Thermal Wind Balance Questions and Answers

Q: Which term describes an atmosphere with intersecting pressure and density surfaces?

In a baroclinic atmosphere the isobaric and isopycnal surfaces intersect, so the solenoidal term is non-zero and circulation can be generated. Equivalently, there is a horizontal temperature gradient on a pressure surface and hence a thermal wind. A barotropic atmosphere is the opposite case with no shear. Isentropic and adiabatic describe constant potential temperature and the absence of heat exchange respectively, which are different ideas.

Q: Which consequence does barotropy have for the vertical wind profile?

With no horizontal temperature gradient the thermal wind vanishes, so the geostrophic wind is identical at every level. This is why the earliest numerical forecast models, which were barotropic, could represent the flow with a single level. The other options describe baroclinic situations with a thermal wind oriented parallel, antiparallel or perpendicular to the flow respectively.

Q: Which energy reservoir is associated with baroclinicity?

A horizontal temperature gradient means that mass could be rearranged to lower the centre of gravity, and the energy released by doing so is the available potential energy. Baroclinic instability is precisely the process that taps it. Total internal energy is enormous but almost entirely unavailable, latent heat is a separate source, and the eddies convert available potential energy rather than drawing directly on the mean kinetic energy. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: Which fraction of the atmosphere's total potential energy is actually available?

Lorenz showed that the available portion is only about 0.5 per cent of the total potential energy, because most of the mass is already close to its lowest attainable arrangement. Even that small fraction is around ten times the kinetic energy of the general circulation, so it is more than sufficient. The calculation is a useful corrective to the intuition that a large reservoir implies a large usable supply. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Q: Which structure sits above the strongest low level temperature gradient?

The thermal wind relation requires the westerly shear to be greatest where the horizontal temperature gradient is greatest, so the jet core lies directly above the polar front. This is why frontal zones and jet streaks are dynamically inseparable. The subtropical high, the convergence zone and the polar vortex all owe their positions to other aspects of the circulation, though the subtropical jet does have its own thermal wind explanation. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: Which seasonal change strengthens the mid-latitude jet?

Winter cooling of the polar regions sharpens the meridional temperature gradient, and by the thermal wind relation this strengthens the westerly shear and hence the jet. A uniform warming of the whole troposphere would produce no gradient and no change in shear. Tropopause height and boundary layer depth do vary seasonally but neither is the mechanism, and the jet also moves equatorward in winter as the gradient shifts. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: Which jet owes its existence to angular momentum conservation as well as thermal wind?

The subtropical jet sits at the poleward edge of the Hadley cell, where air that has conserved angular momentum while moving poleward arrives with a large westerly velocity. It also satisfies the thermal wind relation with respect to the tropical to subtropical temperature gradient. The polar front jet is more purely a thermal wind feature. The nocturnal and African jets have quite different origins in inertial oscillation and in a reversed temperature gradient.

Q: Which sign of thermal wind does the summer polar stratosphere have?

Continuous sunlight warms the summer polar stratosphere above the tropical value, reversing the temperature gradient and producing an easterly thermal wind. The result is the summer stratospheric easterlies, which prevent vertical propagation of planetary waves and so make the summer stratosphere quiescent. The winter hemisphere has the opposite arrangement with a strong westerly polar vortex.

Q: Which effect do stratospheric easterlies have on planetary waves?

The Charney Drazin criterion shows that stationary planetary waves can propagate vertically only through westerlies below a critical speed, so summer easterlies block them entirely. This is why sudden stratospheric warmings occur only in winter, when westerlies allow wave energy to reach the stratosphere. The criterion is one of the cleanest results linking tropospheric and stratospheric dynamics.

Q: Which quantity does the ocean's thermal wind relation use in place of temperature?

In the ocean the thermal wind relates the vertical shear of the geostrophic current to the horizontal density gradient, since density depends on both temperature and salinity. Using temperature alone would be wrong wherever salinity varies appreciably, as at high latitudes and near river outflows. The relation is otherwise identical in form, which is why the dynamic method for computing ocean currents from hydrographic data works at all. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

21.

Which term describes an atmosphere with intersecting pressure and density surfaces?

A.Isentropic
B.Adiabatic, meaning that no heat is exchanged between a parcel and the environment surrounding it
C.Baroclinic
D.Barotropic, in which surfaces of constant pressure and constant density are everywhere parallel

Answer: C

In a baroclinic atmosphere the isobaric and isopycnal surfaces intersect, so the solenoidal term is non-zero and circulation can be generated. Equivalently, there is a horizontal temperature gradient on a pressure surface and hence a thermal wind. A barotropic atmosphere is the opposite case with no shear. Isentropic and adiabatic describe constant potential temperature and the absence of heat exchange respectively, which are different ideas.

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22.

Which consequence does barotropy have for the vertical wind profile?

A.The wind increases with height in proportion to the temperature gradient across the layer concerned
B.The wind reverses direction
C.The wind turns steadily clockwise with height through the whole depth of the layer being considered
D.The geostrophic wind is independent of height

Answer: D

With no horizontal temperature gradient the thermal wind vanishes, so the geostrophic wind is identical at every level. This is why the earliest numerical forecast models, which were barotropic, could represent the flow with a single level. The other options describe baroclinic situations with a thermal wind oriented parallel, antiparallel or perpendicular to the flow respectively.

Discuss in Forum

23.

Which energy reservoir is associated with baroclinicity?

A.Available potential energy
B.Kinetic energy of the zonal mean flow, which is what baroclinic eddies extract as they develop
C.Latent heat
D.The internal energy of the atmosphere, which is far larger than any of the other reservoirs present

Answer: A

A horizontal temperature gradient means that mass could be rearranged to lower the centre of gravity, and the energy released by doing so is the available potential energy. Baroclinic instability is precisely the process that taps it. Total internal energy is enormous but almost entirely unavailable, latent heat is a separate source, and the eddies convert available potential energy rather than drawing directly on the mean kinetic energy. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

24.

Which fraction of the atmosphere's total potential energy is actually available?

A.Essentially all of it, because any horizontal temperature gradient can in principle be removed
B.Less than one per cent
C.About half, since the atmosphere is close to the point at which convection would begin everywhere
D.About ten per cent

Answer: B

Lorenz showed that the available portion is only about 0.5 per cent of the total potential energy, because most of the mass is already close to its lowest attainable arrangement. Even that small fraction is around ten times the kinetic energy of the general circulation, so it is more than sufficient. The calculation is a useful corrective to the intuition that a large reservoir implies a large usable supply. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Discuss in Forum

25.

Which structure sits above the strongest low level temperature gradient?

A.The intertropical convergence zone
B.The polar vortex, which forms in the winter stratosphere over each of the two polar regions in turn
C.The jet stream core
D.The subtropical high pressure belt, which marks the descending branch of the Hadley circulation

Answer: C

The thermal wind relation requires the westerly shear to be greatest where the horizontal temperature gradient is greatest, so the jet core lies directly above the polar front. This is why frontal zones and jet streaks are dynamically inseparable. The subtropical high, the convergence zone and the polar vortex all owe their positions to other aspects of the circulation, though the subtropical jet does have its own thermal wind explanation. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

26.

Which seasonal change strengthens the mid-latitude jet?

A.The summer increase in solar heating, which raises the temperature of the whole troposphere alike
B.The winter decrease in tropopause height
C.The summer reduction in the depth of the frictional boundary layer over land in the middle latitudes
D.The winter increase in the equator to pole temperature contrast

Answer: D

Winter cooling of the polar regions sharpens the meridional temperature gradient, and by the thermal wind relation this strengthens the westerly shear and hence the jet. A uniform warming of the whole troposphere would produce no gradient and no change in shear. Tropopause height and boundary layer depth do vary seasonally but neither is the mechanism, and the jet also moves equatorward in winter as the gradient shifts. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

27.

Which jet owes its existence to angular momentum conservation as well as thermal wind?

A.The subtropical jet
B.The polar front jet, which lies above the boundary between polar and mid-latitude air masses
C.The nocturnal low level jet
D.The African easterly jet, which forms above the temperature contrast at the edge of the Sahara

Answer: A

The subtropical jet sits at the poleward edge of the Hadley cell, where air that has conserved angular momentum while moving poleward arrives with a large westerly velocity. It also satisfies the thermal wind relation with respect to the tropical to subtropical temperature gradient. The polar front jet is more purely a thermal wind feature. The nocturnal and African jets have quite different origins in inertial oscillation and in a reversed temperature gradient.

Discuss in Forum

28.

Which sign of thermal wind does the summer polar stratosphere have?

A.Northerly, reflecting the fact that the temperature gradient there points along a meridian instead
B.Easterly
C.Westerly, as in the troposphere below, since the temperature still falls towards the pole aloft
D.There is none

Answer: B

Continuous sunlight warms the summer polar stratosphere above the tropical value, reversing the temperature gradient and producing an easterly thermal wind. The result is the summer stratospheric easterlies, which prevent vertical propagation of planetary waves and so make the summer stratosphere quiescent. The winter hemisphere has the opposite arrangement with a strong westerly polar vortex.

Discuss in Forum

29.

Which effect do stratospheric easterlies have on planetary waves?

A.They have no effect
B.They shift the waves eastward at a speed determined by the strength of the mean flow at that level
C.They prevent vertical propagation
D.They amplify the waves so that sudden warmings occur most readily during the summer months instead

Answer: C

The Charney Drazin criterion shows that stationary planetary waves can propagate vertically only through westerlies below a critical speed, so summer easterlies block them entirely. This is why sudden stratospheric warmings occur only in winter, when westerlies allow wave energy to reach the stratosphere. The criterion is one of the cleanest results linking tropospheric and stratospheric dynamics.

Discuss in Forum

30.

Which quantity does the ocean's thermal wind relation use in place of temperature?

A.Salinity alone, since it is the dominant influence on seawater density in most of the world ocean
B.Pressure
C.Potential temperature referenced to the surface rather than to the level at which it is measured
D.Density

Answer: D

In the ocean the thermal wind relates the vertical shear of the geostrophic current to the horizontal density gradient, since density depends on both temperature and salinity. Using temperature alone would be wrong wherever salinity varies appreciably, as at high latitudes and near river outflows. The relation is otherwise identical in form, which is why the dynamic method for computing ocean currents from hydrographic data works at all. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Discuss in Forum

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