Atmospheric Waves Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.