Atmospheric Waves Questions and Answers

Answer: C

Both statements correctly identify the restoring mechanism of their respective wave types. Classifying atmospheric waves by what pushes a displaced parcel back is the most reliable organising principle for the whole family, since the dispersion relation of each follows from the physics of its restoring force. Sound waves complete the set, being restored by compressibility.

Answer: D

The intrinsic phase speed is always westward, but the group velocity can be eastward, which is what produces downstream development in Rossby wave trains. This distinction between phase and energy propagation has real forecasting consequences, since it means a disturbance in one region seeds development far downstream several days later, and it also means analysis errors spread faster than the systems themselves.

Answer: A

The first two statements are correct, but the group velocity of an internal gravity wave is perpendicular rather than parallel to the phase velocity. That perpendicularity is the most distinctive property of the wave type and is what allows the direction of energy transport to be inferred from the tilt of the phase lines in an observed time-height section.

Answer: B

The first two statements are correct. Equatorial Rossby waves, like their mid-latitude counterparts, propagate westward relative to the mean flow. The opposing directions of the Kelvin and Rossby responses are precisely what produces the east-west asymmetry of the Gill pattern around a region of tropical heating, so the point is worth fixing firmly.

Answer: C

The Kelvin wave is a gravity wave restored by buoyancy and trapped by rotation against a boundary or the equator, so friction plays no part in it whatever. The other three pairings are correct. Friction damps waves rather than restoring them, and no atmospheric wave type has friction as its restoring mechanism.

Answer: D

Rossby waves have periods of days to weeks, being the slowest of the atmospheric wave types, so the last pairing is wrong by several orders of magnitude. The other three are correct. Arranging the wave types by period from seconds to weeks gives a useful ladder against which any new phenomenon can be placed once its timescale is known.

Answer: A

Mountain waves are generated by flow over topography and are found wherever stable flow crosses a barrier, with no connection to the equatorial waveguide. The other three are members of the family of equatorially trapped solutions derived by Matsuno from the shallow water equations on an equatorial beta plane, all of which decay away from the equator.

Answer: B

Rossby waves survive the hydrostatic approximation untouched, since they depend on the horizontal potential vorticity gradient rather than on vertical acceleration. All the acoustic modes involve vertical accelerations and are removed. Filtering the fast acoustic modes while retaining the slow meteorologically important ones is the central purpose of the approximation. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: C

For internal gravity waves the group velocity is perpendicular to the phase velocity with the opposite vertical sign, so descending phase lines indicate upward energy transport. This is the standard method of identifying tropospheric sources of stratospheric wave activity. A stationary wave would show vertical phase lines, and critical level absorption produces a rapid shortening of the vertical wavelength instead. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

Eastward wavenumber one with a period between 30 and 60 days is the signature of the Madden Julian Oscillation, whose propagation speed of about 5 metres per second is far below that of a dry Kelvin wave. Kelvin waves appear at much shorter periods on the same diagram. The quasi-biennial oscillation has a period of over two years and does not appear in an intraseasonal spectrum at all. Most errors here come from carrying a result across from a neighbouring case where it does not hold.