Growth by Collision and Coalescence Questions and Answers

Answer: A

Numerous small droplets fall together and collide inefficiently, which delays rain. The other three each either broaden the spectrum or give collection more time to work. Spectral breadth and residence time are the two levers available.

Answer: B

Each stage hands over to the next at a fairly well defined size. The transition from the second to the third is the difficult step that the whole subject is organised around. Everything before it is slow and everything after it is fast.

Answer: C

Collection accelerates as the collector grows, so the later stages proceed quickly. This is what reconciles theory with the observation that warm cumulus can rain within half an hour of forming. Condensation alone would need several times that interval.

Answer: D

Equal radii mean equal fall speeds and almost no relative motion, which collapses the kernel. The other three each raise either the swept volume or the fraction of encounters that succeed. The kernel is the product of geometry, motion and efficiency.

Answer: A

Droplets of differing inertia respond differently to a turbulent eddy and so approach one another. The effect also concentrates droplets locally, which raises the encounter rate further still. Both effects grow stronger as the turbulence intensifies.

Answer: B

The first two are correct and are the accepted mechanisms. The third is contradicted by both laboratory work and simulation, though quantifying the effect in real clouds remains difficult. Direct measurement at the relevant scales is not yet routine.

Answer: C

Stokes drag applies only while the Reynolds number is small and the flow remains creeping. Cloud droplets satisfy this, but drizzle and raindrops do not and require empirical drag relations instead. The transition occurs near forty micrometres in radius.

Answer: D

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. Stokes law assumes creeping flow, valid only while the Reynolds number is well below one. A millimetre raindrop has a Reynolds number in the hundreds, with a separated wake behind it, so the drag is far greater than Stokes predicts and empirical relations must be used.

Answer: A

Fall speed rises steeply through the drizzle range and then flattens for larger drops. The limiting value near nine metres per second is approached only by the very largest drops that survive breakup. Fall speed is therefore a poor discriminator among large drops.

Answer: B

Correct answer: an exponential fit to the raindrop size spectrum

Marshall and Palmer found the number of drops falling off exponentially with diameter, with a slope depending on the rainfall rate. It is an empirical fit rather than a derivation, and it remains the standard reference distribution for radar work and for anything requiring an assumed drop spectrum.