Divergence and Vertical Motion Questions and Answers

Answer: C

A Q vector pointing towards warm air indicates frontogenesis, since the geostrophic flow is acting to sharpen the temperature gradient, while one pointing towards cold air indicates frontolysis. The vertical motion is given by the divergence of the field rather than by the orientation of individual vectors. This dual use makes the Q vector one of the most informative single diagnostics available. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: D

The divergent wind is typically a tenth of the rotational wind and carries about one per cent of the kinetic energy, yet by continuity it accounts for essentially all of the vertical motion. This disparity is why the velocity potential receives so much attention in diagnostics of weather despite being dynamically small. Vorticity belongs to the rotational part, and thermal advection involves the full wind. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: A

The Helmholtz decomposition writes the divergent wind as the gradient of a velocity potential, whose Laplacian is the divergence, exactly as the rotational wind is derived from a streamfunction. Charts of velocity potential are widely used to display large-scale divergent circulations such as the Walker cell. The geopotential and the Exner function belong to the mass and thermodynamic fields respectively. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: B

The Walker circulation, with ascent over the maritime continent and descent over the eastern Pacific, appears clearly as a large-scale pattern in the 200 hectopascal velocity potential field. Its variation with the El Nino cycle is monitored in exactly this way. The Hadley cell is meridional and is usually shown as a zonal mean streamfunction, while the convergence zone is a narrower feature better seen in the wind or cloud fields. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

The eastward propagating envelope of enhanced and suppressed convection that constitutes the oscillation is most clearly seen in upper level velocity potential, which responds directly to the divergent circulation above the convection. Sea surface temperature indices and the Darwin Tahiti pressure difference measure the El Nino Southern Oscillation instead, on a much longer timescale.

Answer: D

Descending air warms adiabatically and its relative humidity falls, so cloud evaporates and the sky clears, which is why anticyclones bring settled weather. Subsidence also creates the inversion that caps the boundary layer and traps pollution. Compression warms rather than saturates the air, and subsidence stabilises rather than destabilises the layer it acts upon.

Answer: A

Sinking air warms as it descends and meets the cooler mixed layer beneath, creating a sharp temperature inversion that caps the boundary layer and traps moisture and pollutants below it. The trade wind inversion over the subtropical oceans is the classic large-scale example. Fronts and levels of free convection arise from quite different processes.

Answer: B

An inversion suppresses vertical mixing, so emissions are confined to a shallow layer and concentrations build up, which is the mechanism behind winter smog episodes in many cities. Los Angeles owes its photochemical smog largely to the persistent subsidence inversion of the subtropical high. Long range transport requires the opposite condition of vigorous mixing and strong flow.

Answer: C

With the ground preventing downward escape, converging air can only leave the column upward, so surface convergence necessarily implies ascent. The reason therefore explains the assertion completely. The same argument run in reverse gives descent above a region of surface divergence, which is why anticyclones bring clear skies. Almost every qualitative statement about cloud distribution in a synoptic system rests on this one argument.

Answer: D

The reason gives the correct definition, and because pressure decreases upward that definition makes omega negative for ascent, so the assertion is false. The inverted sign is a frequent source of error when reading model output. Deriving the sign from the definition each time is far more reliable than trying to remember which way round the convention runs.