For the diagnosis of the physical state of the Cold Front, a couple of key parameters and useful key parameter combinations are available which were chosen with respect to the conceptual model (for more information compare Conceptual Models: Cold Front ).

Front Indicator	This is a line indicating the maximum of the thermal front parameter (TFP) 500/850 hPa (*10-1Km-1)
TFP + rel.Top + TA>>=0	This is a combination of the thermal front parameter (TFP) greater than 3 units, the equivalent thickness (rel.Top) 500/850 hPa (K) and warm advection (TA>=0) (K 12h-1); this combination of key parameters is especially indicative of the thermal characteristic of a Cold Front
TFP + Scher300=0	This is a combination of the thermal front parameter (TFP) 500/850 hPa greater then 3 units and the zero line of shear vorticity at the 300 hPa level (Scher300=0); this combination of key parameters is indicative of the relation between front and jet stream
Abs.Top500 + 1000	This is the combination of height contours at 1000 and 500 hPa (gpm); this combination of key parameters is indicative of the flow and pressure distribution near the surface and in the middle of the troposphere
PVA500>=2	This parameter shows maxima of positive vorticity advection (PVA) at 500 hPa exceeding 2 units (*10-9sec-2); this key parameter indicates areas with increased possibility for more severe weather events because of increased vertical motion (compare Conceptual Models: Wave and Conceptual Models: Front Intensification by Jet Crossing )
Relative Stroeme (relative streams)	This parameter shows relative streams on isentropic surfaces; relative streams are indicative of the different air masses involved in the process and of areas with sinking or rising motion; they can be used to explain cloud configurations requiring special explanation
VQS	Gives the location of available vertical cross sections (VQS)

The Cold Front cloud band under consideration reaches from the Atlantic (approximately 38N/10W) across the English Channel into the North Sea approximately up to the Dutch coast. Cloudiness is rather homogeneous with only slightly lower tops at the rear side of the front.

To get an overview, the height contours at 1000 as well as at 500 hPa are interpreted. There is an already well-developed low with a vertical axis between surface and upper troposphere. This is an atypical situation, because in most cases a much earlier stage of development is investigated with a surface low and an upper level trough upstream. Therefore it could be expected that frontal systems connected with such a well-developed low are also in a well-developed stage, or may even be already in dissolution. As the investigations will show the contrary is the case; in particular the cold front represents a very clear example of an Ana Front type. As a next very important step, the thermal frontal conditions should be investigated. The maximum line of the TFP indicated by the solid red line of the front indicator shows the position of the Cold Front, according to the definition that a surface Cold Front can be found where the temperature begins to sink. In the thickness field the zones of high gradients ("crowding zones" of isolines) are a typical characteristic for frontal conditions, representing the quick transition from warm (thick) to cold air (thin layer) at a weather front. Temperature advection (TA) changes its sign in the area of the front, for instance from warm advection (WA) in front of the cloud band to cold advection (CA) behind it. So the zero line of TA should be close to or within the cloud band.

In this case the solid line from the Atlantic across Brittany to the Belgian and Dutch coast, representing the front parameter and indicating the maximum of the TFP (blue lines), is close to the leading edge of the frontal cloud band; consequently the zone with high gradients in the thickness lines (green) can be found within and behind the frontal cloud band. Also the zero line of TA (thick red line) accompanies the leading edge of the frontal cloud band, at least over the land areas, leaving the main cold front cloudiness within CA.

The described appearance of key parameters and their relation to the cloud band is typical for an Ana Cold Front (compare Conceptual Models: Cold Front - Meteorological physical background ) where, because of a back bent Warm Conveyor Belt, the main cloudiness and precipitation events can be found behind the surface Cold Front. This conclusion can be supported by vertical cross sections as well as relative streams.

The black lines represent the equivalent potential temperature. The frontal surface is indicated by the crowding zone of isentropes which are inclined downward from approximately 450 - 400 hPa at 52N/03W to the surface at approximately 47N/00E. The orange lines represent the IR (thin) and WV (thick) pixel values, and it is clearly shown that the highest values are behind the surface front. The relative humidity (green) isolines give still more insight because it can be seen that the maxima of humidity are on top of the crowding zone reaching from the ground with values of 90% up to about 300 hPa with values of 50%. Very dry air (less than 10%) protruding downward can already be seen within the highest region of the frontal crowding zone. This is the explanation for the slightly lower cloud tops at the rear side of the Cold Front cloud band already mentioned with the cloud description. Temperature advection confirms that there is pronounced CA within the frontal crowding zone with the maximal values below it; in this special case CA also partly takes place in front of the frontal zone, especially in middle layers; WA in front of the frontal zone can only be found in the lowest and highest layers. This is not unusual for Ana Fronts but a rather strong example.

Two isentropic surfaces are selected for computation of relative streams: the lower surface is the Theta = 298K surface which is on top of the main frontal crowding zone and reaches from 400 hPa nearly down to the surface; the second is the Theta = 304K surface which represents the layer between 400 and 600 hPa and is a weak upper level front. On both surfaces relative streams are computed with a system velocity of 305° and 18 m/s. System velocities are determined with the help of cross correlation methods.

Both surfaces show the three main relative streams involved in frontal processes:

1. The Warm Conveyor Belt, a relative stream from the region in front of the Cold Front; it originates in the Mediterranean and streams across Spain to west Europe and into the North Sea;
2. A relative stream from behind the Cold Front but originating from more humid regions mostly in the north-west; it can be followed from south of Greenland across the Atlantic into the English Channel where it correlates with the cloud band;
3. A relative stream from behind the Cold Front but originating from very dry areas mostly in the north and north-east which is often called dry intrusion; in this case it is involved in an already closed circulation over and north-west of the British Isles.

A main difference between the two levels in the region of the Cold Front can be seen in the transition area between the Warm Conveyor Belt and the wet relative stream from behind the Cold Front. In the lower level (298K) relative streams of the Warm Conveyor Belt cross the frontal line (which is approximately north-east of Normandy) and remain on the rear side. There the air rises from about 800 hPa up to about 600 hPa. In the higher level the Warm Conveyor Belt remains in front of and above the surface line. Comparing again with the vertical cross section, this is the humid air that reaches up to 600 hPa, with values of 60% - 90%.

The rest of the Cold Front cloud band is, in both levels, under the influence of the humid relative stream from behind the Cold Front with rising motion from 700 up to 400 hPa. The cloud edge of the Cold Front band is at both levels identical with the transition between the humid and the dry relative stream from the rear side of the Cold Front band. Comparing with the vertical cross section the humid relative stream can be identified with the maximum of more than 50% relative humidity on top of the 304K surface which is around 49N/00E, while the very low values north-west of it (around 52N/02W) indicate the dry intrusion. The whole situation is very typical for an ana front (compare Conceptual Models: Cold Front - Meteorological physical background ).

After the evaluation of the thermal characteristics of the Cold Front and of the air masses involved, parameters are discussed which can give some indications about the strength of the connected weather and cloudiness. Maxima of vorticity advection (PVA) superimposed on frontal cloudiness influence weather activity by contributing to upward motion. There are two points of view:

1. PVA maxima indicating the approach and/or deepening of troughs; in these cases curvature vorticity is the dominant parameter and the 500 hPa level is a typical height to look at.
2. PVA maxima indicating the approach of a jet streak; in these cases shear vorticity is the dominant parameter and the 300 hPa level is a typical height to look at.

The PVA maximum at 500 hPa (yellow lines) is very pronounced, which means an active frontal system. At 300 hPa the zero line of shear vorticity (yellow) cuts the TFP (blue lines) approximately over the Netherlands giving room for two conclusions:

• According to classical definitions this could be the area of the transition from Cold Front to Occlusion (the Occlusion point).
• A PVA maximum at 300 hPa can be expected superimposed on the frontal cloudiness on the left side of the zero line; this leads to a different conceptual model: Intensification of frontal cloudiness by jet streak crossing (sub-menu) and will be investigated there.