Transcript Interpretation Guide to MSG WV Channels: Part II

INTERPRETATION GUIDE TO MSG
WATER VAPOUR CHANNELS
Christo Georgiev
Patrick Santurette
National Institute of Meteorology
& Hydrology, Bulgaria
Météo-France
PART II
Interpretation of 6.2 m channel radiance in image format
 WV imagery analysis related to PV concept
 WV imagery features related to synoptic dynamical
structures
Acknowledgments
The Interpretation Guide to MSG WV Channels is developed
by Christo Georgiev and Patrick Santurette through the
Water Vapour Imagery Project of the bilateral cooperation
between Météo-France and the National Institute of
Meteorology and Hydrology of Bulgaria.
The illustrations made by using satellite imagery from
Meteosat satellites of EUMETSAT, other observational data
and numerical model fields are the property of Météo-France,
which has funded the work on the manual.
This training tool is submitted to EUMETSAT as a
contribution of France and Bulgaria to the MSG Interpretation
Guide.
The Interpretation Guide to MSG
WV Channels is drawn on brief
excerpts from Chapters 1-3 and
Appendix A of the book:
Published by Academic Press
Copyright © 2005, Elsevier Inc.
Chapter 4 of the book is
unique in bringing together
the interpretation of
• water vapor images,
• potential vorticity fields and
• model diagnostics
as a guide to validating
numerical model analyses or
short period forecasts.
More information is available at:
http://books.elsevier.com/earthscience/
Patrick Santurette, Météo-France
Christo Georgiev, NIMH, Bulgaria
WV Imagery Analysis
Many pixels are considered as patterns
and features of grey shades.
The interpretation is aimed to relate the patterns
of different moisture distribution and their
changes with time to specific atmospheric
circulation systems and processes.
WV channel radiance in
image format
• Of the two MSG WV channels, the radiation in 6.2
m band is more highly absorbed by water
vapour and, being presented in image format, it
better reflects moisture content of the
troposphere.
• Therefore, the 6.2 m radiation measurements
are the most relevant of the WV channels to be
displayed and used in image format.
Operational use of WV imagery
• The WV channel radiance in 6.2 m of MSG (6.3
m of Meteosat 1-7) is closely correlated with
humidity field in the layer between 600 and 300
hPa and
provides information on the flow
patterns at middle and upper troposphere.
• Therefore, WV imagery may serve as a tool for
operational synoptic scale analysis.
Association of light and dark imagery
features to dynamical structures
• On a water vapour (WV) image being displayed in
grey shades, the areas of dry upper-troposphere
appear darker, and the areas of higher moisture
content appear lighter.
• The basis for synoptic scale applications of WV
imagery is that moist and dry regions as well as
the boundaries between them often relate to
significant upper-level flow features such as
troughs, deformation zones and jet streams.
DYNAMIC APPROACH FOR WV
IMAGERY INTERPRETATION
• An useful approach is interpreting satellite
imagery jointly with various dynamical fields
for the purposes of operational forecasting.
• In order to perform dynamic water vapour
imagery analysis, it calls for applying relevant
dynamical concepts.
WATER VAPOUR IMAGERY ANALYSIS
WATER VAPOUR IMAGERY ANALYSIS
RELATED TO PV CONCEPT
A simple isentropic coordinate version of PV
PV    a
-1
  - g p θ
-1
, where:
is the air mass density
in xy space
  potential temperature, p  pressure, g is the
acceleration due to the gravity.
 a  f   
is the absolute
isentropic vorticity
Potential vorticity is a product of the absolute
vorticity and the static stability.
UNITS for presentation of PV
10-6 m2 s-1 K kg-1

‘PV-unit’ (PVU)
• Potential vorticity is an effective dynamical
parameter for studying the appearance and
evolution of dynamical structures at synoptic scale
due to the following important properties:
• Conservation for adiabatic frictionless
motions, and
• Specific climate distribution in the
atmosphere
CONSERVATION PRINCIPLE FOR PV
If one neglects diabatic and turbulent mixing processes, PV of an
air parcel remains preserved along it’s 3-D trajectory of motion
A vorticity tube between
constant (iso-) surfaces
• When the distance h
increases (decreasing
of the  gradient), the
vorticity increases
• Conversely, when h
decreases (increasing
of the  gradient), the
vorticity decreases.
CLIMATE DISTRIBUTION OF PV
In the middle and
upper troposphere
PV is ranging from
dynamical tropopause
0.5 to 1 PVU
65°N
LATITUDE
25°N
In the stratosphere
PV > 3 PVU ,
due to the strong
increase of the static
stability.
PV discontinuity around the tropopause and its conservation
property allows us to define the surface of constant PV = 1.5
PVU as the “dynamical tropopause” separating the troposphere
with weak PV, from the stratosphere with its strong PV.
A Positive PV anomaly at upper-levels
and associated synoptic development
PV anomaly
=
> 1.5 PVU
 - potential temperature
A PV anomaly is
produced by a
stratospheric
intrusion in the
upper troposphere.
Due to the PV
conservation, the
anomaly leads to
deformations in 
and vorticity of the
surrounding air
In a baroclinic flow increasing with height, the intrusion of PV
anomaly in the troposphere produces vertical motion: the
deformation of the iso- imposes ascending motion ahead of
the anomaly and subsiding motion behind the anomaly.
TROPOPAUSE DYNAMIC ANOMALY
TROPAPUSE
DYNAMIC
ANOMALY
An upper level PV anomaly, advected down to middle
troposphere corresponds to an area of the 1.5 PVU surface
moving down to mid- or low levels. Such a low tropopause area
(moving in a baroclinic environment) is referred to as a
“tropopause dynamic anomaly”.
TROPOPAUSE DYNAMIC ANOMALY (Example 1/2)
6.2 m image & 1.5 PVU surface heights
A tropopause
dynamic anomaly
exhibits:
• a local minimum
of the 1.5 PVU
surface height;
• descending
motions in upper
and middle
troposphere;
• dark grey shades
of the WV image.
TROPOPAUSE DYNAMIC ANOMALY (Example 2/2)
6.2 m image & 1.5 PVU surface heights
Forward to a
tropopause
dynamic anomaly
these are present:
• a local maximum
of the 1.5 PVU
surface height;
• ascending
motion in upper
and middle
troposphere;
• light grey shades
of the WV image.
Relationship between WV image dark/light shades
and low/high geopotential of the 1.5 PVU surface
OPERATIONAL TOOLS
WV imagery is a tool for upper-level diagnosis.
Efficient approaches for imagery analysis in
forecasting environment are:
• Superimposing WV images on upper level
dynamical fields, derived by NWP models, e.g.
1.5 PVU surface heights; 300 hPa wind field.
• Imagery interpretation with reference to
• Vertical cross-sections of NWP modelderived PV and relative humidity.
• Observational data derived by upper-air
soundings.
Dynamic WV imagery analysis
6.2 m water vapour image
Dynamic interpretation
of WV imagery is
efficient to be
performed in areas of
high contrast between
light and dark image
grey shades that is
produced by
significant large-scale
dynamical processes.
WV imagery and mid/upper level dynamical fields
6.2 m image & 1.5 PVU surface heights
Areas of sharp image
grey shade contrast
are associated with
zones of strong height
gradient of the
dynamical tropopause.
• Low tropopause
heights are correlated
with the dark zones in
the imagery.
• High geopotential of
the 1.5 PVU surface
are associated with
light grey shades.
WV imagery and mid/upper level dynamical fields
1.5 PVU surface heights / wind at 300 hPa (only > 100 kt)
A sharp boundary
between different
moisture or cloud
regime on the WV
image is and area of
strong gradient of the
dynamical tropopause
heights.
This boundary is also
aligned with the zone
of the highest upper
level wind speed.
WV imagery and mid/upper level dynamical fields
Contours of wind speed > 100 kt / wind > 80 kt at 300 hPa
Generally, there is
well defined jet
stream axes nearly
coincident with the
moisture boundaries
oriented southwestnortheast.
The jet axis of the
maximum wind speed
is likely along the
most contrast part of
moisture boundary in
the WV image.
WV imagery and vertical cross-sections
6.2 m image & 1.5 PVU surface height
Cross-section of PV
6.2 m
A synoptic-scale dark zone on the
6.2
m
channel
image
is
consistent with low geopotential of
the dynamical tropopause (1.5
PVU surface, solid cross-section
contour).
Tropopause dynamic anomalies
Cross-section of PV (brown)
Relative
humidity (red)
A tropopause dynamic anomaly is associated with
intrusion of very dry stratospheric air down to middle
troposphere along the zone of tropopause folding.
Tropopause dynamic anomalies
6.2 m WV image
Cross-section of Relative humidity
500 hPa
The strip of nearly black WV image gray shades is
produced by very dry air of less than 10% relative
humidity at mid- to upper troposphere along the zone of
a tropopause folding.
WV imagery and upper air soundings
T – air temperature
TD – dewpoint
TD
T
Three upper air soundings
around the WV dark strip reveal
decreasing
of
upper-level
moisture in the direction of
tropopause folding, as seen by
the darkening in the WV image.
TD
T
TD
T
PV concept and upper air soundings
6.2 m image, 1.5 PVU heights
The tropopause level derived by
using temperature lapse rate is not
correct with references to the wind
field and WV image interpretation in
terms of PV concept.
TD
T
PV concept and upper air soundings
6.2 m image, 1.5 PVU heights
Above the sounding release point:
 High gradient of 1.5 PVU surface
heights
 WV image dark zone
 Dynamical tropopause at 600 hPa
analysed by the NWP model.
TD
Dynamical Tropopause
T
PV concept and upper air soundings
Upper-air sounding
Cross-section of PV
TD
When raising through the tropopause folding, the
radio-sound reported strong wind sheer from
north-easterly at 500 hPa to south-westerly flow
at 300 hPa, divided by a zone of low-speed
winds.
T
PV concept and upper air soundings
300 hPa wind
(blue)
500 hPa wind
(yellow)
Upper-air
sounding
release point
Accordingly, the numerical model has analysed south-easterly
wind at 500 hPa (yellow arrows) and south-westerly flow at 300 hPa
(blue arrows) , as being reported by the upper-air sounding.
WV imagery and PV concept
The 6.2 m channel imagery when interpreted in terms of
PV concept is a powerful operational tool for upper-level
diagnosis, due to the ability for easily detecting
tropopause foldings.
WV image & 1.5 PVU surface height
Cross-section of PV
Superimposing WV images on mid- and
upper level dynamical fields
1.5 PVU surface heights and wind at 300 hPa
Areas of low
tropopause (1.5
PVU surface)
height are
associated with
positive PV
anomalies, and
they are well
correlated with
the dark zones
in the imagery.
•The jet axis closely mirrors the shape of the maximum radiance
contrast in the water vapour image.
•The strong gradient of the dynamical tropopause height follows
the jet and the dark/light contrast in the image.
WATER VAPOUR IMAGERY
ANALYSIS
WV IMAGERY FEATURES RELATED TO
SYNOPTIC DINAMICAL STRUCTURES
WV CHANNEL RADIANCE
RELATED TO UPPER-LEVEL DYNAMICS
 Radiance in 6.2 m channel is related to the synopticscale motion field above 600 hPa and thus it is sensitive
to the following upper level dynamical features:
 Upper level jet
 Upper level PV (dynamic tropopause) anomaly
 Synoptic vertical motion
 areas of ascending air = white zones
 areas of subsiding air = dark zones
 upper level PV / dynamic tropopause anomaly = dark zones
 jet streak / strong gradient of 1.5 PVU surface heights = strong
humidity gradient in the imagery (dry air on polar side)
PV – WV image RELATIONSHIP
UPPER-LEVEL DYNAMICAL PERSPECTIVE
1.5 PVU surface heights / wind at 300 hPa (only > 70 kt)
JET-STREAM RELATED PATERNS
• Being upper-level dynamical structures, the
jet systems are related to characteristic cloud
and moisture regimes that are well seen in the
WV imagery.
JET-STREAM RELATED PATERNS
• The jet streams are important dynamical features of
the synoptic scale circulation.
• One of the most efficient use of WV imagery for
weather forecasting is to observe the structure and
evolution of the jet stream zones in association with
the dynamical tropopause behaviour by applying
the PV concept.
• Changes in the jet stream and the height of the
dynamical tropopause may be considered to predict
time changes in the related circulation systems and
may provide early indication of NWP model validity.
JET-STREAM RELATED PATERNS
Wind at 300 hPa (only > 100 kt)
Typically, on a WV
image, there are
many well defined
boundary features,
and only some of
them are associated
with jet stream axes.
JET-STREAM RELATED PATERNS
1.5 PVU surface heights
Typically, on a WV
image, there are
many well defined
boundary features,
and only some of
them are associated
with jet stream axes.
The jet stream axes are present along the boundaries of different
moisture regimes produced by significant tropopause sloping that is
indicated by strong gradient in geopotential of the 1.5 PVU surface.
JET-STREAM RELATED PATERNS
1.5 PVU surface heights
The jet stream
system with a high
amplitude upper-level
through consists of
two branches.
• A jet stream branch
coming from the
upstream ridge.
• A jet stream branch
on the forward side
of the through.
JET-STREAM RELATED PATERNS
Wind-speed at 300 hPa (only > 100 kt)
The axes of
maximum speed of
the two jet stream
branches
are extended along
cloud boundaries or
along moisture
boundaries.
JET-STREAM RELATED PATERNS
Wind-speed at 300 hPa (only > 100 kt)
The axis of a jet
stream branch
coming from the
upstream ridge
is parallel to the
moisture boundary,
located to the east.
JET-STREAM RELATED PATERNS
Wind-speed at 300 hPa (only > 100 kt)
The jet stream axis
on the forward
through side is more
distinct on the WV
image, and
The jet axis is usually
coincident with
moisture boundaries.
The jet streak appears at the cloud boundary, where such a cloud
boundary is in line along with the moisture boundary.
JET-STREAM RELATED PATERNS
Wind-speed at 300 hPa (only > 90 kt)
6.2 m
Ci
Due to difficulties in
distinguishing cloud
boundaries in WV
imagery grey shades,
the jet stream features
are better seen in a
colour image palette.
The axis of the jet
stream branch from
the upstream ridge is
likely to be present
along the boundary
of cirrus clouds,
when such a cirrus boundary is aligned parallel to the moisture
boundary (Ci are colored in cyan colour) .
JET-STREAM RELATED PATERNS
Wind-speed at 300 hPa (only > 90 kt)
6.2 m
At the convex segment
of the jet stream
feature on the forward
side of the through, the
jet axis is present close
to the cirrus boundary,
when this boundary is
aligned with a moisture
boundary.
At the poleward and
the equatorward
segments of the
feature,
the jet axis is often coincident with boundaries between different
moisture regimes.
JET-STREAM RELATED PATERNS
Wind-speed at 300 hPa (only > 90 kt)
6.2 m
Dynamical structure of
the jet stream zone in
the view of the PV
concept may well be
seen in the
Vertical cross-sections
Relative Humidity
Wind speed
The moisture/upper-level cloud
boundary coincides with the
maximum wind speed
JET-STREAM RELATED PATERNS
Wind-speed at 300 hPa (only > 90 kt)
6.2 m
The intrusion into the
troposphere of dry
stratospheric air
with high PV produces
warm brightness
temperatures of the
WV image
Relative Humidity
Potential Vorticity
JET-STREAM RELATED PATERNS
Wind-speed at 300 hPa (only > 90 kt)
6.2 m
The position of the
maximum wind-speed
contour coincides with
the zone of folding of
the tropopause (1.5
PVU constant surface).
Potential Vorticity
Wind speed
Therefore, being in the position of a jet
stream the PV anomaly is in a dynamic
phase of interaction with the jet.
Upper PV anomaly in a dynamical phase:
tropopause anomaly, jet-streak, vertical motion
vertical motion
Interaction between tropopause anomaly and the
jet stream  « forcing» of a jet-streak
INTERACTION OF THE JET STREAM WITH
A TROPOPAUSE DYNAMIC ANOMALY
WV imagery is a tool for observing tropopause
dynamic anomalies and jet streams in the
context of their interaction, which is critical for
the evolution of the synoptic situation.
Interaction of a jet-stream with a tropopause dynamic anomaly:
A jet-streak appears in the southern part of the anomaly (red
arrows), associated with strong tropopause heights gradient
WV image
dark
zone
Tropopause
dynamic
anomaly
Jet-streak
As seen in the WV image
Wind-speed at 300 hPa (only > 80 kt)
Jet-streak
CYCLOGENESIS
• WV imagery is an operational tool for studying the
cyclogenesis, from the very beginning of the
process (about 24 to 48 hours before the onset of
surface deepening) to the decay of the low system,
as well as during any periods of reinforcement.
Cyclogenesis development :
conceptual schema in satellite imagery
• Before the onset of
cyclogenesis, the low
level (blue arrow in the
cold air and orange in the
warm air) and upper level
(brown arrow) flows are in
the same direction.
• There is no clear
organisation of the cloud
mass; the clouds
associated to the moist
warm air are broken and
scattered in several
layers.
Cyclogenesis development :
conceptual schema in satellite imagery
• As the low deepening: at the
lowest part of the cloud mass
this organises a flow with a
direction different from the
upper level flow.
• North of the low, the upward
flow drags low and medium
clouds around the low centre
in a direction perpendicular to
that of the highest clouds: a
cloud head with a cusp or
hook shape appears.
• At the same time, the subsiding cold flow enters the south of the low (dry
intrusion; dark to light blue arrow), while the upstream cold air is
recaptured north of the low by the northerly flow (light blue broken arrow).
CYCLOGENESIS
• WV imagery is a useful operational tool for
interpretation the following different aspects
concerning cyclogenesis:
• Cyclogenesis with upper-level precursors.
• Dry intrusion as an ingredient and a precursor of
cyclogenesis.
• Dry intrusion related to kata- and ana-cold fronts.
CYCLOGENESIS WITH
UPPER-LEVEL PRECURSOR
• Deep tropospheric cyclogenesis is a result of
a baroclinic interaction between a tropopause
dynamic anomaly and a jet-stream in upper
level as well as a low-level baroclinic zone.
• The crucial elements leading to such a
cyclogenesis with an upper level precursor are
observed by means of synoptic and WV
imagery analyses.
UPPER-LEVEL PRECURSOR OF
CYCLOGENESIS
An upper level precursor of deep tropospheric
cyclogenesis is a clear isolated tropopause
dynamic anomaly at an initial phase that is
seen as a dry feature in the WV imagery.
Cyclogenesis with upper-level precursor
WV imagery analysis
tropopause
specific
dark WV
imagery feature
dynamic
anomaly
1.5 PVU heights
A PV anomaly, visible on the WV image as a dry zone,
corresponding to a minimum of the 1.5 PVU surface height.
Cyclogenesis with upper-level precursor
WV imagery analysis
The upper-level forcing at a very initial phase of
cyclogenesis is evident in the superposition of
the WV image and dynamical fields.
Cyclogenesis with upper-level precursor
tropopause
dynamic anomaly
WV imagery analysis
The moist white zone
‘C’ associated with
relatively high
tropopause height is
the sign of ascending
motion downstream
the tropopause
dynamic anomaly.
The baroclinic zone at location ‘B’ is marked by a good relation
between the jet axis (the most inner blue contour), the strong
gradient of 1.5 PVU heights (red contours), and the strong
humidity gradient in WV image dark / white shades.
Cyclogenesis with upper-level precursor
WV imagery analysis
17 February 1997 1200 UTC
18 February 1997 1200 UTC
The cyclogenesis occurs as a result of interaction between the
tropopause dynamic anomaly and the baroclinic zone. The WV
images show the main features at the beginning of the
interaction and during the development phase after 24 hours.
Cyclogenesis with upper-level precursor
WV imagery analysis
17 February 1997 1200 UTC
The dark spot associated with the tropopause dynamic anomaly
(at the green arrow) approaches the white band ‘B’ , which
corresponds to the baroclinic zone, and then interacts with it.
Cyclogenesis with upper-level precursor
WV imagery analysis
17 February 1997 1200 UTC
In the same time, the white band ‘B’ undulates, taking an
appearance of a baroclinic leaf, and becomes clearest as it
is approached by the white zone ‘C’, originally associated
with the tropopause dynamic anomaly.
Cyclogenesis with upper-level precursor
WV imagery analysis
17 February 1997 1200 UTC
18 February 1997 1200 UTC
During the development phase the two white features ‘B’ and ‘C’
merge, contributing to formation of a cloud head ‘H’ to the north
of cyclogenesis area. The dry spot, associated with the PV
anomaly develops as a dry intrusion to the south-west.
WV imagery patterns of cyclone dynamics
A : tropopause
anomaly as a
precursor of
cyclogenesis
B : baroclinic
zone as seen in
moist ascent
P : cloud head
I : dry intrusion
WV imagery patterns of cyclone dynamics
A : tropopause
anomaly as a
precursor of
cyclogenesis
B : baroclinic
zone as seen in
moist ascent
P : cloud head
I : dry intrusion
WV imagery patterns of cyclone dynamics
A : tropopause
anomaly as a
precursor of
cyclogenesis
B : baroclinic
zone as seen in
moist ascent
P : cloud head
I : dry intrusion
WV imagery patterns of cyclone dynamics
A : tropopause
anomaly as a
precursor of
cyclogenesis
B : baroclinic
zone as seen in
moist ascent
P : cloud head
I : dry intrusion
WV imagery patterns of cyclone dynamics
A : tropopause
anomaly as a
precursor of
cyclogenesis
B : baroclinic
zone as seen in
moist ascent
P : cloud head
I : dry intrusion
WV imagery patterns of cyclone dynamics
A : tropopause
anomaly as a
precursor of
cyclogenesis
B : baroclinic
zone as seen in
moist ascent
P : cloud head
I : dry intrusion
WV imagery patterns of cyclone dynamics
A : tropopause
anomaly as a
precursor of
cyclogenesis
B : baroclinic
zone as seen in
moist ascent
P : cloud head
I : dry intrusion
WV imagery patterns of cyclone dynamics
A : tropopause
anomaly as a
precursor of
cyclogenesis
B : baroclinic
zone as seen in
moist ascent
P : cloud head
I : dry intrusion
WV imagery patterns of cyclone dynamics
A : tropopause
anomaly as a
precursor of
cyclogenesis
B : baroclinic
zone as seen in
moist ascent
P : cloud head
I : dry intrusion
WV IMAGERY PATTERNS OF
CYCLONE DYNAMICS
• Moist ascent at the baroclinic zone
• Cloud-head
• Dry intrusion
• Kata- and ana-cold fronts
DRY SPOT and DRY INTRUSION
as characteristic WV imagery patterns of the
upper-level dynamics of cyclogenesis
• Deep tropospheric cyclogenesis is associated
with a dry spot on the WV imagery as an upper
level precursor.
• Dry intrusion appears in the leading zone of the
dry spot when cyclogenesis develops.
2nd 1999 Christmas storm over France
WV
Ingredients of
cyclogenesis
Tropopause
anomaly
Polar jet stream
Undulation of the
baroclinic zone
characteristic WV imagery patterns of
upper-level dynamics of cyclogenesis
2nd 1999 Christmas storm over France
WV
Ingredients of
cyclogenesis
Tropopause
anomaly
Analyse ARPEGE
Polar jet stream
Z 1.5 PVU
Undulation of the
baroclinic zone
’w 850 hPa
Jet
Pmer
characteristic WV imagery patterns of
upper-level dynamics of cyclogenesis
2nd 1999 Christmas storm over France
WV
Appearance of a
«cloud-head»
Strengthening of
undulation and
ascending at the
baroclinic zone
characteristic WV imagery patterns of
upper-level dynamics of cyclogenesis
2nd 1999 Christmas storm over France
WV
Dry Intrusion
characteristic WV imagery patterns of
upper-level dynamics of cyclogenesis
2nd 1999 Christmas storm over France
WV
Dry slot
characteristic WV imagery patterns of
upper-level dynamics of cyclogenesis
DRY SPOT, DRY INTRUSION and DRY SLOT:
characteristic WV imagery patterns of
upper-level dynamics of cyclogenesis
• The dry spot is associated with descending
motions at upper levels linked to a tropopause
dynamic anomaly.
• During the development of cyclogenesis the dry
spot extended in the WV imagery leading to a dry
intrusion associated with further expanding of
subsiding motion.
• A dry slot appears as a part of the descending air
entering to the southwest of the surface low.
Dry intrusion: Very dry air, which comes down to low levels
near cyclones and forms a coherent region of dry air. Although reascending close to the cyclone centre, this dry air have normally
had a long history of descent, the driest parts having been close to
the tropopause upstream about two days earlier.
Conceptual model
(Browning, 1997)
Dry intrusion: Close to the cyclone center, horizontal transport
plays a major role in producing variability of the upper troposphere
flow. As a result, some of the dry air that has originally subsided
also moves horizontally or even rises: This may produce potential
instability and convection near the low center.
Conceptual model
(Browning, 1997)
IDENTIFICATION OF DRY INTRUSION
FROM WV IMAGERY
WV imagery is a tool for identifying dry intrusions
and it is useful for operational purposes:
• Monitoring dry intrusions may help the
forecaster to understand what is happening on
the mesoscale and to anticipate what may
happen over a period of a nowcast.
• This is especially valuable in situations of rapid
cyclogenesis, associated with convective activity,
when local warnings are needed.
Dry intrusion and kata- and
ana-cold fronts
• As regards to the WV imagery analysis of the dry
intrusion, it is appropriate to be considered in
relation to the conceptual models of kata- and
ana-cold fronts.
• By means of a joint interpretation of WV imagery
and model fields, we are able to note important
characteristics of the frontal system.
kata- and ana-cold fronts
pure
kata-front
Intermediate cold-front phase
Sections normal
to the surface
cold front (SCF)
through idealised
ana- and katacold fronts
pure
ana-front
Pure kata-cold front
UCF
CLOUDS
DRY AIR
MOIST AIR
SCF
At the kata-cold front the dry-intrusion air overruns the surface cold
front (SCF) for a distance of 10–200 kilometres. The dry-intrusion
then terminates as an upper cold front (UCF) where the cloudiness
deepens abruptly and often convectively.
At a kata-cold front: Low-level moist air is capped by dry air above.
Pure ana-cold front
MOIST AIR
DRY AIR
SCF
At the ana-front a subsiding circulation transverse to the front is
generated. The leading edge of the dry intrusion then progresses
with the surface cold front below the ascending warm moist air,
which generates a wide cloud band behind the SCF
At the ana-cold front: Low-level dry air is capped by moist/cloudy air.
Kata-/ana-cold fronts in WV image overlaied by
w at 500 hPa (blue) and w 925 hPa (red)
At the kata front: The air
is potentially dryer at 500
hPa (w < 8°C) than it is at
925 hPa (w > 8°C). Lowlevel moist air is capped
by dry air above.
At the ana front: The air
is potentially dryer at 925
hPa (w < 8°C) than it is at
500 hPa (w > 8°C). Lowlevel dry air is capped by
moist air above.
Kata-cold front
WV grey
shade
NEARLY
BLACK
MEDIUM
GREY
NEARLY
WHITE
Upper
CF
WV channel
response
250 hPa
400 hPa
HIGH CLOUDS
800 hPa
LOW CLOUDS
or
MOIST LAYER
Surface CF
MOIST WARM FLOW
Ana-cold front
WV grey
shade
DARK
GREY
NEARLY
WHITE
MEDIUM
GREY
WV channel
response
250 hPa
400 hPa
CELLS
HIGH CLOUDS
LOW CLOUDS
800 hPa
MOIST WARM FLOW
Surface CF
Dry intrusion and kata-cold front
WV channel response
NEARLY
BLACK
MEDIUM
GREY
NEARLY
WHITE
HIGH CLOUDS
LOW CLOUDS
Surface CF
• Rearward the SCF
• Between SCF and UCF
• Forward the UCF
Dry intrusion and ana-cold front
WV channel response
DARK
GREY
NEARLY
WHITE
LIGHT
GREY
HIGH CLOUDS
OPEN CELLS
CLOUDS
Surface CF
• Rearward the SCF
• At the zone of SCF
• Forward the SCF
Summary
• Satellite WV imagery may serve as a significant data
source for inspection different moisture regimes in the
vicinity of synoptic scale weather systems.
• In addition to the pure imagery interpretation, WV
images superimposed with various NWP fields
provide a deep knowledge of the horizontal and
vertical flow patterns.
• Using this essential tool in forecasting environment
is a way to help assessing important ingredients of
the bad weather systems.
Conclusion
• Just after receiving the first water vapor imagery from
Meteosat-1 in 1977, it was recognized as a valuable tool
for synoptic-scale analysis.
• Then, sequences of WV images superimposed with upperlevel dynamical fields have been introduced in forecasting
environment as a significant data source that enables:
• To provide knowledge of the motion field that may help in
interpreting WV imagery focusing on the tropopause
dynamic anomalies and cyclogenesis, and
• To highlight important elements of interaction between
significant dynamical features that may be precursors for
subsequent developments.
• Nowadays, the far more advanced system of MSG allows
to use two WV channels, 6.2 and 7.3 m, with enhanced
capabilities to follow the synoptic situation.
Conclusion
• When interpreting WV imagery for operational forecasting
purposes, the following principles are important:
• To look at an animation of WV images in order to see
changes in the dynamical gray-shade features;
• To superimpose various fields of the forecasting
environment onto the WV image to gain insight into
synoptic ingredients of the atmospheric situation ;
• By joint interpretation of WV imagery and dynamical
fields, to identify the crucial elements responsible for
strong development leading to severe weather.
• To keep a critical mind when considering the model
fields: Priority must always be given to the
observational data and satellite imagery.
References
Browning, K. A. 1997. The dry intrusion perspective of extra-tropical
cyclone development. Meteorol. Appl. 4, 317–324.
Hoskins, B., 1997. A potential vorticity view of synoptic development.
Meteorol. Appl. 4, 325–334.
Santurette, P., Georgiev C. G., 2005. Weather Analysis and
Forecasting: Applying Satellite Water Vapor Imagery and Potential
Vorticity Analysis. ISBN: 0-12-619262-6. Academic Press,
Burlington, MA, San Diego, London. Copyright ©, Elsevier Inc.
179 pp.
Weldon, R. B., Holmes, S. J., 1991. Water vapor imagery:
interpretation and applications to weather analysis and
forecasting, NOAA Technical. Report. NESDIS 57, NOAA, US
Department of Commerce, Washington D.C., 213 pp.