 700 hPa in the area of interest (45N/7.5E). In addition, this vertical cross section indicates the two frontal surfaces better than cross section F.

Concluding these discussions, from the information in the vertical cross section, an additional explanation can be inferred for the distinct form of the Occlusion - like band over the Alps: There is a second frontal structure contained in the total frontal system which might cause and/or intensify the cloudiness there. However, no parameter truly identifies an Occlusion system.

A study of relative streams adds information concerning the vertical structure of the frontal systems as well as the observed double structure.
For 12.00 UTC the surfaces of 300K and 304K represent the main frontal surface as well as the upper level front. For 18.00 UTC, the surfaces of 296K and 300K represent the main frontal system and 304K the upper level front.

At 300K, the limiting streamline between the Warm Conveyor Belt and the upper relative stream is exactly over the W. Alps and identifies the Cold Front there. In addition, the Warm Front Band over Austria and Hungary, as well as the marked Lee Cloudiness over the Adriatic Sea, are in the rising branch of the Warm Conveyor Belt and represent very classical situations. At 304K, the upperrelative stream is already far to the south of the Alps. This vertical distribution of conveyor belts indicates instability. The same isentropic surfaces are computed for 18.00 UTC. They have characteristics very similar to those at 12.00 UTC. At this point in time the lower isentropic surface 296K is also computed. On this surface there is a distinct frontal surface in the Alpine region supporting the analysis of frontal conditions there.

It can be concluded that the investigations of vertical cross sections and relative streams reveal Cold Fronts and their passage across the Alps quite well. However, smaller scale features, such as the lee clearing at 18.00 UTC, are not well represented.

Cold Front crossing without remarkable "Lee clearing"

There are cases where lee clearing cannot be obvserved.
The typical synoptic situation can be summarized as:

• Cold Front crosses the Alpine region accompanied by a distinct thickness gradient
• WA changes to CA during the crossing over the Alps
• PVA existent but impact not clear
• Main cloud features: Cold Front band, Stau features

For the complete sequence of satellite images during this phase see Overview - Phase 9 .