Precipitation Formation Mechanisms Questions and Answers

Answer: A

Both are true and the reason gives the arithmetic, and ocean residence times are measured in thousands of years. The atmosphere is a small and rapidly turning over part of the cycle. Its small size belies its importance to the whole system.

Answer: B

Glaciogenic seeding provides nuclei where the cloud has too few of its own. Hygroscopic seeding of warm clouds with large salt particles is a separate and less common approach. Each targets a different stage of the precipitation process.

Answer: C

The first two are correct and describe the two approaches. The third overstates the evidence, since randomised trials have generally found small and statistically difficult effects. Natural variability is far larger than any claimed increase.

Answer: D

Correct answer: True

The difficulty is that the same cloud cannot be both seeded and left alone, so there is no direct control - and natural rainfall varies so much from one cloud to the next that any effect is buried in that variability. Randomised trials over many events are the only way through, and after decades of them the evidence for glaciogenic seeding of orographic cloud is modest and for hail suppression weaker still.

Answer: A

The physics is reasonably well understood; the difficulty is statistical rather than theoretical. The other three are the standard obstacles to a convincing evaluation. The problem is one of statistics rather than of physics.

Answer: B

Models must parameterise the processes that turn cloud water into rain. Small differences in that parameterisation produce large differences in the forecast rainfall field. Rainfall remains the least skilful common forecast field.

Answer: C

Both are true and the reason gives the cause, and the timing and placement of showers are particularly uncertain. Larger scale fields such as pressure are forecast far more skilfully. Resolved dynamics behaves far better than parameterisation.

Answer: D

The horizontal wind is resolved directly by the model dynamics. The other three occur at scales far below the grid and must be represented by parameterisation. Grid spacing sets the boundary between the two categories.

Answer: A

Correct answer: parameterised rather than resolved explicitly

A convective cell is a few kilometres across, well below the grid spacing of a coarse model, so the model cannot represent the updraught directly. A convection scheme instead estimates how much precipitation and heating the sub-grid convection would produce from the resolved temperature and humidity profile. Those schemes are a principal source of uncertainty, which is why convection-permitting models at grid spacings of a kilometre or two are used where the resources allow.

Answer: B

Grid spacings of about a kilometre begin to represent individual cells directly. Such convection permitting models improve the timing and structure of showers considerably. They remain expensive to run over large domains.