Geostrophic and Gradient Wind Questions and Answers

Answer: A

The first two statements are correct, while the third is false since the balance applies in both hemispheres with the appropriate sign of the Coriolis parameter. In the southern hemisphere the circulation senses reverse but the subgeostrophic character of cyclonic flow does not, because both the Coriolis and centrifugal forces still act outward around a low. Checking a rule in both hemispheres is a good test of whether it has been understood or merely memorised.

Answer: B

The last pairing fails: inertial balance neglects the pressure gradient force, and its trajectory is curved by definition. The other three are correctly matched. Organising the balances by what each discards, rather than by what each retains, makes the family compact and makes it obvious which one to reach for once the scales of a particular problem have been estimated.

Answer: C

A col is a saddle point at which the curvature in one direction largely cancels that in the other, so the vorticity is small rather than large. The other three pairings are correct, following directly from vorticity being the Laplacian of the height field divided by the Coriolis parameter. Reading vorticity from contour shape is a skill that repays practice on real charts.

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.

Answer: A

Gradient wind balance is a three-way balance among pressure gradient, Coriolis and centrifugal forces, with friction entirely absent. Adding friction gives a more complicated balance appropriate to the boundary layer and rarely used analytically. The absence of friction is what makes gradient wind balance a good description of the free atmosphere above about one kilometre.

Answer: B

Cyclonic curvature makes the gradient wind subgeostrophic, so the actual wind will fall short of the geostrophic estimate, typically by ten to twenty per cent for a sharply curved trough. Gradient wind scales on plotting tools apply exactly this correction. Temperature does not enter because pressure is the vertical coordinate, and the sense of the correction is opposite to that stated in the second option.

Answer: C

Veering with height in the northern hemisphere places the thermal wind to the right of the lower level wind, and since the thermal wind blows with cold air on its left, warm air must be being advected into the layer. This is one of the most useful inferences obtainable from a single sounding and follows entirely from the thermal wind relation. No absolute temperatures are needed, only the turning.

Answer: D

At the radius of maximum wind the curvature is strong but the Coriolis term is not negligible, so all three horizontal forces matter and gradient wind balance is the appropriate approximation. Geostrophic balance would overestimate the wind badly by omitting the centrifugal term, cyclostrophic balance is better suited to a tornado, and hydrostatic balance is a vertical relation that constrains the pressure field rather than the tangential wind. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

The geostrophic wind is defined by the two-force balance and blows parallel to the isobars with a magnitude equal to the pressure gradient force divided by the Coriolis parameter. The gradient wind adds curvature, the cyclostrophic wind drops Coriolis, and the thermal wind is a shear rather than a wind at all. Keeping the four definitions distinct is the foundation of this whole topic.

Answer: B

Around a high the pressure gradient and centrifugal forces both act outward while only the Coriolis force acts inward, so a larger wind is required to achieve balance and the flow is supergeostrophic. The centrifugal force always acts outward, which is what the second option gets wrong. The effect is present at any radius, growing as the curvature tightens, until the anticyclonic limit is reached.