17 APRIL 1996 - RAPID CYCLOGENESIS: DIAGNOSIS FOR 17 APRIL 06.00 UTC

by ZAMG


17 April 1996/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models, SatRep menu: key parameters for Cyclogenesis
17 April 1996/06.00 UTC - Meteosat WV image SatRep overlay: names of conceptual models
The cloud configuration named "Zyklogenese WV" describes the typical development of a Rapid Cyclogenesis. In a later stage of the SatRep this conceptual model was renamed "Rapid Cyclogenesis" and a new menu with key parameters has been developed. For the conceptual model see Conceptual Models: Rapid Cyclogenesis .

The satellite image shows a typical configuration for such a development:

  1. There is a frontal cloud band with a Wave - like bulge consisting of cold cloud tops approximately around 51N/25W.
  2. Cloudiness with warmer tops emerges from underneath this cloud bulge; in this case this can be recognized around 52N/26-27W.
  3. In the WV image a Dark Stripe representing very dry air can be found on the cyclonic side of the frontal cloud band up to the area of the cloud bulges described above; often a second more northern Dark Stripe exists at the northern boundary of the lower cloud deck which comes forward below the frontal cloud band.

All three features exist in this special case.

abs.Top1000 + PVA500>=2 + TA>=0: This is a commonly used combination of three parameters typically indicative of a wave:
  • The height contours very close to the ground at 1000 hPa which in the case of a Wave shows a trough
  • PVA at 500 hPa greater than 2 units (*10-9sec-2) which in the case of a Wave contains a maximum indicating the deepening of the low
  • WA 500/1000 hPa (K 12h-1) which, together with a maximum of CA at the rear of the wave bulge, is another sign for the circulation taking place in the Wave area transporting warm and cold air in cyclonic directions
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)
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
PV: Represents isentropic potential vorticity (*10-6m2sec-1K kg-1) on several levels
VQS: Gives the location of available vertical cross sections (VQS)

17 April 1996/06.00 UTC - Meteosat IR image; cyan: height contours 1000 hPa, red: temperature advection - WA 1000 hPa, green: positive vorticity advection (PVA) 500 hPa; SatRep overlay: names of conceptual models
This parameter combination shows a rather typical behaviour of the three key parameters from the numerical model except for the fact that the WA maximum seems to be somewhat displaced to the south-west.
17 April 1996/06.00 UTC - Meteosat WV image; isotachs 300 hPa, black: shear vorticity 300 hPa; SatRep overlay: names of conceptual models
There is a jet streak in the area of the Rapid Cyclogenesis under consideration but it fits neither the southern nor the northern Dark Stripe. This is an indication that the numerical model is not correct there and is not able to resolve the two dry air streams.

For a further clarification additional key parameters have to be inspected e.g. the PV and relative streams (compare Conceptual Models: Rapid Cyclogenesis - Meteorological physical background ).

17 April 1996/06.00 UTC - Meteosat IR image; cyan: potential vorticity (PV) 305K, red: isobars; SatRep overlay: names of conceptual models
17 April 1996/06.00 UTC - Meteosat IR image; cyan: potential vorticity (PV) 320K, red: isobars; SatRep overlay: names of conceptual models
Neither the lower isentropic level of 305K nor the higher isentropic level of 320K show an acceptable correspondence between the dry stratospheric air and the Dark Stripes. In the lower level the high PV values are too far in the north to have any connection with the cloud configuration of the Rapid Cyclogensis. This may be interpreted as indicating that dry stratospheric air has not yet protruded downward to 500 - 400 hPa. In the higher level a small zone with PV values higher than 3 units can be recognized from approximately 48N/35W to 51N/10W. This zone is indeed parallel to the Dark Stripes in the WV imagery but a southward displacement may be reasonable. This could lead to the conclusion that dry stratospheric air has reached a height of 300 hPa in this isentropic layer. But it has generally to be concluded that the physical state in these upper levels described by the numerical model does not satisfactorily coincide with the features in the satellite images. In particular the fine structure of the two dry streams cannot be resolved, and dry air characterized by high PV values can be found only in higher levels than those usually found in this stage of development.
17 April 1996/06.00 UTC - Meteosat IR image; magenta: relative streams 300 K - system velocity 209° 8 m/s, yellow: isobars; SatRep overlay: names of conceptual models
17 April 1996/06.00 UTC - Meteosat IR image; magenta: relative streams 308 K - system velocity 208° 9 m/s, yellow: isobars; SatRep overlay: names of conceptual models
Relative streams in the two isentropic layers of 300K and 308K reveal some of the typical features for a Rapid Cyclogenesis (compare Conceptual Models: Rapid Cyclogenesis - Meteorological physical background ). There is for instance at the 300K level a relative stream from south-south-east heavily rising from 800 up to 500 hPa and splitting there into an eastern and a western branch. At the higher level this splitting does not occur but a stream from north-westerly directions is superimposed on the cloud bulges at 400 hPa. Nevertheless the errors of the model discussed above have to be taken into account so that these features can only be regarded as a rough indication of the ongoing processes.

One can summarize that this is a situation where the satellite images indicate the initial stages of a Rapid Cyclogenesis which is also represented by most of the key parameters of the numerical model, but the latter is neither correct in position nor in the ability to resolve fine structures.


SUB-MENU OF 17 APRIL 1996
CYCLOGENESIS IN THE WESTERN MEDITERRANEAN: FORECAST FOR "SPECIAL INVESTIGATION" 17 APRIL 12.00 UTC TO 18.00 UTC
RAPID CYCLOGENESIS: FORECAST FOR "SPECIAL INVESTIGATION" 17 APRIL 12.00 UTC TO 18.00 UTC