Baroclinic Instability Questions and Answers

Q: Which quantity is used to identify a storm track objectively?

Filtering the height field to retain periods of two to six days and mapping the variance isolates the travelling synoptic disturbances from the slower stationary pattern. This gives a clean objective definition. Seasonal mean pressure shows the stationary waves instead, and precipitation and gale frequency are consequences that are influenced by many other factors as well. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Q: Which stage follows maximum amplitude in the eddy life cycle?

After reaching peak amplitude the eddy tilts eastward with height and horizontally, and the resulting momentum fluxes return energy to the zonal mean flow. This barotropic decay is the second half of the life cycle and is what maintains the surface westerlies. Friction acts throughout but is not the dominant sink at this stage.

Q: Which horizontal tilt characterises the decaying stage?

During barotropic decay the eddies tilt so that their momentum flux converges into the jet, accelerating it and drawing energy out of the eddy. This eddy driven jet maintenance is a central feature of the general circulation. A symmetric eddy transports no momentum, and the flux direction is systematic rather than oscillatory.

Q: Which mean flow feature do these eddy momentum fluxes maintain?

Eddy momentum flux convergence in mid-latitudes drives a westerly acceleration that surface friction balances, which is why the mid-latitude surface westerlies exist at all. Without the eddies the Ferrel cell would drive easterlies there. The trades and the subtropical jet owe their momentum to the Hadley circulation, and the polar easterlies to the polar cell. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Q: Which quantity measures the poleward heat transport by baroclinic eddies?

The eddy heat flux is the time mean of the product of the departures of meridional wind and temperature from their zonal means, and it is positive because warm air moves poleward and cold air equatorward. Mid-latitude eddies carry about five petawatts by this route. Multiplying the two zonal means gives the mean meridional circulation transport instead, which is a separate contribution. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: Which self-regulating behaviour does the eddy transport produce?

Eddies reduce the gradient that created them, so the atmosphere is maintained close to a state of marginal stability in which growth and transport are in rough balance. This baroclinic adjustment idea explains why observed mid-latitude lapse rates and gradients vary so little. The regulation is not perfect, which is why individual storms and quiet spells still occur. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Q: Which view of cyclogenesis complements the normal mode picture?

The potential vorticity view describes cyclogenesis as the mutual amplification of an upper level anomaly and a surface thermal anomaly, each inducing a circulation that reinforces the other. It explains individual cases far better than a normal mode calculation, which describes the statistically preferred structure. Convective, frictional and radiative processes contribute but do not constitute alternative frameworks for the development itself.

21.

Which quantity is used to identify a storm track objectively?

A.Bandpass filtered variance of geopotential height
B.The mean sea level pressure averaged over the whole of the winter season across the region concerned
C.The total precipitation
D.The number of days on which the surface wind exceeds gale force at coastal stations along the track

Answer: A

Filtering the height field to retain periods of two to six days and mapping the variance isolates the travelling synoptic disturbances from the slower stationary pattern. This gives a clean objective definition. Seasonal mean pressure shows the stationary waves instead, and precipitation and gale frequency are consequences that are influenced by many other factors as well. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

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22.

Which stage follows maximum amplitude in the eddy life cycle?

A.A prolonged period of constant amplitude during which the eddy neither grows nor decays for several days
B.Barotropic decay returning energy to the mean flow
C.Continued growth at an accelerating rate until the eddy fills the whole depth of the troposphere completely
D.Immediate frictional dissipation

Answer: B

After reaching peak amplitude the eddy tilts eastward with height and horizontally, and the resulting momentum fluxes return energy to the zonal mean flow. This barotropic decay is the second half of the life cycle and is what maintains the surface westerlies. Friction acts throughout but is not the dominant sink at this stage.

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23.

Which horizontal tilt characterises the decaying stage?

A.A tilt that spreads momentum away from the jet
B.A tilt that reverses direction every few hours as the disturbance oscillates about its equilibrium state
C.A tilt whose momentum flux converges into the jet
D.No horizontal tilt at all, the eddy remaining symmetric about its own axis throughout the whole life cycle

Answer: C

During barotropic decay the eddies tilt so that their momentum flux converges into the jet, accelerating it and drawing energy out of the eddy. This eddy driven jet maintenance is a central feature of the general circulation. A symmetric eddy transports no momentum, and the flux direction is systematic rather than oscillatory.

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24.

Which mean flow feature do these eddy momentum fluxes maintain?

A.The trade winds, which blow from the east across both of the tropical oceans throughout the entire year
B.The polar easterlies
C.The subtropical jet, whose westerly momentum is instead supplied by the poleward branch of the Hadley cell
D.The surface westerlies of mid-latitudes

Answer: D

Eddy momentum flux convergence in mid-latitudes drives a westerly acceleration that surface friction balances, which is why the mid-latitude surface westerlies exist at all. Without the eddies the Ferrel cell would drive easterlies there. The trades and the subtropical jet owe their momentum to the Hadley circulation, and the polar easterlies to the polar cell. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

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25.

Which circulation is maintained as a residual of the eddy fluxes?

A.The Ferrel cell
B.The Hadley cell, which is a thermally direct circulation driven by the excess of heating at the equator
C.The Walker circulation
D.The Brewer-Dobson circulation, which transports ozone poleward within the stratosphere above the tropopause

Answer: A

The Ferrel cell is thermally indirect, with cold air rising and warm air sinking, and it exists only because the eddy fluxes force it, which is why it appears in the zonal mean but corresponds to no coherent physical circulation. The Hadley cell is thermally direct and drives itself. The Walker and Brewer-Dobson circulations belong to the tropics and the stratosphere respectively. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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26.

Which quantity measures the poleward heat transport by baroclinic eddies?

A.The correlation between the meridional wind and the vertical velocity within the disturbance concerned
B.The covariance of meridional wind and temperature
C.The product of the zonal mean meridional wind and the zonal mean temperature at that latitude and level
D.The eddy kinetic energy

Answer: B

The eddy heat flux is the time mean of the product of the departures of meridional wind and temperature from their zonal means, and it is positive because warm air moves poleward and cold air equatorward. Mid-latitude eddies carry about five petawatts by this route. Multiplying the two zonal means gives the mean meridional circulation transport instead, which is a separate contribution. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

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27.

Which magnitude does that transport reach in mid-latitudes?

A.About 5 gigawatts
B.About 500 petawatts, which would exceed the total solar radiation intercepted by the Earth as a whole
C.About 5 petawatts
D.About 5 terawatts, which is comparable with the total rate of primary energy consumption by humanity

Answer: C

Baroclinic eddies transport roughly \( 5 \times 10^{15} \) watts poleward across 40 degrees latitude, which is a substantial fraction of the total required to balance the radiative imbalance between equator and pole. The ocean carries much of the remainder. The other values are too small by orders of magnitude or exceed the total solar input entirely.

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28.

Which self-regulating behaviour does the eddy transport produce?

A.The temperature gradient increases steadily until convection begins across the whole of the middle latitudes
B.No regulation occurs
C.The eddies grow without limit until the available potential energy of the atmosphere is entirely exhausted
D.The atmosphere is held near marginal baroclinic stability

Answer: D

Eddies reduce the gradient that created them, so the atmosphere is maintained close to a state of marginal stability in which growth and transport are in rough balance. This baroclinic adjustment idea explains why observed mid-latitude lapse rates and gradients vary so little. The regulation is not perfect, which is why individual storms and quiet spells still occur. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Discuss in Forum

29.

Which view of cyclogenesis complements the normal mode picture?

A.The potential vorticity interaction view
B.The convective view, in which the storm is understood as an organised ensemble of individual cumulus towers
C.The frictional view
D.The radiative view, in which the development is driven by the differential cooling of cloud tops over time

Answer: A

The potential vorticity view describes cyclogenesis as the mutual amplification of an upper level anomaly and a surface thermal anomaly, each inducing a circulation that reinforces the other. It explains individual cases far better than a normal mode calculation, which describes the statistically preferred structure. Convective, frictional and radiative processes contribute but do not constitute alternative frameworks for the development itself.

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30.

Which arrangement of anomalies produces mutual amplification?

A.The two anomalies displaced in the north-south direction rather than along the direction of the mean flow
B.The upper anomaly upstream of the surface anomaly
C.The upper anomaly directly above the surface anomaly, so that their circulations are perfectly aligned
D.The upper anomaly downstream

Answer: B

Growth requires the upper anomaly to lie upstream, which in westerly flow means to the west, so that its induced circulation advects warm air poleward beneath it and the surface anomaly in turn advects the upper anomaly. Vertical alignment is the mature non-growing configuration. This tilt criterion is one of the most useful practical signatures of an actively deepening system. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

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