Atmospheric Waves Questions and Answers

Answer: C

The mixed Rossby-gravity or Yanai wave behaves like a gravity wave at high frequency and like a Rossby wave at low frequency, and it propagates westward in phase while carrying energy upward. It is one of the two wave types driving the quasi-biennial oscillation. Kelvin, inertia-gravity and Lamb waves each belong to a single family rather than bridging two. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

The Lamb wave is a horizontally propagating acoustic mode with no vertical velocity, trapped against the surface with an amplitude decaying exponentially upward. The pressure wave from the 1883 Krakatoa eruption and the 2022 Tonga eruption circled the globe several times as Lamb waves. It is the only acoustic mode of any meteorological interest.

Answer: A

The Hunga Tonga eruption generated a pressure pulse recorded by barometers worldwide, circling the globe several times and arriving at predictable intervals consistent with the acoustic speed. It was the clearest such observation since Krakatoa. The other eruptions were meteorologically significant for their aerosol and ash but did not produce a comparable global pressure wave. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

Including rotation modifies the gravity wave dispersion relation so that the frequency is bounded below by the Coriolis parameter and above by the buoyancy frequency. In the troposphere that band runs from a period of about 17 hours to about 10 minutes. Waves at the low frequency end are strongly affected by rotation and those at the high end are essentially pure gravity waves. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

Rapid changes in the flow near jet streaks and fronts leave the atmosphere temporarily out of balance, and the adjustment radiates inertia-gravity waves. These are routinely observed near jet exit regions and are an important source of stratospheric wave activity. Convection and topography are the other principal sources, while the processes named in the other options are too slow or too smooth. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: D

An unbalanced state radiates inertia-gravity waves that carry away the imbalance within a few hours, leaving a balanced residual whose scale is set by the Rossby radius of deformation. This is why the atmosphere is observed to be close to balance almost everywhere despite continual forcing. Friction and diabatic heating act far more slowly and instability generates rather than removes departures from the mean state. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

For disturbances much larger than the deformation radius the mass field adjusts to the wind field, and for much smaller ones the wind adjusts to the mass field. The deformation radius is precisely the scale at which rotation and stratification are equally important. The scale height and Ekman depth describe vertical structure, and the baroclinic wavelength is several times larger than the deformation radius. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

Richardson's initial field contained a small imbalance whose gravity wave response dominated the computed pressure tendency, giving a change of 145 hectopascals in six hours. The episode led directly to the development of initialisation techniques and filtered equation sets. The other problems listed are genuine numerical difficulties but arose from quite different causes. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: C

A digital filter runs the model briefly forward and backward and applies a low pass filter in time, removing the high frequency gravity wave components while leaving the balanced flow untouched. Incremental analysis update achieves a similar result within the assimilation. Resolution and truncation do not address balance, and time averaging would smear the meteorological signal as well. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Group velocity is the derivative of frequency with respect to wavenumber and gives the velocity at which a wave packet and its energy travel. For a dispersive wave it differs from the phase velocity, sometimes dramatically, as with Rossby waves where phase moves west while energy moves east. Distinguishing the two is essential for understanding downstream development. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.