Occlusions and the Frontal Life Cycle - ATPL 050 — Meteorology

The previous lesson ended with a cold front travelling faster than the warm front ahead of it, and this lesson is about what happens when it catches up: the occlusion, in its cold and warm varieties, with both frontal weather systems stacked into one; the stationary front, where nothing advances at all; and the whole life story of a mid-latitude depression, from the first ripple on the polar front to dissolution. It closes with the rules for moving fronts and depressions across tomorrow's chart, and with the changes in pressure, temperature and wind that an observer reads as a frontal wave passes overhead.

What an occlusion is

The chase always ends the same way. The cold front gains on the warm front, fastest where the warm sector is narrowest (at the centre of the depression), and there it catches up first. An occlusion, marked on the chart as an occluded front, is the front formed when the cold front overtakes the warm front and lifts the warm-sector air off the surface: at the ground, cold air now meets cold air, while the warm air is held aloft in a wedge above the junction. The chart symbol says exactly that: half-circles and triangles together on the same side of the line, one front carrying both identities.

The occlusion starts at the centre and grows outward. The occlusion point, also called the triple point, is the junction where the warm front, the cold front and the occluded front meet: the outer end of the occlusion, beyond which the warm sector still reaches the ground. As the cold front continues to gain, the occluded section lengthens and the occlusion point travels along the front, away from the depression's centre: exactly the movement the three panels of the sketch show.

Cold occlusions and warm occlusions

Cold air meets cold air at the surface, but the two are almost never equally cold, and which one is colder decides the occlusion's character:

Cold occlusion — the air behind the front is colder than the air ahead: the advancing air undercuts the cool air in front of it, and the occlusion behaves like a cold front at the surface. This is the usual variety over the Atlantic and northwestern Europe, where fresh maritime polar air chases air that has already been modified on its journey.
Warm occlusion — the air behind the front is less cold than the air ahead: the overtaking air rides up over the denser wedge in front, and the occlusion behaves like a warm front at the surface. Typical where a frontal system runs into very cold continental air: continental Europe in winter.

In both varieties the weather is the two frontal systems pushed into one stack. A cold occlusion serves warm-front weather first and cold-front weather straight after: cirrus, cirrostratus and altostratus running ahead, then deep nimbostratus with prolonged precipitation, with cumulonimbus embedded near the occlusion point throwing heavier, showery bursts into the steady rain. Ground visibility is poor throughout the precipitation belt. The hazards: prolonged icing in the deep layered cloud, turbulence and thunderstorm risk in the embedded cumulonimbus, and low cloud around the front. True to its cold-front character it also ends like one: the cloud shield is narrower than a pure warm front's, and it clears comparatively quickly into bright, showery air once through.

A warm occlusion is the slower, murkier version. The same combined cloud stack arrives, but the surface front behaves like a warm front: cloud lowering over hours, a long belt of continuous precipitation, and ground visibility deteriorating slowly and recovering slowly. Its particular menace is the very cold wedge ahead: precipitation falling from the warm air aloft into air well below freezing sets up a serious freezing-rain risk, on top of prolonged airframe icing in the layered cloud and cumulonimbus embedded along the upper cold front.

Occlusion weather and seasons

Season sets the flavour. In winter, occlusions mean long-lived layered cloud, snow or freezing precipitation, prolonged icing and grey murk down to low level, among the most persistent producers of instrument weather in the European year. In summer, the extra heating destabilises the lifted warm air: embedded thunderstorms become the main threat, hidden inside layered cloud where they cannot be seen and avoided (more in the flight-hazards module), though the cloud sheet itself breaks up sooner after passage.

The stationary front

Not every front advances. A stationary front (quasi-stationary, in the careful language of charts) is a front showing little or no movement: neither air mass is replacing the other, and the warm air flows parallel to the front rather than across it, so there is nothing to drive it forward. Its weather is weak warm-front weather that refuses to leave: extensive layered cloud, intermittent light rain or drizzle, and poor ground visibility, lingering over the same area for days. The hazards are patience-testing rather than violent: prolonged instrument conditions and icing in the layered cloud. But a stationary front is also a loaded spring: a wave forming on it can grow into the next depression, which is exactly the process the next section describes.

The life of a depression

The polar front is rarely straight for long. The life cycle of a mid-latitude depression (the Norwegian wave model) runs in four acts. First, a wave forms on the polar front: warm air bulges poleward, cold air digs in behind, and pressure at the tip of the wave begins to fall. Second, the open wave: a young depression with warm front, cold front and warm sector, deepening as it runs east-northeastward; the northern-hemisphere track along the polar front, and the stage of the previous lesson, its cloud and rain belts broad along the warm front and narrow and violent along the cold front. Third, occlusion: the cold front catches the warm front at the centre, the low is near its deepest, and the occlusion point works outward. Fourth, fully occluded, the depression is cut off from the warm moist air that fed it: it slows, fills and the fronts dissolve, the whole story running its course in a few days, typically under a week.

Depressions also travel in company. On the long trailing cold front of one system a new wave often forms, so that a family of depressions follows one another along the polar front, each centre typically passing farther equatorward than the last, until an outbreak of polar air finally sweeps the sequence away and the front reforms.

Rules of movement

Forecasting begins with moving what is already on the chart. The rules the examination wants:

A front moves at right angles to its own line, at the speed of the geostrophic wind component perpendicular to the front, read from the isobar spacing across it, using the geostrophic rules of the wind module.
A warm front travels at about two thirds of that geostrophic component.
A cold front travels at about the full geostrophic component: roughly one and a half to two times the warm front's speed, which is why every warm sector eventually closes.
A frontal depression moves parallel to the isobars of its warm sector, at about the speed of the geostrophic wind in the warm sector.
Young, deepening depressions move fastest; as a depression occludes it slows down and begins to fill.

The frontal wave, read off the chart

One learning objective asks for the whole system in two drawings: a plan view of the open wave, and the cross-sections through its fronts. The cross-sections are the previous lesson's warm-front and cold-front figures; here is the plan view that completes the pair, with the surface observations written on:

Let the wave pass over a stationary observer in the northern hemisphere, and every instrument tells the same story in its own language:

Pressure — falls steadily as the warm front approaches, levels off in the warm sector, and rises sharply behind the cold front.
Temperature — cool ahead, a step up at the warm front, a sharp drop at the cold front into the coldest air of the whole crossing.
Surface wind — southerly ahead, veering to southwesterly at the warm front and veering again to northwesterly at the cold front: in the northern hemisphere the wind veers at each frontal passage.
Wind aloft — ahead of the warm front, warm air is advancing overhead and the wind veers with height; behind the cold front, cold air is advancing aloft and the wind backs with height: veering with height means warm advection, backing with height means cold advection, again in the northern hemisphere.

What the exam asks

Occlusion questions test the cold-versus-warm distinction and the triple point; life-cycle and movement questions test the Norwegian stages and the speed rules as plain numbers. The overview, following the learning objectives:

Define 'occlusion' and 'occluded front', and sketch the development of an occlusion and the movement of the occlusion point

Describe the cloud, weather, ground visibility and aviation hazards in a cold occlusion and in a warm occlusion, and sketch their cross-sections

Explain the seasonal differences in the weather at occlusions

Define a stationary front and describe its cloud, weather, ground visibility and aviation hazards

Describe the genesis, development and life cycle of a frontal depression, with its cloud and rain belts

State the rules for predicting the direction and speed of fronts, the speed difference between cold and warm fronts, and the rules for frontal depressions

Sketch a frontal wave in plan and cross-section and illustrate the changes of pressure, temperature, surface wind and wind in the vertical axis