Transcript Frentes 3ª parte - Previsores (SENEAM 2009)

Frentes 3ª parte
M en C Marcial Orlando Delgado D
SENEAM
Meteorología Sinóptica y Análisis I
Trimestre enero marzo 2010
Parameters
by ZAMG
•
Equivalent thickness:
The cloud field of the Detached Warm Front is within the high gradient zone at
the eastern branch of a pronounced ridge of the (equivalent) thickness.
• Absolute topography at 500 hPa:
It is very similar to the (equivalent) thickness, therefore the cloudiness of the
Detached Warm Front is also within the high gradient zone at the eastern
branch of a pronounced ridge of absolute topography.
•
Warm advection (WA):
The cloudiness of the Detached Warm Front is superimposed upon a distinct
(claro) WA maximum. But often two WA maxima can be observed. In this case
the northern maximum is associated with the original Warm Front, and the
southern one with the Detached Warm Front.
•
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Wind Vectors at 500 hPa:
The wind field has, in the area of the cloud field of
the Detached Warm Front, a more or less strong
southern component, and blows normal to the
displacement of the whole frontal system of the
ridge and frontal zones. Consequently, cloud
elements of the Detached Warm Front are moving
quickly southward while the complete cloud
configuration is displaced eastward, much more
slowly.
Shear vorticity at 300 hPa:
The zero line coincides with the leading edge of the
Warm Front cloud shield.
Isotachs at 300 hPa:
The leading edge of the Warm Front cloud shield is
superimposed upon a jet streak with intensities
varying from case to case.
04 January 2005/00.00 UTC Meteosat 8 IR 10.8 image;
blue: geopotential height 500
hPa, green: equivalent
thickness 500/850 hPa
04
January
2005/00.00
UTC
Meteosat 8 IR 10.8
image;
yellow:
isotachs
300
hPa,
black: zero line of
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The parameter distribution is very similar to
the ideal situation described above. If the
wind vectors at 500 hPa (green arrows) are
compared to the relative streams
(see Meteorological Physical Background),
differences in the directions can be observed
at a height close to 500 hPa (296K
isentrope). While the absolute winds are
coming from a north-westerly direction the
relative streams are coming from a more
north-easterly direction, which is very close
to the orientation of the cloud structure.
WARM FRONT
 Detached Warm Front
 Warm Front Band
 Warm Front Shield
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Appearance In Vertical Cross Sections
by ZAMG
Vertical cross sections of the Detached Warm Fronts do not
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differ from the classical band-type Warm Front.
As described before, the isentropes of the equivalent
potential temperature across the Detached Warm Front
show a high gradient zone through the whole troposphere,
which is inclined upwards from low to high levels.
The colder air can be found in front of and below the high
gradient zone, the warmer air being in front, and above
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 high gradient zone of
the equivalent potential temperature.
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Like the distribution of humidity, the field
of temperature advection can also be
separated into two parts. WA exists above
and within the high gradient zone of the
Warm Front. The maximum of the WA can
be found within the high gradient zone
where it often has several maxima from
low up to high levels. On the other hand
CA can be found below and in front of the
high gradient zone. In actual cases the
isentropes forming the lower boundary of
the frontal surface do not represent the
transition from WA to CA, but WA can also
mostly be found far below the frontal
surface while CA exists only at a larger
distance from the surface front.
At the leading part of the system, above
the frontal surface at approximately 300
hPa, a pronounced isotach maximum can
be observed.
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.
In the ideal case, the satellite radiance
values across the Warm Front are
characterized by typical distributions.
While the IR image shows continuously
increasing values of grey shades from the
rear to the leading edge across the frontal
area, the distribution of the grey shades in
the VIS image is reversed (see Cloud
structure in satellite image). In contrast,
the WV image shows high pixel values
within the frontal cloudiness and a
pronounced minimum associated with the
dry air in front. In reality, these variations
of grey shades for Detached Warm Fronts
are, by far, not as clear as in the ideal
conceptual model.
04 January 2005/00.00
UTC - Meteosat 8 IR
10.8 image; position of
vertical cross section
<04 January 2005/00.00 UTCVertical 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
04 January 2005/00.00 UTCVertical cross section; black:
isentropes (ThetaE), blue:
relative humidity, orange thin: IR
04 January 2005/00.00 UTCVertical 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
04 January 2005/00.00
UTC - Vertical cross
section; black:
isentropes (ThetaE),
yellow: isotachs, orange
thin: IR pixel values,
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The first cross section shows the high gradient zones of
the equivalent potential temperature of a pronounced
surface Warm Front with intense WA, especially through
the whole troposphere.
The IR radiance values are characterized by high pixel
values in the centre of the Warm Front cloudiness (pixel
values between approximately 190 and 200 units)
. Lower pixel values, in connection with the dry
(anticyclonic) side (and the superimposed jet), can be
found in the leading part of the Warm Front (pixel values
between approximately 160 and 180 units).
The field of relative humidity shows high values within
the lower to mid-levels, also immediately behind the
surface front (above 80%).
The third cross section shows, in the lower and midlevels, intense upward motion within the high gradient
zone which is caused by a zone of convergence in low
and mid-levels of the troposphere situated within and
below the frontal surface.
Divergence can be found above the frontal surface.
The fourth cross section is characterized by a pronounced
isotach maximum within the leading part of the Detached
Warm Front at approximately 200 hPa. This maximum is
accompanied by a decrease of the WV pixel values.
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Warm Front Bands are accompanied by cloud
bands which usually are shorter than Cold Front
cloud bands
Chapters:
Cloud Structure In Satellite Images
Meteorological Physical Background
Key Parameters
Typical Appearance In Vertical Cross
Sections
Weather Events
References
WARM FRONT BAND
by ZAMG
Warm Front Band - Cloud Structure In
Satellite Images
The satellite image shows an anticyclonically curved synoptic scale
cloud band which is connected to a Cold Front cloud band.
 In ideal cases:
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◦ in the VIS image the grey shades are generally white at the rear edge becoming
increasingly grey towards the forward edge;
◦ 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.
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In reality:
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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 a sharp
gradient from white to black indicating dry air along the cyclonic side of
the jet.
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 clouds can
exist associated with processes in the lowest layers .
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◦ very often no continuous cloud band exists but rather several cloud layers with
broken cloudiness, or sometimes even only high cloudiness;
◦ in the IR image several white cloud areas are superimposed on grey lower cloud
layers (see Meteorological physical background).
15 September
2004/06.00 UTC Meteosat 8 IR 10.8
image
15 September
2004/06.00 UTC Meteosat 8 WV 6.2
image
This case has an appearance very close to the
classical description.
The Warm Front cloud band can be observed
over the Atlantic (just east of approximately
25W) extending to Northern Ireland. Several
features mentioned above can be observed: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) along the cyclonic
side of the above-mentioned cloud edge;
 the cloudless warm sector.
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14 November
2004/00.00 UTC Meteosat 8 IR 10.8
image
14 November
2004/06.00 UTC Meteosat 8 WV 6.2
image
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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 Iceland across the Atlantic towards
Norway. 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. Over Iceland the mountains trigger some
lee clouds, embedded by the Warm Front Band. In the WV
image a broad white band in the area of the Warm Front
indicates high water vapour content bounded by Dark Stripes
on the northern as well as the southern edges. The northern
stripe represents the dry air along the cyclonic side of the jet.
The dark grey area extending over Great Britain is an
extensive area of Fog and Stratus.
n the case of a Warm Front, warm moist
air moves against colder dry air. At the
boundary of these two air masses the
warm air tends to slide up over the wedge
of colder air (see Typical appearance in
vertical cross section). This process
causes the frontal cloud band, and the
associated precipitation, found mainly in
front of the surface front (or the TFP)
(see Weather
events).
Warm
Front Band
- Meteorological
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Physical Background
The idealized structure and physical
background of a Warm Front can be
explained with the conveyor belt
theory:
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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.
Discussion
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In addition to this idealized structure, the experience from a series of case
studies carried out at ZAMG differs somewhat from the one described
above, allowing more differentiation.
In the case of a band type the Warm Conveyor Belt can be observed
within the warm sector up to the Cold and Warm Front line.
If there is high cloudiness in front of and parallel to the Warm Front line,
it is situated within an (at least in the area of the fronts) ascending
conveyor belt from the rear side of the Cold Front extending from southwest to north-east, the so-called upper relative stream.
While the high cloudiness can thus be explained, the Warm Front
cloudiness in the lower levels of the troposphere develops, as in the ideal
case described above, within the Cold Conveyor Belt.The conveyor belt
situation, especially of the Warm Conveyor Belt and the upper relative
stream, described above can be found in a thick layer of isentropic
surfaces, but there is some tendency for the Warm Conveyor Belt in lower
layers to overrun the surface Warm Front to a small extent.
The ascending Warm Conveyor Belt in the warm sector is not
accompanied by appreciable cloudiness either because of too dry air
masses or/and too little lifting. But it can be observed that in the
ascending Warm Conveyor Belt cloudiness may develop leading to a
second Warm Front type, the Warm Front Shield.
14 November 2004/00.00 UTC - Vertical cross section; black:
isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV
pixel values
The 302K isentropic surface chosen for the relative streams in the
figure below is very close to the upper boundary of the higher
gradient, inclined Warm Front zone.
14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image;
magenta: relative streams 302K - system velocity: 295° 10
m/s, yellow: isobars 300K, position of vertical cross section
indicated
The analysis shows two Conveyor Belts:
there is one from eastern directions across The Netherlands,
Belgium, France, turning northward in an anticyclonic direction
over the Atlantic. This is a typical example of a Warm Conveyor
Belt extending across the warm sector to the leading edge of the
Cold Front and the rear edge of the Warm Front cloudiness.
 A second relative stream originates from the cold air mass over
the Atlantic behind the Cold Front and approaches the Warm
Conveyor Belt over Iceland, extending from there on across the
Atlantic (approx. 66N/2W) towards Norway parallel to the Warm
Conveyor Belt.
 Generally speaking, the Warm Conveyor Belt rises from about
800 hPa over the Atlantic up to 400 hPa over Norway. The relative
stream from the Atlantic exists in a higher layer, rising from about
600 to 450 hPa. The high cloudiness of the Warm Front exists in
this latter relative stream.
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15 September 2004/06.00 UTC - Vertical cross section; black:
isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV
pixel values
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Equivalent thickness:
The cloud band of the Warm Front Band is within the higher
gradient zone of 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 lies 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 upon a jet
streak
Warm Front Band - Key Paramete
14 November 2004/00.00 UTC - Meteosat 8
IR 10.8 image; thermal front parameter
(TFP) 500/850 hPa, green: equivalent
thickness 500/850 hPa
14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; red:
temperature advection 500/1000 hPa, green: equivalent thickness
500/850 hPa
14 November 2004/00.00 UTC - Meteosat 8
IR 10.8 image; yellow: isotachs 300 hPa,
black: zero line of shear vorticity 300 hPa