GG3069:  Climate and Weather Systems

 

Lecture 8: Mid-latitude Weather: Fronts, Jet Streams and Wave Cyclones  

 

Introduction

Mid Latitude climates are dominated by the interaction of air masses, specifically cold air masses originating in high latitudes and warm air masses originating in the sub-tropical Highs (see the bottom of this page for a summary of the main air masses). As a result of this interaction, mid-latitude climates are commonly characterised by vigorous circulation resulting from high rates of energy transfer. The most active circulation occurs in the vicinity of fronts, where temperature, pressure, and other characteristics change rapidly over a few tens of kilometres in the zones between air masses. The importance of air mass interaction was first recognised by Admiral Robert Fitzroy, who compiled weather maps of Britain in the mid-19th C. (Fitzroy was the first superintendent of the UK Met Office. Earlier in his varied career he was captain of the Beagle during Darwin’s historic voyage.) Further advances were made between 1910 and 1930 by a small research group in Norway, including Vilhelm Bjerknes, Halvor Sollberg and Tor Bergeron. Cut off from Europe by the 1st World War, this group looked closely and perceptively at what observations they had (only surface observations), deriving a model of frontal development which still has value as a teaching tool today. Recent ideas of frontal evolution and mid-latitude weather depend on knowledge of 3- dimensional patterns of air flow and atmospheric characteristics, including those in the upper troposphere.

To achieve a balanced understanding of mid-latitude weather systems, we need to look at processes occurring throughout the troposphere. In this lecture, we will focus on the evolution of travelling depressions (or wave cyclones), and their relationship with air flow in the mid- and upper troposphere. At all levels in the atmosphere, mid-latitude circulation consists of a series of waves developed along fronts. We will look first at the large-scale patterns of these waves in the upper atmosphere, then zoom in to examine their detailed evolution at all levels. 

Mid Latitude Circulation: Planetary Scale

Warm atmospheres expand vertically to occupy a greater thickness than cold atmospheres. As a result, air pressure in the upper troposphere will be higher at any given altitude for a warm atmosphere than for a cold atmosphere, because a greater mass of air will lie above that altitude. Thus, the mid- to upper troposphere is characterised by low pressure and temperature in the polar regions, and higher pressures and temperatures closer to the equator. This trend is apparent, for example, in the shape of the 500 hPa surface, which dips closer towards sea level at higher latitudes. The elevation of the 500 hPa surface is typically 5,800 metres at low latitudes, but may be less than 5,000 metres at the poles. The equator to pole gradient in pressure and temperature is not uniform, but is steeper in narrow zones, known as fronts. In the mid- to upper troposphere, fronts consist of a wave-like boundaries between cold, low-pressure air, and warmer, high-pressure air. There is thus a sharp pressure gradient across fronts. This pressure difference drives high winds, known as Jet Streams, which are located near the top of the troposphere (c. 200 hPa: c. 10,000 metres above sea level). The jet stream winds blow parallel to the isobars, as the main driving mechanisms are the pressure gradient force and the Coriolis effect, and friction is unimportant: the jet streams are therefore geostrophic (or nearly so). Where colder, low pressure air lies on the poleward side of the front and warmer, high pressure air lies on the equatorward side of the front, the resulting jet stream winds are westerlies. The most persistent jet streams are the subtropical jet streams, located at the poleward extent of the Subtropical Highs. Additional, transient jet streams occur at the polar fronts (polar front jet streams) and the arctic fronts (arctic front jet streams). Seasonal easterly jet streams develop in the tropics, most notably as part of the South Asian summer monsoon. The easterly airflow reflects the fact that the warmest upper tropospheric air in this region is on the poleward side (Indian subcontinent and Tibetan Plateau), and the coolest air over the Indian Ocean.

The jet streams, especially the polar front jet streams, are very dynamic features, which exhibit long wavelength meanders known as Long Waves or Planetary Waves. These are also commonly known as Rossby Waves after the Swedish meteorologist Carl-Gustav Rossby, who first modelled their role in global atmospheric circulation. Strictly speaking, Rossby Waves are a particular kind of long wave, which migrates upstream relative to the fluid flow (i.e. from east to west in the case of the sub-tropical and polar jet streams). In fact, planetary waves may be stationary or migrate slowly eastward or westward. Superimiposed on planetary waves are shorter wavelength meanders known as Short Waves; these waves propagate from west to east, with the air flow. Planetary Waves tend to have preferred positions, and commonly develop downwind (east) of mountain barriers such as the Rocky Mountains and the Tibetan Plateau in the northern hemisphere, and the Andes in the southern hemisphere.

The meandering jet streams are very clear in the above image, which shows the wind speed at the 200 hPa (mb) level in the northern hemisphere on the 14th November 2000. Note the almost continuous ring of the Subtropical Jet around the planet, and the more fragmentary, highly meandering Polar Front Jet in the mid-latitudes. The long wavelength meanders in the subtropical jet are Long or Planetary Waves, whereas the polar front jet is highly contorted in a series of Short Waves. Check out the latest 200 hPa wind map at the COLA-IGES website.

Wave instability

The position, wavelength and amplitude of Long and Short waves in the atmosphere are constantly changing. This is because such waves are subject to two kinds of instability. First , barotropic instability, is caused by topography and variations in the Coriolis parameter f with latitude (Lecture 5). Consider a wind blowing across a high mountain barrier such as the Rockies, which rises up through a substantial thickness of the troposphere. As the air flows against, then over, then past the mountains its vertical thickness is forced to change. This in turn affects the horizontal extent of the flowing air (from mass continuity), and consequently patterns of horizontal convergence and divergence. As we saw in earlier lectures, convergence and divergence affect the spin, or vorticity of the air. Changes in relative vorticity then produce a wave.

In detail, the chain of events is as follows:

A, B: As a westerly airstream flows against the mountains, lower air is blocked and stagnates. Upper air flows over the blocked air. Thus, just ahead of the mountains, the atmosphere is thickened compared to its upstream value. This vertical stretching is balanced by horizontal convergence. The conservation of absolute vorticity means that converging air acquires positive, or cyclonic relative vorticity (zr+). The air stream thus begins to twist anti-clockwise (in the northern hemisphere), so becomes more south-westerly.

C. Over the mountains themselves, the airstream is constrained between the mountains and the tropopause, which acts as a lid on vertical motions of the atmosphere. Because the air is restricted vertically, it must spread outwards or diverge. The conservation of absolute vorticity means that diverging air acquires negative or anticyclonic relative vorticity (zr-), a clockwise rotation in the northern hemisphere. Thus an anticyclonic ridge develops over the mountain crest , and the air stream turns into a westerly wind, then north-westerly.

D. As the air descends on the lee side of the mountains, it can expand vertically again, and therefore undergoes horizontal convergence. Again, vorticity must be conserved, so the air develops positive or cyclonic relative vorticity (zr+), anticlockwise rotation in the northern hemisphere). Thus, an upper level low-pressure trough develops in the lee of the mountains, and the air stream is diverted anticlockwise around its southern perimeter.

E: Further waves develop downstream of the initial disturbance because northward and southward moving air masses acquire anticyclonic and cyclonic relative vorticity, respectively due to the change in the Coriolis parameter f with latitude. As air flows south, it moves into areas where the planetary vorticity is smaller. To conserve absolute vorticity, relative vorticity must increase. Thus the air flow gains positive (cyclonic) vorticity, and so becomes westerly then south-westerly. Conversely, northward-flowing air (southerly winds) move into areas with higher planetary vorticity. Conservation of absolute vorticity dictates that the relative vorticity must decrease: the air must lose positive vorticity. In other words, it begins to turn anticyclonically (clockwise). Thus air flowing northward around the eastern side of a low-pressure trough will acquire anticyclonic relative vorticity and begin to turn back east then southwards, whereas air flowing southward around the eastern side of a high pressure ridge will acquire cyclonic vorticity and begin to turn back east then northwards. The original orographically-induced wave is thus propagated downwind.

Note for the perplexed: the reasons for the successive left- and right deflections of meandering winds may seem confusing. In particular, you may wonder why the winds are not continually deflected to the right by the Coriolis deflection. The answer is that the Coriolis effect has already been taken into account: the westerly flow of the jet streams arises from the Coriolis deflection of air that would otherwise flow poleward under the pressure-gradient force. Thus the jet streams are already in (approximate) geostrophic balance. Waves represent further modification of the flow by oscillations in relative vorticity, triggered by some initial disturbance. Make sure you understand the concept of vorticity, and how it relates to the spin of the planet at different latitudes.

 

The second type of instability is baroclinic instability. This type of instability arises when thermal gradients in the atmosphere cut across pressure gradients (i.e. isotherms and isobars plotted on a map cross each other). Baroclinic instability is due to essentially the same mechanisms as barotropic instability, but in this case the initial vertical constraint of the air occurs at a frontal zone, where warm air is forced to rise along a sloping boundary above lower, cold air, and is constrained between the cold air and the tropopause. The vertical constraint causes divergence as before, and anticyclonic vorticity. The wave extends downstream as before, with the vorticity oscillating due to the latitudinal variations in the Coriolis parameter. Baroclinic instability tends to form short wavelength meanders superimposed upon planetary waves. Baroclinic instability is encouraged by large horizontal temperature gradients in the atmosphere. When temperature gradients are high, energy needs to flow from low latitudes to high latitudes. Baroclinic instability arising from the thermal gradient leads to greater waviness in the atmosphere, providing a mechanism for warm air to flow polewards and cold air to flow equatorward. This eventually decreases the thermal gradient, reducing the baroclinic instability and reducing the waviness of the atmosphere. In this case, the jet streams are predominantly zonal (west - east). There is therefore no tendency for warm or cold air to be transported across the mid-latitudes, allowing thermal gradients to build up once more. There is thus a feedback between thermal transfer mechanisms and the vorticity of the atmosphere.

The waviness of the mid-latitude atmosphere is measured by the zonal index, which is the pressure gradient between given latitudes (e.g. 35° - 55°) at a given level in the atmosphere (e.g. 500 hPa). A low zonal index is associated with high waviness, high meridional (north-south) energy transfer, and sluggish zonal flow (westerlies), whereas a high zonal index is associated with low meridional energy transfer, and vigorous zonal flow. The oscillation of the zonal index is known as the index cycle. Cycles generally last 3-8 weeks, although they do not exhibit a regular period.

Wave Cyclones

Low-level airflow at frontal zones is characterised by transient instabilities and the development of mobile low-pressure zones known as depressions. Some of the characteristics of depressions can be seen on this map, for the 14th November 2000.

The grey dashed lines are isobars, joining areas of equal surface pressure. A low pressure centre, with a central pressure of 979 hPa, is located west of Iceland. South of the centre, is a wave-like trough of low pressure. Over Britain and western Scandinavia is a second, weaker trough. In the south of the map, over the Azores, is a ridge of high pressure (central pressure 1024 hPa). The bold hachured lines are surface fronts: cold (pointed hachures), warm (rounded hachures) and occluded (pointed and rounded hachures).

The pressure gradient force will drive wind towards areas of low pressure, but the Coriolis effect turns it to the right, so the geostrophic wind should flow along the isobars, with low pressure on the left, high pressure to the right. However the effect of surface friction is such that the air flows obliquely to the isobars, spiralling anticlockwise towards the centres of depressions, and spiralling anticlockwise out from the centres of High Pressure zones.

The evolution of depressions tends to follow a characteristic sequence:

(1) The surface front is elongated roughly west-east, and marks the line of convergence between cold and warm air.

(2) The front develops into a wave with a low pressure centre, characterised by advancing warm air at the leading edge of the wave (warm front), and advancing cold air at the trailing edge (cold front). Temperature gradients of 5° C / 100km are commonly observed at cold fronts. The slope of fronts is not great,  and is only 0.5 - 1° for warm fronts and c. 2° for cold fronts. Winds spiral in towards the centre of the depression (i.e. there is a pattern of surface convergence). As the depression develops, central pressure falls, and can fall 10-20 hPa in 12-24 hrs.

(3) The cold front moves faster than warm front, so a wedge of warm air (the warm sector) is lifted aloft, bringing continued cloud formation and precipitation. As the warm sector is lifted, the front at ground level is known as an occluded front. Occluded fronts can be seen over western Scotland and to the west of Iceland in the map above.

(4) The system becomes exhausted and the low pressure area decays or‘fills’, as can be seen over Finland in the map above.

Relationship between depressions and Jet Stream waves

We have noted that developing surface depressions

  1. have low pressure centres;
  2. are areas of surface convergence; and
  3. deepen as they develop.

This appears to result in a contradiction; how can deepening low pressure be associated with surface convergence? The key lies in consideration of upper level winds. Oscillations in the jet streams have important implications for surface pressure and weather, by determining where convergence and divergence occur in the upper atmosphere. In Lecture 2, surface air pressure was shown to be a function of the overlying atmospheric mass, which is controlled by patterns of upper level divergence and convergence. In the mid-latitudes, these turn out to be strongly influenced by the dynamics of the jet streams, in the following way.

Where air is flowing in a curved path, the local centrifugal force influences wind speed (Lecture 5). In the case of flow around a low pressure depression or trough (anticlockwise in the northern hemisphere) the centrifugal force (acting outward from the centre of rotation) opposes the pressure gradient force (which acts in towards the centre of low pressure). Thus, the wind speed is lower than it would be in the absence of the centrifugal force. Conversely, when air flows around a high pressure centre or ridge, the centrifugal force - again acting outwards from the centre of rotation - acts in the same direction as the pressure gradient force, giving higher wind speeds. Now, in the case of a jet stream meandering around troughs of low pressure and ridges of high pressure, the winds will be accelerated as they flow around the ridges, and decelerated as they flow around the troughs. Thus, accelerating flow will characterise the poleward-flowing limbs of jet stream waves (passing from trough to ridge) and decelerating flow will characterise the equatorward-flowing limbs (passing from ridge to trough). If the flow lines remain parallel, accelerating flow is associated with divergence, or 'stretching out' of the air, and conversely, decelerating flow is associated with convergence, or 'shrinking' of the flow volume. (Note that this context, divergence and convergence do not mean the same as diffluence and confluence of flow lines). Importantly, divergence is associated with a reduction in surface pressure and confluence with an increase in surface pressure. Thus, the poleward-flowing, (divergent) limbs of jet waves are associated with falling pressure at the surface. Such locations (on the eastern side of an upper level trough) favour low level convergence, ideal for the formation and deepening of surface depressions.

Thus, the surface low is located to the east of the upper tropospheric low. The relative location of lower- and upper-level lows is easy to remember, because the upper level low relates to cold air, and this is located above the cold sector at the surface, behind the surface cold front. This is in contrast with tropical cyclones, which are vertically stacked with an upper level high located directly above the surface low.

Three-dimensional air flow

Surface convergence and upper level divergence are linked by vertical motions, through which air is carried from low levels to higher levels. This vertical motion of air within depressions is visualised as conveyor belts, or zones of air and moisture transfer from one level to another. Two basic types of conveyor belt are recognised: warm conveyor belts and cold conveyor belts. Warm conveyor belts transfer warm, moist air from the warm sector of a depression over cooler air above the surface warm front. The precise form of these varies from depression to depression, and sometimes two distinct warm conveyor belts can be recognised. Cold conveyor belts transfer air from the cold sector ahead of the surface warm front. Both wam and cold conveyor belts are associated with cloud bands, which reflect the uplift and cooling of air. The interpretation of cloud patterns seen on satellite images in terms of conveyor belts allows the 3-D visualisation of air movement through a depression, and is an important part of weather forecasting. An important point to bear in mind about conveyor belts is that they represent the movement of air through the depression: the depression itself moves more slowly. Thus the depression is not an object, consisting of a finite mass of air with distinct properties, but is a dynamic disturbance through which air flows. A given air packet may thus enter the system at low altitude and leave the system aloft, while the system itself continues to evlove.

Cyclogenesis

Depressions form in preferential locations. In the northern hemisphere, these regions are: (1) the western Atlantic; (2) in the lee of the Appalachians; (3) in the lee of the Rockies; (4) the western Pacific, near Japan. These locations are favoured for the following reasons:  

Families of depressions

Frontal waves do not occur as isolated events, but in families of three or four. The following depressions are known as secondaries along the trailing edge of the extended cold front. Each depression follows a course to the south of the one before as the polar air pushes further and further equatorwards to the rear of each depression in the series. Eventually, polar air forms a large wedge of high pressure which terminates the sequence.  

Weather patterns associated with depressions

Knowledge of the structure of depressions and the associated cloud types can allow surface observers to anticipate the arrival of depressions, and so use the sky to make weather forecasts. There are many variants on the weather sequence associated with the passage of fronts, but the basic pattern is as follows.

(1) Ahead of an advancing warm front is a ridge of high pressure with clear sky conditions. The first sign of an advancing front is seen in form of wispy cirrus, or high, ice clouds. Extensive, elongate cirrus tends to be associated with a jet stream core, and the orientation of the cirrus gives information on the direction of upper level winds. Extensive NW-SE oriented cirrus commonly indicates the approach of a vigorous front.

(2) Cirrus gives way to stratiform cirrostratus: a high skin of grey cloud through which the watery sun is still visible. A halo pattern may be formed around the sun, due to the presence of ice crystals. The speed of cirrostratus development gives an indication of upper-level wind speeds and the speed of approach of the front.

(3) The cirrostratus thickens steadily as the bottom edge lowers, and eventually forms a multilayered sandwich of cirrostratus and altostratus. In plan form, this cloud indicates the geometry of the front. As the cloud thickens and uplift of air continues along the front, precipitation is formed. The resulting extensive, stratiform rain clouds are known as nimbostratus. Heavy rain with light winds usually indicates the presence of an occlusion.

(4) The nimbostratus deck eventually passes as more stable air in centre of warm sector passes over. (5) The advance of the cold front is marked by towering nimbus and cumulo-nimbus, bringing brief sharp rainstorms due to the rapid uplift of cold air, especially if the ground surface is warm. The cloud mass associated with the cold front is much narrower than that at the warm front, so this zone generally passes over rapidly to be replaced by cool north-westerly winds and occasional showers.  

Anticyclones and Blocking Highs

Anticyclones are areas of high pressure which rotate slowly in the opposite direction to cyclones (clockwise in the northern hemisphere). Temporary anticyclones, lasting days to weeks, may form in mid-latitude areas in association with low zonal index conditions. Such ‘blocking highs’ are associated with stable air conditions, warm in summer, cold in winter. In Britain,the lowest air temperatures are experience in high pressure conditions, under clear skies. For example, in December 1995, clear, high-pressure conditions were associated with temperatures as low as -25° C over much of Scotland.

Air Masses

Air masses can be defined as large bodies of air with physical characteristics (especially temperature, moisture content and vertical temperature profile) which are more or less uniform over great horizontal distances (100s km). Air masses form if air remains in the same location for 3-7 days, during which time the air gradually takes on the characteristics of the surface (e.g. land masses or ocean surface). The chief air mass source regions are areas of extensive, uniform surface overlain by more or less stationary pressure systems. Air masses classified on basis of two main factors: (1) whether the surface is land or ocean (continental / maritime), (2) temperature.  

Primary Air Masses

Warm Air Masses originate in the sub-tropical high pressure cells or over the summer continents. Maritime tropical, or mT air masses form in the subtropical highs (e.g. Tropical Atlantic and Pacific), and have high surface pressure, and high moisture content in the boundary layer.

Continental tropical, or cT air masses form over the summer continental interiors (e.g. the interior of Asia, South west USA, and North Africa in the northern hemisphere summer). They are characterised by high temperature in the lower layers of the atmosphere, declining with altitude at the Dry Adiabatic Lapse Rate (i.e. potential temperature in the lower troposphere is tends to be relatively constant, due to efficient mixing by dry thermals). However, deep convection is discouraged by low humidity.

Cold Air Masses originate over the polar regions and the winter continents. Continental arctic or cA air masses form over the Arctic basin and Antarctica Here, extensive snow and ice cover chills the lower layers of air, forming a marked temperature inversion up to 850 mb. The cold air has a low moisture content. The Antarctic air mass is more persistent than the Arctic, which tends to decay in summer when sea-ice becomes less extensive. Continental polar or cP air masses form in the winter anticyclones of Siberia and Canada. These are very extensive in winter, but shrink northwards in summer. The high pressure is a shallow feature, confined to the lower layers of the atmosphere.  

Secondary air masses

As air masses move away from source areas, they undergo changes, producing secondary air masses. These changes result from heat and moisture exchanges between the air masses and the surfaces they pass over. Air may be either warmed or cooled by the underlying surface, or gain or lose moisture (evaporation or precipitation), associated with gains or losses of latent heat. Secondary air masses have horizontal temperature gradients, and so behave differently to primary air masses.  

Main secondary air masses:

Cold air: Maritime Arctic (mA) and Maritime polar (mP) air masses form in the n.w. Pacific and Atlantic, and in the southern ocean as cold air streams out from cP and cA sources. Warming over warmer ocean surfaces (e.g. North Atlantic Drift) encourages instability and increase in moisture content, with associated cloud formation. These secondary air masses are very important in mid latitude circulation.

Continental polar (cP) air is also modified by equatorward movement as it is heated by warmer land surfaces, but the lack of moisture sources limits cloud formation and precipitation. Large lakes and epeiric seas can be very important moisture sources for such air: e.g. Hudson's Bay and Great Lakes yield moisture to cold air moving southwards in early winter. These moisture sources can lead to local heavy snowfalls, until the lakes freeze over.

Cold air moving across the Great lakes, gaining moisture and feeding snowfall across the eastern USA and Canada. Note the cloud streets:these make visible the convection in the lower troposphere, triggered by air movement over the lakes.

 

Warm air: the modification of warm air masses is usually a gradual process, as cooling occurs mainly by radiative heat loss, which is much less efficient than the convective heat transfer associated with warming from the surface. Maritime tropical (mT) air moving polewards gradually cools, forming extensive stratus decks or fog: air of this type is common in the western approaches to the English Channel in spring and early summer.

 

Internet Resources:

The best current and forecast weather charts (surface and upper air) can be found at: http://grads.iges.org/pix/

A good collection of links to other weather sites is Richenda’s weather page (University of Birmingham) on: http://sun1.bham.ac.uk/reh589/wxlinks.htm

Satellite images of Britain’s weather are available from the Dundee Satellite Receiving Station: http://www.sat.dundee.ac.uk/

 

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