Vorticity and Circulation Questions and Answers

Q: Which quantity does helicity measure?

Helicity is the integral of the dot product of velocity and vorticity, so it is large when the flow moves along its own vortex lines. Storm relative helicity, computed over the lowest few kilometres, is the standard measure of the potential for rotating updraughts in severe storm forecasting. The other options describe the stretching term, the vorticity magnitude and the mean vorticity respectively. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: Which application does storm-relative helicity have?

Values of storm relative helicity above about 150 square metres per square second in the lowest three kilometres indicate an environment favourable for rotating updraughts. The quantity captures the streamwise vorticity that the updraught will tilt and stretch. Tropical cyclone intensity, frontogenesis and boundary layer depth are assessed by quite different diagnostics.

Q: Which curve on a hodograph indicates high streamwise vorticity?

Curvature of the hodograph means the shear vector turns with height, which produces vorticity aligned with the storm relative flow and hence large helicity. A straight hodograph gives crosswise vorticity and split storms rather than a dominant rotating updraught. A hodograph of zero length means no shear at all, and a closed hodograph implies no net shear across the layer.

Q: Which quantity is the divergence of the velocity field?

Divergence measures the fractional rate of increase of the area of a horizontal fluid element, so positive divergence means the element is spreading out. Rotation is vorticity, circulation is the loop integral of velocity, and shape change at constant area is deformation. Together these four describe every way a fluid element can be affected by the local velocity gradient.

Q: Which relationship connects divergence at upper levels to surface pressure?

Surface pressure measures the weight of the overlying column, so if more mass leaves the top than enters at the bottom the column lightens and the pressure falls. This is the mass budget argument behind the deepening of surface lows beneath upper divergence. Static stability affects the vertical motion profile but not the sign of this relationship.

Q: Which level typically separates convergence below from divergence above in a developing low?

In a typical developing system convergence occupies the lower troposphere and divergence the upper, with the change of sign at a level of non-divergence around 500 to 600 hectopascals. That level is where the barotropic vorticity equation is most nearly valid, which is why 500 hectopascals was chosen for the first numerical forecasts. The tropopause is well above the divergence maximum rather than at the transition. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Q: Which historical reason explains the choice of 500 hectopascals for the first forecast model?

The barotropic model assumes non-divergent flow, so it is most nearly valid at the level where the divergence actually vanishes, and 500 hectopascals is close to that level. It is also the level with half the atmospheric mass beneath it, which reinforces the choice. The strongest winds are near 250 hectopascals, and observational convenience did not drive the decision.

Q: Which effect does friction have on the vorticity of a surface cyclone?

Convergence in the frictional boundary layer forces air upward out of it, and the associated divergence within the layer reduces the vorticity, spinning the vortex down over about four days. The inward turning of the wind is the mechanism by which this happens rather than a source of vorticity. Planetary and relative vorticity are exchanged by the beta effect rather than by friction.

Q: Which timescale characterises Ekman spin-down of a synoptic vortex?

The spin-down time is the depth of the troposphere divided by the Ekman pumping velocity, which works out at roughly 4 days for typical mid-latitude values. That is far shorter than molecular diffusion would give and matches the observed lifetime of decaying systems, which is the empirical check that makes the theory convincing. Boundary layer adjustment and turbulent mixing operate on much shorter timescales. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

71.

Match each theorem with its content.

List I

A. Stokes

B. Kelvin

C. Ertel

List II

P. circulation conserved in a barotropic fluid

Q. circulation equals the flux of vorticity

R. potential vorticity conserved for adiabatic motion

Select the correct match.

A.A-R, B-P, C-Q
B.A-Q, B-R, C-P
C.A-Q, B-P, C-R
D.A-P, B-Q, C-R

Answer: C

Stokes' theorem is the mathematical identity relating circulation to the flux of vorticity, Kelvin's theorem is the dynamical statement of conservation in a barotropic fluid, and Ertel's theorem generalises conservation to stratified adiabatic flow. Bjerknes' theorem completes the family by giving the rate of change of circulation in the baroclinic case.

Discuss in Forum

72.

Which quantity does helicity measure?

A.The rate at which vorticity is being generated by the convergence of the flow within a rotating column
B.The magnitude of the vorticity
C.The circulation around a closed loop divided by the area enclosed by that loop within the flow
D.The alignment of vorticity with velocity

Answer: D

Helicity is the integral of the dot product of velocity and vorticity, so it is large when the flow moves along its own vortex lines. Storm relative helicity, computed over the lowest few kilometres, is the standard measure of the potential for rotating updraughts in severe storm forecasting. The other options describe the stretching term, the vorticity magnitude and the mean vorticity respectively. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

73.

Which application does storm-relative helicity have?

A.Assessing the potential for supercell development
B.Estimating the maximum wind speed that a mature tropical cyclone can attain at its given latitude
C.Diagnosing frontogenesis
D.Determining the depth of the boundary layer through which the surface stress is being transmitted

Answer: A

Values of storm relative helicity above about 150 square metres per square second in the lowest three kilometres indicate an environment favourable for rotating updraughts. The quantity captures the streamwise vorticity that the updraught will tilt and stretch. Tropical cyclone intensity, frontogenesis and boundary layer depth are assessed by quite different diagnostics.

Discuss in Forum

74.

Which curve on a hodograph indicates high streamwise vorticity?

A.A hodograph that doubles back on itself so that the wind at the top matches the wind at the bottom
B.A strongly curved hodograph in the lowest kilometres
C.A straight hodograph, since a uniform shear vector gives the greatest possible vorticity in the layer
D.A hodograph of zero length

Answer: B

Curvature of the hodograph means the shear vector turns with height, which produces vorticity aligned with the storm relative flow and hence large helicity. A straight hodograph gives crosswise vorticity and split storms rather than a dominant rotating updraught. A hodograph of zero length means no shear at all, and a closed hodograph implies no net shear across the layer.

Discuss in Forum

75.

Which quantity is the divergence of the velocity field?

A.The circulation
B.The rate at which the shape of a fluid element is distorted without any change to its total area
C.The rate of areal expansion of a fluid element
D.The rate at which a fluid element rotates about a vertical axis as it travels along with the flow

Answer: C

Divergence measures the fractional rate of increase of the area of a horizontal fluid element, so positive divergence means the element is spreading out. Rotation is vorticity, circulation is the loop integral of velocity, and shape change at constant area is deformation. Together these four describe every way a fluid element can be affected by the local velocity gradient.

Discuss in Forum

76.

Which relationship connects divergence at upper levels to surface pressure?

A.Divergence aloft always raises the surface pressure by drawing additional mass into the column beneath
B.There is no relationship
C.Divergence aloft has an effect whose sign depends on the static stability of the intervening layers
D.Divergence aloft exceeding convergence below lowers the surface pressure

Answer: D

Surface pressure measures the weight of the overlying column, so if more mass leaves the top than enters at the bottom the column lightens and the pressure falls. This is the mass budget argument behind the deepening of surface lows beneath upper divergence. Static stability affects the vertical motion profile but not the sign of this relationship.

Discuss in Forum

77.

Which level typically separates convergence below from divergence above in a developing low?

A.The level of non-divergence near 600 hectopascals
B.The tropopause, above which the flow becomes divergent as it enters the stably stratified stratosphere
C.The surface
D.The 850 hectopascal level, which lies just above the top of the frictional boundary layer over land

Answer: A

In a typical developing system convergence occupies the lower troposphere and divergence the upper, with the change of sign at a level of non-divergence around 500 to 600 hectopascals. That level is where the barotropic vorticity equation is most nearly valid, which is why 500 hectopascals was chosen for the first numerical forecasts. The tropopause is well above the divergence maximum rather than at the transition. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Discuss in Forum

78.

Which historical reason explains the choice of 500 hectopascals for the first forecast model?

A.It is the level at which the temperature is closest to the mean temperature of the whole troposphere
B.It lies close to the level of non-divergence
C.It is the level at which the strongest winds are found within the mid-latitude westerly flow aloft
D.It is easiest to observe

Answer: B

The barotropic model assumes non-divergent flow, so it is most nearly valid at the level where the divergence actually vanishes, and 500 hectopascals is close to that level. It is also the level with half the atmospheric mass beneath it, which reinforces the choice. The strongest winds are near 250 hectopascals, and observational convenience did not drive the decision.

Discuss in Forum

79.

Which effect does friction have on the vorticity of a surface cyclone?

A.It has no effect
B.It converts relative vorticity into planetary vorticity as the system moves polewards across the flow
C.It destroys vorticity through Ekman pumping
D.It generates vorticity by turning the wind across the isobars towards the low pressure at the centre

Answer: C

Convergence in the frictional boundary layer forces air upward out of it, and the associated divergence within the layer reduces the vorticity, spinning the vortex down over about four days. The inward turning of the wind is the mechanism by which this happens rather than a source of vorticity. Planetary and relative vorticity are exchanged by the beta effect rather than by friction.

Discuss in Forum

80.

Which timescale characterises Ekman spin-down of a synoptic vortex?

A.About four hours, which is comparable with the time taken for the boundary layer to adjust to a change
B.About four weeks
C.About four minutes, which is the timescale on which turbulent eddies mix momentum within the layer
D.About four days

Answer: D

The spin-down time is the depth of the troposphere divided by the Ekman pumping velocity, which works out at roughly 4 days for typical mid-latitude values. That is far shorter than molecular diffusion would give and matches the observed lifetime of decaying systems, which is the empirical check that makes the theory convincing. Boundary layer adjustment and turbulent mixing operate on much shorter timescales. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Discuss in Forum

Loading…

Topics

Other Subjects

Tags: Vorticity and Circulation Questions and Answers || Vorticity and Circulation MCQ Questions and Answers || Vorticity and Circulation GK Questions and Answers || Vorticity and Circulation GK MCQ Questions || Vorticity and Circulation Multiple Choice Questions and Answers || Vorticity and Circulation GK || GK on Vorticity and Circulation || Atmospheric Dynamics Questions and Answers || Atmospheric Dynamics MCQ Questions and Answers || Atmospheric Dynamics GK Questions and Answers || GK on Atmospheric Dynamics

Vorticity and Circulation Questions and Answers

Topics

Other Subjects

About Us

Stay informed about all important Dates & Notifications