Divergence and Vertical Motion Questions and Answers

Answer: A

A synoptic system lifting 1 centimetre per second over a million square kilometres moves a mass flux comparable with a large convective complex lifting several metres per second over a few thousand square kilometres. Comparing area times velocity rather than velocity alone is what makes the two commensurable, and the exercise is a useful corrective to judging importance by intensity.

Answer: B

Mass continuity requires the strong narrow updraught to be balanced by gentle subsidence spread over a much larger surrounding area, typically a hundred times the cloud's own cross-section. This compensating subsidence warms and dries the environment, which is how convection stabilises the atmosphere. Convective downdraughts do exist but are driven by evaporative cooling and precipitation drag rather than by continuity. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: C

Descending air warms adiabatically and its relative humidity falls, which stabilises the environment and suppresses further convection nearby. This is why convective cells tend to be separated by clear gaps and why an isolated storm often inhibits neighbours. The process is the principal means by which convection adjusts the large-scale thermal structure towards a moist adiabatic profile. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

A grid box of tens of kilometres contains both the updraught and its compensating subsidence, so the scheme must represent the net effect of the pair on the box mean temperature and humidity rather than either separately. This is the essence of mass flux convection parameterisation. Predicting individual cloud properties exactly is neither possible nor necessary for that purpose.

Answer: A

Mass flux schemes represent the ensemble of clouds within a grid box by a single upward mass flux and its associated entrainment and detrainment, from which the effect on the box mean profile is computed. Individual cloud properties are neither tracked nor needed. The closure, which determines the magnitude of the flux, is the most uncertain part of such schemes and a longstanding source of model error. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

Convection permitting models run at grid spacings of about one kilometre, which resolves the largest updraughts well enough to dispense with a deep convection scheme, though shallow convection is still parameterised. At 100 metres the model becomes large eddy simulation, resolving the turbulence within the cloud as well, which remains too expensive for operational forecasting over any large domain. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

At one kilometre spacing the aspect ratio of the resolved motion approaches unity, so vertical accelerations are comparable with the residual of gravity and the pressure gradient and the hydrostatic assumption fails. Convection permitting models are therefore non-hydrostatic. Quasi-geostrophic and barotropic systems are far too heavily filtered to represent convection at all.

Answer: D

Repeated convection carries air upward along a moist adiabat and the compensating subsidence warms the environment until the mean profile approaches that same moist adiabat. This is why tropical soundings lie so close to a moist adiabat through the depth of the troposphere. A dry adiabatic profile would be far too steep to be maintained, an isothermal profile is characteristic of the lower stratosphere instead, and a warming profile is what an inversion looks like over a shallow layer rather than the whole troposphere.

Answer: A

A moist adiabat is a surface of constant equivalent potential temperature, so a convectively adjusted profile has that quantity almost independent of height through the troposphere. Relative humidity falls sharply in the subsiding air between clouds, mixing ratio decreases rapidly as temperature falls, and convective momentum transport is real but far less effective than the transport of heat and moisture. Recognising equivalent potential temperature as the conserved variable of moist adiabatic ascent is the key to reading any tropical sounding.

Answer: B

Because the Coriolis parameter is small, any temperature gradient is rapidly removed by gravity waves, so tropical soundings hundreds or thousands of kilometres apart are remarkably similar. This weak temperature gradient approximation underlies much of tropical dynamics. Tropical pressure gradients are correspondingly weak, winds are generally light outside disturbances, and the tropical tropopause is in fact higher and colder than the mid-latitude one.