Geostrophic and Gradient Wind Questions and Answers

Answer: A

Because imbalance is radiated away within hours while the balanced flow evolves over days, the atmosphere spends almost all of its time close to balance. This justifies balanced models such as the quasi-geostrophic system and explains why unbalanced initial conditions ruined Richardson's forecast. Gravity waves carry the adjustment but little of the energy, and friction remains important on its own longer timescale.

Answer: B

An upper air or constant pressure chart contours the geopotential height of a chosen pressure surface, most often 850, 700, 500, 300 or 250 hectopascals. The contours act exactly as isobars do on a surface chart, so the geostrophic wind is read from their spacing and blows along them. A thickness chart shows the difference between two such surfaces, a surface chart uses mean sea level pressure, and a cross section is a vertical rather than a horizontal display. Knowing which chart answers which question is a basic operational skill.

Answer: C

The 500 hectopascal surface lies at about 5.5 kilometres, with half the mass of the atmosphere below it, which makes it the natural single level to represent the steering flow. The 850 level is used for low level thermal advection, 250 or 300 for the jet stream and 1000 for surface conditions. The special status of 500 follows directly from hydrostatic balance and is worth understanding rather than merely accepting, since it explains why so much synoptic reasoning is conducted at that one level.

Answer: D

The jet core sits near the tropopause, around 250 to 300 hectopascals in mid-latitudes, so charts at those levels display it best. Lower levels show progressively weaker westerlies because the thermal wind has not yet accumulated. The choice of level therefore follows directly from the thermal wind relation rather than being an arbitrary convention, and it shifts with season and latitude as the tropopause height changes. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

Because thickness is proportional to layer mean temperature, and the thermal wind is proportional to the horizontal temperature gradient, the spacing of thickness contours gives the thermal wind directly. The thermal wind blows along the thickness contours with low thickness, and therefore cold air, on the left in the northern hemisphere. This makes a thickness chart a direct display of the vertical shear, which is why it is such a useful analytical tool.

Answer: B

The thermal wind obeys the same geometry as the geostrophic wind, with thickness playing the role of height, so it blows along the thickness contours with low values on the left in the northern hemisphere. Since low thickness means cold air, this is the familiar statement that the thermal wind blows with cold air on its left. The rule reverses in the southern hemisphere along with the sign of the Coriolis parameter, which is worth checking explicitly. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

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.

Answer: D

A barotropic layer has no horizontal temperature gradient, so the thermal wind vanishes and the geostrophic wind is the same at top and bottom. Strong wind with no directional change still implies a thermal wind if the speed changes with height. Perpendicular isotherms and contours describe strong thermal advection rather than the absence of a thermal wind, which is very nearly the opposite of the situation described. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

If the wind turns without changing speed, the thermal wind is perpendicular to the mean wind, which means the isotherms are perpendicular to the flow and thermal advection is at its maximum. Veering then means warm advection and backing cold advection. A barotropic layer has no directional change at all, and neutral stability and cyclostrophic balance concern quite different aspects of the flow.

Answer: B

Replacing the actual momentum by the geostrophic momentum in the acceleration term gives the semi-geostrophic equations, which describe frontogenesis far better than the quasi-geostrophic system does. The cross-frontal ageostrophic circulation is retained and the fronts sharpen realistically. Coriolis is certainly retained, and neither gravity wave filtering nor density removal is what the approximation accomplishes. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.