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Conceptual Models of Cold Fronts:

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.

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Conceptual Models of Cold Fronts:

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  1. Conceptual Models ofCold Fronts: Anacoldfront Katacoldfront

  2. Cloud Structures in Satellite Images

  3. 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

  4. Typical substructures • Wave bulges at the rear cloud edge • Increased cloud area with embedded convective cloud

  5. VIS grey: thin cloud bright: thick cloud

  6. IR white: cold tops grey: warm tops

  7. WV black: dry air

  8. 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

  9. 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

  10. VIS white: thick white to grey: multilayered

  11. grey: warm tops white: cold tops

  12. dry air above crossing jet axis

  13. Meteorological Physical Background

  14. 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

  15. 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 Upper relative stream Warm conveyor belt

  16. 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

  17. Warm conveyor belt and upper relative stream are the reason for cloudiness and precipitation frontal cloud

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

  19. dr.i. w.c.b. u.r.s

  20. 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

  21. 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

  22. 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

  23. 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

  24. frontal cloud band

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

  26. u.r.s.

  27. dr. i.

  28. Key Parameters:Relevant numerical parameters and their typical distribution

  29. Thickness and TFPAnacoldfront • High gradient of thickness lines • mostly within the cloud band • TFP (maximum line) • at the leading edge of the cloud band TFP high thickness gradient

  30. TFP TFP TFP

  31. 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 high thickness gradient TFP

  32. TFP

  33. 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 CA CA WA

  34. TA=0 TA=0

  35. 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

  36. Zeroline of TA:

  37. PVA maximumAnacoldfront • Behind, partly also superimposed on cloud band; • the latter is a consequence of curvature and appears for instance in the wave bulges

  38. PVA PVA PVA

  39. 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

  40. PVA PVA

  41. Jet streakAnacoldfront • mostly strong jet streak existing • Jet axis: • represented by zeroline of shear vorticity • along the rearward edge of the cloud band

  42. Jet axis Jet axis

  43. 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

  44. 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

  45. 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

  46. Typical Appearance inVertical Cross Sections

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

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