Lecture 21 Midlatitude Cyclones Observation Homework Due 11/24 1 2 Midlatitude Cyclones Midlatitude Cyclone or Winter Storm Cyclogenesis Energy Source Life Cycle Air Streams Vertical Structure Storm Hazards Air masses and Fronts Air mass formation Types of air masses Types of Fronts Identifying Fronts Formation of Fronts Previous Lecture 3 4
Warm Front Cold Front Stationary Front Occluded Front Four Types of Fronts Frontal symbols are placed pointing in the direction of movement of the front (except in the case of the stationary front). Identifying Fronts Across the front - look for one or more of the following: 1. Change of Temperature 2. Change of Moisture characteristic (RH, Td) 3. Change of Wind Direction 4. Change in pressure readings (falling vs rising pressure 5. Characteristic Precipitation Patterns 6. Characteristic Cloud Patterns 5 6 Typical Cold Front Structure Cold air replaces warm; leading edge is steep in fastmoving front shown below due to friction at the ground Strong vertical motion and unstable air forms cumuliform clouds Upper level winds blow ice crystals downwind creating cirrus and cirrostratus Slower moving fronts have less steep boundaries and less vertically developed clouds may form if warm air is stable Typical Warm Front Structure In an advancing warm front, warm air rides up over colder air at the surface; slope is not usually very steep. Lifting of the warm air produces clouds and precipitation well in advance of boundary. At different points along the warm/cold air interface, the precipitation will experience different temperature histories as it falls to the ground (snow, sleet, fr.rain,& rain). 7 8
Midlatitude Cyclone More commonly known as a Winter Storm Cyclogenesis: the Formation of a Cyclone Cold, heavy air sinks, displacing warm air, which rises, thus converting potential energy into kinetic energy in the form of a cyclonic wind circulation. Cyclones develop along frontal zones because denser, cold air is located at the same height as nearby, less-dense, warm air. 9 10 Energy Source for Winter Storms Life Cycle of Midlatitude Cyclone 1. Incipient Stage 2. Mature Stage 3. Occluded Stage 4. Dissipating Stage Temperature Gradients Fuel Cyclogenesis 11 12
Stationary Front Stationary Front cp cp mt mt Isobars Isotherms Cyclone begins with a stationary polar front that separates cold easterlies and warm westerlies. Note the two air masses, cp and mt, that are involved in the early formation of this front. 13 14 Incipient Stage Mature Stage A kink forms on the front and cold air starts to move southward. Warm air starts to move northward. Cold air continues to move south, and warm air north. Low pressure develops in the center and converging air strengthens the fronts. 15 16
Weather Map of a Mature Storm Mature Stage Temperature - dashed lines Pressure - solid lines Fonts - heavy lines with barbs 1016 1021 10 17 20 1022 1021 1023 14 1019 19 25 34 1019 1020 16 10 1024 171023 1021 22 1024 21 1023 18 1022 23 1022 1023 24 22 1020 48 18 1025 21 1026 22 1021 25 24 1013 41 1016 35 1014 1012 1022 33 19 1025 27 1023 20 24 1019 29 1014 19 1024 1006 1023 41 1002 49 1005 25 1023 33 1011 35 29 30 1021 1016 25 32 1023 33 1023 1005 42 1009 49 1004 72 1011 76 1008 72 1005 30 1016 45 49 63 1013 1016 32 1021 32 1024 351024 38 1021 38 1020 29 1023 37 1024 35 1026 31 1020 39 1021 38 1023 42 1025 48 1022 53 1022 70 1017 64 12 1024 64 1020 74 45 14 1020 1025 13 28 Mature Wave Cyclone 55 1013 66 1010 55 1013 75 77 1017 17 18 Occluded Stage Occluded Stage Cyclone is mature, precipitation and winds are most intense. Cyclone matures, precipitation and winds become more intense. 19 20
Occluded Stage Dissipating Stage Cyclone is mature, precipitation and winds are most intense. Cyclone continues to occlude (end of life cycle) and begins to dissipate or weaken. 21 22 Formation of Occluded Fronts East of the Rockies - Cold Occluded Fronts Formation of Occluded Fronts West of the Rockies - Warm Occluded Fronts 23 24
Air Mass Modification and the Dry Line Air Steams in Midlatitude Cyclones are Three Dimensional Warm air stream brings warm moist (mt) air in the warm sector and lifts it over the warm front. Cold air stream brings cold moist (mp) air westward to the north and beneath the warm front to the low pressure center. Dry air entering eastern Texas from the west encounters warm moist air moving north from the Gulf of Mexico, resulting the formation of a dry line. Dry air stream brings cold dry (cp) air from the north west and descends behind the cold front. 25 26 Warm Air Stream Warm Air Stream 27 28
Cold Air Steam Cold Air Steam 29 30 Dry Air Stream All Three Air Streams Warm, Cold, Dry 31 32
Another view of air streams in cyclones Warm -red Cold - blue Dry - yellow All Three Air Streams What Causes the Surface Low to Form? Relationship of Surface and Upper-level Lows When upper-level divergence is greater than lower-level convergence, more air is taken out at the top than is brought in at the bottom. Surface pressure drops, and the low deepens. What maintains the surface low? Imagine a surface low forming directly below an upper level low. 33 What Causes the Surface Low? 34 Low High convergence and divergence aloft When upper-level divergence is stronger than lower-level convergence, more air is taken out at the top than is brought in at the bottom. Surface pressure drops, and the low intensifies, or deepens. Surface convergence fills in the low Surface divergence undermines the high Upper level pressure contours 35 36
Vertical Structure of Midlatitude Cyclones Upper-level divergence initiates and maintains a surface low. Upper-level low is tilted westward with height with respect to the surface. Surface Pressure Changes 37 Cyclogenesis Upper level shortwave passes. Upper level divergence leads to sfc low. Cold advection throughout lower troposphere. Cold advection intensifies upper low. Leads to more upper level divergence. Temperature advection is key! Deepening Lows Tilt Westward with Height 38 Cold air moving in behind the cold front causes the pressure to rise. Warm air moving over the warm front causes pressure to fall. 39 Lows at surface are located east of the corresponding upper-level troughs. 40
Deepening Lows Tilt Westward with Height Deepening Lows Tilt Westward with Height Lows at surface are located east of the corresponding upper-level troughs. Vertical Structure of Cyclone 41 Lows at surface are located east of the corresponding upper-level troughs. Storm Track 42 cold cold cold warm warm warm a) Incipient stage, b) mature stage, c) occluded stage. Thin contours are sea level pressure, thick arrow show jet-stream level flow. Dashed lines show temperature, with cold air to the NW and warm to the SE. Storms are steered by flow in the upper troposphere. The location and strength of the jet-stream flow is governed in part by the distribution of sea surface temperature. Thus, el niño influences the storm track. 43 44
Questions? 45