Atmospheric Stability Questions and Answers

Answer: A

Antarctic surface inversions of twenty to thirty kelvin are commonly confined to the lowest few hundred metres, above which the profile returns to a more normal lapse rate. The intensity rather than the depth is what is remarkable. That shallow intense structure drives the katabatic winds that flow off the plateau.

Answer: B

Radiatively cooled air adjacent to the sloping ice becomes denser than the free air at the same level and drains downhill, reaching hurricane force where the terrain funnels it. The winds at Cape Denison average over 70 kilometres per hour throughout the year. Barrier winds are a genuine phenomenon in Antarctica too but arise from blocking rather than from drainage. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: C

Suppressed mixing means momentum is not transferred efficiently, so the wind at the surface falls light while the air a hundred metres up retains its speed, producing very strong shear. This is why nocturnal low level jets form and why aircraft encounter shear on approach in stable conditions. Unstable conditions mix momentum downward and reduce the shear. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

An aircraft descending through a shear layer experiences a change in headwind that translates directly into a change of airspeed, which must be anticipated and corrected. Low level wind shear alert systems monitor for exactly this. The stable layer is typically smooth rather than turbulent, which is why the hazard is the airspeed change rather than the buffeting. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Stable stratification suppresses vertical motion, so an aircraft flying in a stable layer with little shear encounters very smooth air. Convective boundary layers give a bumpy ride from the thermals. A Richardson number below the critical value indicates that the layer is about to become turbulent, which is the least smooth condition of all. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: B

The nocturnal stable layer persists until surface heating begins to erode it, so the hours around dawn offer the smoothest conditions at low level. Glider pilots wait for the opposite conditions in the afternoon. Late evening can also be smooth once the convection has decayed, which makes that a reasonable secondary answer.

Answer: C

The buoyancy frequency appears directly in the dispersion relation for internal gravity waves, so a more strongly stratified atmosphere supports faster waves. That does bear on the time step through the Courant condition, which makes the last option partly defensible. The Coriolis parameter depends on latitude alone and resolution is a design choice. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

A nocturnal inversion only fifty metres deep requires several levels within it, which is why models place their levels very close together near the surface and stretch them with height. Uniform spacing would waste resolution aloft and provide none where it is needed. Poor representation of the stable boundary layer remains a recognised weakness of many models. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

Models commonly apply more mixing than the physics warrants in stable conditions, partly to avoid runaway cooling and numerical difficulties, which leaves the modelled inversion too weak and the surface too warm. Nocturnal minimum temperatures are correspondingly overpredicted. The problem has proved stubborn and is an active area of parameterisation development. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

Excessive mixing brings warm air down to the surface and prevents the strong radiative cooling that actually occurs, so minima are forecast too high and frost and fog are underpredicted. Forecasters routinely adjust for this known bias. Surface winds are forecast too strong rather than too light for the same reason.