The Cold Front cloud band has already been discussed in part 1 of this case study (compare Cold Front ), but the part in that section concerning the cloud band situated over south France and the western Mediterranean shows a more complicated structure and has been shifted to the "special investigation" under discussion.

The diagnosis in the SatRep indicates three different conceptual models:

• A "Front Intensification by Jet Crossing", called "Jet Intensification" (compare Conceptual Models: Front Intensification by Jet Crossing ) in the area of enhanced cloudiness over south France and the Pyrenees
• A "Cold Front in Cold Advection" (compare Conceptual Models: Cold Front in Cold Advection ) for the short front-like branch south of the Pyrenees along the Spanish east coast
• A "Cold Front in Warm Advection" (compare manual, Conceptual Models: Cold Front in Warm Advection ) for the eastern branch in the western Mediterranean, from the French south coast to the North African coast.

This is a rather unusual diagnosis and has to be enhanced.

The satellite image shows an area with brighter grey shades reaching from central France across the easternmost parts of the Pyrenees approximately to the Balearic Islands. There is a broadened cloud bulge close to the Pyrenees. Such a configuration is typical for a Wave or an intensification produced by the crossing of a jet streak. Because of the cellular structure the latter conceptual model seems to be more likely. The following key parameters and their combinations are regarded as relevant for the conceptual model "Front Intensification by Jet Crossing" (short "Jet Intensification"):

Front Indicator:	This is a line indicating the maximum of the thermal front parameter (TFP) 500/850 hPa (*10-1K m-1)
Isot + PVA300>=3:	This is a combination of isotachs greater than 30 units (m sec-1) and positive vorticity advection (PVA) at 300 hPa greater than 3 units (*10-9sec-2)
Isot + PVA500>=2:	This is a combination of isotachs greater than 30 units (m sec-1) and positive vorticity advection (PVA) at 500 hPa greater than 2 units (*10-9sec-2); both parameter combinations locate the exit region and the PVA maximum in relation to the frontal cloud band
Isot300 + Scher300=0:	This parameter combination shows the isotachs greater than 30 units (m sec-1) and the zero line of shear vorticity which marks the jet axis; one can get information about the existence and strength of jet streaks and the course of the jet axis
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 of sinking or rising motion; they can be used for cloud configurations requiring special explanation

Both levels show that there is a jet streak over the Iberian Peninsula which can still be seen at 500 hPa but is fully developed at 300 hPa. PVA maxima in the left exit region are very pronounced, but inclined with height so that the maximum at 500 hPa is more to the south-west influencing the short Cold Front - like branch over east Spain. As can be concluded from the WV image, the isotach maximum is almost completely in a very dry (black) area, which is a rather unusual situation. Mostly, the different physical state on either side of the jet axis can easily be noticed by the transition from bright grey shades on the anticyclonic side to black grey shades on the cyclonic side of the jet axis. The parameter fields in these images are from the ECMWF forecast fields (12 hourly forecast), just as they are used in the operational SatRep. In this case, an inspection of the same parameters in the analysis fields can enlighten the situation. The parameter fields computed from analysis data differ noticeably from those above, computed from the forecast data:

1. There is no separate jet streak above the Iberian Peninsula, but only a southward oriented secondary branch belonging to the main jet streak over the Atlantic along 50N; there is no indication of a jet streak at 500 hPa at all. Nevertheless, at 300 hPa the PVA maximum fits to the left exit region and accompanies the intensified frontal cloud part over France.
2. At both levels a second PVA maximum to the south of the one just discussed, which accompanies the short branch of the Cold Front over east Spain, can be seen. This PVA maximum has the same strength as the northern one and should, since it is not combined with a jet streak, be a consequence of a propagating trough. This strong PVA maximum may also play an important role in the formation of the short cloud band which will be discussed further below.

So, from the comparison of the relevant parameter fields computed from ECMWF forecasts and diagnoses, the conceptual model "Jet Intensification" is restricted to the northern part of the cloud intensification over France and the Pyrenees, while a clear indication of the development of frontal cloudiness south of it can be drawn at both height levels. This will be enlightened by the key parameters typical for Cold Fronts.

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 (*10-1K m-1)
TFP + rel.Top + TA>=0:	This combination contains the thermal front parameter (TFP) 500/850 hPa greater than 3 units, the equivalent thickness (rel.Top) 500/850 hPa (K) and warm advection 500/1000 hPa (TA>=0) (K 12h-1); this parameter combination contains the information about the thermal situation in the area of the 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 + abs.Top1000:	This is the combination of height contours at 1000 and 500 hPa (gpm); this combination of key parameters is indicative of the stream 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 of more severe weather events because of increased vertical motion (compare manual 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 of sinking or rising areas; they can be used to explain cloud configurations requiring special explanation
VQS:	Gives the location of available vertical cross sections (VQS)

For the detailed discussion of the whole frontal cloud band see Cold Front . There are two branches of maximal values of the TFP: a western one related to the short cloud band at the Spanish east coast and an eastern one on the rear side of a cyclonically curved cloud band. While the western one is in CA but close to the zero line of TA and within a zone of high thickness gradients, the eastern one is completely under the influence of WA and in the thickness zone of high gradients in front of the thickness ridge. The latter configuration would be highly indicative of a Warm Front but several conditions contradict this. The most important one is the genesis of the whole cloud system, which clearly shows that this cloud part has developed from the Cold Front; the cyclonic curvature is a sign for this development. Another indication can be drawn from the vertical cross sections discussed further below.

The height contours at 1000 and 500 hPa reveal an atypical situation which may be the main reason for the untypical substructures. There is a low at the 1000 hPa surface exactly at the rear of the eastern cloud band, which is the one that shows features of a Warm Front in the thickness and temperature advection field. In contrast to this, there is a newly developed short Wave trough and ridge system at the 500 hPa level which leads to a distribution of a height ridge superimposed on the eastern cloud band, but a pronounced trough at the rear of the western cloud band. The latter is connected with the PVA maximum mentioned above in the paragraphs about "Jet Intensification".

These cross sections have already briefly been mentioned in part 1 of this case study (compare Cold Front ).

The short frontal cloud band along the Spanish east coast is accompanied by a configuration of isentropes that shows a very unstable area from the surface up to 600 hPa, with very dry air above 850 hPa and CA in a layer from the earth surface up to about 550 hPa. A zone of strong convergence accompanied by strong upward motion has much more the shape of a frontal zone than the configuration of equivalent potential temperature. From this distribution of parameters it seems to be clear that no thick cloud band can be expected, but rather some convective effects according to the existing instability. In addition to this it has to be remembered that this cloud band is connected with the small scale upper level trough and the PVA maximum in front of it.

The eastern frontal cloud band shows indeed a configuration of isentropes which is typical for a Warm Front accompanied by maximal WA. But it has to be remembered that there are serious doubts about this and that this cloud band is regarded rather as a modified cold front than as a Warm Front. In the vertical cross section, there are some smaller indications supporting this idea:

1. There is a small upper level Cold Front between 400 and 500 hPa on top of the Warm Front zone (38N/06E - 38N/07E) which is accompanied by a pronounced maximum of CA. Such a configuration and intensity is unusual for Warm Fronts.
2. The distribution of relative humidity shows only low values within the supposed Warm Front zone, but a narrow zone of high values of 60 - 70 % between 700 - 400 hPa, which has a distinct connection to the upper level Cold Front.

So, with help of the vertical cross sections some more details of the vertical distribution, especially concerning instability, can be derived. However, still the question about the kind of front type of the eastern cloud band cannot be clarified completely. It seems that there is a pronounced change in the complete state of the troposphere, mainly because of the development of the upper level trough which can be followed further in the forecast.