Entropy and Potential Temperature Questions and Answers

Answer: A

A front is a boundary between air masses, so it appears as a concentrated gradient of wet bulb potential temperature, often more sharply than in the temperature field itself. Locating fronts this way is more objective than by the temperature alone. Jet streaks, anticyclones and troughs are features of the wind and pressure fields rather than of the air mass distribution. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: B

A front separates air masses that differ in both temperature and humidity, and combining the two into one conserved variable produces a sharper and more reliable gradient than either alone. It is also less affected by diurnal heating, which makes the last option a genuine secondary advantage. The quantity must be computed rather than measured directly.

Answer: C

The tropopause is not itself an isentropic surface but cuts across many of them, and it coincides with the region of sharp potential vorticity gradient separating the two reservoirs. That is why the dynamical definition uses a potential vorticity value. Isentropic surfaces pass from troposphere to stratosphere as they rise polewards, which is exactly what makes them so useful for studying exchange between the two. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Because a single isentropic surface can lie in the troposphere at low latitudes and in the stratosphere at high latitudes, adiabatic motion along it transports air between the two reservoirs. This isentropic exchange dominates in the extratropics. Slow diabatic ascent through the tropical tropopause is the other route and operates on a much longer timescale. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

The extratropical transition layer or middleworld is the region between the tropical and polar tropopause levels where isentropic surfaces span both reservoirs. Hoskins named the three regions the underworld, middleworld and overworld according to whether the surfaces intersect the ground, the tropopause or neither. The tropical tropopause layer is the corresponding region in the tropics with a quite different dynamics.

Answer: B

The overworld comprises surfaces above about 380 kelvin, which lie in the stratosphere at all latitudes, and air enters it only through slow diabatic ascent in the tropics. The middleworld spans both reservoirs and the underworld intersects the ground. The threefold classification organises stratosphere troposphere exchange very economically.

Answer: C

Air ascends slowly through the tropical tropopause layer, being freeze dried in the process, and that is the sole significant entry route into the overworld. Isentropic exchange operates in the middleworld only. Convective overshooting does inject some material and matters for the water vapour budget, which makes it a genuine secondary route.

Answer: D

The tropical cold point acts as a freeze drier, dehydrating the ascending air to the saturation mixing ratio there, which is a few parts per million. This is the Brewer mechanism explaining the extreme dryness of the whole stratosphere. Ozone and potential vorticity are acquired gradually thereafter rather than at the moment of entry.

Answer: A

The annual cycle of cold point temperature imprints alternating moist and dry layers on the ascending air, and these propagate upward over several years like a signal on a tape. Tracking them gives a direct measurement of the tropical ascent rate. It is one of the few direct measurements of a large scale vertical velocity anywhere in the atmosphere.

Answer: B

The tape recorder signal ascends about ten kilometres in a year, which corresponds to roughly 0.3 millimetres per second. That is far slower than tropospheric synoptic vertical motion, reflecting the strong stratification and the weakness of the wave driving. The measurement is a valuable constraint on models of the Brewer-Dobson circulation.