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Lecture 9

EARTH 108 Lecture Notes - Lecture 9: Wind Shear, Landfall, Hurricane Hanna (2008)

Earth and Environmental Sciences
Course Code
Eric Hetland

of 3
Lecture 9: Cyclones: Preparing & Warning
Lecture outline
1) forecasting cyclone paths
2) cyclone (early) warning
3) responding to warning (more in L11)
4) hurricanes & climate:
a) global warming
b) El Niño & La Niña
c) North Atlantic Oscillation
-Forecast = early warning
-Hurricane Hanna in 2008: There was a forecast to remain category 1 and make
land-fall around 8am on Friday, but it quickly degraded into a tropical depression
by the next morning. However, the hurricane made landfall earlier than
expected. There were 7 deaths and losses of $160 million.
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Two of the conditions that dictate a cyclones fate are:
-wind shear (needs low)
-ocean temperature (needs high)
Are Cyclones becoming more intense?
North Atlantic hurricanes appear to be becoming more intense as the sea surface
temperature increases.
-Hurricane risk is not constant
-Under El Nino conditions, there is low risk of hurricanes.
-Under La Nina conditions, there is high risk of hurricanes.
When the North Atlantic Oscillation is positive
-there are stronger trade winds
-polar jet stream more direct
-cooler seas off Africa
-warmer off E. US
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When NAO negative
-weaker trade winds
-polar jet stream more sinuous
-warmer seas off Africa
-cooler off E. US
-always expect the unexpected. 1992 hurricane Andrew, the 4th most powerful
Atlantic hurricane recorded (behind Katrina) had 75 fatalities, $40bn (2009$).
1992 was an El Niño year!
-global climate warming: more intense but not more
-El Niño generally less active, La Niña more N. Atlantic hurricanes
-size & strength of Atlantic atmos. pressure steers hurricanes
-several days of warning possible from forecasts of storm track
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