21 Cyclones and Anti-Cyclones

D K Khan

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1. Learning Outcome
2. Introduction
3. Cyclones
3.1 Formation
3.2 Characteristics
3.3 Wind Flow
3.4 Energy Source
3.5 Asymmetries
3.6 Primary and Secondary Circulation
3.7 Buoyancy
3.8 Dissipation
4. Types of Cyclones
4.1 Tropical Cyclone
4.2 Mid-latitude Cyclone
5. Effects of Cyclones
6. Anticyclones
6.1 Cold Anti-cyclone
6.2 Warm Anti-C-cyclone
7. Summary

 

  1. Learning Outcomes
  • After studying this module, you will be able to
  • Differentiate cyclones and anticyclones
  • Understand the origin of cyclones and anticyclones Identify the energy source of cyclones
  • Learn about the tropical and extra-tropical cyclones Evaluate the effects of cyclones
  1. Introduction

Atmospheric motions are triggered by geographic variations in heating of the surface caused by meridional gradient of insolation, albedo variations and other factors. Winds generally offset the effects of the variation by transporting energy. Local energy balance of a unit atmospheric column includes the effects of radiation, sensible heat exchange with the surface, condensation heating and horizontal flux energy in the atmosphere. Heating of the atmosphere by transfer of sensible heat from the surface is relatively small. Largest contribution to balance the radiative loss is the release of latent heat during precipitation. Motions in the atmosphere is however associated with diverse physical phenomena with a large variety of space and time scale. Turbulence and thunderstorms are primarily effective in transporting momentum, moisture and energy vertically. Besides, large scale phenomena like cyclones extending over hundreds of kilometers are effective in transporting momentum, heat and moisture horizontally between the tropics and the poles. Cyclones and anticyclones produce large meridional transport of momentum, heat and moisture.

  1. Cyclones

Cyclones are intense, low-pressure weather systems forms over the oceans. It is normally associated with warm core, large scale convective wind system, heavy rainfall and sustained wind speed. The warm core converts heat energy to potential energy and potential energy to kinetic energy. The inward spiraling moisture laden air is forced upwards out of the boundary layer in the inner core region. It expands and cools. Condensation rapidly ensues as the air continues to rise in the eye wall clouds and latent heat is released due to precipitation. The latent heat is responsible for the warm core in the cyclone. Satellite image of tropical cyclone is shown in Fig.1.

 

3.1. Formation

 

Cyclones originate from pre-existing disturbances containing abundant deep convection sometimes designated as cloud clusters. As the clusters evolve from a loosely organized state into mature, intense storms they pass through several characteristic stages. However six environmental factors are associated with the formation of a cyclone. They are i) large value of low level relative vorticity, ii) a location at least a few degrees poleward of the equator, iii) weak vertical shear of the horizontal winds, iv) sea surface temperature exceeding 26º C with deep thermocline, v) conditional instability through a deep atmospheric layer and vi) large values of relative humidity in the lower and middle troposphere. The first three factors are functions of horizontal dynamics while the remaining three are thermodynamic in nature. The thermodynamic parameters vary slowly in time and would be  expected to remain above any threshold values necessary for cyclone development throughout the cyclone season. On the other hand the dynamic potential can change dramatically through synoptic activity. Thus it could be hypothesized that cyclones form only during periods when the dynamic potential attains a magnitude above its regional climatological mean. However the above six environmental parameters are not independent. In the tropics, regions of high sea surface temperatures are invariably correlated with conditional instability due to the weak horizontal temperature gradients in the lower troposphere. High humidity in the middle levels also tend to occur in convective clusters over warm waters and virtually all areas with widespread deep convection are associated with mean ascending motion.

 

3.2. Characteristics

 

Mature cyclone consists of a horizontal quasi-symmetric circulation over laying a thermally direct vertical (transverse) circulation. These are referred as primary and secondary circulations. The combination of these two circulations results in spiraling motion. Air spirals into the storms at low levels with much of the inflow confined to a shallow boundary layer typically 500 m to 1km deep and it spirals out of the storm in the upper troposphere where the circulation outside a radius of a few hundred kilometers is anti-cyclonic. The spiraling motions are often evident in cloud patterns as is evident in satellite imagery. The primary circulation is strongest at low levels in the eye wall cloud region and decreases in intensity with both radius and height. Fig. 2 shows the typical characteristics of tropical cyclone.

 

3.3. Wind Flow

 

The low level circulation comprises the distinct features of wind patterns. The wind speed increases from periphery to the core. Annulus of maximum winds surrounding the eye is the most outstanding feature of the mature cyclone. It is about 10 to 20 km wide and coincides with the wall cloud, site of the most vigorous convection and heaviest rain in the storm. The eye is the innermost part of the storm, where the wind weakens rapidly towards the eye centre. The circulation is divided into three layers. The inflow layer extends from the surface to about 3km. It contains a pronounced component of motion toward the storm centre. This inflow is largely confined to the planetary boundary layer below 1km. In the middle layer between 3to7.6km the flow is mostly tangential while the outflow layer extends from 7.6km to the top of the storm with maximum outflow near 12km. Winds are typically strongest to the right of the direction of movement and in the eye wall. In the upper troposphere outflow is cyclonic. This circulation is much smaller than near the surface and is surrounded by anti-cyclonic flow. Strongest winds with strongest divergence extend outward of the storm. Such a pattern is often reflected in the cirrus clouds in satellite pictures. A storm’s outflow pattern depends on the environmental winds and on the dynamics of the storms itself. The three-dimensional wind structure of cyclones comprises air flowing into the cyclone in the lower layers rising primarily in the eye wall cloud and other rain bands and finally flowing outward from the cyclone top and sinking some distance away. Forced sinking inside the eye by warming the air through adiabatic compression contributes to the low surface pressure. Fig. 3 shows the wind flow in tropical cyclone.

 

3.4. Energy Source

 

Cyclones of at least tropical storm intensity are warm-core direct atmospheric circulation converts heat energy to potential energy and potential energy to kinetic energy. Their primary energy sources are latent heat of condensation released in the eye wall and in the spiral rain bands, and sensible heat supplied from the ocean surface. As surface winds spiral in toward the wall cloud of cyclone speed increases along the trajectory. Evaporation from the ocean and the rate at which latent heat is added to the air also increase. Along the trajectory, pressure decreases. The resulting adiabatic cooling is counteracted by sensible heat added from the ocean. In the annulus surrounding the eye, the pressure gradient is very large and the winds are strong. In this zone, the air receives most latent and sensible heat from the ocean. This causes the wall cloud to be much warmer than the peripheral air and so maintains a direct energy producing circulation. The upper troposphere warmed most with temperatures 10ºC or more above normal. The largest horizontal temperature gradients occur in mid  troposphere and are concentrated in a narrow band extending across the wall cloud. Outside the eye-wall temperature in the lower troposphere are slightly below normal especially in the rear quadrant of the cyclone where the surface waters have been cooled by evaporation and upwelling as the eye passed. The ultimate source of both latent heat and sensible heat is the warm sea surface.

 

3.5.  Asymmetries

 

Normally the inner-core regions of intense tropical cyclones show a significant degree of axial symmetry. The axisymmetric core is typically surrounded by a less symmetric vortex that merges into the synoptic environment. In the lower troposphere, the cyclonic circulation may extend nearly 1000 km from the centre. The boundary between cyclonic and anti-cyclonic circulation slopes inward with height, so that the circulation in the upper troposphere is primarily anti-cyclonic except near the centre. The flow asymmetries in this region have a significant effect on the vortex motion. In tropical cyclones that originate in the monsoon trough, the symmetric flow is often associated with a band of convection that joins the cyclone to the trough (Holland, 1984).

 

3.6. Primary and Secondary Circulation

 

The mature tropical cyclone consist of a horizontal quasi-symmetric circulation on which is superposed a thermally-direct vertical circulation. These are sometimes referred to as primary circulation and secondary circulation respectively. The former refers to the tangential flow rotating about the central axis and the latter to the in-up-and-out circulation. When these two components are combined the picture emerges of air parcels spiraling inwards, upwards and outwards. The combined spiraling circulation is energetically direct because the rising branch of the secondary circulation near the centre is warmer than the subsiding branch, which occurs at large radial distance.

 

3.7. Buoyancy

 

Tropical cyclones intensify when as a direct or indirect result of latent heat release the buoyancy in the core increases. The direct effect of latent heat release in saturated ascending air such as in the eye wall clouds or in the cores of individual convective clouds is to maintain the air close to the moist adiabat from which the updraught originates. The indirect effect of latent heat release is to  produce subsidence in clear air regions adjacent to deep convection. The extent to which local buoyancy is produced will depend amongst other things on the rate at which the buoyancy is generated and on the scale on which it is generated.

 

3.8. Dissipation

 

The cyclone dissipates as the buoyancy of the system is destroyed due to the non- availability of the surface supply of sensible heat to the core. Only a uniformly warm ocean surface can supply this heat. The circulation weakens rapidly as it moves over the land surface. The isothermal inflow at the land surface stops and the air is cooled by adiabatic expansion resulting disappearance of horizontal buoyancy gradient. The same effect is produced when relatively cold or dry surface air penetrates the core. Alternatively when a cyclone moves beneath a strongly shearing layer, the low level centre may become detached from the rest of the circulation. It weakens by subsidence and quickly dissipates.

  1. Types of Cyclones

Depending upon the place of origin, size, intensity and strength, the cyclones may be categorized as i) tropical cyclone and ii) mid-latitude cyclone.

 

Tropical cyclones form in many parts of the world from initial convective disturbances. Each year approximately 80 tropical cyclones occur throughout the world. Two thirds of these cyclones reach the severe tropical cyclonic stage. Preferred regions of formation of these cyclones are over the tropical ocean with high sea surface temperature.

 

In contrast to the tropical cyclone mid-latitude cyclone derives energy from horizontal temperature differences which are typical in higher latitude. Mid-latitude cyclones form as a result of the release of baroclinic instability. The preferred zone of formation f these cyclones are i) downwind of major mountain ranges and ii) near the eastern coast lines of continents.

 

4.1. Tropical Cyclone

 

Tropical cyclones are intense, cyclonically-rotating, low pressure weather systems that form over the tropical oceans. The formation of tropical cyclone is the topic of most extensive ongoing research today. There are several factors that help the formation of tropical cyclone. In general sea surface temperature of more than 26º C are needed down to a depth of at least 50m. Water of this temperature causes the overlying atmosphere to be unstable enough to sustain convection and thunderstorms. Rapid cooling with height allows the release of heat of condensation to supply the energy to the cyclone. High humidity is another factor particularly in the lower-to mid- troposphere. The high moisture content in the atmosphere favours the development of disturbances. Low amount of wind shear also favours storm’s circulation. Tropical cyclones generally form 5º of latitude away from the equator allowing the Coriolis effect to deflect winds blowing towards the low pressure center to create a circulation. Pre-existing disturbances of weather system also needed to form tropical cyclone.

 

Any tropical system with a sustained wind force more than 30 knots is known as tropical cyclone. The low level circulation comprises three distinct areas. In the outer portion, extending from the storm periphery inward to the edge of the zone of maximum winds which increases towards the center. The annulus of maximum winds surrounding the eye is the most outstanding feature of the mature tropical cyclone. It is about 10 to 20km wide and coincides with the wall cloud, the site of the most vigorous convection and heaviest rain in the storm. The eye is the inner most part of the storm. Winds weaken rapidly towards the eye center.

 

The circulation of intense tropical cyclones extends upward to around 14 to 15km close to tropical tropopause. The warm cored cyclonic circulation weakens with height. Up to a height of 6km the shear of the wind in the vertical is small. The circulation however could be divided into three layers. The inflow layer extends from the surface to about 3km and contains a pronounced component of motion towards the storm center. This inflow is largely confined to the planetary boundary layer while the flow is mostly tangential in the middle layer in between 3 to 7 km. The outflow layer extends from 7.6 km to the top of the storm. Typically winds are strongest to the right of the direction of movement and in the eye wall. In the upper troposphere outflow is cyclonic. This circulation is much smaller than near the surface and is surrounded by anti-cyclonic flow. A storm’s outflow pattern depends on the environmental winds and on the dynamics of the storm itself.

 

Tropical cyclones are warm-core direct atmospheric circulation that converts heat energy to potential energy and potential energy to kinetic energy. Their primary energy sources are latent heat of condensation released in the eye wall and in the spiral rain bands while the sensible heat is supplied from the ocean. In the annulus surrounding the eye the pressure gradient is very large and the winds are strong. In this zone the air receives most latent heat and sensible heat from the ocean. This causes the wall cloud to be much warmer than the peripheral air and maintains a direct energy producing circulation.

 

The major convective cloud systems (rainbands) in tropical cyclones lie along the spirals. Upward motion is concentrated in the rainbands and especially in the wall cloud. Vertical transport of heat and conversion of potential to kinetic energy are concentrated in the rainbands.

 

4.2. Mid-latitude Cyclone

 

Mid-latitude cyclones are formed as a result of the release of baroclinic instability. This baroclinically unstable westerly flow is associated with a large north-south thermal contrast with warm air towards the equator and cold air towards the pole. At the initial stage this thermal contrast is manifest by a stationary front. As the cyclone intensifies and matures a well defined cold and warm fronts develop. Mid-latitude cyclones are often associated with frontal boundaries. From the thermal wind relationship the presence of a large north-south thermal contrast implies strong vertical wind shear nominally characterized by westerly flow that increases in speed with increasing altitude. This westerly flow peaks in magnitude near the tropopause at the level of jet stream. Indeed vertical wind shear is typically maximized in the presence of an upper tropospheric jet stream. Mid-latitude cyclone typically develops in the presence of a jet stream. The mid-latitude cyclones intensify as it extracts energy from the vertically-sheared flow and begin to decay once there is no further energy that can be extracted.

 

Clouds and precipitation associated with birth stage of a mid-latitude cyclone is typically oriented in a linear fashion, not acquiring the archetypal comma shape until the development stage. Well defined cold and warm fronts develop as the mid-latitude cyclone intensifies. The rotational flow of the cyclone leads to cold air advection to its south and west and warm air advection to its north and  east. Concurrently, the magnitude of the cross-front thermal contrast associated with the cyclone’s cold and warm fronts strengthen which is known as frontogenesis. At all stages mid-latitude cyclones are tilted to the west or against the vertically-sheared westerly flow with increasing altitude. The westerly-tilted structure fosters warm air advection, cyclonic voticity advection and middle to upper tropospheric diffluence atop the surface cyclone. The magnitudes of each advection decreases as the cyclone’s tilt decreases, eventually becoming zero once the cyclone reaches its decay stage. As the mid-latitude cyclone reaches maturity and reaches its peak intensity an occluded front develops. Finally as the mid-latitude cyclone reaches its decay stage it is separated from the fronts and isolated in relatively homogenous air (Fig. 4).

 

The mid-latitude cyclones typically have a comma-shaped appearance (Fig. 5). Precipitation along the mid-latitude cyclone’s cold front is often convective in nature, whereas precipitation along and poleward of the mid-latitude cyclone’s warm front is predominantly stratiform in nature.

 

Fig. 4. Idealized view of the birth and decay of the mid-latitude cyclone (blue line denotes cold front and red line warm front, black line with arrows indicate streamlines at the surface)

 

  1. Effects of Cyclones

Tropical cyclones cause large waves, heavy rains, flood and high winds disrupting public utility services. On land strong winds damage or destroy buildings, bridges and other outside objects turning them into loose debris. Storm surge is typically the worst effect from landfalling of the tropical cyclone resulting large number of deaths. Over the past two centuries tropical cyclones have been responsible for deathsof more than 1.9 million people worldwide. Although cyclones take an enormous toll of lives and personal property they may be important factors in precipitation regimes as they may bring much needed precipitation to otherwise dry regions. Tropical cyclone also helps maintain the global heat balance by moving warm and moist tropical air to mid-latitudes and polar regions and by regulating the thermohaline circulation through upwelling. The storm surge stir up the waters of coastal estuaries which are important fish breeding areas.

  1. Anti-cyclones

An anti-cyclone is a pressure system wherein the pressure in the central region is higher than in the surrounding parts. In anti-cyclone the winds circulate around the centre in a clockwise sense in the northern hemisphere and anticlockwise in the southern hemisphere.

 

The weather typically associated with an anti-cyclone is in direct contrast with cyclone. In the central region of the anti-cyclone is characterized by light winds and fair weather. This is true for the sub-tropical high pressure areas and for anti-cyclones in temperate latitudes in summer. Cloudy skies are frequent in anti-cyclone over the oceans in temperate latitudes precipitation is rather uncommon near the middle of an anti-cyclone because active fronts do not penetrate such regions. In winter over the continents in temperate latitudes the weather can seldom be described as fair.

 

The anti-cyclone is an area of horizontal divergence at its lowest level. Horizontal divergence at the earth’s surface implies a downward motion of the air accompanied by adiabatic heating. Hence the relative humidity of air in the lower level is lowered, clouds tend to be evaporated resulting a frequent occurrences of fair weather.

 

6.1. Cold Anti-cyclone

 

Cold anti-cyclone is one in which the air at the surface and in the lower layer of the troposphere is colder than the air in the adjacent region. The air in this type of anti-cyclone is denser than the surrounding air level for level. The high pressure of a cold anti-cyclone is primarily due to the density of the lower layer of the troposphere being greater than the density of the same layer in the area surrounding the anti-cyclone. Though cold anti-cyclones are of limited vertical extent they play a very important role in low level atmospheric circulation in winter.

 

6.2. Warm Anti-cyclone

 

In a warmer anti-cyclone the air for the greater part of the troposphere is warmer than that of the surrounding region. Warm anti-cyclones are the oceanic sub-tropical belts of high pressure systems which are characterized by subsiding air mass. The weather is generally fine with little or no cloud and good visibility. The sub-tropical anti-cyclones are the source regions of the maritime-tropical air masses providing the warm air that feeds travelling depressions of the temperate latitudes. Clouds forming in the lower layers of an anti-cyclone tend to spread out at the temperature inversion giving a layer of stratocumulus which is more typical of the boundaries of the anti-cyclone than the center where the subsidence prevents cloud formation.

  1. Summary
  • Cyclones are intense low pressure systems formed over the ocean surface.
  • It is associated with large scale convective wind system, heavy rainfall and sustained wind flow.
  • Anti-cyclone is an area of horizontal divergence at its lower level. Cyclones are characterized by warm core.
  • Latent heat is the source of energy for the cyclone.
  • Low level circulation is the distinct feature of wind pattern.
  • Cyclones dissipate as the buoyancy of the system is destroyed.
  • Circulation weakens rapidly as the cyclone moves over the land surface. 
  • Tropical cyclones form over the ocean with sea surface temperature > 26º C.
  • Tropical cyclones generally form 5º of latitude away from the equator to allow deflection. Mid-latitude cyclones form as a result of baroclinic instability.
  • Mid-latitude cyclones have typically comma shaped appearance.
  • Storm surge is typically the worst effect of landfalling of the tropical cyclone. Tropical cyclone also maintains the global heat balance.
  • Weather system in anti-cyclone is in direct contrast with cyclone Anti-cyclone is characterized by fair weather and light wind.
  • In anti-cyclone downward motion of the wind is accompanied by adiabatic heating.