WV imagery analysis related to PV concept WV imagery analysis related to PV concept WV imagery features related to synoptic dynamical structures WV imagery

WV imagery analysis related to PV concept WV imagery analysis related to PV concept

WV imagery features related to synoptic dynamical WV imagery features related to synoptic dynamical structuresstructures

PART IIPART IIInterpretation of 6.2Interpretation of 6.2 m m channel radiance in image format channel radiance in image format

INTERPRETATION GUIDE TO MSG INTERPRETATION GUIDE TO MSG WATER VAPOUR CHANNELSWATER VAPOUR CHANNELS

Christo GeorgievNational Institute of Meteorology

& Hydrology, Bulgaria

Patrick SanturetteMétéo-France

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.

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.

Published by Academic PressCopyright © 2005, Elsevier Inc.

More information is available at:http://books.elsevier.com/earthscience/

The Interpretation Guide to MSG WV Channels is drawn on brief

excerpts from Chapters 1-3 and Appendix A of the book:

Patrick Santurette, Météo-France

Christo Georgiev, NIMH, Bulgaria

http://books.elsevier.com/earthscience/default.asp?isbn=0126192626

WV WV Imagery Analysis 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 channelWV channel radiance in radiance in image formatimage 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 imageryOperational 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 Association of light and dark imagery featuresfeatures to to dynamical dynamical structuresstructures

• 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 DYNAMIC APPROACH FOR WV IMAGERY INTERPRETATIONIMAGERY 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 WATER VAPOUR IMAGERY

ANALYSISANALYSIS

WATER VAPOUR IMAGERY ANALYSIS WATER VAPOUR IMAGERY ANALYSIS RELATED TO PV CONCEPTRELATED TO PV CONCEPT

A simple isentropic coordinate version of PVA simple isentropic coordinate version of PV

a-1 PV , where:

1 θp - g - is the air mass density in xy space

potential temperature, p pressure, g is the acceleration due to the gravity.

fais the absolute isentropic vorticity

Potential vorticity is a product of the absolute vorticity and the static stability.

UNITSUNITS for presentation of PV 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 FORCONSERVATION PRINCIPLE FOR PV PV

If one neglects diabatic and turbulent mixing processes, PV of an air parcel remains preserved along it’s 3-D trajectory of motion

• When the distance h increases (decreasing

of the gradient), the vorticity increases• Conversely, when h decreases (increasing

of the gradient), the vorticity decreases.

A vorticity tube between constant (iso-) surfaces

CLIMATE DISTRIBUTION OFCLIMATE DISTRIBUTION OF PV PV

In the middle and upper troposphere

PV is ranging from

0.5 to 1 PVU

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.

In the stratosphere PV > 3 PVU ,

due to the strong increase of the static

stability. 65°N 25°N LATITUDE

dynamical tropopause

A PositiveA Positive PV anomaly PV anomaly at at upper-level upper-levelss anandd associated associated synopticsynoptic development development

PV anomaly = > 1.5 PVU

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.

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 - potential temperature

TROPOPAUSE DYNAMIC ANOMALYTROPOPAUSE 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”.

TROPAPUSE TROPAPUSE DYNAMIC DYNAMIC ANOMALYANOMALY

6.2 m image & 1.5 PVU surface heights 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 1/2)TROPOPAUSE DYNAMIC ANOMALY (Example 1/2)

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.


TROPOPAUSE DYNAMIC ANOMALY (Example 2/2)TROPOPAUSE DYNAMIC ANOMALY (Example 2/2)

Relationship betweenRelationship between WV image dark/light shades WV image dark/light shades and low/high geopotential of the and low/high geopotential of the 1.5 PVU surface1.5 PVU surface

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 model-

derived PV and relative humidity.

• Observational data derived by upper-air

soundings.

OPERATIONAL TOOLSOPERATIONAL TOOLS

DynamicDynamic WV WV imagery analysis 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.

WVWV imagery and imagery and mid/upper level dynamical fieldsmid/upper level dynamical fields


• 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.

Areas of sharp image

grey shade contrast

are associated with

zones of strong height

gradient of the

dynamical tropopause.


1.5 PVU surface heights 1.5 PVU surface heights // wind at 300 hPawind at 300 hPa (only > 100 kt)(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.


Contours of wind speed > 100 ktContours of wind speed > 100 kt // wind > 80 kt at 300 hPawind > 80 kt at 300 hPa

Generally, there is

well defined jet

stream axes nearly

coincident with the

moisture boundaries

oriented southwest-

northeast.

The jet axis of the

maximum wind speed

is likely along the

most contrast part of

moisture boundary in

the WV image.

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).

6.2 m image & 1.5 PVU surface height1.5 PVU surface heightCross-section of PVCross-section of PV

WVWV imagery and imagery and vertical cross-sectionsvertical cross-sections

Tropopause dynamic anomaliesTropopause dynamic anomalies

A tropopause dynamic anomaly is associated with intrusion of very dry stratospheric air down to middle troposphere along the zone of tropopause folding.

PV (brown)PV (brown)Relative Relative humidity (red)humidity (red)

Cross-section ofCross-section of

Tropopause dynamic anomaliesTropopause dynamic anomalies

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.

6.2 m WV imageWV image Cross-section of Relative humidityCross-section of Relative humidity

500 hPa

WVWV imagery and imagery and upper air soundingsupper air soundings

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 TTD

T – air temperatureTD – dewpoint

PV conceptPV concept and and upper air soundingsupper air soundings

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.

6.2 m image, 1.5 PVU heights1.5 PVU heights

Tropopause by lapse rate

TD T

PV conceptPV concept and and upper air soundingsupper air soundings6.2 m image, 1.5 PVU heights1.5 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.

Tropopause by lapse rate

Dynamical Tropopause

TD T


Cross-section of PVCross-section of PV

Up

per

-air

so

und

ing

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.

TD T


300 hPa wind 300 hPa wind (blue)(blue)

500 hPa wind 500 hPa wind (yellow)(yellow)

Upper-air Upper-air sounding sounding release pointrelease 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 image WV image && 1.5 PVU surface height1.5 PVU surface height Cross-section of PVCross-section of PV

WVWV imagery and imagery and PV conceptPV 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.

SuperimposingSuperimposing WVWV image imagess on on mid- and mid- and upper level dynamical fieldsupper level dynamical fields

1.5 PVU surface heights 1.5 PVU surface heights andand wind at 300 hPawind at 300 hPa

•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.

JET STREAM

JET STREAMAreas 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.

WATER VAPOUR IMAGERY WATER VAPOUR IMAGERY

ANALYSISANALYSIS

WV IMAGERY FEATURES RELATED TO WV IMAGERY FEATURES RELATED TO SYNOPTIC DINAMICAL STRUCTURESSYNOPTIC DINAMICAL STRUCTURES

Radiance in 6.2 m channel is related to the synoptic-scale 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) anomalySynoptic 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)

WVWV CHANNEL RADIANCE CHANNEL RADIANCERELATED TO UPPER-LEVEL DYNAMICSRELATED TO UPPER-LEVEL DYNAMICS

PV – WV image RELATIONSHIPPV – WV image RELATIONSHIP UPPER-LEVEL DYNAMICAL PERSPECTIVEUPPER-LEVEL DYNAMICAL PERSPECTIVE

1.5 PVU surface heights 1.5 PVU surface heights // wind at 300 hPa (only > 70 kt)wind at 300 hPa (only > 70 kt)

JET-STREAM RELATED PATERNSJET-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.


• 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.


Wind at 300 hPaWind at 300 hPa (only > 100 kt)(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.


Typically, on a WV

image, there are

many well defined

boundary features,

and only some of

them are associated

with jet stream axes.

1.5 PVU surface heights1.5 PVU surface heights

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.


1.5 PVU surface heights1.5 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.

Wind-speed at 300 hPa (only > 100 kt)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.



The axis of a jet

stream branch

coming from the

upstream ridge

is parallel to the

moisture boundary,

located to the east.



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.

when such a cirrus boundary is aligned parallel to the moisture

boundary (Ci are colored in cyancyan colour) .

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,

Ci

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.

the jet axis is often coincident with boundaries between different

moisture regimes.



At the poleward and

the equatorward

segments of the

feature,

6.2 m



Wind speedWind speed

Relative HumidityRelative Humidity

Vertical cross-sectionsVertical cross-sections

Dynamical structure of the jet stream zone in the view of the PV concept may well be seen in the

The moisture/upper-level cloud boundary coincides with the maximum wind speed

6.2 m


Wind-speed at 300 hPa (only > 90 kt)Wind-speed at 300 hPa (only > 90 kt)The intrusion into the troposphere of dry stratospheric air

Potential VorticityPotential Vorticity

warm

Relative HumidityRelative Humidity

with high PV produces warm brightness temperatures of the WV image

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 VorticityPotential Vorticity

Wind speedWind speed

Therefore, being in the position of a jet stream the PV anomaly is in a dynamic phase of interaction with the jet.

6.2 m

Upper PV anomaly in a dynamical phase:Upper PV anomaly in a dynamical phase:

Interaction between tropopause anomaly and the jet stream « forcing» of a jet-streak

vertical motion

tropopause anomaly,tropopause anomaly, vertical motionvertical motionjet-streak,jet-streak,

IINTERACTION OF THE JET STREAM WITH NTERACTION OF THE JET STREAM WITH A TROPOPAUSE DYNAMIC ANOMALYA 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 anomalyInteraction 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

JET- STEAK

WV image darkzone

Tropopause dynamic anomaly

JJet-streaet-streakk As seen in the WV image


Jet-streak

CYCLOGENESISCYCLOGENESIS

• 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 : Cyclogenesis development : conceptual schema in satellite imagery 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.

• 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 development : Cyclogenesis development : conceptual schema in satellite imagery conceptual schema in satellite imagery

CYCLOGENESISCYCLOGENESIS

• 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 CYCLOGENESIS WITH UPPER-LEVEL PRECURSORUPPER-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 UPPER-LEVEL PRECURSOR OF CYCLOGENESISCYCLOGENESIS

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.

specific dark WV imagery feature

1.5 PVU heights1.5 PVU heights

Cyclogenesis with upper-level precursorCyclogenesis with upper-level precursorWV imagery analysis

A PV anomaly, visible on the WV image as a dry zone, corresponding to a minimum of the 1.5 PVU surface height.

tropopause dynamic anomaly

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 precursorCyclogenesis with upper-level precursor

The moist white zone ‘C’ associated with relatively high tropopause height is the sign of ascending motion downstream the tropopause dynamic anomaly.

WV imagery analysis

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.


tropopause dynamic anomaly

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.

WV imagery analysis17 February 1997 1200 UTC

18 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.



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.



BAROCLINIC LEAF

BAROCLINIC LEAF

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.


18 February 1997 1200 UTC


CLOUD HEAD

CLOUD HEADDRY INTRUSION

DRY INTRUSION

DRY SPOT

DRY SPOT

WV imagery patterns ofWV imagery patterns of cyclone cyclone dynamicsdynamics

A : tropopause anomaly as a precursor of cyclogenesis

B : baroclinic zone as seen in moist ascent

P : cloud head

I : dry intrusion




P : cloud head

I : dry intrusion




P : cloud head

I : dry intrusion




P : cloud head

I : dry intrusion




P : cloud head

I : dry intrusion




P : cloud head

I : dry intrusion




P : cloud head

I : dry intrusion




P : cloud head

I : dry intrusion




P : cloud head

I : dry intrusion

• Moist ascent at the baroclinic zone

• Cloud-head

• Dry intrusion

• Kata- and ana-cold fronts

WV IMAGERY PATTERNS OF WV IMAGERY PATTERNS OF CYCLONECYCLONE DYNAMICSDYNAMICS

DRY SPOT and DRY INTRUSIONDRY SPOT and DRY INTRUSION

• 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.

as characteristic WV imagery patterns of the as characteristic WV imagery patterns of the upper-level dynamics of cyclogenesisupper-level dynamics of cyclogenesis

22ndnd 1999 Christmas storm over France 1999 Christmas storm over France

Undulation of the baroclinic zone

Polar jet stream

Tropopause anomaly

WV

characteristic WV imagery patterns of characteristic WV imagery patterns of upper-level dynamics of cyclogenesisupper-level dynamics of cyclogenesis

Ingredients of Ingredients of cyclogenesiscyclogenesis

Undulation of the baroclinic zone

Polar jet stream

Tropopause anomaly

Z 1.5 PVU

’w 850 hPa

Jet

Pmer

Analyse ARPEGE

WV



Ingredients of Ingredients of cyclogenesiscyclogenesis

Strengthening of undulation and ascending at the baroclinic zone

Appearance of a «cloud-head»

WV



Dry Intrusion

WV



Dry slot

WV



DRY SPOT, DRY INTRUSION and DRY SLOT: DRY SPOT, DRY INTRUSION and DRY SLOT:


• 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.

Conceptual model (Browning, 1997)

Dry intrusionDry intrusion:: Very dry air, which comes down to low levels near cyclones and forms a coherent region of dry air. Although re-ascending 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 intrusionDry 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.

IDENTIFICATION OF IDENTIFICATION OF DRY INTRUSIONDRY INTRUSION FROM WV IMAGERYFROM 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 Dry intrusion and kata- and ana-cold frontsana-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 frontskata- and ana-cold fronts

Sections normal to the surface cold front (SCF) through idealised ana- and kata- cold fronts

purekata-front

pureana-front

Intermediate cold-front phase

Pure kata-cold frontPure kata-cold front

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.

DRY AIRDRY AIR

CLOUDSCLOUDS

UCF

MOIST AIRMOIST AIR

SCFSCF

DRY INTRUSIONDRY INTRUSION

Pure ana-cold frontPure ana-cold front

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.

MOIST AIRMOIST AIRC L O U D B A N D

C L O U D B A N D

DRY INTRUSION

DRY INTRUSION

DRY AIRDRY AIR

SCFSCF

Kata-/ana-cold fronts in WV image Kata-/ana-cold fronts in WV image overlaied by overlaied by w w at 500 hPaat 500 hPa (blue) and (blue) and w w 925 hPa925 hPa (red) (red)

Ana cold front

Ana cold front

Kata cold front

Kata cold front

Upper cold front

Upper cold frontAt the ana front:At the ana front: The air is potentially dryer at 925 hPa (w < 8°C) than it is at

500 hPa (w > 8°C). Low-

level dry air is capped by moist air above.

At the kata front:At the kata front: The air is potentially dryer at 500 hPa (w < 8°C) than it is at

925 hPa (w > 8°C). Low-

level moist air is capped by dry air above.

Kata-cold frontKata-cold front

MOIST LAYER

LOW CLOUDS

HIGH CLOUDS

800 hPa

400 hPa

250 hPa

Surface CF

Upper CF

DRY INTRUSION

DRY INTRUSION

MOIST WARM FLOWMOIST WARM FLOW

WV channel WV channel responseresponseWV grey

shadeNEARLY BLACK

MEDIUM GREY

or

NEARLY WHITE

Ana-cold frontAna-cold front

LOW CLOUDS

HIGH CLOUDS

800 hPa

400 hPa

250 hPa

Surface CF

DRY INTRUSION

DRY INTRUSION

MOIST WARM FLOWMOIST WARM FLOW

WV channel WV channel responseresponseWV grey

shadeDARK GREY

NEARLY WHITE

MEDIUM GREY

CELLS

Dry intrusion and Dry intrusion and katakata-cold front-cold front

Kata cold front

Kata cold front

Upper cold front

Upper cold front

NEARLY BLACK

MEDIUM GREY

NEARLY WHITEWV channel responseWV channel response

Surface CF

HIGH CLOUDS

• Rearward the SCF• Between SCF and UCF

• Forward the UCF

LOW CLOUDS

Dry intrusion and Dry intrusion and anaana-cold front-cold front

Ana cold front

Ana cold front

WV channel responseWV channel responseDARK GREY

NEARLY WHITE

LIGHT GREY

Surface CF

HIGH CLOUDS

• Rearward the SCF

OPEN CELLS

• At the zone of SCF• Forward the SCF

CLOUDS

• 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.

SummarySummary

ConclusionConclusion

• 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 upper-level 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.

• 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.

ConclusionConclusion

ReferencesReferences

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.

Documents

WV imagery analysis related to PV concept WV imagery analysis related to PV concept WV imagery features related to synoptic dynamical structures WV imagery