Conceptual Models of Cold Fronts: Anacoldfront Katacoldfront

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 tops grey: 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 Warm conveyor belt Upper relative stream

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 warm conveyor 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 Upper relative stream

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 Dry intrusion

frontal cloud band

w.c.b u.r.s

u.r.s.

dr. i.

Key Parameters: Relevant numerical parameters and their typical distribution

Thickness and TFP Anacoldfront High gradient of thickness lines –mostly within the cloud band TFP (maximum line) –at the leading edge of the cloud band high thickness gradient TFP

Thickness and TFP Katacoldfront 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 high thickness gradient TFP

Temperaturadvection Anacoldfront 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 WA CA

TA=0

Temperaturadvection Katacoldfront 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 maximum Anacoldfront Behind, partly also superimposed on cloud band; the latter is a consequence of curvature and appears for instance in the wave bulges

PVA

PVA maximum Katacoldfront 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

Jet streak Anacoldfront mostly strong jet streak existing Jet axis: –represented by zeroline of shear vorticity –along the rearward edge of the cloud band

Jet axis

Jet streak Katacoldfront –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 in Vertical Cross Sections

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

fronal zone 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

frontal zone frontal zone humid dry

Temperature advection: –WA in front and above frontal crowding zone –CA below frontal crowding zone

´frontal cloud frontal zone WA CA

Divergence and Vertical motion: –Convergence within frontal zone, divergence above frontal zone –Upward motion on top of frontal zone

frontal zone frontal cloud

Vorticity advection: –Maximum of PVA at high levels ( hPa) –backbent in respect to lower frontal zone