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Appearance in Satellite Data

Definition: A Split Front is accompanied by a cyclonically curved cloud band, which, contrary to a classical Cold Front (see Cold Front ), contains a distinct double banded structure with cold cloud top temperatures at the leading edge and warmer cloud top temperatures at the rear edge:

  • In the thick cloud band at the leading part of the Split Front VIS signals are white to grey, IR and WV signals are white, representing a thick, multilayered cloud band;
  • In the low cloud band at the rear part of the Split Front VIS signals are white, IR signals are dark grey to grey and WV signals are either black (if one is using the Meteosat 8 WV 6.2 µm , which shows higher level water vapour) representing a low cloud band with very dry air above or fairly white/light grey if the Meteosat 8 WV 7.3 µm representing the lower atmosphere is used;
  • In the ideal case the boundary between both cloud bands is marked by a sharp gradient of IR and WV pixel values. In reality only a gradual change of IR and WV grey shades exists;
  • Some features of a life cycle can be observed:
    • often EC shaped cellular cloudiness develops within and above the low cloud band on the cyclonic side of the jet axis;
    • in the WV imagery a black area sometimes develops on the anticyclonic part of the jet axis over the low cloud band of the front, indicating sinking dry air connected with relative streams (see Meteorological physical background);
    • in a multilayered leading cloud band the higher cloud fibres are shifted downstream and the high and low cloud bands become decoupled.

Note: In literature (especially US) the name Split front has been used in relation to an upper level Cold Front. This is comparable to "Frontal delay by mountains" and "Decoupling of cloud layers at different heights" in this manual (see Orographic Effects on Frontal Cloud ).

30 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image
30 August 2005/06.00 UTC - Meteosat 8 WV 6.2 image
30 August 2005/06.00 UTC - Meteosat 8 VIS 0.8 image

The leading high cloud band can be observed in the IR and WV images over the Atlantic from 30W/45N to 18W/57N; in contrast to the schematics this is a case where the leading cloud band consists predominantly of high level clouds.

The long rear cloud band, with warm tops, can be seen from 32W/46N to 20W/57N. The jet axis is indicated in the WV image by the two dark lines behind the front. In this case the jet axis is almost perpendicular to the front. The convective cloudiness in the rear cloud band on the cyclonic side of the jet or within the jet is not very striking (but it is there!), but there is dry sinking air above the rear cloud band on the anticyclonic side of the jet.

Surface and upper level fronts are in accordance with the schematic, as shown in the vertical cross section described in more detail later on.

Meteorological Physical Background

The conceptual model of a Split Front is strongly associated with jet streaks and sinking of very dry stratospheric air.

The initial stage of a Split Front is generally an Ana Cold Front type (see Cold Front - Meteorological physical background ). In contrast to the Ana Cold Front, the Warm Conveyor Belt is overrun aloft by the relative stream of the dry intrusion. This process takes place as the warm air ascends ahead of the surface cold front with a forward component relative to the frontal system.

Looking at the situation on isentropic surfaces, the meteorological process which leads to the typical appearance of a Split Front in the satellite images can be explained as follows: together with a jet streak approaching the frontal cloud band, dry stratospheric air is advected on the cyclonic side, and dry tropospheric air on the anticyclonic side of the jet axis. Both air streams are sinking at this stage of development. Relative streams on the isentropic surface are parallel to the jet axis within the jet streak but show a characteristic splitting in the exit region into a northwards oriented and a southwards oriented component. While the southern branch is still sinking, the northern branch is rising again. During an interaction of jet streak and frontal cloud band this configuration of relative streams causes dissolution of cloudiness from above and the Split Front character then appears.

One way to classify the rear edge of the low cloud band is to regard it as a surface front and the rear edge of the high cloud band as an upper level front (see Cloud structure in satellite image). Between these frontal surfaces a shallow moist zone remains (see Weather events). A characteristic feature of the upper level front is that this frontal surface is a moisture boundary and not a thermal boundary (see Typical appearance in vertical cross sections).

Associated with the approaching jet streak, a PVA maximum situated in the left exit region may be superimposed upon the low cloud band of the Split Front. Within this area the development of the above mentioned EC-like cloudiness can often be observed (see Front Intensification By Jet Crossing - Cloud structure in satellite image ).

30 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 318K - system velocity 236°15 m/s, yellow: isobars 318K, position of vertical cross section indicated
30 August 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

Looking at the vertical cross section, the humidity maximum in front and above the 318K surface between 500 and 350 hPa represents the warm conveyor belt (accompanied by peaks in IR and WV pixel values) while on this isentropic surface further upstream, near 350 hPa one can see drier air, which is connected to the relative streams bringing dry stratospheric air over the frontal region.

Key Parameters

  • Temperature advection (WA):
    The ridge of WA is superimposed on the high level cloudiness representing the warm air rising on the upper level frontal surface.
  • Jet streak and positive vorticity advection (PVA):
    A jet streak approaches the cloud band at a large acute angle accompanied by a PVA maximum in the left exit region.
  • Humidity:
    Very dry values in the upper and middle troposphere above the low level cloud band and a strong gradient between the two cloud bands at different heights can be observed.
  • Relative streams and potential vorticity (PV):
    Typical configuration of relative streams as described in the Meteorological physical background, PV anomaly on relevant isentropic surfaces indicating stratospheric air.

30 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image; red: positive vorticity advection (PVA) 300 hPa, yellow: isotachs 300 hPa

Typical Appearance In Vertical Cross Sections

In the ideal case the isentropes of the equivalent potential temperature show two frontal gradient zones, an upper level front and a surface front. Both zones have Cold Front inclinations. While the upper level front is connected to the high leading cloud band, the surface front represents the low-level cloudiness to the rear of the frontal system. Whereas the zone of the surface front is pronounced, that of the upper level front is quite weak. Therefore, the upper level front is not characterized as a thermal boundary but rather as a moisture boundary (see Meteorological physical background).

The field of temperature advection often shows pronounced WA in front of and above the upper level frontal zone, which is connected with the upper level cloudiness. CA, typical for Cold Fronts, is situated below the surface front.

The most characteristic feature of the humidity distribution is a dry area at higher levels between the two frontal zones. High values of humidity can be found in front of the frontal zones.

In the case of superimposed EC cloudiness, a distinct isotach and PVA maximum can be found above the surface front at approximately 300 hPa (see Front Intensification By Jet Crossing - Typical appearance in vertical cross section ).

Looking at the distribution of humidity the satellite signals, IR and WV images show the highest pixel values in front of the upper level front and, if existing, within the EC - like cloud. To the rear of the upper level front the VIS signals are usually higher while IR and WV signals are much lower than those in front of the upper level front (see Cloud structure in satellite image).

Compare the chapter Cloud structure in satellite image, where the cross section is indicated in the image. In contrast to the ideal case the surface Cold Front has a superadiabatic layer in the lower levels of the troposphere. The upper level Cold Front is well developed. The distribution of humidity shows the described insertion of drier air between the two frontal zones. The IR and WV images show the pronounced decrease of temperature from the low to the high cloud part.

30 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated
30 August 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
30 August 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), green thick: vorticity advection - PVA, green thin: vorticity advection - NVA, orange thin: IR pixel values, orange thick: WV pixel values
30 August 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), yellow: isotachs, orange thin: IR pixel values, orange thick: WV pixel values

Weather Events

Weather events are highly variable and have to be split into two parts: the upper level front and the shallow moist area.

 

Upper front

Parameter Description
Precipitation
  • Moderate to heavy showery precipitation
  • Quite often thunderstorms are observed.
Temperature
  • No significant change
Wind (incl. gusts)
  • Around embedded Cb's strong gusts are possible.
Other relevant information
  • The upper front at the transition between high and low cloud part
  • Hail and thunderstorms possible during whole year
  • Risk of moderate to severe icing and turbulence

 

Shallow moist area, including the surface front

Parameter Description
Precipitation
  • Slight to moderate rain or drizzle
  • Sometimes thunderstorms are observed.
Temperature
  • Falls after the passage of the surface front
Wind (incl. gusts)
  • Veering of the wind at the front passage
Other relevant information
  • Precipitation in area of shallow moist zone behind upper front
  • Risk of moderate icing
  • With superimposed PVA-max showers and thunderstorms are possible.
  • With showers moderate to severe icing and turbulence
28 December 2004/12.00 UTC - Meteosat 8 IR 10.8 image; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm with precipitation, purple: freezing rain, orange: hail, black: no actual precipitation or thunderstorm with precipitation)

References

 

General Meteorology and Basics

  • BROWNING K. A. (1985): Conceptual models of precipitation systems; Quart. J. R. Meteor. Soc., Vol. 114, p. 293 - 319
  • BROWNING K. A. (1986): Conceptual models of precipitation systems; Weather&Forecasting, Vol. 1, p. 23 - 41
  • CONWAY B. J., GERARD L., LABROUSSE J., LILJAS E., SENESI S., SUNDE J. and ZWATZ-MEISE V. (1996): COST78 Meteorology - Nowcasting, a survey of current knowledge, techniques and practice - Phase 1 report; Office for official publications of the European Communities
  • GREEN J. S., LUDLAM F. H. and MCLVEEN J. F. R. (1966): Isentropic relative-flow analysis and the parcel theory; Quart. J. R. Meteor. Soc., Vol. 92, p. 210 - 219
  • HUBER-POCK F. and KRESS CH. (1989): An operational model of objective frontal analysis based on ECMWF products; Met. Atmos. Phys., Vol. 40, p. 371 - 382
  • KURZ M. (1990): Synoptische Meteorologie - Leitfäden für die Ausbildung im Deutschen Wetterdienst; 2. Auflage, Selbstverlag des Deutschen Wetterdienstes
  • ZWATZ-MEISE V. (1987): Satellitenmeteorologie; Springer Verlag, Berlin - Heidelberg - New York - London - Paris - Tokyo

 

General Satellite Meteorology

  • BADER M. J., FORBES G. S., GRANT J. R., LELLEY R. B. E. and WATERS A. J. (1995): Images in weather forecasting - A practical guide for interpreting satellite and radar imagery; Cambridge University Press

 

Specific Satellite Meteorology

  • BROWNING K. A. and MONK G. A. (1982): A simple model for the synoptic analysis of cold fronts; Quart. J. R. Meteor. Soc., Vol. 108, p. 435 - 452
  • BRENNAN M. J., LACKMANN G. M. and KOCH S. E.: An Analysis of the Impact of a Split-Front Rainband on Appalachian Cold-Air Damming. pages 712-731, Vol 18, No 5, Oct 2003.
  • HOBBS P. V., LOCATELLI J. D. and MARTIN J. E., 1996: A new conceptual model for cyclones generated in the lee of the Rocky Mountains. Bull. Amer. Meteor. Soc., 77, 1169-1178, 1996.