Thunderstorms

Question 1

Conditions required for Cb development

Answer 1

1. An adequate amount of water vapour at low levels
2. Conditional instability through depth (= release of latent heat)
3. Trigger mechanism

Question 2

Types of trigger mechanisms

Answer 2

1. Convection
2. Orography
3. Frontal lifting (widespread ascent)

Question 3

Classifications of Thunderstorms

Answer 3

1. Airmass (parcel of air with similar characteristics)

2. Frontal (boundary b/t two different types of airmasses, triggered by 3 main mechanisms)

Question 4

How can Airmass Thunderstorms be triggered?

Answer 4

1. Convection (localised/stationary and traveling TS)
2. Orographic
3. Nocturnal lifting (tropics - islands etc)

Question 5

How can Frontal Thunderstorms be triggered?

Answer 5

1. SFC Frontal/convergence TS (cold fronts e.g. southerly moving North is more likely to cause TS, due heating from SFC upwards)
2. SFC trough TS
3. Upper trough TS
4. Warm front embedded TS

Question 6

The lifecycle of a TS

Answer 6

1. Cumulus stage
2. Mature stage
3. Anvil/dissipating stage
   Overall time of life cycle is 1.5 hrs.

Question 7

Describe the cumulus stage in the life cycle of a TS

Any dangers at this stage? Time taken?

Answer 7

1. Trigger initiates lifting, a TCU begins to form.
2. There are only updrafting winds in a TCU, which carries SCWDs upwards.
3. Some SCWDs freeze, creating snow and ice. Eventually enough of these form at the top for it to fall, melt and create the first rain at the SFC.
   - Nil dangers except severe icing at this stage (no downdrafts to worry about). 30 mins.

Question 8

Describe the mature stage in the life cycle of a TS

Any dangers? Time Taken?

Answer 8

1. First heavy rain at SFC
2. An anvil will begin to form at the top of the cloud as the SCWDs begin to spread out horizontally beneath the tropopause.
   - Any of the EIGHT hazards now exist, tornadoes are unlikely however from stationary Cbs. 30 mins.

Question 9

Describe the Anvil/Dissipating stage of a TS

Any dangers? Time taken?

Answer 9

1. The top of the anvil will start to become fibrous (due to formation of cirrus)
   - Updrafts cease and hazards start to dissipate. 30 mins.

Question 10

List the hazards created by Cb (8)

Answer 10

1. Severe turbulence (due up/down drafts - centre to base should be avoided)
2. Severe icing (in clusters of Cb, area above FZL 0 to -12 degrees should be avoided)
3. Electrical phenomena (Lightning/static charges)
4. Hail (ice crystals cycled in up/down drafts)
5. Poor Visibility (due heavy precip)
6. Tornadoes (in severe TS, over water they are called waterspouts, both are rare in NZ)
7. Microbursts
8. First gust/gust front. (sudden strong downdraft of cold, dense air = Low level wind shear, characterised by roll clouds)

Question 11

Define Microbursts

Answer 11

• Localised severe wind pattern driven by extremely strong downdrafts from dense, cooled air. Can exceed 100kts in the vertical.
• Microburst winds intensify 5 mins after touch down, general lifespan is 15 mins.
• Can be Dry (occur with high based TS, rain evaporates before reaching SFC. More dangerous)
• Can be Wet (When precip accompanies microburst to SFC. More common in NZ due moist lower atmosphere)

Question 12

Formation speeds for:

1. Non-severe TS
2. Severe TS
3. Super Cells

Answer 12

1. 1200 - 2500ft per minute
2. 5500ft per minute
3. 12,000 - 15,000ft per minute

Question 13

Enhancing factors of glaze ice

Answer 13

When air is forced to rise (from frontal/lee waves/orographic, CONVECTION - TS)

Question 14

TS Characteristics in NZ

Answer 14

• Ave about 35,000ft in height, about that of the tropopause.
  (20,000ft in S in winter, 40,000ft in N in summer)
• Less hazardous/destructive compared to TS in the US

Question 15

Lightning in NZ

Answer 15

• Cloud to ground strikes are only 10% of all strikes.
• Other 90% is cloud to cloud or within the cloud.
• In general, lightning can be either positively or negatively charged.

Question 16

Shapes of Microbursts

Answer 16

• Symmetrical, striaght down out of a stationary Cb
• Asymmetrical, due strong winds ahead of a travelling TS.
• Twisting (can precede tornadoes)
• Radial

Question 17

Development of Hailstones

Answer 17

• Only occur within Cb clouds with up/down drafting air.
• Number of times the ice crystal cycles up and down the Cb before it gets too heavy and falls out defines size and number of layers of:
  Rime ice (upper Cb)
  Clear ice (lower Cb)
• Hail can fall out of: Anvil/sides/bottom of Cb.

Question 18

List the characteristics of Super cell TS (8)

Answer 18

1. The Cb leaning over as it develops = travelling, extremely unstable Cb. Strong vertical and horizontal wind shears increasing with height.
2. Rain and microburst occur ahead of Cb
3. Separation of updrafts (behind)/downdrafts (in front). These don’t cancel each other.
4. Strong updraft punching through into stratosphere
5. Interaction b/t updrafts and downdrafts produces rotating horizontal eddy
6. Rotating eddy can get caught in updrafts, tilting to the vertical
7. Small % of Cbs produce rotating wall clouds
8. 10% of these produce tornadoes/water spouts/funnel clouds.

Question 19

How to avoid TS

Answer 19

1. Avoid in planning phase! (use radar in flight)
2. have an alternate route
3. Don’t fly under orographic TS (downdrafts)
4. Cross frontal TS at right angles

Question 20

Characteristics of Stationary TS

Answer 20

1. (An Airmass Cb). Upright stance and develop in situ. However can move slowly.
2. Little to no wind shear aloft = no tilt.
3. Results in updrafts/downdrafts being in line, therefore destroying each other and cloud mass at the same time. (Anvil not affected)
4. Has all hazards bar tornadoes.

Question 21

Characteristics of Travelling TS

Answer 21

1. (An Airmass Cb).
2. Have increasing windshear with increasing height, causing the ‘tilt’ of the Cb in direction of travel.
3. Up and downdrafts are separated, therefore don’t cancel each other out, and can therefore last longer than stationary Cb with enough moisture, can also develop into Super cell TS

Question 22

Characteristics of Orographic TS

Answer 22

1. Airmass or frontal
2. Occur when moist air is forced to lift by terrain (e.g. the west coast of South Island where atmosphere is conditionally unstable)
3. Low cloud bases ensure plenty of latent heat is release early in lifting process = TS

Question 23

Characteristics of Nocturnal TS

Answer 23

1. An Airmass Cb
2. Tops of Cumulus clouds cool at night due loss of Longwave radiation, increasing instability (steepens lapse rate).
3. The cumulus clouds therefore grow and become Cb.
4. more common in the tropics (warmer sea temps)

Question 24

Characteristics of frontal/convergence TS

Answer 24

1. Frontal TS
2. Trigger is lifting of warm, unstable air at frontal SFC
3. Tends to form multiple Cb along boundary of front, which move with the front.
4. Convergence TS tend to form in clusters

Question 25

Characteristics of upper trough TS

Answer 25

1. Frontal TS
2. Occur when cold air is transferred horizontally at higher ALTs, which steepens the lapse rate through depth. The atmosphere then tends towards instability.

Question 26

Characteristics of Warm front embedded TS

Answer 26

1. Frontal TS

2. Stable fronts, therefore TS are rare.