Which depth do superadiabatic layers typically occupy?
Answer: C
Convection removes instability so efficiently that superadiabatic gradients survive only in the thin surface layer where molecular and turbulent transfer must carry the whole heat flux. Above that the mixed layer is very nearly adiabatic. Measuring a superadiabatic gradient over any greater depth would indicate an instrument error.
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Which profile does the convective boundary layer show in its bulk?
Answer: D
The mixed layer is close to neutral with potential temperature nearly uniform, capped by an entrainment zone and underlain by a thin superadiabatic surface layer. That three part structure is the standard picture of a daytime boundary layer. Condensation occurs only in the cumulus that form at the top, not throughout the layer.
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Which process warms the mixed layer from above?
Answer: A
Thermals overshooting into the stable air above draw warm air down into the mixed layer, and that entrainment flux typically supplies about a fifth of the surface heat flux. It is what causes the layer to warm and deepen simultaneously. Conduction is negligible and direct solar absorption by the air is small compared with the surface flux.
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Which ratio characterises the entrainment flux at the top of the mixed layer?
Answer: B
The entrainment flux is downward and warming, and its magnitude is about 0.2 times the surface flux, giving the characteristic minus 0.2 ratio used in mixed layer models. The negative sign reflects the opposing direction of the two fluxes in the usual convention. The relationship is remarkably robust across a wide range of conditions.
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Which model represents the boundary layer as a single well mixed slab?
Answer: C
A slab model treats the layer as uniform in potential temperature and humidity, with jumps at the top, and predicts its depth from the surface and entrainment fluxes. It captures the essential behaviour with very little computation. Higher order closures and large eddy simulation resolve progressively more of the turbulence at correspondingly greater cost. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.
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Which technique resolves the largest turbulent eddies explicitly?
Answer: D
Large eddy simulation resolves eddies larger than the grid and parameterises only the smaller ones, which is feasible for the boundary layer where the energy containing eddies are hundreds of metres across. Direct numerical simulation resolves everything but is limited to laboratory Reynolds numbers. Reynolds averaging parameterises the whole spectrum. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.
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Which quantity does the Obukhov length measure?
Answer: A
The Obukhov length compares the turbulence generated mechanically by shear with that generated thermally by buoyancy, and its sign indicates whether the surface layer is stable or unstable. Above that height buoyancy dominates. It is the fundamental scaling parameter of Monin-Obukhov similarity theory, which underlies every surface layer parameterisation. Framing the question as which quantity the change acts upon makes the answer follow without calculation.
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Which sign does that length take in an unstable surface layer?
Answer: B
The Obukhov length is negative when the surface heat flux is upward, which is the unstable daytime case, and positive at night when the flux is downward. Neutral conditions correspond to an infinite length. The sign convention is arbitrary but universal, and remembering that daytime is negative saves confusion when reading the literature.
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Which theory describes the surface layer profiles?
Answer: C
Monin and Obukhov showed that the dimensionless wind and temperature gradients in the surface layer are universal functions of height divided by the Obukhov length. The theory reduces the whole surface layer problem to a pair of empirically determined functions. Mixing length ideas are one ingredient and Kolmogorov theory concerns the inertial subrange of the turbulence spectrum. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.
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Which profile does the wind follow in a neutral surface layer?
Answer: D
In neutral conditions the wind increases as the logarithm of height divided by the roughness length, which follows from the constant stress assumption and dimensional analysis. Stability modifies the profile through the Monin-Obukhov functions. The logarithmic law is one of the most robust results in boundary layer meteorology and underpins the estimation of surface stress. The shared properties explain why these belong to one family, and the differences explain why more than one exists.
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