DARGAN M. W. FRIERSON DEPARTMENT OF ATMOSPHERIC SCIENCES DAY
10: 04/29/2010 ATM S 111, Global Warming: Understanding the
Forecast
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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
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Huracan: evil Taino & Mayan god of winds &
destruction
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Eye of Katrina from UW Research Flight
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Profile of a Tropical Cyclone Hurricane = typhoon = cyclone All
different words for the same thing Rising motion has rain &
clouds, sinking motion is dry
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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
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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 Cant be too cold below the surface
either
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Hurricanes and Global Warming Warmer temperatures means: Warmer
ocean More water vapor in the air Shouldnt these mean stronger
storms? Yes, but its not so simple
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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
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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
Saffir-Simpson Hurricane Scale Higher category = higher winds =
more storm surge = lower pressure in the eye
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Tracks Colored by Category Many strongest storms are around
Taiwan, Philippines
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1991 Bangladesh cyclone: 144,000 fatalities, left 10 million
homeless
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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
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Winds
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Storm surge
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Winds push water towards land with no place else to go
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Flooding
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Worldwide Hurricane Damage Recent study claims the strongest
have increased Claims Cat. 4 & 5 have increased 50%
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Paper by Chris Landsea argues that recording instruments
changed, not the hurricanes
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Are hurricanes becoming more intense? 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. Ocean temperatures (solid) and PDI (dashed) From
Emanuel (2007)
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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 arent high enough resolution to capture
hurricanes well
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Recent model results are suggesting fewer but more intense
storms Stronger Weaker
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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
