Convection and Buoyancy Questions and Answers

Answer: C

Storm relative helicity integrates the streamwise vorticity available to the updraught and correlates well with the occurrence of mesocyclones. Values above about 150 square metres per square second in the lowest three kilometres are considered supportive. Buoyancy measures determine the intensity of the updraught rather than its rotation, and both are needed together. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

Doppler velocity data show a rotational couplet with strong inbound velocities adjacent to strong outbound ones, which is the direct signature of rotation. The hook echo on reflectivity is an associated feature produced by precipitation wrapping around the mesocyclone, which makes it a good but indirect indicator. The bright band and hail spike indicate melting and large hail respectively. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

An updraught strong enough to carry droplets aloft before they grow to detectable size produces a region of low reflectivity surrounded by high values above and around it, which is the bounded weak echo region or vault. Its presence indicates a very intense storm. The bright band is a stratiform signature and is absent in vigorous convection. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

A rear inflow jet descending into a squall line accelerates the outflow and bows the line forward, producing a swath of damaging winds tens of kilometres wide. A long lived family of such events is called a derecho. Supercell winds are damaging but over a narrower path, and the tornado is a separate and much narrower phenomenon.

Answer: C

A derecho is defined as a swath of convectively generated wind damage extending at least 400 kilometres with gusts above 26 metres per second along most of its length. It is produced by a progressive family of bow echoes. Microbursts are much smaller and haboobs are the dust raised by such an outflow rather than the wind event itself.

Answer: D

Fujita defined a microburst as a downburst with an outflow diameter under four kilometres and a macroburst as anything larger. The distinction matters because the smaller scale gives less warning and produces a more abrupt wind change. Wet and dry microbursts are a separate classification based on whether precipitation reaches the ground. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

Precipitation falling into a deep dry adiabatic layer evaporates completely, cooling the air and driving a strong downdraught that reaches the ground as a violent gust with little or no rain. The inverted V sounding is the classic signature. High plains summer conditions produce them regularly, and they are particularly hazardous to aviation because there is no visual warning. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

An aircraft entering the outflow meets an increasing headwind, then a strong downdraught, then a tailwind, and the resulting loss of airspeed close to the ground can exceed the aircraft's ability to recover. Several major accidents established the hazard and led to the installation of terminal Doppler radars. The wind shear rather than any precipitation is the danger. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: C

Terminal Doppler weather radars measure the radial velocity field close to the runway and detect the divergent signature of an outflow directly, issuing automatic alerts. Low level wind shear alert systems using anemometer networks serve the same purpose. The combination has substantially reduced the accident rate from this cause.

Answer: D

The gust front is the leading edge of the outflow, behaving as a density current with a raised head, and its passage brings a sudden wind shift, a temperature drop and a pressure rise. It is often marked by a shelf or roll cloud. Convergence along it frequently triggers new cells, which is how multicell storms propagate.