A typical drizzle drop has a diameter of about
Answer: C
The conventional boundary between drizzle and rain is placed near half a millimetre. Below about a tenth of a millimetre the particle is better regarded as a cloud droplet still. The boundaries are conventions rather than physical transitions.
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A disdrometer measures
Answer: D
Correct answer: the sizes and fall speeds of individual hydrometeors
A disdrometer records each particle that passes through its sampling volume, giving the drop size distribution directly rather than only the accumulated depth a rain gauge provides. That distribution is what a radar rainfall relationship has to assume, so disdrometers are used to derive and validate those relationships and to distinguish rain from drizzle, snow and hail.
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Assertion (A): Virga can be accompanied by a strong downdraught at the surface.
Reason (R): Evaporation cools the sub-cloud air and makes it negatively buoyant.
Answer: A
Both are true and the reason gives the mechanism, and dry microbursts arise in exactly this way. A hazard can therefore reach the ground even when no rain does. Dry microbursts are among the most dangerous aviation hazards.
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A microburst is
Answer: B
Evaporative cooling of precipitation drives air downward and it spreads violently on reaching the ground. The resulting wind shear is a recognised hazard to aircraft during take off and landing. Detection systems at major airports exist for this reason.
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Consider the following statements about dry microbursts.
1. They form where precipitation evaporates in a deep dry layer.
2. Little or no rain reaches the ground beneath them.
3. They require heavy surface rainfall to develop.
Answer: C
Correct answer: 1 and 2 only
Statements 1 and 2 describe the dry microburst: precipitation falls into a deep, dry sub-cloud layer, evaporates, and the evaporative cooling makes the air negatively buoyant so that it accelerates downward and spreads violently at the surface. Statement 3 inverts the requirement - the rain must *not* reach the ground, since it is the evaporation that drives the downdraught. Virga beneath a high cloud base is the visible signature.
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Precipitation efficiency is defined as
Answer: D
It accounts for water retained aloft, evaporated below cloud or carried away by the wind. Values vary from a few per cent in dry continental convection to well over half in maritime systems. Efficiency is a property of the environment as much as the cloud.
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All of the following reduce precipitation efficiency EXCEPT
Answer: A
A moist environment reduces both evaporation and the effect of entrainment. The other three each remove condensate before it can reach the ground. Losses can exceed the amount that actually falls. Efficiency below a tenth is observed in dry continental storms.
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Assertion (A): Precipitation efficiency is generally higher in maritime than in continental convection.
Reason (R): The maritime environment is moister, so less condensate is lost to evaporation.
Answer: B
Both are true and the reason gives the principal cause, and maritime microphysics also favours earlier rain. Both factors work in the same direction towards a higher efficiency. Maritime systems therefore rain from much shallower clouds.
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The drop size distribution of rain is most commonly represented by
Answer: C
The Marshall Palmer form expresses drop number as decaying exponentially with diameter. Gamma distributions with an extra shape parameter are now often preferred for their greater flexibility. The choice of form affects the derived rainfall appreciably.
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Assertion (A): A gamma distribution is often preferred to a pure exponential for raindrop spectra.
Reason (R): The extra shape parameter represents the observed shortage of the very smallest drops.
Answer: D
Both are true and the reason gives the improvement, and the exponential form overpredicts small drops. The additional parameter must itself be estimated, which is the cost of the better fit. Extra parameters need extra observations to constrain them.
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