Divergence and Vertical Motion Questions and Answers

Answer: C

A synoptic omega of 5 pascals per second corresponds to a vertical velocity of about half a metre per second, which is more than an order of magnitude above anything the large scale produces. Accumulated error in the vertically integrated divergence is by far the most likely cause, which is exactly the weakness the O'Brien correction addresses. Deep convection would not be resolved by a synoptic wind analysis in any case. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: D

Omega is the vertical velocity in pressure coordinates and is negative for ascent because pressure decreases with height. The symbol w denotes the geometric vertical velocity, psi is the streamfunction of the rotational wind, and chi is the velocity potential of the divergent wind. Keeping the four symbols distinct saves a great deal of confusion when reading the literature.

Answer: A

The level of non-divergence lies near 500 to 600 hectopascals in a typical developing system and is where the vertical velocity reaches its maximum. The level of free convection and the lifting condensation level are thermodynamic quantities from parcel theory, and the level of no motion is the oceanographic reference surface used in the dynamic method.

Answer: B

The wind field is analysed first, the divergence is computed from it at each level, the profile is integrated upward from the lower boundary condition, and finally the result is corrected so that omega vanishes at the top, giving 4, 2, 3, 1. Setting the sequence out explicitly makes clear where the error enters and why the final correction is needed rather than optional.

Answer: C

The kinematic method uses wind divergence, the adiabatic method uses the observed temperature tendency together with the advection, and the omega equation uses vorticity and thermal advection. Because the three draw on different observations with different error characteristics, agreement between them is a genuine check on the derived vertical motion rather than a mere consistency of arithmetic. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

For adiabatic motion the thermodynamic equation relates the vertical velocity to the difference between the observed local temperature change and the change explained by horizontal advection. Where the observed cooling exceeds the advective cooling, ascent must be responsible. The method fails wherever latent heat release or radiation is significant, which limits its usefulness in precisely the situations of greatest interest. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: A

Latent heat release in a precipitating system violates the adiabatic assumption on which the method rests, and since precipitation accompanies most large-scale ascent the limitation bites hard. Radiative cooling is a smaller but persistent additional problem. The method uses temperature observations rather than dense wind data, works anywhere soundings exist, and gives a quantitative result. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: B

Satellite imagery and radar provide an independent indication of where air is actually rising, so comparing the diagnosed field with the observed cloud shield is the natural arbiter. Averaging two results of unknown reliability has no justification, and there is no reason to suppose errors act systematically in either direction. Confronting a diagnostic with an independent observation is a sound general principle. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

Widespread gentle ascent produces the extensive layered cloud and steady precipitation of the warm conveyor belt, which is the visible signature of synoptic scale vertical motion. Convective towers indicate small-scale vertical motion of a much greater magnitude over a much smaller area. Clear skies and a capping inversion both indicate subsidence, which is the opposite condition.

Answer: D

Continuity requires that the same mass flux be achieved either by weak motion over a wide area or by strong motion over a narrow one, so a convective updraught a kilometre across can reach several metres per second while a synoptic system a thousand kilometres across manages only centimetres. The inverse relationship between area and intensity runs through the whole of atmospheric science. Framing the question as which quantity the change acts upon makes the answer follow without calculation.