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Correct Answer : B

Deep convection becomes common above roughly 27 degrees, which is why the western Pacific warm pool and not the cooler eastern Pacific supports the ascending branch of the Walker circulation. The threshold is not a sharp physical limit but reflects the temperature at which the boundary layer moist static energy is sufficient. Tropical cyclone formation uses a similar criterion. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

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Correct Answer : C

A rotating annulus of fluid heated at its outer wall and cooled at its inner one develops either a symmetric overturning or a regime of travelling waves, depending on the rotation rate and temperature difference. The wave regime is a laboratory analogue of baroclinic instability. Hide and Fultz established the regime diagram in the 1950s.

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Correct Answer : C

The Northern Annular Mode, closely related to the North Atlantic Oscillation, describes the north-south seesaw of mass that accompanies a strong or weak polar vortex and westerly flow. Its positive phase brings mild wet winters to northern Europe. The other indices listed describe tropical phenomena on interannual and intraseasonal timescales.

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Because the stratospheric influence on the surface persists for several weeks after a major warming, forecasting the stratospheric state adds skill in the two to six week range where tropospheric memory has been lost. This is now exploited operationally at several centres. The onset of a warming is itself predictable only a week or two ahead, so long lead prediction of the event is not available.

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Correct 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.

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Correct 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.

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Correct Answer : C

Only in winter are the stratospheric winds westerly and weak enough for the longest planetary waves to propagate upward, which is why sudden stratospheric warmings occur only then. Summer easterlies block propagation completely, making the summer stratosphere quiescent. The equinoctial transitions are brief and the winds are changing rapidly through them.

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Correct 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.

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Correct Answer : A

Despite continuous sunlight, the summer mesopause reaches about 130 kelvin, far colder than the winter mesopause, because the wave-driven circulation forces strong ascent and adiabatic cooling there. It is the coldest place in the Earth's atmosphere and is where noctilucent clouds form. This departure from radiative expectation is the clearest evidence of the wave driving. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

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Correct Answer : B

The geostrophic wind is non-divergent on an f-plane, so every bit of the divergence and therefore of the vertical motion is carried by the small ageostrophic component. This is why a quantity roughly a tenth the size of the total wind receives so much attention. The thermal wind is a shear rather than a flow, and vertical averaging is not part of the formulation.

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Correct Answer : B

Convection permitting models run at grid spacings of about one kilometre, which resolves the largest updraughts well enough to dispense with a deep convection scheme, though shallow convection is still parameterised. At 100 metres the model becomes large eddy simulation, resolving the turbulence within the cloud as well, which remains too expensive for operational forecasting over any large domain. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

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Correct Answer : B

Inverting an elliptic operator spreads the influence of a local source over the whole domain, so a region of forced ascent is necessarily accompanied by subsidence elsewhere. This is why clear skies so often accompany a nearby area of heavy rain. The forcing certainly matters, and the equation is routinely solved numerically as part of diagnostic analysis packages.

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Correct Answer : C

Log-pressure height increases linearly with geometric height in an isothermal atmosphere, so vertical derivatives look familiar, while the horizontal pressure gradient force retains the simple geopotential gradient form of pressure coordinates. It is therefore the natural coordinate for middle atmosphere dynamics. Temperature is still required for thickness, the Coriolis term is untouched, and the continuity equation acquires a density weighting rather than losing one.

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Correct Answer : C

Subtracting the geostrophic wind vectors read from two constant pressure charts gives the thermal wind for the layer between them directly. Equivalently, the thickness chart for that layer gives the same information in one field, which is why thickness charts are drawn. Two surface charts give a tendency rather than a shear, and a satellite image carries no wind information of the required kind.

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Correct Answer : C

The thermal wind is by definition the vector difference between the geostrophic winds at two levels, so adding it to the lower level wind gives the upper level wind. Drawing this triangle on a chart is the classical way of estimating upper winds from a surface analysis and a thickness field. The density ratio and the cross-isobaric rotation address quite different questions, and no averaging construction gives the correct result. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

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Correct Answer : D

Ekman balance includes friction and therefore describes a dissipative rather than a balanced flow in the usual sense, since balanced flow approximations concern the inviscid equations. Geostrophic, gradient wind and cyclostrophic balances are all inviscid balances distinguished by which of the inertial terms they retain. The distinction matters because balanced flow can be inverted from a single scalar field while frictional flow cannot. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

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Correct Answer : A

The thermal wind is proportional to the horizontal temperature gradient, which vanishes in a barotropic atmosphere, so the geostrophic wind is the same at every level. That is why the earliest numerical forecasts, which used a barotropic model, could represent the flow at a single level. The other options describe baroclinic situations in which the thermal wind is non-zero and oriented in various ways relative to the flow. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

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Correct Answer : B

Relative vorticity is positive for anticlockwise rotation in the usual convention, so northern hemisphere anticyclonic flow has negative relative vorticity. In the southern hemisphere anticyclonic flow is anticlockwise and so has positive relative vorticity, which is why cyclonic is the safer word than any statement about sign. Divergence is an independent property that does not affect the sign of the vorticity.

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Correct Answer : D

Air moving horizontally against sloping terrain must acquire a vertical velocity equal to the wind dotted with the gradient of the surface height, which is the origin of orographic lift, lee waves and mountain-induced vorticity changes. The Coriolis parameter changes only if the flow crosses latitudes, which is a consequence rather than the mechanism, and surface pressure over a range is lower rather than higher because the column above is shorter. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

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Correct Answer : D

Differentiating the position vector once introduces one rotation term, and differentiating the resulting velocity again introduces a second identical term, so the two combine into a factor of two. The derivation is short and worth performing once, since the factor is otherwise a detail that looks arbitrary. The sidereal and solar day differ by only one part in three hundred and sixty five, and the two components of the rotation vector make quite unequal contributions.