Download Warm front band theory

1. WARM FRONT BAND - CLOUD STRUCTURE
IN SATELLITE IMAGES
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The satellite image shows an anticyclonically curved synoptic scale cloud
band which is connected with a Cold Front cloud band.
In ideal cases:
o in the VIS image the grey shades are generally white at the rear
edge becoming gradually more and more grey towards the forward
edge;
o in the IR image the grey shades of the cloud band are grey to
white, where the brighter values appear in the ideal case towards
the forward and downstream cloud edge.
In reality:
o very often no continuous cloud band exists but rather several cloud
layers with broken cloudiness, or sometimes even only high
cloudiness;
o in the IR image several white cloud areas are superimposed on
grey lower cloud layers (see Meteorological physical background).
High bright WV pixel values can be observed in the area of the frontal
cloud band.
At the leading edge of the cloud band the WV image shows in connection
with the jet axis a sharp gradient from white to black indicating dry air at
the cyclonic jet side.
In contrast to the Warm Front Shield (see Warm Front Shield), the warm
sector of the Warm Front Band is usually cloudless, except in winter and
spring time when extended fields of fog and low level clouds can exist
which relate to processes in the lowest layers (see Fog and Stratus).
Figure 1: Typical grey shades from warm front bands in VIS, IR and WV satellite images
Figure 2:
10 January 1995/06.00 UTC - Meteosat IR
image
Figure 3:
10 January 1995/06.00 UTC - Meteosat WV
image
This case has an appearance very close to the classical description.
The Warm Front cloud band can be observed on the Atlantic (east of
approximately 25W) extending to Scotland and Northern Ireland. Several
features mentioned above can be observed:
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the gradual increase of cloud top temperatures from the rear to the leading
edge (in IR image);
the distinct leading boundary of the cloud band (in IR and WV images);
the Dark Stripe (in WV image) at the cyclonic side of the above-mentioned
cloud edge;
the cloudless warm sector.
Also the relation of the cloud band and the key parameters is very close to the
ideal case (see Key parameters).
Figure 4:
01 March 1995/06.00 UTC - Meteosat IR
image
Figure 5:
01 March 1995/06.00 UTC - Meteosat WV
image
This
case
shows
some
deviations
from
the
ideal
cases.
The cloud band of the Warm Front can be located in the IR and WV images from
the south-eastern part of Sweden across the Baltic Sea and the Baltic countries
to the Ukraine. In the IR image it consists of several high, i.e. cold, cloud areas
and cloud lines superimposed on dark grey, i.e. warmer, cloud tops. In the WV
image a broad white band in the area of the Warm Front indicates high water
vapour content which is bounded by dark stripes on the northern as well as the
southern edges. The northern stripe represents the dry air on the cyclonic side of
the jet. The dark grey area extending from the Benelux countries across
Germany and Denmark to Poland is an area of Fog and Stratus cloudiness.
2. WARM FRONT BAND - METEOROLOGICAL
PHYSICAL BACKGROUND
In the case of a Warm Front, warm air (moist air) moves against colder air (dry air). At
the boundary of these two air masses the warm air tends to glide up over the wedge of
colder air (see Typical appearance in vertical cross section). This process causes the
frontal cloud band, and therefore also the precipitation, to be found in front of the surface
front (or the TFP) (see Weather events).
Figure 6: Schematic picture of the physical processes belonging to a warm front band
The idealized structure and physical background of a Warm Front can be explained with
the conveyor belt theory as follows:
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Frontal cloud band and precipitation are in general determined by the ascending
Warm Conveyor Belt, which has its greatest upward motion between 700 and
500 hPa. The Warm Conveyor Belt starts behind the frontal surface in the lower
levels of the troposphere, crosses the surface front and rises to the upper levels
of the troposphere. There the Warm Conveyor Belt turns to the right
(anticyclonically) and stops rising, when the relative wind turns to a direction
parallel to the front. If there is enough humidity in the atmosphere, the result of
this ascending Warm Conveyor Belt is condensation and more and more higher
cloudiness.
The Cold Conveyor Belt in the lower layers, approaching the Warm Front
perpendicularly in a descending motion, turns immediately in front of the surface
Warm Front parallel to the surface front line. From there on the Cold Conveyor
Belt ascends parallel to the Warm Front below the Warm Conveyor Belt. Due to
the evaporation of the precipitation from the Warm Conveyor Belt within the dry
air of the Cold Conveyor Belt, the latter quickly becomes moister and saturation
may occur with the consequence of a possible merging of the cloud systems of
Warm and Cold Conveyor Belt to form a dense nimbostratus.
3. WARM FRONT BAND - KEY PARAMETERS
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Equivalent thickness:
The cloud band of the Warm Front Band is within the crowding zone of the
equivalent thickness.
Thermal front parameter (TFP):
The TFP can be found close to and parallel to the rear edge of the Warm
Front cloud band. This is in contrast to the Ana Cold Front where the TFP
accompanies the leading edge of the cloud band.
Warm advection (WA):
The whole frontal cloudiness is within a WA maximum, which increases
towards the occlusion point. Consequently the maximum is in front of the
frontal line.
Shear vorticity at 300 hPa:
The zero line coincides with the leading edge of the Warm Front cloud
band.
Isotachs at 300 hPa:
The leading edge of the Warm Front is superimposed by a jet streak with
intensities varying from case to case.
Figure 7:
10 January 1995/06.00 UTC - Meteosat IR image; thermal
front parameter (TFP) 500/850 hPa, green: equivalent
thickness 500/850 hPa
Figure 8:
10 January 1995/06.00 UTC - Meteosat IR image;red:
temperature advection 500/1000 hPa, green: equivalent
thickness 500/850 hPa
Figure 9:
10 January 1995/06.00 UTC - Meteosat IR image; yellow:
isotachs 300 hPa, black: zero line of shear vorticity 300
hPa
4. WARM FRONT BAND - TYPICAL
APPEARANCE IN VERTICAL CROSS
SECTIONS
The isentropes of the equivalent potential temperature across the Warm Front
Band show a crowding zone through the whole troposphere which, looking
downstream, is inclined from low to high levels. The colder air can be found in
front of, and below, the warmer air in front of and above the crowding zone. (see
Meteorological physical background).
The field of humidity shows high values immediately behind and above the frontal
surface of the Warm Front. Low values can be found below the crowding zone of
the equivalent potential temperature.
Like the distribution of the humidity, the field of temperature advection can also
be separated into two parts. Therefore WA exists above and within the crowding
zone of the Warm Front. The maximum of the WA can be found within the
crowding zone in the mid-levels of the troposphere at approximately 500 hPa. On
the other hand CA can be found below and in front of the crowding zone.
At the leading part above the frontal surface, at approximately 300 hPa, the
atmosphere is characterized by a pronounced isotach maximum.
Well developed fronts are accompanied by a zone of distinct convergence within
and divergence above the frontal zone. Consequently, upward vertical motion
can be found above the frontal zone, responsible for cloud development.
The satellite images across the Warm Front in the ideal case are characterized
by typical distributions. While the IR image shows across the frontal area
continuously increasing values of grey shades from the rear to the leading edge,
the distribution of the grey shades in the VIS image is inverse (see Cloud
structure in satellite image). In contrast to this the WV image shows high pixel
values within the frontal cloudiness and a pronounced minimum connected with
the dry air.
Figure 10:
22 November 1995/06.00 UTC - Meteosat IR
image; position of vertical cross section
indicated
The first cross section shows the crowding zones of the equivalent potential
temperature of a surface and upper level warm front with intense WA within the
whole troposphere. In the IR signal the sharp leading edge can be clearly
discriminated. This decrease coincides with the decrease of the relative humidity
field in the second cross section, while values higher than 80% accompany the
frontal zones. The third cross section shows a zone of convergence within the
lower level front, followed by intense upward motion within and above it. The
fourth cross section finally shows the jet streak around 250 hPa connected with
the pronounced decrease of WV pixel values.
Figure 11:
22 November 1995/06.00 UTC - Vertical cross section;
black: isentropes (ThetaE), red thick: temperature
advection - WA, red thin: temperature advection - CA,
orange thin: IR pixel values, orange thick: WV pixel
values
Figure 12:
22 November 1995/06.00 UTC - Vertical cross section;
black: isentropes (ThetaE), blue: relative humidity,
orange thin: IR pixel values, orange thick: WV pixel
values
Figure 13:
22 November 1995/06.00 UTC - Vertical cross section;
magenta thin: divergence, magenta thick: convergence,
cyan thick: vertical motion (omega) - upward motion,
cyan thin: vertical motion (omega) - downward motion,
orange thin: IR pixel values, orange thick: WV pixel
values
Figure 14:
22 November 1995/06.00 UTC - Vertical cross section;
black: isentropes (ThetaE), yellow: isotachs, orange
thin: IR pixel values, orange thick: WV pixel values
5. WARM FRONT BAND - WEATHER EVENTS
Warm Front Bands are connected with multi-layered cloudiness, with clearances
in the warm sector. Precipitation can be found well ahead of the surface front
until just after front passage.
Parameter
Description
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Precipitation
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Temperature
Wind (incl. gusts)
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Rises after the passage of the front
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Veering of the wind at the front passage
Sometimes in warm sector increasing and more gusty
winds
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Precipitation areas move faster than surface front
(embedded in WCB).
Sometimes moderate to severe icing ahead of surface
front
In wintertime in the warm sector great risk of fog
In warm sector low level turbulence possible
In summer thunder possible, most likely at the warm
side of the front
Apart from fog in winter and spring, the warm sector a
dry area with clearings
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Other
relevant
information
Rainbands typically 50 kilometres wide and a few
hundred kilometres, usually orientated at a small angle
to the surface front
Slight, moderate or heavy precipitation ahead of the
surface front
In winter ahead of surface front snow, freezing rain
(drizzle) is possible.
After the front passage small areas with drizzle (light
snow in winter)
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Figure 15: Schematic picture from warm front band