Growth by Collision and Coalescence Questions and Answers

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. A larger collector sweeps a wider path and falls faster relative to the droplets it meets, so its rate of collection accelerates as it grows. Condensation does the opposite: the same vapour flux spread over a larger surface makes the radius grow ever more slowly, which is why condensation alone cannot produce rain.

Answer: D

Correct answer: sweeps up droplets at a steady average rate

The continuous model treats collection as a smooth deterministic process, so every collector of a given size grows identically. It is simple and analytically tractable, and its weakness is exactly that uniformity - real collection is a sequence of discrete chance events.

Answer: A

Both are true and the reason gives the advantage, and it is those lucky drops that reach precipitation size first. Treating collection as a random rather than an averaged process shortens the predicted time to rain considerably.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 give the essential difference: the continuous model has every collector following one trajectory, while the stochastic model lets a lucky few collect far more than average and run ahead of the population. Statement 3 has it backwards - it is those fortunate collectors that produce rain sooner than the continuous model predicts.

Answer: C

Bulk microphysics schemes represent warm rain through terms of exactly this kind. Autoconversion is the slowest and most uncertain of them, and models differ considerably in how they parameterise it. The other three terms are comparatively well constrained.

Answer: D

It is the bottleneck term, since accretion cannot proceed until some rain water exists. Its formulation strongly influences how quickly a modelled cloud begins to precipitate and how sensitive that is to aerosol loading.

Answer: A

Both are true and the reason gives the sensitivity, and this is the modelling route by which aerosol influences rainfall. Accretion depends chiefly on the amount of rain water already present. Aerosol therefore acts mainly on the initiation rather than the growth.

Answer: B

Correct answer: a few millimetres in diameter

Beyond about five or six millimetres the aerodynamic forces flatten the drop and then hollow it into a bag which bursts. That sets a ceiling on drop size regardless of how much water is available, and it is why heavy rain consists of many drops rather than a few very large ones.

Answer: C

Both are true and the reason gives the limiting process, and collisional breakup on impact with another drop contributes as well. The resulting fragments then act as new collectors themselves. The population settles towards a characteristic size distribution.

Answer: D

Correct answer: True

Breakup returns a large drop as several smaller ones, each of which is a fresh collector sweeping its own path through the cloud. The total collecting surface is increased, so the chain reaction of collection and breakup can convert cloud water to rain faster than growth alone would.