Basic comments about this case study: compare Introduction and short case description . For the diagnosis of the physical state of the Occlusion, 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: Occlusion: Warm Conveyor Belt Type ).

Front Indicator	This is a line indicating the maximum of the thermal front parameter (TFP) 500/850 hPa (*10-1Km-1)
rel.Top + TA>=0	This is a combination of the equivalent thickness 500/850 hPa (rel.Top) and warm advection (TA>=0) 500/1000 hPa; this combination of key parameters is especially indicative of the thermal qualities of an Occlusion
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 Front Intensification by Jet Crossing )
Scher300=0	The zero line of shear vorticity (Scher300=0) is a key parameter indicative of the relation between front and jet stream; in this connection the basic rule that the jet stream crosses the cloud band with its axis close to the Occlusion point is included.

The parameter distribution indicates a typical situation for an Occlusion band, namely a pronounced ridge in the thickness field (white) extending from the Netherlands coast into the North Sea, and an area of WA (red) within this ridge and superimposed on the cloud band. As mentioned in connection with the Warm Front (compare Conceptual Model: Warm Front Band , Conceptual Model: Warm Front Shield and Conceptual Model: Detached Warm Front ) and Occlusion (compare Conceptual Model: Occlusion: Warm Conveyor Belt Type ) during this stage of development the maximum of WA is not in the area of the Warm Front Band but shifted to the Occlusion band and close to the Occlusion point.

There is another very typical configuration fulfilled in the key parameters: the zero line of shear vorticity at 300 hPa representing the jet axis (black line) crosses the cloud band exactly at the Occlusion point indicated by the front indicator (thick red line).

As can be seen in the second image, this point as well as the southern part of the Occlusion band are under the influence of PVA.

Looking at these typical diagnoses there should be no doubt about the physical state of an Occlusion in the region under discussion. However, there are some indications that contradict this classical model. The cloud spiral in the satellite image is not as distinct as is usually the case with Occlusion bands, and there are some dissolution effects in the northern part over the North Sea. But the main difference appears in the vertical cross section.

The vertical cross section extends from Sweden across the Occlusion band over the North Sea to central England. The typical appearance of an Occlusion band in the vertical cross section can be seen in the manual (compare Conceptual Model: Occlusion: Warm Conveyor Belt Type - Typical appearance in vertical cross section ). Equivalent potential temperature forms a pronounced trough with a Warm Front - like zone at the leading and a Cold Front - like zone at the rearward edge of the cloud band. In the centre of this trough high humidity and a maximum of WA indicate cloudiness as well as the Occlusion process.

In this case equivalent potential temperature shows a completely different structure. The main feature is a pronounced thick zone typical for a Cold Front, inclined from about 400 hPa over Sweden down to gridpoint 27/17, which is close to the rear end of the cloud band immediately east of the Scottish coast. A trough-like configuration in the isentropes only exists higher than approximately 600 hPa, and may indicate an Occlusion there.

Wet air is in front and on top of the frontal zone with very high values in high levels above 600 hPa; there is very dry air (less than 30%) within and below the frontal zone and an even drier maximum of less than 20% just approaching on top of the frontal zone from north-east. WV and IR pixel values are very similar with a jump from typically cloud-free to cloudy values near 61°N/08°E, and a rather broad band extends south-westwards over the trough feature in the isentropes. The vertical distribution of temperature advection shows the typical appearance of an Occlusion band with a maximum of WA centred around the trough in the isentropes, typical for an Occlusion but with maximal values of WA below it. The whole frontal zone north-eastward from the gridpoint 29/15 is under CA, an area which is already outside the main part of the cloud band.

Summarizing these different features, one can conclude on the basis of a couple of key parameters, that an Occlusion cloud band exists but is undergoing transformation to a new Cold Front. This process will be evaluated in more detail.

Two characteristic isentropic surfaces are selected: ThetaE = 290K, a surface which is within the frontal zone of the Cold Front close to its top and appears only in this cloud system, and ThetaE = 300K. This surface is relatively high in relation to this cloud system, but the connection to the main Warm and Cold Front cloud bands is more evident.

On the 290K surface in the area of the cloud band, the layer from the ground up to about 600 hPa is considered. Relative streams are oriented normally to the vertical cross section and flow through it at about 700 hPa with a rising component, while further north-westward the relative streamlines keep approximately the same height. Comparing with the other parameters in the vertical cross section, this is the point where maximal WA exists, but where also already the transition between wet and dry air masses takes place. From the point of view of conceptual models this relative air stream should be characteristic for the Cold Conveyor Belt (compare Conceptual Models: Occlusion: Warm Conveyor Belt Type - Meteorological physical background ) which does not contribute much to the cloudiness.

Another relative stream merges with the Cold Conveyor Belt from more north-eastern directions exactly in the region of the cross section. This stream is sinking from about 500 hPa down to 650 hPa where it meets the relative stream from the south which has been discussed above. Comparing again with the vertical cross section, this relative stream from east and north-east transports much drier air.

The higher 300K isentropic surface describes a layer between 400 and 350 hPa in the area of the vertical cross section. In this layer a relative stream from the humid side on the back of the Cold Front crosses (compare Cold Front and Conceptual Models: Cold Front - Meteorological physical background ) the vertical cross section, rising strongly over the North Sea but only moderately in the more northern part of the Occlusion. Describing the qualities of air masses at this height of the vertical cross section clearly shows that this is the region of highest humidity, connected with the brightest peak in the IR and WV imagery. So this relative stream is responsible for the formation of the high level cloudiness.

Similarly to the lower 290K surface, a sinking relative stream from north-eastern directions forms a limiting stream line for the north-eastern edge of the cloud band.

In addition to the contents of the image just discussed, the blue lines indicate isentropic potential vorticity (IPV) on this surface. According to theory, IPV values lower than 1 unit are indicative of tropospheric air, values higher than 2 IPV units of dry stratospheric air, while between 1 and 2 IPV units a transition area is expected. In this situation it can be concluded that very dry stratospheric air is transported by relative streams from the Norwegian coast under sinking motion in a south-eastward direction towards the Occlusion cloud band.

This may be a typical situation for the start of a transformation from Occlusion to Cold Front cloud band. For a further development see Forecast for "Special Investigation": 18 February 12.00 to 19 February 06.00 UTC .