Lecture 5 - Tropical Cyclones

Question 1

Tropical cyclones are typically categorized using the … Wind Scale which is a scale based on …

Answer 1

Saffir-Simpson
Wind speed

Question 2

When are tropical storms given a name?

Answer 2

> 39 mph

Question 3

What is the definition of a tropical cyclone?

Answer 3

“Warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation around a well-defined center”

Question 4

How large can be a cyclone become in diameter?

Answer 4

500-1000 km

Question 5

How is a tropical cyclone formed?

Answer 5

1. Tropical disturbance
   - Group of thunderstorms over warm tropical waters
   - Tropical waves
   - Intertropical Convergence Zone
2. The Coriolis Force
   - Force of Earth’s rotation causes the inflowing air to start spiraling
   - This is why tropical cyclones do not form on the Equator, but approximately 5° (~550 km) North or South!
3. Warm ocean waters
   - At least 26.5°C in the ocean top layer (~50 m)
   - Fuels the heat engine of the tropical cyclone
4. Low vertical wind shear (~ 10 m/s)
   - Difference in wind speed between surface and upper troposphere
   - High wind shear will disrupt the deep convection and prevent genesis or weaken/destroy a developed cyclone

Question 6

What is the percentage of tropical disturbances which eventually become a tropical cyclone?

Answer 6

Approx. 7%

Question 7

Explain when we speak of rapid intensification. Why is this so dangerous?

Answer 7

Definition: Intensification of 30 knots (= 15 m/s) in 24 hr. Primarily caused by warm ocean waters.

Extremely dangerous in particular when this happens right before landfall as no warming and this adaptation have been implicated.

Question 8

What are the five ways a tropical cyclone can weaken?

Answer 8

Landfall takes away ocean interaction (and fuel!)

Wind shear can blow the tropical cyclone apart

Extratropical transition means the tropical cyclone transitions into an extratropical cyclone

Loss of angular momentum, can be caused by one of the above, causes a cyclone to weaken

Dry entrainment can kill the moisture transport in the eye.

Question 9

What is the motion of cyclones? How is it affected by low and high pressure systems?

Answer 9

General direction of movement driven by Coriolis force

Cyclones are steered by high and low pressures – pushed around by HP, pulled toward LP areas (+ Jet stream!)

In general: slower translation speed near Equator, higher near poles

Fastest hurricane: Nate (45 km/hr)

Question 10

What are the main impacts of tropical cyclones?

Answer 10

• Wind
• Storm surge
• Rainfall

Question 11

In which part of the cyclone can the highest surges be found (on the NH)?

Answer 11

Depends on where the landfall is but mostly in the direction the cyclone moves and than the third quarter.

Question 12

What are the impact of rain?

Answer 12

Dump a lot of rain: flooding, landslides, but also water contamination

No clear relationship between intensity and rainfall totals: forward speed seems to be more important factor.

Question 13

By what is the hazard of a tropical cyclone impacted?

Answer 13

Landfall location: metropolitan area vs rural area, but also wealthy vs poor (exposure)
Adaptation measures: USA vs Asia (vulnerability)

Question 14

How are tropical cyclones expected to change with further global warming/ climate change? Also its characteristics and not only the intensity and occurrence.

Answer 14

Frequency: No consensus – some models project increase, some models project decrease. Caused by specific model characteristics (model resolution, model setup ..).

On average: most basins showing decrease due to enhanced atmospheric stability (less tropical disturbances), increase for others.

Observations show little change in frequency over the last decades.

Increase in frequency of most intense (Cat 4 – 5) tropical cyclones.
Tracks: Poleward shift due to increasing sea-surface temperatures

Forward speed: Unclear, there seems to be a slowing down of TCs over the past decade (Kossin, 2018), but discussion remains on whether that’s due to anthropogenic forcing or climate variability

Storm surge: increase – stronger TCs + sea-level rise (sea-level rise seems to be the most important contributor)

Precipitation: Increase (warm air can hold more moisture!

Question 15

How are hazards further impacted?

Answer 15

Increase wealth/ income, intensity TC’s and population.

Question 16

What is the definition of an extratropical cyclone?

Answer 16

Extratropical cyclone (or midlatitude cyclone), a type of storm system formed in middle or high latitudes, in regions of large horizontal temperature variations called frontal zones.

Question 17

What are two types of extratropical cyclones? And how do they differ?

Answer 17

SHAPIRO – KEYSER
NORWEGIAN

For Norwegion wave intensifies as warm air pushes north, cold air pushes south. Occlusion (cold/warm air mix) forms, cold front catches up with warm front. Cold core has formed.

For the Sharpie Keyser, Warm air fills the core, cold front detaches from the warm front. No occlusion! Cold air wraps around warm core.

Question 18

What are post-tropical cyclones.

Answer 18

Can occur more often under future warming -> warmer ocean waters can sustain a TC further poleward + stronger TC’s = more chance TC can survive passage over Atlantic

Question 19

As known, extratropical cyclones achieve high wind speeds, which damage mainly to rooftops and trees. Can you explain sting jets?

Answer 19

Typically occurs in Shapiro – Keyser cyclones (= warm core): cold dry air wraps around warm core, forcing the air downward (cold air = heavier!). This air evaporates, becomes even cooler, and the air “falls down” with an immense force/

Question 20

What is a medicane?

Answer 20

A mediterranean hurricane