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Frontal Systems Lessons 35/36/37

Frontal Systems Lessons 35/36/37. Definition of a Front. A front is a zone of transition between two contrasting air masses. Within the frontal zone exist strong horizontal gradients of temperature and humidity.

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Frontal Systems Lessons 35/36/37

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  1. Frontal SystemsLessons 35/36/37

  2. Definition of a Front • A front is a zone of transition between two contrasting air masses. • Within the frontal zone exist strong horizontal gradients of temperature and humidity. • The interaction of two air masses at the frontal surface frequently gives rise to a great deal of frontal cloud and weather.

  3. Definition of a Front, cont’d • A frontal surface separating two air masses slopes upwards over the colder air. The surface itself is about 3000 feet in thickness. • Typical frontal surface slopes are between 1:100 to 1:150.

  4. Warm Front • Warm air over rides cold air ahead. • Gradual lifting results in layer cloud. • NS,AS,CS,CI.

  5. Warm Front Weather Frontal slope 1:125 • Wind veers & increases slightly • Temperature/Dewpoint rise. • Pressure steadies • Ppn changes from RA to DZ or DZRA. • Cloud changes from NS to ST or SC. Vis. good but reduces in ppn. Poor Vis

  6. Cold Front • Cold air undercuts the warmer air ahead • Rapid lifting causes instability and CB clouds

  7. Global Distribution of Fronts • Polar Fronts • The Arctic/Antarctic Fronts • Mediterranean Front • Inter-tropical Convergence Zone (ITCZ), or Inter-tropical Front (FIT).

  8. North Atlantic Polar Front • The North Atlantic Polar Front is the boundary between the cold polar air masses to the north and the warmer tropical air masses to the south. • In summer the NAPF lies further to the north from about Newfoundland to North of Scotland. • In winter it lies further south from Florida to towards the SW of the British Isles. • Lows form along the line of the front in association with jet streams.

  9. Classification of Fronts • Generally classified as cold/warm and as kata/ana fronts. • Ana-fronts: • are active fronts associated with pronounced vertical motion (lifting) along the frontal surface. • pronounced temperature contrast across the frontal surfaces and substantial cloud layers with associated precipitation.

  10. Classification of Fronts con’t • Katafronts: • associated with subsidence aloft and a downward components at the frontal surface. • Thus any lifting is confine close to the surface and vertical cloud development is curtailed • Most of the cloud will be SC and ST with associated drizzle and patchy light rain. • The temperature contrast across katafronts are small and they are less active and slow moving.

  11. WARM ANAFRONT

  12. Summary of Weather ahead of a Warm Front • Increasing cloud cover (CI,CS,AS,NS and CU/SC ahead with BKN ST in rain band) base gradually lowering. • Pressure falls then steadies on frontal passage. • Wind gradually backs to SW and freshens ahead of front, then veers on frontal passage. • Temp and Dew point steady ahead, but rise rapidly to meet each other on passage of front.

  13. Summary of Weather ahead of a Warm Front, cont’d • Visibility good but moderate in rain with prob of frontal fog just ahead of front. • Low level wind shear just ahead of front • Light to moderate continuous rain 100 - 150 nm (200-400 km) ahead of warm front. • Probability of rain ice below frontal inversion.

  14. WARM KATAFRONT

  15. Warm Kata-Front Cloud and Weather • The weather hazards will be less severe due to the fact that it is less active. • Main precipitation is drizzle or a mix of drizzle and rain. Drizzle is far more restrictive for visibility.

  16. Warm Kata-Front Cloud and Weather • Cloud layers have less vertical development giving thick SC and thus • Icing is mostly light but on occasions may be moderate. • Prolonged low level wind shear will also be less of a problem as the wind shift at low level is less.

  17. Cold Front Weather Frontal Slope 1:50 • Pressure rises rapidly • Wind veers and decreases slightly • Temp/Dew point fall. • Vis improves • Int Ra/Hvy Sh begin from NS/Embd Cb Good vis and SHRA.

  18. COLD ANAFRONT

  19. Summary of Weather Behind a Cold Front • Low NS, AC/AS CC,CI with embedded CU/CB. • Pressure rises on frontal passage. • Wind veers to NW (squalls) on frontal passage then decreases slightly. • Temp and Dew point fall rapidly and spread on passage of front.

  20. COLD KATAFRONT

  21. Cold Kata-Front Weather • Occurs when there is a relative downward component along the frontal surface. • This happens when the air ahead has a greater velocity than the air behind.

  22. Cold Kata-Front Weather.cont’d • Extensive layers of medium and upper layer cloud are absent and • Mostly stratocumulus and low altocumulus layers up to about 3 to 4 km altitude. • The shallow cloud tends to spread out on either side of the frontal zone rather than being mostly in the warm air mass.

  23. Cold Kata-Front Weather (contd) • Any precipitation is light and patchy. • Temperature contrasts may still be high across the front even if rain is slight. • Only a small change in wind direction across the front. • Turbulence and icing are much less due to the lack of strong vertical motion in the cloud structures.

  24. Summary of Weather Behind a Cold Front • Visibility moderate in heavy rain but good behind front. • Low level wind shear just behind front • Mod/heavy intermittent rain or rain showers 50 - 100 NM wide. • Probability of rain ice below frontal inversion. • Clear slot behind rain band. • TCU/CB and SHRA follow about behind cold front.

  25. PLAN VIEW OF WARM SECTOR POLAR FRONTAL DEPRESSION

  26. Summary of Weather in the Warm Sector • Low ST or SC • Pressure steady. • Surface wind generally steady westerly. • Temp and Dew point steady, little or no spread. • Visibility poor especially in DZ, possibility of hill fog. • Light to moderate intermittent DZ or DZRA.

  27. Typical Cross Section Through a Frontal System

  28. Mature Frontal Depression • The frontal wave now has an extensive N/S depth as: • The cold front begins to catch up with the warm front.

  29. Occluding Frontal Depression • The cold front over takes the warm front forming either a: • Warm Occlusion or • Cold Occlusion. • Section BA will show typical profiles through these.

  30. Cross section through occlusions

  31. Warm Occlusion • Less cold air over-rides colder air ahead.

  32. Cold Occlusion • Colder air under-cuts less cold air ahead

  33. Additional Frontal Facts • Secondary lows may form at triple point of an occlusion. • Weather in these may be worse than primary low.

  34. Additional Frontal Facts • Secondary lows may also form on trailing cold fronts. • Secondary lows tend to rotate anti-clockwise around the primary low.

  35. Occluding Frontal Depression • The point of occlusion ‘O’ (triple point) now moves to the SE.

  36. Quasistationary Front • Is a front whose position is almost unchanged on successive synoptic charts. • There is a strong tendency for wave like disturbances to form due to cyclonic windshear. Cyclonic Windshear L

  37. Developing Low • Pressure disturbances are caused by the cyclonic windshear along the line of the quasi-stationary front. • These pressure disturbances may develop into frontal lows (depressions) if conditions are favourable.

  38. Why do surface lows deepen? • Lows are convergent at the surface which produce: • Wide spread ascent within the low. • An area of divergence must exist aloft. • If upper level divergence exceeds surface convergence then the low will deepen. • If not then the low will quickly fill. • The upper airflow, mainly the PF Jetstream provides the upper level divergence and convergence to sustain the surface pressure systems.

  39. Deepening Low • If the upper level flow is favourable then the surface low will: • continue to deepen and, • will follow the direction of the jetstream aloft.

  40. Development of Surface Pressure • Upper winds largely dictate the development of weather systems. • Upper winds may take away more air than they bring, leading to ascent of air to replace it and falling surface pressure. • Upper winds can bring more air than they remove, leading to descending (subsiding) air and rising surface pressure. L H

  41. Cyclonic Development • The strong upper level (300mb) flow usually the jet stream provides areas of: • Divergence aloft to deepen surface lows and, • Convergence aloft to sustain surface highs (anti-cyclones).

  42. How do fronts form? • A front is a boundary between air masses of different temperature properties. • The warm air to the east of the low moves northward over-riding the cold air to the north causing a: • Warm Front. • The cold air to the west of the low moves southward undercutting the warm air ahead causing a: • Cold Front.

  43. Developed Frontal Wave • At this stage the fronts are now well defined, and • the distinctive cloud formations, and • associated weather patterns are now in evidence. • Section AB will show a typical profile through a frontal depression

  44. Typical Cross Section Through a Frontal System

  45. Tracks of North Atlantic Lows

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