Atmospheric Waves Questions and Answers

Answer: A

Dispersion means the phase speed varies with wavenumber, so a packet spreads and the group velocity differs from the phase velocity. Rossby and gravity waves are dispersive while sound and equatorial Kelvin waves are not. Trapping and evanescence describe how the amplitude varies in space rather than how the speed varies with wavelength.

Answer: B

At a critical level the intrinsic frequency vanishes, the vertical wavelength shrinks and the wave is absorbed, depositing its momentum into the mean flow. Reflecting levels turn waves back by a different mechanism, the steering level is a synoptic concept for system movement, and the level of non-divergence concerns the vertical structure of the divergence field. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: C

Acoustic waves have periods of seconds, gravity waves minutes to hours, inertial oscillations about a day, and Rossby waves days to weeks, giving the order 2, 4, 3, 1. The ladder spans some six orders of magnitude, and placing any newly encountered phenomenon on it from an estimate of its timescale is a quick way to identify which family it belongs to.

Answer: D

Acoustic waves are restored by compressibility, internal gravity waves by buoyancy, and Rossby waves by the meridional gradient of potential vorticity. Organising the wave family by restoring force rather than by name makes the dispersion relations far easier to reconstruct, since each follows directly from the physics of what returns a displaced parcel towards its original position. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

The Kelvin wave travels eastward only, the equatorial Rossby wave westward, and the mixed Rossby-gravity wave has westward phase propagation while carrying energy upward. The opposing directions of the first two produce the east-west asymmetry of the Gill response to tropical heating, and the momentum carried by the first and third drives the quasi-biennial oscillation. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

Unresolved gravity waves generated by orography and convection deposit momentum in the stratosphere and mesosphere, and without a parameterisation of that drag models produce a polar vortex far too strong and a mesosphere far from observation. The other processes are important elsewhere in the model but do not control the middle atmosphere circulation in the same way. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

The cold pole problem, in which the modelled winter polar vortex is too strong and too cold, was a longstanding defect traced directly to missing wave drag. Adding an orographic gravity wave drag scheme largely resolved it. The other errors listed arise from quite different deficiencies in convection, microphysics and numerical conservation.

Answer: D

The scheme launches a wave momentum flux at the source level and computes where it is deposited, the vertical divergence of that flux being the drag applied to the resolved flow. Saturation criteria determine the deposition level. Counting individual crests is neither possible nor meaningful at model resolution, and the surface amplitude is an input rather than the output of the calculation. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

The wavenumber is defined as two pi divided by the wavelength and gives the number of radians of phase per unit distance. The number of oscillations per second is the frequency, and the distance travelled per oscillation is the wavelength itself. In meteorology a zonal wavenumber is often quoted instead as the number of complete waves around a latitude circle, which is a different convention worth noticing when reading the literature. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: B

Meteorologists count the number of complete waves fitting around the globe, so wavenumber three means three ridges and three troughs at that latitude. At 45 degrees north the corresponding wavelength is about 9400 kilometres. This planetary convention differs from the mathematical wavenumber and both appear in the literature, so checking which is meant is worthwhile. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.