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What aviation hazards are associated with thunderstorms, and how can they be avoided? ■ Where does turbulence occur around a thunderstorm? ■ How do microbursts develop? ■ What conditions are favorable for the formation of hail? ■ How does lightning affect flight? ■ Unit 22 Thunderstorm hazards Turbulence TS Aviation Weather Hazards Due to strong up- and downdrafts and latent heat release within the thunderstorm ■ Two different scales within thunderstorms ■ turbulence ■ downbursts / microbursts ■ gust fronts / outflow boundaries ■ tornadoes ■ airframe icing ■ hail ■ lightning ■ flashfloods ■ ♦ large eddies such as the downdrafts and updrafts ♦ small scale gusts produced by strong wind shear on the edges of the vertical drafts. ■ In the clear air surrounding storms: ♦ gravity waves above storm tops ♦ mid/upper level eddies downstream of a storm ♦ below cloud base, due to updrafts, downbursts & gust fronts Turbulence regions near a TS Airflow Above and downstream from overshooting top Within cloud Near upand downdrafts Beneath overhanging anvil on downstream side of storm Downbursts and microbursts Downburst - a strong downdraft which includes an outburst of damaging winds on or near ground ■ Leading edge of cold air called gust front ■ Microburst - a downburst with horizontal dimensions less than 4 km ■ ♦ ■ Small means more shear Caused by ♦ Near downburst or microburst Near gust front Falling precipitation dragging air down with it Evaporative cooling making air more dense ♦ Often augmented by violent storm dynamics ♦ 1 Microbursts ■ ♦ ♦ ■ ■ they are small (large wind shear) they develop suddenly (lasting less than 30 min) Microbursts can be either wet or dry After microburst touchdown, a vortex ring spreads out over the ground ♦ ■ Hazards - microbursts Microbursts are dangerous to aviation because Often made visible by dust Vortex ring Dry microbursts are largely due to evaporative cooling within the virga below the cloud base ♦ A thunderstorm with a high cloud base and rain evaporating before it reaches the ground can produce severe DRY microbursts cloud base. Photo Moller After touchdown, a vortex ring spreads out over the ground. Sometimes this ring is made visible by dust. Gust front vortex along landing approach to Denver’s Stapleton Airport Microbursts - cont'd ■ On 9 July 1982 at 2:15 pm, PANAM flight 759 took off at New Orleans. ♦ Soon after take-off, the plane crashed, killing all people on board ♦ Crash was attributed to sudden change from headwind to tailwind ♦ What should a pilot do if caught in a microburst? 45 kt downburst 45 kt tailwind Flight path of plane 45 kt headwind Hazards - hail Airframe icing Stronger vertical motions Þ greater potential for hail ■ Hail is recycled several times within the cloud before it falls out ■ Strong updrafts needed to suspend large hailstones and keep them within the cloud for a long time ■ Strong updrafts can produce large concentrations of large supercooled cloud droplets, which freeze on impact ■ Ice accumulation potential is more severe for large systems (squall lines and MCCs) ■ ♦ ♦ Difficult to fly out of or around these systems Icing distorts airfoil shape, increases drag and reduces lift ♦ ■ Large hail can A golfball-size hailstone seriously damage has a fall speed of 22 m/s aircraft (40 kt) so needs an updraft (mainly airfoil and of at least this speed propellors). 2 Distribution of large hail deep layer of moisture in the low levels with drier air in the mid & upper levels ■ Lapse rates with strong conditional instability at lower levels Baseball, anyone? ■ Need Hazards - lightning ■ ■ ■ ■ ■ Lightning kills more people in the USA every year than tornadoes and hurricanes combined. A lightning stroke is very hot (~30,000C), and it triggers a sound wave in the air : the thunder. Both cloud-to-ground and intra-cloud lightning occurs. In a strongly charged environment, the wings of aircraft may start glowing: St Elmo's fire (a type of corona discharge). Lightning may disturb radio communications. 3