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ATM S 111, Global Warming: Understanding the Forecast

ATM S 111, Global Warming: Understanding the Forecast. Dargan M. W. Frierson Department of Atmospheric Sciences Day 10: 04/ 29/ 2010. Assignments. Should have read “Impacts on Agriculture and Ecosystems”: p.147-168 Read “Climate Records” p.171-192 for next time HW 3 due tomorrow 11:59 PM

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ATM S 111, Global Warming: Understanding the Forecast

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  1. ATM S 111, Global Warming: Understanding the Forecast Dargan M. W. Frierson Department of Atmospheric Sciences Day 10: 04/29/2010

  2. Assignments • Should have read “Impacts on Agriculture and Ecosystems”: p.147-168 • Read “Climate Records” p.171-192 for next time • HW 3 due tomorrow 11:59 PM • Quiz 2 in class on Tuesday • On “Floods and Droughts” through “Hurricanes” • Bring a scantron sheet and pencil

  3. Huracan: evil Taino & Mayan god of winds & destruction

  4. Eye of Katrina from UW Research Flight

  5. Profile of a Tropical Cyclone • Hurricane = typhoon = cyclone • All different words for the same thing • Rising motion has rain & clouds, sinking motion is dry

  6. What drives a hurricane? • Turns out water vapor is the fuel • Heat is released when condensation of water vapor occurs • Evaporation from warm ocean waters provides the water vapor • Warmer water means more water vapor, more fuel for hurricanes

  7. Requirements for Hurricanes • Sea surface temperatures must be sufficiently warm • Must be at least 5 degrees off the equator • Not much wind shear • Wind shear: when the winds change with height • This rips hurricanes apart • Can’t be too cold below the surface either

  8. Hurricanes and Global Warming • Warmer temperatures means: • Warmer ocean • More water vapor in the air • Shouldn’t these mean stronger storms? • Yes, but it’s not so simple…

  9. Shears Can Change… An example of a prediction from a computer model of global warming Increased shear over the Gulf of Mexico would act to weaken hurricanes Such a change would offset some of the increase in strength due to increased temperatures in this part of the world…

  10. Have Hurricanes Been Changing? • In 2005 in the Atlantic, there were: • 3 of 6 strongest storms ever (Wilma, Rita, and Katrina) • 27 named storms (smashing the previous record of 21) • Ran out of letters in the alphabet • Q, U, X, Y and Z are not used… • Had to use Alpha, Beta, Gamma, Delta, Epsilon, and Zeta • But we know better to say that one season is due to global warming… • Plus, it turns out that only 10% of all hurricanes occur in the Atlantic

  11. Worldwide Hurricane Tracks Worldwide hurricane tracks

  12. Saffir-Simpson Hurricane Scale Higher category = higher winds = more storm surge = lower pressure in the eye

  13. Tracks Colored by Category Many strongest storms are around Taiwan, Philippines

  14. 1991 Bangladesh cyclone: 144,000 fatalities, left 10 million homeless

  15. Hurricane Damages • Damages in hurricanes are caused by: • Winds • Storm surge • High winds pushing water towards the land • Flooding • Financial damages from hurricanes is increasing, but this is primarily due to more people living on the coast

  16. Winds

  17. Storm surge

  18. Winds push water towards land with no place else to go

  19. Flooding

  20. Worldwide Hurricane Damage Recent study claims the strongest have increased Claims Cat. 4 & 5 have increased 50%

  21. Paper by Chris Landsea argues that recording instruments changed, not the hurricanes

  22. Are hurricanes becoming more intense? Ocean temperatures (solid) and PDI (dashed) From Emanuel (2007) PDI stands for “power dissipation index” a measure of the intensity of storms that combines frequency, intensity, and duration. Thought to be a better measure than just categories, and this suggests warmer ocean means stronger storms.

  23. Observed Hurricane Changes • Debate was rather heated for a while • Results are still somewhat contentious • Fundamental problem is that the observed record is short, and hurricanes are relatively rare events • How about computer modeling results? • These are difficult too: global climate models aren’t high enough resolution to capture hurricanes well

  24. Recent model results are suggesting fewerbut more intense storms Stronger Weaker 

  25. Summary of Hurricanes • Data seems to show an increase in strength globally • Some argue that data quality is not good enough to make the case though • Models suggest the strongest storms will become more frequent and stronger • But also a reduction in the total number of storms • Should be much scientific progress in this field over the next decade! • As model resolution increases and we get more data

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