Baroclinic Instability Questions and Answers

Q: Which value marks the dynamical tropopause on such a map?

The two unit surface separates tropospheric air of low potential vorticity from stratospheric air of high potential vorticity and is the conventional dynamical definition of the tropopause. Some authors use 1.5 or 3 units, so the choice is a convention rather than a physical constant. Tropospheric values are typically a few tenths of a unit rather than zero.

Q: Which unit is a potential vorticity unit equal to?

One potential vorticity unit is defined as \( 10^{-6} \) kelvin metres squared per kilogram per second, a combination chosen so that typical stratospheric values are a few units. Using a named unit avoids repeatedly writing an awkward combination. The other values listed are ordinary vorticities, which have units of inverse seconds rather than the combined form.

Q: Which process erodes an upper level potential vorticity anomaly?

Latent heat release beneath an anomaly generates negative potential vorticity above the heating maximum, which can erode the anomaly from below over the course of a day or two. Adiabatic motion and horizontal advection both conserve the quantity and merely move it about. This diabatic erosion is one reason forecasts of upper features degrade in regions of heavy precipitation. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Q: Which model deficiency does this identify as important?

Because diabatic potential vorticity generation feeds back onto the large scale flow, errors in convection and cloud schemes limit medium range predictability even for the largest scales. This has motivated substantial effort on moist physics in global models. Resolution, coordinates and time stepping all matter but none has the same direct route into large scale forecast error. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Q: Assertion (A): Storm tracks are most active in winter. Reason (R): The equator to pole temperature contrast is greatest in winter. Select the correct answer from the options below.

A larger temperature contrast means a larger vertical shear through the thermal wind relation, and the baroclinic growth rate is proportional to that shear. The reason therefore explains the assertion completely. The chain from radiative forcing through temperature gradient and thermal wind to growth rate is a good illustration of how several results in this subject link together into a single argument.

Q: Assertion (A): Blocking events are easy to forecast. Reason (R): Once established a block persists for a week or more. Select the correct answer from the options below.

The reason is correct, and persistence does help the forecast once the block exists, but the onset and breakdown remain among the least predictable events in mid-latitude meteorology. The assertion is therefore too strong. Models also show systematic biases in block frequency and duration, so the difficulty is one of both initial condition and model error.

Q: Statement I: The eddy heat flux is directed poleward. Statement II: The eddy momentum flux converges into the jet during barotropic decay. In the light of the above statements, choose the correct answer.

Both statements are correct and together they summarise the two halves of the eddy life cycle. The poleward heat flux accomplishes the baroclinic energy conversion during growth, while the momentum flux convergence returns energy to the jet during decay. Between them the two fluxes explain how eddies both feed on and maintain the mean flow.

Q: Consider the following statements about blocking. 1. It is associated with an inverse energy cascade. 2. It can produce heatwaves and cold spells. 3. Its onset is well forecast by current models. Which of the statements given above is or are correct?

The first two statements are correct, but the onset of blocking remains poorly forecast and models show systematic errors in its frequency. The persistence of a block once established is what makes it consequential for surface weather, since ordinary conditions maintained for two weeks become an extreme event.

Q: Consider the following pairs of feature and location. Storm track entrance - western ocean in winter Blocking - eastern ocean and Europe Subtropical jet - about thirty degrees latitude Baroclinic instability - the equator How many of the pairs given above are correctly matched?

Baroclinic instability requires an appreciable Coriolis parameter as well as a temperature gradient, and neither is available at the equator, so the last pairing is wrong. The other three are correct. Mapping the main features of the general circulation onto their preferred locations is a compact way to hold the whole picture together.

91.

Which value marks the dynamical tropopause on such a map?

A.Ten units
B.Zero units, since the potential vorticity of the troposphere is negligible compared with that above
C.Two potential vorticity units
D.One potential vorticity unit, which is the value found within the middle troposphere at most latitudes

Answer: C

The two unit surface separates tropospheric air of low potential vorticity from stratospheric air of high potential vorticity and is the conventional dynamical definition of the tropopause. Some authors use 1.5 or 3 units, so the choice is a convention rather than a physical constant. Tropospheric values are typically a few tenths of a unit rather than zero.

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92.

Which unit is a potential vorticity unit equal to?

A.10 to the minus 4 per second, which is the same order as the Coriolis parameter in the middle latitudes
B.1 kelvin per second
C.10 to the minus 5 per second, which is the typical relative vorticity of a mid-latitude weather system
D.10 to the minus 6 kelvin metres squared per kilogram per second

Answer: D

One potential vorticity unit is defined as \( 10^{-6} \) kelvin metres squared per kilogram per second, a combination chosen so that typical stratospheric values are a few units. Using a named unit avoids repeatedly writing an awkward combination. The other values listed are ordinary vorticities, which have units of inverse seconds rather than the combined form.

Discuss in Forum

93.

Which process erodes an upper level potential vorticity anomaly?

A.Diabatic heating beneath it
B.The adiabatic ascent of air within the warm conveyor belt, which conserves potential vorticity exactly
C.Horizontal advection
D.The geostrophic adjustment of the surrounding flow to the circulation that the anomaly itself induces

Answer: A

Latent heat release beneath an anomaly generates negative potential vorticity above the heating maximum, which can erode the anomaly from below over the course of a day or two. Adiabatic motion and horizontal advection both conserve the quantity and merely move it about. This diabatic erosion is one reason forecasts of upper features degrade in regions of heavy precipitation. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Discuss in Forum

94.

Which consequence does that erosion have for the forecast?

A.The system deepens more rapidly than it otherwise would because the erosion releases additional energy
B.The downstream flow evolution becomes less predictable
C.The forecast improves because the anomaly is removed and the flow becomes simpler to represent numerically
D.There is no consequence

Answer: B

Errors in the representation of diabatic processes translate directly into errors in the upper level potential vorticity field, which then propagate downstream at the group velocity and degrade the forecast far from the original storm. This coupling between moist processes and large scale predictability is an active research area. A simpler flow does not follow from a poorly represented one. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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95.

Which model deficiency does this identify as important?

A.The vertical coordinate choice
B.The time stepping scheme, whose truncation error accumulates over the course of a multi-day integration
C.The representation of moist processes
D.The horizontal resolution of the model grid, which must be fine enough to resolve individual frontal bands

Answer: C

Because diabatic potential vorticity generation feeds back onto the large scale flow, errors in convection and cloud schemes limit medium range predictability even for the largest scales. This has motivated substantial effort on moist physics in global models. Resolution, coordinates and time stepping all matter but none has the same direct route into large scale forecast error. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Discuss in Forum

96.

Assertion (A): Storm tracks are most active in winter.

Reason (R): The equator to pole temperature contrast is greatest in winter.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: D

A larger temperature contrast means a larger vertical shear through the thermal wind relation, and the baroclinic growth rate is proportional to that shear. The reason therefore explains the assertion completely. The chain from radiative forcing through temperature gradient and thermal wind to growth rate is a good illustration of how several results in this subject link together into a single argument.

Discuss in Forum

97.

Assertion (A): Blocking events are easy to forecast.

Reason (R): Once established a block persists for a week or more.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: A

The reason is correct, and persistence does help the forecast once the block exists, but the onset and breakdown remain among the least predictable events in mid-latitude meteorology. The assertion is therefore too strong. Models also show systematic biases in block frequency and duration, so the difficulty is one of both initial condition and model error.

Discuss in Forum

98.

Statement I: The eddy heat flux is directed poleward.

Statement II: The eddy momentum flux converges into the jet during barotropic decay.

In the light of the above statements, choose the correct answer.

A.Statement I is incorrect but Statement II is correct
B.Both Statement I and Statement II are correct
C.Both Statement I and Statement II are incorrect
D.Statement I is correct but Statement II is incorrect

Answer: B

Both statements are correct and together they summarise the two halves of the eddy life cycle. The poleward heat flux accomplishes the baroclinic energy conversion during growth, while the momentum flux convergence returns energy to the jet during decay. Between them the two fluxes explain how eddies both feed on and maintain the mean flow.

Discuss in Forum

99.

Consider the following statements about blocking.

1. It is associated with an inverse energy cascade.

2. It can produce heatwaves and cold spells.

3. Its onset is well forecast by current models.

Which of the statements given above is or are correct?

A.2 and 3 only
B.1, 2 and 3
C.1 and 2 only
D.1 and 3 only

Answer: C

The first two statements are correct, but the onset of blocking remains poorly forecast and models show systematic errors in its frequency. The persistence of a block once established is what makes it consequential for surface weather, since ordinary conditions maintained for two weeks become an extreme event.

Discuss in Forum

100.

Consider the following pairs of feature and location.

Storm track entrance - western ocean in winter

Blocking - eastern ocean and Europe

Subtropical jet - about thirty degrees latitude

Baroclinic instability - the equator

How many of the pairs given above are correctly matched?

A.Only one
B.Only two
C.All four
D.Only three

Answer: D

Baroclinic instability requires an appreciable Coriolis parameter as well as a temperature gradient, and neither is available at the equator, so the last pairing is wrong. The other three are correct. Mapping the main features of the general circulation onto their preferred locations is a compact way to hold the whole picture together.

Discuss in Forum

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