Entropy and Potential Temperature Questions and Answers

Answer: C

During the decay phase the eddies tilt so that their momentum fluxes converge into the jet, accelerating the mean flow at their own expense. This is what maintains the surface westerlies against friction. Frictional dissipation removes energy from the system entirely rather than transferring it between reservoirs.

Answer: D

In a steady state the generation of available potential energy by differential heating must equal the frictional dissipation of kinetic energy, since the intermediate conversions merely move energy between reservoirs. Tracing the energy from the ultimate source to the ultimate sink through the four reservoirs is the point of the diagram. The eddy heat transport is one of the internal conversions rather than a sink.

Answer: A

Turbulent friction, mostly in the boundary layer, dissipates the kinetic energy of the general circulation at roughly 2 watts per square metre, which is about one per cent of the absorbed solar radiation. That figure sets the scale of the whole mechanical energy budget. It also gives an upper bound on the wind energy that could in principle be extracted globally. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: B

The total dissipation of about a thousand terawatts is far larger than global energy demand, though only a small fraction is accessible near the surface and over land. Estimates of the extractable resource nonetheless exceed present consumption by a wide margin. Very large extraction would alter the boundary layer locally, which is a legitimate concern at regional scale rather than a global constraint. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: C

Water evaporates at the warm surface and condenses at a colder level, so the latent heat is transferred across a temperature difference and entropy is generated. The frictional dissipation of falling raindrops adds a further contribution. Equal quantities of heat absorbed and released at different temperatures do not give zero entropy change, which is exactly the point the last option misses. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Raindrops falling at terminal velocity dissipate their potential energy as heat through drag, generating entropy at a rate comparable with the mechanical dissipation of the wind. It is a surprisingly large term that is often overlooked. The apparent order of the falling drops is irrelevant, since entropy accounting concerns heat and temperature rather than visual arrangement. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: A

Estimates place the dissipation by falling precipitation at a few watts per square metre globally, comparable with or exceeding the dissipation of the wind. Both are small compared with the radiative terms but they dominate the mechanical energy budget. Recognising the comparability is a useful corrective to the assumption that the wind is the only significant mechanical dissipation. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

Moist entropy is approximately proportional to the logarithm of the equivalent potential temperature, so a parcel conserving that quantity conserves its moist entropy. This is why equivalent potential temperature is conserved for both dry and saturated adiabatic motion. The parallel with the dry case, where entropy is tied to potential temperature, is exact enough to be a useful guide. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: C

In the reversible process the condensed water stays with the parcel and can be re-evaporated on descent, whereas removing it as precipitation makes the process irreversible. The pseudo-adiabatic process is therefore adiabatic but not isentropic. Latent heat release itself is reversible, and expansion and cooling are both reversible in the absence of precipitation removal. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

The reversible and pseudo-adiabatic lapse rates differ by only a few tenths of a kelvin per kilometre, because the heat capacity of the condensed water is small compared with that of the air. The distinction matters for precision work and for the thermodynamic definition rather than for practical forecasting. Real ascent lies somewhere between the two idealisations. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.