Conceptual Models ofCold Fronts:

AnacoldfrontKatacoldfront

Cloud Structures in Satellite Images

Typical cloud configurations:Anacoldfront

• A multilayered cloud band

• White in VIS, white/grey in IR and WV

• A black stripe at the cold side in the WV image

CF

Typical substructures

• Wave bulges at the rear cloud edge

• Increased cloud area with embedded convective cloud

VIS

bright: thick cloud

grey: thin cloud

IR

grey: warm tops

white: cold tops

WV

black: dry air

Typical cloud configurations:Katacoldfront

• Double structure:• Leading part:

– multilayered cyclonically curved cloud band in all three channels

• Rearward part: – darker grey shades in IR

continuously increasing northward to the point of occlusion

– same area is bright in the VIS but dark in the WV image

CF

• Often:– A high cloud fibre representing the jet axis crosses the

frontal cloud band

• Small to meso scale areas of enhanced and mostly cellular cloudiness can be superimposed on the lower cloud part – on the cyclonic side of the jetaxis

VIS

white: thick

white to grey: multilayered

white: cold topsgrey: warm tops

crossing jet axis

dry air above

Meteorological Physical Background

Physical Background Anafront: General Ideas

• Downward inclined frontal zone (isentrops)

• Upgliding of warm air on top of the frontal zone

• Downgliding of cold air below the frontal zone

• Cloudiness and precipitation at the rear of the surface front

Physical Background Anafront:Conveyor Belt Theory

• Rising warm conveyor belt: – backbent in relation to

surface front, at least in lower layers

• Rising upper relative stream: – in the rearward part of the

cloud band; – from behind the frontal

zone but from the more humid anticyclonic jet side

Warmconveyor

belt

Upperrelativestream

Physical Background Anafront:Conveyor Belt Theory

• Sinking dry intrusion – at the rear of the cold

front band – in connection with

black WV stripe; – air from the cyclonic

jet side

Dry intrusion

frontal cloud

Warm conveyor belt and upper relative stream are the reason for cloudiness and precipitation

w.c.b.u.r.s.

w.c.b.u.r.s

dr.i.

Physical Background Katafront: General Ideas

• Downward inclined frontal zone (isentropes)

• Downgliding of cold air below the frontal zone

• Circulation cell in front of the frontal zone

• Restriction of upward motion in this cell by sinking dry air above

• Dry air originates from behind the frontal zone

• Cloudiness and precipitation in front of the surface front

Physical Background Katafront:Conveyor Belt Theory

• Rising warm conveyor belt – in front of the surface

front, at least in the leading parts of the frontal cloud band and the lower layers

– The warm conveyor belt is rather parallel to the cloud band

warmconveyor

belt

Physical Background Katafront:Conveyor Belt Theory

• A rising upper relative stream – in the upper layers above

the warm conveyor belt; – originates from behind the

frontal cloud band,but from the more humid anticyclonic jet side

– The upper relative stream crosses the cloud band and the warm conveyor belt

Upperrelativestream

Physical Background Katafront:Conveyor Belt Theory

• Sinking dry intrusion – in upper levels; – originates from behind

the frontal cloud band but from the cyclonic jet side

– crosses the cloud band and is rather parallel to the upper relative stream

Dryintrusion

frontal cloud band

w.c.bu.r.s

u.r.s.

dr. i.

Key Parameters: Relevant numerical parameters and their

typical distribution

Thickness and TFPAnacoldfront

• High gradient of thickness lines– mostly within the

cloud band • TFP (maximum

line) – at the leading edge

of the cloud band

highthicknessgradient

TFP

TFP

TFP

TFP

Thickness and TFPKatacoldfront

• High gradient of thickness lines– mostly at the rear of

the cloud band • TFP (maximum line)

– at the rear edge of the Katafront cloud band

highthicknessgradient

TFP

TFP

TemperaturadvectionAnacoldfront

• Zeroline of TA – close to the leading

edge of the cloud band – weak WA in front of

the cloud.– strong CA behind the

frontal cloud band

• More intensive WA maxima within wave bulges

WACACA

TA=0

TA=0

TemperaturadvectionKatacoldfront

• Temperature Advection (TA) – zeroline within and

close to the rear edge of the cloud band

– (rather weak) WA within the cloud band

– CA behind

WA

CA

Zeroline of TA:

PVA maximumAnacoldfront

• Behind, partly also superimposed on cloud band;

• the latter is a consequence of curvature and appears for instance in the wave bulges

PVA

PVA

PVA

PVA maximumKatacoldfront

PVA maximum on the cyclonic side of the jet axis

very often superimposed on the low top cloud part

can be connected with enhanced cellular cloudiness

PVA

PVA

PVA

Jet streakAnacoldfront

• mostly strong jet streak existing

• Jet axis: – represented by

zeroline of shear vorticity

– along the rearward edge of the cloud band

Jet axis

Jet axis

Jet streakKatacoldfront

– Jet axis crosses the Katafront cloud band

– very often accompanied by a jet streak

– This leads to the separation into higher and lower cloud tops of the Katafront cloud band

Typical weather events:Anacoldfront

• Moderate to heavy showery precipitation; in winter snow

• Precipitation immedeately at surface front; enhanced area of precipitation behind surface front

• Often thunderstorms observed• Temperture falls rapidly after the

front passage;• veering of the wind at the frontal

passage• At thunderstorms gusts possible• Risk of moderate to severe icing• Risk of moderate to sever

turbulence

Typical weather events:Katacoldfront

• Moderate to heavy precipitation; in winter snow

• Precipitation ahead of surface front• Precipitation sometimes in narrow

bands• Sometimes thunderstorms observed• Temperture falls rapidly after the

front passage;• veering of the wind at the frontal

passage• At thunderstorms gusts possible• Risk of moderate to severe icing• Risk of moderate to sever

turbulence

Typical Appearance inVertical Cross Sections

Downward inclined zone of high gradient of isentropes ("crowding zone");If "bulge" feature in low layer: sign for unstable layer

front front

unstable

x

p

x

p

Typical distribution of parameters in vertical cross sections: Anacoldfront

• Isentrops:– Downward inclined

zone of high gradient• Humidity:

– Moist area on top of isentropic frontal zone; backbent;

– dry area below isentropic frontal zone

fronalzone

humid

humid

dry

Typical distribution of parameters in vertical cross sections: Katacoldfront

• Isentrops:– Downward inclined

zone of isentropes

• Humidity:– Moist zone in front of

frontal crowding zon– very dry air in high

and middle layers of frontal crowding zone

frontalzone

frontalzone

humid

humid

dry

dry

• Temperature advection:– WA in front and

above frontal crowding zone

– CA below frontal crowding zone

´frontal cloud

frontalzone

WA

CA

• Divergence and Vertical motion: – Convergence within

frontal zone, divergence above frontal zone

– Upward motion on top of frontal zone

frontalzone

frontal cloud

• Vorticity advection: – Maximum of PVA at

high levels (500 - 300 hPa)

– backbent in respect to lower frontal zone