Atmospheric Waves Questions and Answers

Answer: A

A dispersive wave has a phase speed that varies with wavenumber, so a packet spreads out as it travels and its group velocity differs from its phase velocity. Rossby and gravity waves are dispersive while sound and shallow water gravity waves are not. Trapping and evanescence describe the spatial structure of a wave rather than the relationship between its speed and its wavelength. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

The equatorial Kelvin wave has a phase speed independent of wavenumber for a given vertical mode, so a packet travels without changing shape. This is why Kelvin wave signals cross an ocean basin as coherent pulses. Rossby and inertia-gravity waves are both strongly dispersive, and the mixed wave inherits dispersion from both of its parents.

Answer: C

Plotting the spectral power of cloud or outgoing longwave radiation against zonal wavenumber and frequency reveals distinct ridges corresponding to Kelvin, Rossby, mixed and inertia-gravity waves, matching the theoretical dispersion curves. Wheeler and Kiladis published the classic diagram. Single soundings and pressure traces cannot separate the wave types in this way. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: D

The observed spectral peaks lie remarkably close to the dispersion curves predicted by Matsuno's shallow water theory of equatorial waves, which is a striking confirmation of a theory developed decades earlier. Both eastward and westward propagating modes appear clearly. The agreement is one of the more satisfying instances of theory anticipating observation in this subject. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: A

The Madden Julian Oscillation is an eastward propagating envelope of enhanced and suppressed convection that circles the tropics with a period of 30 to 60 days. It modulates monsoon onset, tropical cyclogenesis and even mid-latitude weather. The other oscillations listed operate on interannual, biennial and synoptic to seasonal timescales respectively.

Answer: B

The convective envelope moves eastward at roughly 5 metres per second across the warm pool, far slower than a dry Kelvin wave, which is the central puzzle the theory must explain. The slowing is attributed to the interaction between the wave and the moisture field. Once past the dateline the circulation signal speeds up as it decouples from the convection. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

Simulating the oscillation with the correct amplitude and propagation speed has been a longstanding difficulty, and it is closely tied to the treatment of convection and its coupling to moisture. Improvements in convective parameterisation have gradually improved matters. The phenomenon is large enough to be well resolved and is thoroughly documented by satellite observation, so neither resolution nor data is the limitation. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: D

Because the group velocity vector is perpendicular to the wavenumber vector and directed with an opposite vertical component, energy travels upward when phase lines descend. The reason therefore explains the assertion exactly. This property allows the source of stratospheric wave activity to be identified from a time-height section, which is how tropospheric generation of stratospheric waves was first established observationally.

Answer: A

Summer stratospheric easterlies block vertical propagation entirely, so no wave activity reaches the summer stratosphere to break there. Only in winter are the winds westerly and weak enough for the longest waves to ascend. The reason therefore explains the assertion completely, and the Charney-Drazin criterion that supplies it is one of the cleanest links between tropospheric and stratospheric dynamics.

Answer: B

The reason correctly states the defining property, and it is exactly that property which forces eastward propagation: only the eastward solution decays away from the equator, the westward one growing without bound. The assertion is therefore false. This is a good example of a structural constraint determining a direction of travel, which is the same argument that fixes the sense of coastal Kelvin waves.