Divergence and Vertical Motion Questions and Answers

Answer: C

For adiabatic motion a parcel is confined to its isentropic surface, so where those surfaces slope upward along the flow the parcel must rise. This is the natural way to visualise frontal ascent, since frontal surfaces are close to isentropic. Isentropic analysis makes the vertical motion visible as flow along a sloping surface rather than as a small residual computed from divergence.

Answer: D

Because adiabatic parcels stay on their isentropic surfaces, a chart of pressure on an isentropic surface shows immediately where air must be rising or sinking as it flows across the contours. The wind field is still required to determine the flow direction. Latent heating moves parcels between surfaces, which is a limitation rather than something the method eliminates, and the units of the derived motion depend on how it is expressed. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

Latent heat release, radiation and turbulent mixing all change a parcel's potential temperature, so its trajectory departs from the isentropic surface exactly where the interesting weather is occurring. Neutral layers do make surfaces vertical and hard to use, which is a real practical difficulty, and boundary layer mixing makes the method unreliable near the ground, but the diabatic limitation is the fundamental one. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: B

Surface pressure measures the weight of the overlying column, so if more mass leaves the top than enters at the bottom the column lightens and the pressure falls. This mass budget argument is the dynamical basis of the rule that surface lows deepen beneath and downstream of upper troughs. Static stability affects how the vertical motion is distributed but not the sign of the pressure change. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

The tendency of surface pressure equals minus the integral of the horizontal mass divergence through the whole column, which is why it is so hard to compute accurately. The integral is a small residual between large convergence at low levels and large divergence aloft, so small errors in either dominate the answer. This is precisely the difficulty that wrecked Richardson's first attempt at numerical forecasting. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

Richardson computed a six hour surface pressure change of 145 hectopascals because his initial wind field contained a small imbalance whose divergence dominated the vertical integral. The episode led directly to the development of initialisation and to the use of filtered equation sets. The ENIAC forecast was in fact a success, and the barotropic limitation is a genuine but quite separate matter. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: A

Charney's insight was that the vorticity equation does not contain the troublesome residual, so forecasting vorticity and recovering the height field by inversion avoids the problem entirely. That is why the first successful numerical forecast integrated the barotropic vorticity equation. Finer grids and longer averaging do not remove a cancellation that is intrinsic to the formulation.

Answer: B

Air accelerating into the jet entrance becomes subgeostrophic, so the ageostrophic component points towards low pressure, producing divergence in the right entrance quadrant and convergence in the left. Divergence aloft implies ascent beneath, so the right entrance is a favoured region for cloud and development. The exit region reverses the pattern.

Answer: C

Air leaving the jet core decelerates and becomes supergeostrophic, so the ageostrophic component points towards high pressure, giving divergence in the left exit quadrant. Together with the entrance pattern this produces a thermally direct circulation at the entrance and an indirect one at the exit. The four quadrant model is one of the most used diagnostics in operational forecasting.

Answer: D

The entrance circulation has warm air rising and cold air sinking, which is thermally direct and releases energy, while the exit circulation has the opposite sense and consumes it. This pairing is a robust feature of jet streaks and explains why cloud shields often appear beneath the right entrance and left exit quadrants simultaneously. The two circulations do not cancel because they occupy different parts of the jet. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.