Water Vapour and Humidity Variables Questions and Answers

Answer: A

Attenuation increases as the wavelength shortens, so X band at about 3 centimetres is strongly attenuated, C band at 5 centimetres moderately and S band at 10 centimetres very little. This is why national networks in regions of heavy rainfall use S band despite the larger and more expensive antenna it requires. X band is used for short range gap filling.

Answer: B

Operational rainfall products adjust the spatial pattern given by radar using the accurate point values supplied by gauges, obtaining both the coverage of the one and the accuracy of the other. Kriging and similar geostatistical methods perform the merging. Nowcasting and satellite estimation are separate products addressing different questions, and reanalysis is a model based dataset rather than an observational one. The merged analysis is what hydrological models are normally driven with.

Answer: C

Microwave radiometers detect emission from raindrops over the radiometrically cold ocean and scattering by ice above land, giving a much more direct estimate than infrared cloud top temperature. Their weakness is the poor temporal sampling of polar orbiting satellites. Combined products blend the frequent infrared imagery with the more accurate but less frequent microwave retrievals. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

The Tropical Rainfall Measuring Mission carried the first spaceborne precipitation radar in 1997, and the Global Precipitation Measurement core observatory extended the capability to higher latitudes and lighter rain. An active radar gives the vertical structure that passive instruments cannot. The mission transformed estimates of tropical rainfall and of latent heating. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: A

An active radar resolves the precipitation in the vertical, revealing the bright band, the depth of the rain layer and the structure of convective cores. Passive radiometers integrate through the whole column and cannot separate the levels. Dual frequency radar does constrain the drop size distribution statistically, though not drop by drop.

Answer: B

The saturation vapour pressure at 20 degrees is about 23 hectopascals, and multiplying by epsilon and dividing by the total pressure gives a saturation mixing ratio close to 15 grams per kilogram. Doing the calculation once fixes the relationship between the two ways of expressing saturation. The value roughly doubles for each 10 kelvin of warming, so 30 grams per kilogram corresponds to about 30 degrees and 4 grams per kilogram to about zero, which is why those figures make plausible distractors.

Answer: C

A rate of about 7 per cent per kelvin compounds to roughly a doubling over 10 kelvin, which is the rule of thumb worth carrying. It explains the enormous contrast between polar and tropical moisture contents and the corresponding contrast in rainfall potential. Compounding the percentage rather than adding it is the step most often got wrong.

Answer: D

Mean rainfall is constrained by the surface energy budget to about 2 per cent per kelvin while extremes draw on the available moisture and scale closer to 7 per cent. Observations of the heaviest hourly rainfall in several regions show rates even above the thermodynamic value, attributed to dynamical intensification of the convection. The divergence between mean and extreme is among the more confidently projected consequences of warming. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: A

Plants draw water from the soil through their roots and release it as vapour through pores in the leaves, and over a closed canopy this transpiration dominates the total flux. Soil evaporation matters where the canopy is sparse and interception loss can be a substantial fraction in forests during wet weather. Together the three components make up evapotranspiration, and separating them matters for understanding how land use change alters the water balance of a catchment.

Answer: B

Stomata open in light to admit carbon dioxide and close under water stress or high vapour pressure deficit, giving the plant active control over its water loss. This physiological regulation is what distinguishes transpiration from evaporation from an open water surface. Root depth determines the water available rather than the rate at which it is used.