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Unformatted text preview: Weather Systems • Air Masses • Fronts: warm, cold, occluded • Cyclonic storms • Anticyclonic weather • Tropical Weather Systems Weather Systems
• Weather system: a recurring pattern of at mospheric circulation (e. g., high pressure system, low pressure system) associated with characteristic weather events. • Occur at a wide range of scales, fro m a few kilo meters (e.g. a tornado) to more than a thousand kilometers (e.g. cyclones and anticyclones) • Weather patterns are typically a response to the move ment of large bodies of air called air masses, 5 comprising 10 107 km of air. 3 1 Air Masses • An air mass is a large body of air with fairly uniform temperature and moisture characteristics. • Air masses acquire their characteristics in source regions and are classified on the basis of: 1. Latitudinal position (Polar, Tropical) 2. Nature of source region surface (maritime, continental) Air Mass Types Fig. 8.2a Lat. of origin: Arctic (A) Polar (P) Tropical (T) Surface type: marit ime (m) continental (c) 2 Airmasses Affecting Vancouver Area: Winter • Over the Vancouver region in winter, we typically see the following air masses and weather conditions: • Maritime Polar (mP): cool, moist, 5° to 8° C (rain) This is the dominant air mass over Vancouver. • Maritime Arctic (mA): cold, moist, 5° to 5° C (rain/snow) • Continental Arctic (cA): very cold, very dry, 5° to 15° C (snow or clear, very cold weather) Airmasses Affecting Vancouver Area: Summer • Over the Vancouver region in summer, we typically see the following air masses and weather conditions: • Maritime Tropical (mT): warm, moist 15° to 25° C (dry weather if anticyclonic flow, otherwise showery if cyclonic). This is the typical midsummer airmass. • Maritime Polar (mP): cool, moist 10° to 15° C (showery). This is the cooler airmass which causes cooler, wetter interludes in summer. 3 Fronts • A given air mass usually has a welldefined boundary between itself and a neighbouring air mass. This boundary is termed a front. • Fronts are drawn on a weather map when there are notable temperature differences between air masses. • Three types of fronts: cold, warm, occluded • Frontal precipitation: precipitation occurring where air is lifted and cooled adiabatically at the frontal boundary between air masses; extensive clouds occur. How and Why Fronts Form Fronts form because a cold airmass (blue) and a warm airmass (red) converge towards each other towards a lowpressure centre. The front is the interface between the air masses drawn at the surface. High cool mP old C nt ro f cool mP Low W
war m sector mT High ar m F n ro t High Typical midlatitude cyclone with warm and co ld fronts 4 Development of a MidLatitude Wave Cyclone warm sector These diagrams show a 3D view of a frontal surface separating two air masses. Convergence of warm and cold air masses causes the frontal surface to buckle, so that the warm sector is surrounded by colder air. Typical Winds and Clouds in MidLatitude Cyclone cool mP Cs High C. Cu layer clouds cool mP Low Ns High B. A. Sc warm mT Point A: advancing warm air is about to override colder air to east. Point B: located in cold air; warmer air has overr idden co ld air. Point C: advancing co ld air is about to undercut warmer air to south. 5 Vertical CrossSection Through a Warm Front Fig 8.13 mT mP A B • At a warm front, less dense, warmer air overrides denser, colder air below. (Points A and B are as depicted in Slide 10). • Extensive layer clouds (thiker below, thinner alo ft) form along the frontal interface as the warm air mass is lifted and cooled. Vertical CrossSection Through a Cold Front C • At the cold front, the advancing denser, colder air has undercut and lifted the less dense, warmer air mass causing condensat ion as cumulus (Cu) and cumulonimbus (Cb) clouds. (Point C is as depicted in Slide 10). 6 Typical Weather Map of a Wave Cyclone Section at the bottom is drawn along the line A – A’ on the weather map. Weather Changes with Passage of a MidLatitude Cyclone: 1 Ahead of the warm front, Point A: easterly to southeasterly flo w of cool or cold air; high layer clouds overhead, though thickening to the west, heralding the approach of the warm front; no precipitation because only very high layer clouds are overhead; steadily decreasing pressure as the low centre approaches. Low A 7 Weather Changes with Passage of a MidLatitude Cyclone: 2 At the warm front, Point B: Wind has veered from SE to SW at the warm front; extensive lowlevel layer cloud now overhead (Ns, Sc, St), yielding abundant precipitation; temperature has increased with arrival o f warm sector air plus release of latent heat in clouds; pressure is now steady. Wind Veer: ahead of front behind front L B Weather Changes with Passage of a MidLatitude Cyclone: 3 In the warm sector, Point C: wind is steady at SW; clouds have thinned out because there is now no frontal lift ing – typically a broken cover of stratocumulus and fair weather cumulus occurs; temperature is st ill warm since we’re st ill in the warm sector air; pressure is steady. Low C 8 Weather Changes with Passage of a MidLatitude Cyclone: 4 At the cold front, Point D: Wind has veered again at the cold front, this time fro m SW to NW; cumuliform clouds exist along the cold front (cumulus and perhaps cumulo nimbus), giving showery weather, with a chance of thundershowers in summer when strong temperature contrasts exist across cold fronts; temperature has dropped with the arrival of the colder air; pressure is now rising again, since the low centre has passed through to the east. Wind Veer: ahead of front Low D behind front Weather Changes with Passage of a MidLatitude Cyclone: 5 Well behind the cold front, Point E: wind is NW, backing to W; skies have cleared since no frontal lift ing in the cold air, but expect scattered Cu clouds with showers, since the southmo ving cold air is picking up heat from below and beco ming more unstable; temperature remains cool because we’re still in the co lder airmass; pressure is steadily rising because of the approach of a travelling ridge of higher pressure behind the low. E Backing Wind H L 9 Occluded Stage of MidLatitude Wave Cyclone Y X More rapid advance if co ld air relat ive to the warmer air results in the warm sector air gradually being lifted completely o ff the ground. This process is known as occlusion. Note that the occluded warm air maintain a counterclockwise, cyclo nic circulation alo ft. Vertical CrossSection Through Occluded Front: 1 In Stage 1, no actual occlusio n has occurred yet, but themore rapidly advancing co ld front is ‘catching up’ wit h the warm front. Note the developing wedge of Nimbo stratus (Ns) and Stratocumulus (Sc) clouds. Red arrows depict lift ing and condensation of warmer, mo ist air. Some showers, and it’s overcast and gloomy! warm mT air As and Cs clouds cold mP Co ld Ns and Sc clouds fr on
nt fro rm a t cold air advancing rapidly W cold mP air This is the situation at Point X on Slide 19. 10 Vertical CrossSection Through Occluded Front: 2 In Stage 2, occlusio n of the warm sector air is just beginning, meaning that the warm sector air is about to be bodily lifted off the ground. Wedge of cloud has thickened because of stronger lift ing and adiabat ic cooling. An extensive blanket of Nimbostratus (Ns) cloud has developed. It’s raining! Cirrus cloud Ns cloud Cirrus cloud cold mP cold mP This is the situation at Point Y on Slide 19. Vertical CrossSection Through Occluded Front: 3 In Stage 3, occlusio n of the warm air is at an advanced stage, and a ‘trough of warm air alo ft’ with very thick Ns, Sc and As cloud has developed. It’s raining hard now! Notice that there is now no real air mass temperature contrast at the surface because the warm air has been occluded. Dashed line indicates lack of a temperature contrast at the surface. Cirrus cloud Thick wedge of Ns, Sc, and As cloud Trough axis Cirrus cloud cold mP cold mP This is the situation in the righthand diagram, Slide 19. 11 Depiction of Occluded Fronts on Weather Maps From the preceding sketches we see that the most intense precipitation will generally occur at or close to the axis o f the trough of warm air alo ft (or TROWAL), because it is here that the clouds are thickest, and thus the potential for collis io n coalescence and Bergeron process formation of raindrops or snowflakes is at a maximum. For this reason, Canadian analysts have always drawn the trace of this axis on a weather map, rather than the location of the dashedline blue front at the surface. Because there is no real temperature contrast at the surface after occlusion has occurred, it makes sense not to draw the latter line. Depiction of Occluded Fronts on Weather Maps mP Cs As Low mP Cu Ns mP Axis o f trough of warm air alo ft Sc Remnant of warm sector mT Note the distinctive ‘comma’ shaped cloud formation which develops after occlusion. Note that the low centre is now located completely within the cold air. Also note that, with the slow elimination of the warm sector, the cold and warm winds are no longer sharply convergent; now they’re more parallel to one another. Occluded systems are very common over Western Canada in winter. 12 Weather changes associated with occluded fronts Since occluded fronts invo lve the complete lift ing of the warm sector off the ground, the classic warm/cold front sequence is not experienced. Instead, there are no sharp, veers of wind as the low centre passes through, and since the air ahead of and behind the occluded front is cool, no large temperature changes occur at the surface eit her. Instead, we tend to see the fo llowing: slowly veering winds as the low approaches and passes through; a thickening wedge of layer cloud with increasing rainfall as the axis of the occluded front trough approaches; then a thinning wedge of layer cloud with declining rainfall as the front passes over Vancouverto the east. Eventually skies clear as the next travelling ridge of higher pressure approaches, and slowly backing winds develop. The typical airmass at the surface with occluded fronts over Vancouver is mP (cool, mo ist), and at midwinter mA (cold, mo ist). Vertical Structure of Cyclones: schematic All cyclo nes (midlat itude type, tropical t ype, a.k.a. hurricane) invo lve convergence of air into a surface low pressure cell, lift ing and adiabatic cooling of air, and divergence of air aloft (810 km level). Upperlevel divergence causes removal of air and thus a drop in pressure at the surface, which then drives the surface convergence. upperlevel divergence lift ing, cooling, condensation surface convergence Low 13 Ultimate Causes of MidLatitude Cyclones: 1 The development of a zone of upperlevel divergence is crit ical to the development of a surface lowpressure cell. If upperlevel divergence is stronger than surface convergence of air, then pressure continues to drop at the surface (the low ‘deepens’), the pressure gradient at the surface steepens, the winds speed up and both convergence of air masses and frontal develo pment are more vigorous. Latent heat release when clouds are actively forming gives the rising air in the low a further convective boost. If upper level divergence weakens, then the inflo w of air at the surface can ‘catch up’ with divergence alo ft, the low starts to fill, the pressure gradient slackens, the winds slow down, the rate of lift ing of air declines, the rate of latent heat production declines, and the who le cyclo ne system starts to weaken and dissipate. Ultimate Causes of MidLatitude Cyclones: 2 A feature of all midlat itude cyclones is that the low occurs in the colder air. So we cannot use the landsea breeze argument or the ITCZ reasoning to explain the development of a low pressure centre in co ld air. Something else is obviously at work! Careful study of upper level winds has shown that the westerly flo w invo lves both largescale oscillat ions (Rossby waves) and smaller scale oscillations (frontal waves). The air invo lved in these wavelike motions experiences significant changes in lat itude as the waves sweep around the planet at midlat itudes, and lat itude change means a change in the Corio lis force: decreasing Coriolis force as it sweeps Equatorward increasing Corio lis force as it sweeps poleward 14 Ultimate Causes of MidLatitude Cyclones: 3 To compensate for the changing Corio lis force, the moving upperair stream undergoes divergence when moving polewards and convergence when moving Equatorwards. A zone of upper level divergence will cause a pressure drop at the surface and the development of a low. Conversely, a zone of upperlevel convergence will cause a surface high to develop. lis rio o g C s in Rossby Wave convergence de e cr rea i nc divergence g in as lis rio Co divergence Ultimate Causes of MidLatitude Cyclones: 4 H surface highpressure cell (anticyclogenesis) L surface lowpressure cell (cyclogenesis) Equatorward moving upper air streams converge and develop clockwise circulation. This will promote subsidence of air and the devel opment of high pressure at the surface with divergence. Conversely, poleward moving air develops counter clockwise circulation, plus the streams diverge, which causes rising air and low pressure at the surface. This then causes convergence and frontal development. dive ridge of warmer air aloft rgen trough of cooler air aloft ce co er nv ge e nc 15 Anticyclonic Weather Conditions • As the ter m suggests, anticyclonic weather is the opposite of that found under cyclonic conditions. • Cyclones have low pressure with strong pressure gradients, conver gence of airstreams, lifting, and cooling of air and generally cloudy conditions with precipitation. • Anticyclones are high pressure cells with much wea ker pressure gradients and light winds or calm conditions, subsiding, warming air, which diver ges at the surface, and generally clear sky conditions. Therefore precipitation is rare. • Clear skies and calm conditions under anticyclones at night can cause a negative longwa ve budget, hence temperature inversions with fog, if air the is moist enough, or frost in midwinter. Vertical Structure of anticyclones: schematic All ant icyclo nes, irrespective of air temperature, invo lve divergence of air fro m a surface high pressure cell, subsidence and adiabatic warming of air, and convergence of air aloft (810 km level). Upperlevel convergence puts more mass in the air column, causing a r ise in pressure at the surface, which then drives surface divergence. upperlevel convergence subsidence, warming, cloud evaporation surface divergence High 16 Cold Anticyclonic Weather Associated with cold continental Arctic air over interior Canada in winter. cA air is a very co ld (20 to 40° C) and very dry (Td = 20 to 30° C). Extreme co ld in this airmass is because of clear skies due to subsidence, which causes a negative longwave budget. The air tends to be very stable, typically wit h a temperature inversion layer in the lowermost 300 m. H Pacific Ocean very cold, very dry air Note clockwise circulation causing outflow of cold, dry air. Cold Anticyclonic Weather Over Vancouver, outflow from a cold anticyclone causes dr y, easterly or northeasterly flow conditions. The air picks up heat and moisture as it approaches the coast, because the gr ound is initially unfr ozen and moist, and so the air becomes slightly unstable because of this heat and moisture gain in the lower la yers. Precipitation tends to be as light snow, though if the air undercuts moister mP/mA air to the west, then much hea vier snow can fall, especially over the mountains on the ma inla nd and along the east coast of Vancouver Isla nd. Ci Ns, Sc
warmer mP/mA air cA air outflow cold front 17 Cold Anticyclonic Weather: 2 Spells of co ld ant icyclo nic weather can persist for one to three weeks and, because winds are light to calm and skies are generally clear, air stagnation and radiat ion fog may occur in the inversio n layer, especially where air is mo ister near coasts. Radiat ion fog forms when air is chilled to the dewpoint temperature as a result of a negat ive longwave budget overnight in the inversio n layer. Fogs can persist in coastal areas for several days since K↓ is reflected off the top of the fog layer. Wit hout adequate ground heating the fog will persist because the near surface air is still at the dewpoint temperature. Fogs are usually removed by the arrival of winds which ‘stir up’ the saturated air with warmer, unsaturated air alo ft. This happened in Vancouver area in midJanuary 2009, ending two weeks of fog! LowLevel Temperature Inversion Under cA conditions 18 Warm Anticyclonic Weather
This is associated with marit ime Tropical (mT) air in summer. The air is warm (17 – 25° C) and mo ist (T = 12 – 15° C). Over d Vancouver, the subtropical Hadley Cell circulat ion moves north as the ITCZ moves into the Northern Hemisphere. This causes a large warm ant icyclo ne to move over much of the west coast of North America in July and August. As a result, Vancouver receives a steady onshore flow of west to northwest air. L H mT air Warm Anticyclonic Weather
Like the cold winter ant icyclo ne, the warm anticyc lo ne is also associated with stable air, but stabilit y in this case is caused not by a nearsurface inversion but by the development of an upper level inversio n of temperature under prolonged subsidence conditions. As air parcels descend they warm at DALR, and since the ELR is usually less than the DALR in the lowest 5 km, descending parcels mo ve diagonally downwards to the right on a temperatureheight diagram (see next slide). The result is the development of a strong temperature inversio n at about 400 – 600 m above the surface within which the air temperature increases with height for a couple of 100 metres. This provides a vertical ‘cap’ for convect ion. Warm, dry ant icyclo nic weather can persist for up to 6 weeks in Vancouver and lead to drought condit ions (mid July – end August). 19 Development of an upperlevel inversion subsidence Final ELR curve after subsidence warming DA LR height Initial ELR < DALR inversion layer smog accumulation temperature Develop ment of Photochemical Smog in Summer Although the air near the ground is strongly heated under clear skies, and ma y even be unstable near the gr ound, ther e is a strong vertical cap for convection, since rising air parcels cannot break through the upper level inversion. As a result, air circulation is restricted to the lower most 500 m or so, and consequently air quality deteriorates rapidly over urban areas when these conditions persist. Ver y few clouds can for m (only fairweather cumulus). Cu i n v e r s i o n l a y e r Cu 500 m smoggy air trapped under inversion smog air unstable at surface 20 Develop ment of Photochemical Smog in Summer Beneath the inversion, photochemical smog builds up because of the persistence of the ant icyclo ne over urban areas such as Vancouver. Smog consists of nitrogen oxides and particulates fro m vehicle exhaust and industrial emissio ns. These co mpounds manifest themselves as a brownish haze within the lower atmosphere. Since K↓ is very strong in summer, photochemical react ions occur within the smog layer and produce groundlevel ozone as a biproduct. Needless to say, all of these co mpounds are irritants to human respirat ion and can have deleterious effects on plants as well. Vancouver smog has greatly worsened over the past 30 years. Tropical Weather Systems Tropical and Equatorial weather systems show some major differences fro m midlatitude weather systems: 1. Weak upper air winds so air mass mo vement is slow. 2. Air masses are warm and mo ist everywhere, so there are no clearly defined fronts, unlike the midlatitudes. 3. High mo isture content of the air leads to intense convect ional activit y because o f the huge latent heat releases which occur when condensat ion occurs. 4. The Corio lis force is much weaker at low latitudes, so air moves more directly alo ng the pressure gradient line, and thus lows tend to fill quite readily (except in the case of hurricanes). 21 Tropical Cyclones Tropical cyclones are the most powerful and destructive type of cyclonic storm. • There are three terms used to these features: 1. Hurricane (western hemisphere) 2. Typhoon (western Pacific Ocean off the coast of Asia) 3. Cyclone (Indian Ocean) Most tropical cyclones originate at about 15° latitude as minor disturbances (weak lows) embedded in the Trade Wind flow and require high sea surface temperatures (above 27° C) to intensify. Factors Influencing the Development of Hurricanes • Hurricanes require very warm, mo ist air at the surface (latent heat energy source), and so typically form and beco me more intense over tropical oceans off the east side o f the continents. (Hurricanes are much rarer on west coasts, but do occur). • They cannot form equatorwards of about 15°N/S latitude because the Corio lis deflecting force is too weak. Close to the Equator a low pressure centre will quickly fill, and so cannot intensify. Fig 8.26 22 Tropical Cyclones (Hurricanes) • Tropical cyclo nes have centres of very low pressure (down as low as 920 mb), causing very high winds (strong PGF), very strong convergence, and heavy rainfall. • Tropical cyclo ne intensit y is measured by the central pressure of the storm, mean wind speed, and storm surge height. • Note the lack of fronts on the weather map. mT air mT air Hurricane Katrina •Hurricane Katrina is the most devastating natural disaster to ever hit the U.S., and was more destructive than any natural disaster ever to have occurred in Canada: over 1,800 deaths over $200 billio n in damage over 1 millio n people displaced over 5 millio n without power • Katrina made landfall wit h winds over 235 km/hr (Cat. 4) Source: NASA 23 Vertical Structure of Hurricanes subsidence Cb ‘wall clouds’ 15 km rising condensing air strong convergence of warm, moist air Source: ‘Wikimedia Commons’ Low Hurricanes draw their ener gy source from rising, condensing air involving huge latent heat releases. The convergence and inward spiraling of air into the low causes it to accelerate as its radius of rotation decreases (Conser vation of Angular Momentum). When it passes over land the moisture source is cut off, and surface wind speeds also are reduced because of incr eased friction with the ground. Thus the low centre starts to fill, and convergence and convection are greatly reduced. Hurricanes Can Develop into Strong MidLatitude Lows Hurricanes which stay over water tend not to weaken so much, then strengthen as the Corio lis force increases in midlatitudes to become intense frontal systems, as occurred in fall 1987 in N.W. Europe. L Typical tracks of mid latitude cyclone centres are shown with thin blue arrows. Tracks of hurricanes are shown in thinner red arrows. Note the ‘steering’ of hurricane tracks around the Atlantic Hadley Cell high. Note the fronts developing around the hurricane low off eastern Canada. 24 ...
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This note was uploaded on 11/03/2009 for the course ECON 210 taught by Professor James during the Spring '09 term at UBC.
- Spring '09