Water Vapour and Humidity Variables Questions and Answers

Q: Which wavelength suffers most from attenuation in heavy rain?

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.

Q: Which product combines radar and gauge measurements?

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.

Q: Which satellite technique estimates rainfall over the open ocean?

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.

Q: Which quantity does spaceborne precipitation radar provide that passive sensors cannot?

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.

Q: Which quantity is the specific humidity of saturated air at 20 degrees and 1000 hectopascals?

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.

Q: Which change in saturation mixing ratio accompanies a rise of ten kelvin?

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.

Q: Which consequence follows for extreme rainfall in a warming climate?

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.

Q: Which process transfers water from the surface to the atmosphere over vegetation?

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.

Q: Which control do plants exert over transpiration?

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.

91.

Which wavelength suffers most from attenuation in heavy rain?

A.X band
B.S band, which is the longest of the common weather radar wavelengths and therefore the most severely affected
C.C band
D.L band, which is used for wind profiling and is attenuated strongly by the drops within a rainfall event

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.

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92.

Which product combines radar and gauge measurements?

A.A satellite precipitation estimate, which uses the temperature of the cloud tops to infer the rate beneath them
B.A merged or calibrated rainfall analysis
C.A nowcast, which extrapolates the current radar echoes forward in time to predict the rainfall over the next hour
D.A reanalysis

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.

Discuss in Forum

93.

Which satellite technique estimates rainfall over the open ocean?

A.Visible imagery
B.Radar altimetry, which measures the height of the sea surface and therefore the rainfall falling upon it
C.Passive microwave sensing of emission and scattering by hydrometeors
D.Infrared imagery of the cloud top temperature, which is the only technique available over the remote oceans

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.

Discuss in Forum

94.

Which mission carried a precipitation radar into space?

A.The Aqua satellite, which carries an infrared sounder and a microwave radiometer but no active instrument at all
B.The Meteosat series
C.The Argo float programme, which measures the temperature and salinity of the upper ocean rather than rainfall
D.The Tropical Rainfall Measuring Mission and its successor

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.

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95.

Which quantity does spaceborne precipitation radar provide that passive sensors cannot?

A.The vertical profile of the precipitation
B.The total accumulation over a month, which requires the continuous sampling that only an active radar allows
C.The surface rainfall rate
D.The size distribution of the individual drops, which is measured directly by the returned signal from each one

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.

Discuss in Forum

96.

Which quantity is the specific humidity of saturated air at 20 degrees and 1000 hectopascals?

A.About 1 gram per kilogram, which corresponds to the saturated value at a temperature well below freezing
B.About 15 grams per kilogram
C.About 4 grams per kilogram, which is closer to the saturated value at a temperature near zero degrees Celsius
D.About 30 grams per kilogram

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.

Discuss in Forum

97.

Which change in saturation mixing ratio accompanies a rise of ten kelvin?

A.It is unchanged
B.It increases by a factor of ten, which is why tropical air is able to hold so much more water than polar air
C.It roughly doubles
D.It increases by about ten per cent, following the rate of seven per cent per kelvin given by the relation

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.

Discuss in Forum

98.

Which consequence follows for extreme rainfall in a warming climate?

A.Extreme events become less frequent as the additional moisture is spread more evenly throughout the year
B.Nothing changes
C.Extreme events intensify at exactly the same rate as the mean, since both depend upon the same water supply
D.Extreme events intensify faster than mean rainfall

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.

Discuss in Forum

99.

Which process transfers water from the surface to the atmosphere over vegetation?

A.Transpiration through the stomata of the leaves
B.Direct evaporation from the soil surface, which supplies the great majority of the flux over any vegetated area
C.Sublimation
D.Interception loss, which is the evaporation of water held on the leaf surfaces after rainfall has ceased

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.

Discuss in Forum

100.

Which control do plants exert over transpiration?

A.The temperature of the leaf, which the plant regulates independently of the temperature of the air around it
B.Stomatal opening in response to light and water stress
C.The albedo of the leaf surface, which determines how much of the available energy is absorbed for evaporation
D.Root depth alone

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.

Discuss in Forum

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