Atmospheric Waves Questions and Answers

Q: Which condition allows lee waves to form downstream of a ridge?

Trapped lee waves require a stable layer with the wind increasing with height, which creates a duct that reflects wave energy downward and allows a train of waves to persist far downstream. Neutral stratification supports no waves at all, light winds cannot force them, and strong directional shear disrupts the coherence of the train. Glider pilots exploit exactly these conditions. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Q: Which cloud marks the crest of a trapped lee wave?

Lenticular cloud forms in the ascending part of each wave crest and dissipates in the descending part, so it appears stationary while the air moves through it. The clouds mark the wave train clearly from the ground and from satellite. Deep convection requires conditional instability rather than wave lifting, and the other cloud types have no particular association with lee waves. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Q: Which hazard do mountain waves present to aircraft?

Rotor turbulence beneath the wave crests and vertical currents of several metres per second can exceed the climb or descent capability of an aircraft, and several accidents have been attributed to them. Lenticular cloud is generally smooth and not a major icing hazard. Navigation and air density are not affected in the way suggested by the other options.

Q: Which level acts as a critical level for a gravity wave?

At a critical level the intrinsic frequency of the wave vanishes, its vertical wavelength shrinks and the wave is absorbed, depositing its momentum into the mean flow. Critical level absorption is a principal mechanism of wave-mean flow interaction. A neutral layer does prevent propagation but by a different mechanism, and the tropopause acts as a partial reflector rather than as an absorber. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Q: Which quantity vanishes at a critical level?

The intrinsic frequency is the frequency seen by an observer moving with the mean flow, and it is the Doppler shifted quantity that vanishes when the flow speed equals the phase speed. The vertical wavelength shrinks rather than growing, which is why the wave is absorbed. Amplitude and buoyancy frequency behave quite differently and are not what defines the level. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Q: Which restoring mechanism gives a Rossby wave?

A parcel displaced poleward conserves absolute vorticity and therefore acquires anticyclonic relative vorticity, which turns it back, giving a wave whose phase propagates westward relative to the mean flow. The gradient may come from beta or from a gradient in the mean flow. Buoyancy and compressibility give gravity and sound waves, and the Coriolis force acting alone gives inertial oscillations rather than a propagating wave. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Which direction does Rossby wave phase always travel relative to the mean flow?

The intrinsic phase speed of a Rossby wave is always westward, with a magnitude proportional to beta and to the square of the wavelength divided by four pi squared. Short waves are carried eastward by the mean flow because their intrinsic speed is small, while long waves may be stationary or retrogressive. Understanding the observed eastward movement as advection overcoming an intrinsic westward propagation is the key point. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: Which wavelength makes a Rossby wave stationary in a westerly flow?

Setting the intrinsic westward phase speed equal and opposite to the advection by the mean flow gives the stationary wavelength, typically several thousand kilometres in mid-latitudes. Orographically forced waves adopt this wavelength because they must be stationary relative to the mountains. The deformation radius sets a different scale altogether.

Q: Which orographic feature forces stationary planetary waves in the northern hemisphere?

The two great northern hemisphere mountain barriers force stationary waves of zonal wavenumber two and three, which is why the northern hemisphere has a much stronger stationary wave pattern than the southern. Land-sea thermal contrast contributes as well. The southern hemisphere topography is much smaller in extent, which is why its flow is more nearly zonally symmetric. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

11.

Which source generates gravity waves in the troposphere?

A.Radiative cooling
B.The horizontal advection of temperature by the geostrophic wind across a region of strong thermal gradient
C.Flow over mountains
D.The condensation of water vapour into cloud droplets within a slowly ascending layer of stratiform cloud

Answer: C

Stably stratified flow forced over a mountain range generates waves whose phase lines are stationary relative to the ground, and these mountain waves are a principal source of momentum flux into the stratosphere. Convection and jet stream imbalance are the other two major sources. Radiative cooling and thermal advection act too slowly and too smoothly to excite waves of any significant amplitude. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

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12.

Which condition allows lee waves to form downstream of a ridge?

A.Neutral stratification through a deep layer so that a displaced parcel experiences no restoring force
B.Very light winds
C.A wind direction that changes rapidly with height so that the flow crosses the ridge at varying angles
D.Stable stratification with wind increasing with height

Answer: D

Trapped lee waves require a stable layer with the wind increasing with height, which creates a duct that reflects wave energy downward and allows a train of waves to persist far downstream. Neutral stratification supports no waves at all, light winds cannot force them, and strong directional shear disrupts the coherence of the train. Glider pilots exploit exactly these conditions. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Discuss in Forum

13.

Which cloud marks the crest of a trapped lee wave?

A.Altocumulus lenticularis
B.Cumulonimbus, which develops where the ascending branch of the wave lifts air beyond its level of free convection
C.Stratus
D.Cirrus uncinus, whose trailing ice crystals indicate strong vertical shear at the level where they form

Answer: A

Lenticular cloud forms in the ascending part of each wave crest and dissipates in the descending part, so it appears stationary while the air moves through it. The clouds mark the wave train clearly from the ground and from satellite. Deep convection requires conditional instability rather than wave lifting, and the other cloud types have no particular association with lee waves. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Discuss in Forum

14.

Which hazard do mountain waves present to aircraft?

A.A sudden increase in air density in the descending part of the wave that overloads the wing structure
B.Severe turbulence and strong vertical currents
C.Rapid airframe icing caused by the very high liquid water content of the lenticular cloud at the crest
D.Loss of navigation signal

Answer: B

Rotor turbulence beneath the wave crests and vertical currents of several metres per second can exceed the climb or descent capability of an aircraft, and several accidents have been attributed to them. Lenticular cloud is generally smooth and not a major icing hazard. Navigation and air density are not affected in the way suggested by the other options.

Discuss in Forum

15.

Which level acts as a critical level for a gravity wave?

A.The tropopause
B.Where the wind direction reverses so that the wave begins to propagate back towards its source below
C.Where the mean flow equals the wave phase speed
D.Where the buoyancy frequency falls to zero because the layer has become neutrally stratified there

Answer: C

At a critical level the intrinsic frequency of the wave vanishes, its vertical wavelength shrinks and the wave is absorbed, depositing its momentum into the mean flow. Critical level absorption is a principal mechanism of wave-mean flow interaction. A neutral layer does prevent propagation but by a different mechanism, and the tropopause acts as a partial reflector rather than as an absorber. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Discuss in Forum

16.

Which quantity vanishes at a critical level?

A.The vertical wavelength, which becomes infinite as the wave approaches the level from below it
B.The buoyancy frequency
C.The amplitude of the wave, which is dissipated completely before the level is actually reached at all
D.The intrinsic frequency

Answer: D

The intrinsic frequency is the frequency seen by an observer moving with the mean flow, and it is the Doppler shifted quantity that vanishes when the flow speed equals the phase speed. The vertical wavelength shrinks rather than growing, which is why the wave is absorbed. Amplitude and buoyancy frequency behave quite differently and are not what defines the level. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Discuss in Forum

17.

Which restoring mechanism gives a Rossby wave?

A.The meridional gradient of potential vorticity
B.Buoyancy acting on a parcel displaced vertically within a stably stratified layer of the atmosphere
C.Compressibility
D.The Coriolis force acting alone on a parcel that has no horizontal pressure gradient acting upon it

Answer: A

A parcel displaced poleward conserves absolute vorticity and therefore acquires anticyclonic relative vorticity, which turns it back, giving a wave whose phase propagates westward relative to the mean flow. The gradient may come from beta or from a gradient in the mean flow. Buoyancy and compressibility give gravity and sound waves, and the Coriolis force acting alone gives inertial oscillations rather than a propagating wave. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

18.

Which direction does Rossby wave phase always travel relative to the mean flow?

A.In whichever direction the mean flow itself is travelling, since the wave is carried along with it
B.Westward
C.Eastward, which is why the short waves of the upper westerlies move rapidly around the hemisphere
D.Poleward

Answer: B

The intrinsic phase speed of a Rossby wave is always westward, with a magnitude proportional to beta and to the square of the wavelength divided by four pi squared. Short waves are carried eastward by the mean flow because their intrinsic speed is small, while long waves may be stationary or retrogressive. Understanding the observed eastward movement as advection overcoming an intrinsic westward propagation is the key point. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

19.

Which wavelength makes a Rossby wave stationary in a westerly flow?

A.The shortest wavelength present
B.The wavelength corresponding to the Rossby radius of deformation at the latitude concerned in the flow
C.The one whose westward intrinsic speed equals the eastward flow speed
D.The wavelength equal to the circumference of the latitude circle at which the wave is being observed

Answer: C

Setting the intrinsic westward phase speed equal and opposite to the advection by the mean flow gives the stationary wavelength, typically several thousand kilometres in mid-latitudes. Orographically forced waves adopt this wavelength because they must be stationary relative to the mountains. The deformation radius sets a different scale altogether.

Discuss in Forum

20.

Which orographic feature forces stationary planetary waves in the northern hemisphere?

A.The Andes and the Southern Alps, which together dominate the topography of the southern hemisphere instead
B.The Sahara desert
C.The Gulf Stream, whose sea surface temperature gradient provides a strong thermal forcing of the flow
D.The Rocky Mountains and the Tibetan Plateau

Answer: D

The two great northern hemisphere mountain barriers force stationary waves of zonal wavenumber two and three, which is why the northern hemisphere has a much stronger stationary wave pattern than the southern. Land-sea thermal contrast contributes as well. The southern hemisphere topography is much smaller in extent, which is why its flow is more nearly zonally symmetric. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

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