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Planetary-scale tropospheric systems
Rev. 17 — page content was last changed 12 November 2013
Planetary-scale systems are atmospheric phenomena that last from several days to weeks, and extend over several thousand kilometres.
As the Earth rotates at a constant rate and the winds continue, the transfer of momentum between Earth–atmosphere–Earth must be in balance and the angular velocity of the system maintained. (The atmosphere is rotating in the same direction as the Earth but westerly winds move faster and easterly winds move slower than the Earth's surface. Remember, winds are identified by the direction they are coming from not heading to!)
The broad and very deep band of fast-moving westerlies in the westerly wind belt, centred around 45°S (but interrupted at intervals by small, migrating lows moving east — not shown in the diagram above) lose momentum to the ocean through surface friction, resulting in the Southern Ocean's west wind drift surface current. The equatorial easterlies or trade winds and, to a lesser extent the polar easterlies, gain momentum from the Earth's surface. That gain in momentum is transferred, to maintain the westerlies, via large atmospheric eddies and waves — the sub-tropical high and the sub-polar low belts.
These eddies and waves are also part of the mechanism by which excess insolation heat energy is transferred from the low to higher latitudes.
Globally, the equatorial low pressure trough is situated at about 5°S during January and about 10°N during July. Over the Pacific Ocean the trough does not shift very far from that average position — but due to differential heating it moves considerably further north and south over continental land masses. In Australia the trough will sometimes approach Alice Springs — latitude 23°S in the hot centre of the continent. The average summer msl pressure chart shows the position of the three most intense low pressure areas of the trough over South America, Africa and Australia/Papua-New Guinea.
The low-level air moving towards the trough from the sub-tropical high belts at about 30°S and 30°N is deflected by Coriolis, and forms the south-east and north-east trade winds. Coriolis effect deflects air moving towards the equator to the west and air moving away from the equator to the east. Thus, when the north-east trade winds cross the equator in the southern summer, they turn to become the north-west monsoon which brings the 'Wet' to northern Australia.
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The trade winds converging at a high angle at the equatorial trough, the 'doldrums', form the intertropical convergence zone [ITCZ]. The air in the trade wind belts is forced to rise in the ITCZ and large quantities of latent heat are released as the warm, moist, maritime air cools to its condensation temperature. About half the sensible heat transported within the atmosphere originates in the 0–10°N belt, and most of this sensible heat is released by condensation in the towering cumulus rising within the ITCZ.
A secondary convergence zone of trade wind easterlies — the South Pacific convergence zone — branches off the ITCZ near Papua-New Guinea, extends south-easterly, and shows little seasonal change in location or occurrence.
Over land masses the trade winds bring convective cloud, which develops into heavy layer cloud with embedded thunderstorms when the air mass is lifted at the ITCZ.
The ITCZ is the 'boiler room' of the Hadley tropical cells, which provide the circulation that forms the weather patterns and climate of the southern hemisphere north of 40°S. The lower-level air rises in the ITCZ then moves poleward at upper levels — because of the temperature gradient effect — and is deflected to the east by Coriolis, at heights of 40 000 – 50 000 feet, while losing heat to space by radiative cooling.
The cooling air subsides in the sub-tropical region, warming by compression and forming the sub-tropical high pressure belt. Part of the subsiding air returns to the ITCZ as the south-east trade winds thus completing the Hadley cellular cycle. (The system is named after George Hadley [1685-1768], a British meteorologist who formulated the trade wind theory.)
At latitudes greater than about 30°S the further southerly movement of Hadley cell air is limited by instability, due to conservation of momentum effects, and collapses into the Rossby wave system. The Hadley cell and the Rossby wave system — combined with the cold, dry polar high pressure area over the elevated Antarctic continent — dominate the southern hemisphere atmosphere. Fifty per cent of the Earth's surface is contained between 30°N and 30°S, so the southern and northern Hadley cells directly affect half the globe.
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The subsiding high-level air of the Hadley cells forms the persistent sub-tropical high pressure belt, or ridge, that encircles the globe and which is usually located between 30°S and 50°S. Within the belt there are three semi-permanent year-round high-pressure centres in the South Indian, South Pacific and South Atlantic oceans. In summer, anticyclonicity also peaks in the Great Australian Bight.
In winter the high-pressure belt moves northward, the high in the Bight extends and migrates into a large, semi-permanent winter anticyclone over southern Australia.
The Indian Ocean centre produces about 40 anticyclones annually which, as they develop, slowly pass from west to east, with their centres at about 38°S in February and about 30°S in September. The anticyclones, or warm-core highs, are generally large, covering 10° of latitude or more, roughly elliptical, vertically extensive and persistent, and with the pressure gradient weakening towards the centre. The anticyclones are separated by lower-pressure troughs.
Winds move anticlockwise around the high, with easterlies on the northern edge and westerlies on the southern edge. Air moving equatorward on the eastern side is colder than air moving poleward on the western side. The high-level subsiding air spreads out, chiefly to the north and south of the ridge due to the higher surface pressures in the east and west. Thus the position of the sub-tropical high belt dominates Australian weather. In summer, when it is centred just south of the continent, sub-tropical easterlies cover much of Australia, with monsoonal movement in the north. In winter the belt, being further north, allows the strong, cold fronts that are embedded in the westerlies to affect southern Australia (refer to section 5.2).
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The lowest surface temperatures on Earth occur at the Antarctic continent, at minus 80 °C or less. The very dry air allows any long-wave radiation to escape without any appreciable atmospheric warming. The cold-core Antarctic polar high is quite shallow — 5000 to 10 000 feet deep — which decreases in intensity with height, and has a very steep inversion and an extensive upper-level low aloft; the combination of high pressure and low temperatures producing very dense air.
The air moving in an anti-clockwise direction around the anticyclone produces the surface outflow belt of polar easterlies. But, over the high-altitude icecap, tropospheric circulation consists of mid and upper-level inflow and katabatic outflow in a shallow surface layer. (A monthly mean katabatic wind of 58 knots has been recorded at Commonwealth Bay.) Very cold air masses and minor highs can split off the main Antarctic air mass — following passage of a major cyclone — and move northwards in winter, bringing the very cold Antarctic continental/maritime air towards Australia. By contrast, due to the Antarctic ice cap elevation of 6000 to 13 000 feet, Southern Ocean storms usually do not penetrate the Antarctic region south of Australia and surface pressure mainly depends on elevation.
A series of deep lows — usually centred between 50°S and 60°S and tending further south during the equinoctial periods (the Antarctic sub-polar low belt) — surround the Antarctic polar high, the boundary between the two systems is formed by the polar easterlies. This boundary between the intensely cold continental air and the warmer, moister polar maritime air is termed the Antarctic front.
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Upper westerlies blow over most of the troposphere between the ITCZ and the upper polar front. They are concentrated in the westerly wind belt where they undulate north and south in smooth, broad waves. These waves comprise one, two or three semi-stationary, long wave, peaks and troughs. They occur during each global circumnavigation and have a number of distinct mobile short waves; each about half the length of the long waves.
The amplitude of these mobile Rossby waves, as shown on upper atmosphere pressure charts, varies considerably and can be as much as 30° of latitude. Then the airflow, rather than being predominantly east/west, will be away from or towards the pole. The gradient wind speed in the equatorward swing will be super-geostrophic and the speed in the poleward swing will be sub-geostrophic.The poleward swing of each wave is associated with decreasing vorticity and an upper-level high pressure ridge and the equatorward swing is associated with increasing vorticity and an upper trough.
Downstream of the ridge, upper-level convergence occurs, with upper-level divergence downstream of the trough. This pattern of the Rossby waves in the upper westerlies results in compensating divergence and convergence at the lower level. This is accompanied by vorticity and the subsequent development of migratory surface depressions — lows or cyclones (cyclogenesis) — and the development of surface highs or anticyclones (anticyclogenesis).
The long waves do not usually correspond with lower-level features, as they are stable and slow moving, stationary or even retrograding. However, they tend to steer the more mobile movement of the short waves which, in turn, steer the direction of propagation of the low-level systems and weather.
The swings of the Rossby waves carry heat and momentum towards the poles, and cold air away from the poles. The crests of the short waves can break off, leaving pools of cold or warm air, which assist in the process of heat transfer from the tropics. Wave disturbances at the polar fronts perform a similar function at lower levels.
An upper-level pool of cold air — an upper low or cut-off low or upper air disturbance — will lead to instability in the underlying air. The term cut-off low is also applied to an enclosed region of low surface pressure that has drifted into the high pressure belt, i.e. cut off from the westerly stream, or is cradled by anticyclones and high pressure ridges. Similarly the term cut-off high is also applied to an enclosed region of high surface pressure cut off from the main high pressure belt (refer to 'blocking pairs') and to an upper-level pool of warm air that is further south than normal — also termed upper high.
Air thickness charts show the vertical distance between two isobaric surfaces. Usually, 1000 hPa is the lower, and the upper may be 700 hPa, 500 hPa or 300 hPa. The atmosphere in regions of less thickness — upper lows — will be unstable and colder, whereas regions of greater thickness — upper highs — tend to more stability. On these charts, winds blow almost parallel to the geopotential height lines.
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The polar fronts, a series of separate fronts globally distributed in the Southern Ocean, are the major frontal zones of the southern hemisphere. They mix between polar air, mid-latitude air and returning tropical air (refer to diagram 4.2). The very cold, dense air moving from the Antarctic high pressure cell and which is deflected by Coriolis into easterlies, contacts the warmer, moister Southern Ocean air moving away from the sub-tropical high pressure belt and which is deflected by Coriolis into westerlies. The returning tropical air is the upper-level air flowing from the Hadley cell, which subsides behind the front and returns to the sub-tropical region at lower levels. Polar fronts are quasi-stationary and generally located about 45°S, but move with the seasons.
Upper air flow in the Hadley cell moves to about 30°S latitude while cooling and eventually subsiding, forming the sub-tropical high pressure belt or ridge. Applying the principle of conservation of momentum: the rotation at the equator is 464 metres/second while at 30°S the surface rotation is 402 m/sec. Thus at 30°S a molecule of upper air transported from the equator has a surplus momentum of 62 m/sec or 122 knots. This surplus momentum forms the westerly sub-tropical jet stream, with an average velocity of 120 knots — the upper stream represented in the following diagram from The Weather Company www.weatherzone.com.au.
The polar front jet streams are embedded in the upper-level westerlies, snaking north and south daily and seasonally with the movement of the polar front depressions. They exist because of the strong thermal gradient in that area and they are regions of maximum upper-level air mass transport. As they meander polewards and equatorwards with the general upper air waves, they tend (by their sheer mass) to steer the movement of major low-level air masses. This encourages development of surface pressure features, and intensification of pre-existing features, by the concentrated convergence/divergence within the jet stream. The jet streams are stronger in the winter when the polar front is closest to the equator. The image indicates the position of the sub-tropical and polar front jet streams on 29 August 2009.
Jet streams are not continuous but can be as much as 3000 – 5000 km long, 100 – 300 km wide and 7000 – 10 000 feet deep. About 60% of the width tends to be on the equatorial side of the core, which is located near the tropopause. Over Australia, core wind speeds normally range from 60 – 150 knots, but occasionally reach 200 knots. The wind speeds usually decrease by 3 – 6 knots per 1000 feet above and below the core, but the rate may reach 20 knots per 1000 feet. Horizontally, the wind speeds are diminished by about 10 knots per 100 km distance from the core. Jet stream cirrus may form on the equatorial side of the core.
|The next section of the Aviation Meteorology ground school covers synoptic scale tropospheric circulation|
Aviation meteorology guide modules
| Meteorology guide contents | The atmosphere and thermodynamics (part 1) | Thermodynamics (2) and dynamics |
| Effects of altitude — contained in the Flight Theory Guide module 2 & module 3 |
| Cloud, fog and precipitation | Planetary-scale tropospheric systems | Synoptic scale systems |
| Southern hemisphere winds | Mesoscale systems | Micrometeorology — atmospheric hazards |
| Airframe and engine icing | Atmospheric electricity | Atmospheric light phenomena |
| Aviation weather reports and forecasts |
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