Fronts and Mid-latitude Cyclones
- 2. Fronts The boundary between two different air masses is called a front. It is a region of significant horizontal gradients in temperature or humidity. Typically 100 to 200 km wide – very sharp transitions are uncommon. Fronts are a dominant feature of mid-latitudes. In particular fronts associated with low pressure systems (mid-latitude cyclones, extra-tropical cyclones, depressions). The movement of fronts is responsible for much of the day- to-day variability in weather conditions. Northwest Europe receives many different air mass types, with frequent frontal passages – results in very variable weather.
- 3. Warm Front warm air cool air movement of front cool air warm air nimbo-stratus alto-stratus cirro-stratus cirrus ~300 km ~500 km • Warm air flows up over denser cold air • Inclination of frontal surface is very shallow: 0.5 to 1° • Approach of front signalled by high cirrus or cirrostratus, cloud base lowering as surface front approaches. • Rain starts ahead of surface front, is widespread and persistent • Skies clear quickly after passage of surface front ~10km
- 4. Cumulo- nimbus Cold Front cold air warm air movement of front cold air warm air ~70 km ~200 km • Dense cold air pushes forward into warmer air, which is forced upward • Steeper than warm front: ~2° • Deep convective clouds form above surface front, heavy rain in narrow band along surface front • Behind front cloud base lifts, eventually clearing • Near the surface the cold air may surge forward, producing a very steep frontal zone ~10km
- 5. Stationary Fronts • There is no fundamental difference between the air masses either side of warm and cold fronts – the front is defined by the direction of motion • When the boundary between air masses does not move it is called a stationary front • Note that the wind speed is not zero – the air individual masses still move, but the boundary between them does not cold air warm air
- 6. Occluded Fronts movement of front • In general cold fronts move faster than warm fronts, and may thus catch up with a warm front ahead – the result is an occluded front • There are two types of occluded fronts: warm and cold, depending on whether the air behind the cold front is warmer or cooler than the air ahead of the warm front • Cold occlusions are the more common type in the UK • Occlusion is part of the cycle of frontal development and decay within mid-latitude low pressure systems
- 7. cold air warm air cool air Warm Occlusion • In both warm and cold occlusions, the wedge of warm air is associated with layered clouds, and frequently with precipitation • Precipitation can be heavy if warm moist air is forced up rapidly by the occlusion
- 8. cold air warm air cool air Cold Occlusion
- 9. Mid-latitude Cyclones • Low pressure systems are a characteristic feature of mid- latitude temperate zones • They form in well defined zones associated with the polar front – which provides a strong temperature gradient – and convergent flow resulting from the global circulation 31-08-2000
- 10. • Low pressure forms at surface over polar front due to divergence aloft • As rotation around initial low starts, a ‘wave’ develops on the polar front • Mass balance: inward flow compensated by large-scale lifting ⇒ cooling ⇒ cloud formation cloud
- 11. • Surface low is maintained (or deepens) due to divergence aloft exceeding convergence at surface • Flow is super-geostrophic: cold sector air pushes cold front forward; warm sector air flows up warm front – warm front moves slower than cold • Cold front overtakes warm front to form an occlusion, which works out from centre • Depression usually achieves maximum intensity 12-24 hours after the start of occlusion
- 12. • Low starts to weaken as inflowing air ‘fills up’ the low pressure • Low continues to weaken, clouds break up
- 14. A BA B
- 15. A BA B
- 17. Ana-Fronts • Air is rising with respect to both frontal surfaces • Clouds are multi-layered and deep, extending throughout the troposphere tropopause cold warm cold
- 18. Kata-Fronts tropopause Sc Sc subsidence inversion • Air aloft in the warm sector is sinking relative to the fronts • Restricts formation of medium & high- level clouds. Frontal cloud is mainly thick stratocumulus, it’s depth limited by the subsidence inversion • Precipitation is mostly light rain or drizzle. cold warm cold
- 19. Ana-cold fronts may occur with kata-warm fronts, and vice-versa. Forecasting the extent of rain associated with fronts is complicated – Most fronts are not ana- or kata- along whole length, or at all levels within the troposphere
- 22. 30 60 80 Major Frontal Zones Northern Hemisphere Winter Atlantic Polar Front Pacific Polar Front Canadian Arctic Front Atlantic/Asiatic Arctic Front Mediterranean Front