Growth by Collision and Coalescence Questions and Answers

Answer: A

Correct answer: 30 per cent

\[ \frac{30}{100} = 30\ \mathrm{per\ cent}. \] The remaining seventy per cent is re-evaporated, detrained into the surrounding air, or left as cloud that never falls. Efficiencies of this order are normal, and they are why the water actually condensed in a cloud is a poor guide to the rain it will produce.

Answer: B

Water carried downwind falls into air that is descending or subsaturated and evaporates. Shear also organises storms in ways that can raise efficiency, so the net effect is not always negative. Shear acts differently on a single cell and on a system.

Answer: C

Moderate shear organises storms into longer lived structures that can rain for hours. The effect on any single updraught differs from the effect on the system as a whole. Organisation can more than offset the loss from a single cell.

Answer: D

Efficiency is the net outcome of several competing influences rather than a property of the cloud alone. The environment surrounding the cloud matters as much as its own microphysics. Efficiency is a property of the cloud and its surroundings together.

Answer: A

A grazing collision between two large drops can rotate and separate after a temporary union. The resulting satellite droplets are an additional product of such encounters. Satellite drops add to the small end of the spectrum.

Answer: B

Both are true and the reason gives the outcome, and this is collisional breakup viewed from the number perspective. It is one of the processes that maintains the equilibrium spectrum. Breakup is a source of drop number as well as a sink of size.

Answer: C

Observed warm rain onset in shallow maritime clouds is remarkably rapid. Reproducing this timing was the principal test that stochastic collection theory had to pass. Continuous collection predicted far longer times than observed.

Answer: D

Correct answer: the warming of the cloud by released latent heat

Latent heat release drives the updraught and so affects the cloud’s dynamics, but it is not a microphysical mechanism for producing rain quickly. The other three all act directly on collection: chance in the stochastic model, giant nuclei as ready-made collectors, and turbulence increasing the rate at which droplets encounter one another.

Answer: A

The first two are correct and describe the mechanism. The third is wrong, since high latitude winter precipitation forms overwhelmingly through the ice phase instead. Warm rain requires a cloud entirely above freezing.

Answer: B

Once part of a cloud is colder than zero, ice processes may operate alongside or instead of collection. The two routes are not exclusive and many clouds precipitate by a combination of both. Deep clouds commonly run both routes simultaneously.