5 Things You Never Knew About Hurricanes - Original PDF
نویسندگان: Dr. Mark Guishard Director, Bermuda Weather Service
خلاصه: 5 Things You Never Knew About Hurricanes Dr. Mark Guishard Director, Bermuda Weather Service Background Hurricanes are: Large low pressure systems triggered by pre-existing disturbances Fueled by the energy released from the ocean surface Source: NOAA Background Most of the energy and mass in a hurricane is focused quite near the centre Strongest in a concentric ring of severe weather, called the “eyewall” Surrounds a calm central “eye” Source: NASA Background June September Birthplaces of Hurricanes Background June September Birthplaces of Hurricanes Source: NOAA Background Hurricane Season: Jun. 1- Nov. 30 Background Sea Surface Temperature shows a direct relationship with hurricane activity – Represents the ocean energy source for fueling the storms Background Sea Surface Temperature shows a direct relationship with hurricane activity – Represents the ocean energy source for fueling the storms L H Heating In order to keep the heating in the core above the surface disturbance, there must be little or no shear. Strong shear in the medium levels would rip the storm apart. Background Sea Surface Temperature shows a direct relationship with hurricane activity – Represents the ocean energy source for fueling the storms Vertical wind shear has a destructive effect on storm formation L H Heating In order to keep the heating in the core above the surface disturbance, there must be little or no shear. Strong shear in the medium levels would rip the storm apart. L H Heating 1) El Niño Influences Atlantic Hurricane Numbers What is El Niño? –A climate cycle that occurs every few years. –A shift of warm surface water across the tropical Pacific from west to east. –Caused by the relaxation of the trade winds. 1 Average conditions - January - March El Niño conditions - January - March El Niño conditions Sea Surface Temperature 1 Named by South American fishermen for the Christ child. Warming of waters off Peru coincided with Christmas. Source: NOAA Average conditions - January - March El Niño conditions - January - March El Niño conditions Sea Surface Temperature 1 Named by South American fishermen for the Christ child. Warming of waters off Peru coincided with Christmas. The opposite effect is named “La Niña” La Niña conditions - January - March Source: NOAA El Niño Effects Increased wind shear downstream – suppressed hurricane formation in the main Atlantic development region 1 Source: NOAA Effects on Hurricane season The disruption of the upper winds translates to the Atlantic, suppressing Hurricane activity for El Niño years in this part of the world. The opposite occurs in a La Niña year, when Atlantic Hurricane Season tends to be very active. 1 1987 - an El Niño year Only 8 Named Storms in the Atlantic. But what else happened in 1987? Hurricane Emily! 1 Source: Unisys 1998 - A La Niña Year 14 Named Storms in the Atlantic. Cat 5 Hurricane Mitch dumped up to 800mm of rain within 48 hours in Honduras & Nicaragua. Mitch caused over 9,000 deaths. 1 Source: Unisys 2) Vulnerability to Hurricanes Vulnerability is the susceptibility to physical or emotional injury or attack. “Disasters occur when hazards meet vulnerability". A natural hazard will hence never result in a natural disaster in areas without vulnerability, e.g. strong earthquakes in uninhabited areas. Some factors influencing vulnerability: – Social – Physical 2 Source: Wikipedia Social aspects of vulnerability Awareness/Preparedness 1900 Galevston Hurricane – The Galveston Hurricane of 1900 is to date the deadliest natural disaster ever to strike the United States: estimated 8,000 fatalities. – Large death toll may be attributed to lack of awareness that the storm was coming 2 Source: Wikipedia Social aspects of vulnerability Miami Beach 1926 Wendler Collection Miami Beach 2006 Joel Gratz © 2006 2 50000 52000 54000 56000 58000 60000 62000 64000 66000 68000 1980 1991 2000 2010 2020 Projected Bermuda Population Growth Our own poulation density well exceeds 1000 people per km 2 Only forecast to get higher… 50000 52000 54000 56000 58000 60000 62000 64000 66000 68000 1980 1991 2000 2010 2020 2 Source: Government of Bermuda Dept. of Statistics Physical aspects of vulnerability Geography Hurricane Katrina, 2005 – City of New Orleans lies largely below sea level 2 Source: FEMA Events themselves drive vulnerability 2 Intensity: Andrew 1992 Mitch 1998 Fabian 2003 Frequency: Combinations of factors can maximize vulnerability 1970 Bhola Cyclone: Deadliest known tropical cyclone of all time – A powerful cyclone that hit Bangladesh on November 13, 1970 killed 500,000 people from storm surge, high winds, and flooding. 2005 Hurricane Katrina Factors: – Lack of Awareness &/or Preparedness – Geography – Population – Intensity 2 3) Global Warming and Hurricanes 3 Source: UEA NHC Best Track Storm Database 3 Source: Landsea, 2006 Bermuda Best Track Storms Historically Bermuda is affected on average ~1 every 2 years (not necessarily direct hits) 0 1 2 3 4 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 More recently… 3 Source: BWS Global Warming & Increase in Hurricane Frequency? 0 1 2 3 4 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Two camps – fierce debate. Problems with conclusions made on both sides of argument. 3 Source: Landsea, 2006 4) Water is the biggest killer, not wind “If the projected rise in sea level due to global warming occurs, then the vulnerability to tropical cyclone storm surge flooding would increase.” Storm Surge is a dome of water driven by the persistent strong winds in a tropical system (not by the low central pressure!) Source: NOAA Storm Tide 4 Ocean WavesOcean Waves MWL Wave Setup ExtremeExtreme WindsWinds Wave Runup MSL datum Expected High Tide StormStorm TideTide Surge after Harper (2001) Storm Surge - Continental Effects Continental Shelf Ocean Direction of wave movement As waves move into a shallower environment, their amplitudes increase and wavelengths decrease. 4 Waves - Island effects On a small, isolated island such as Bermuda, this effect is small; Large waves are deflected around the sea mount, in the surrounding deep water. Amplitudes and wavelengths stay fairly constant. However, sea states can still be quite significant in tropical systems... Bermuda Sea Floor Direction of wave movement 4 Before and After the Hurricane Richelieu Apartments Pass Christian, MS Hurricane Camille 17 Aug 1969 4 Source: NOAA 4 Katrina Storm Surge Model 4 Source: LSU Katrina Storm Surge Map Source: FEMA 4 5) A Hurricane is not always a Hurricane… 5 Subtropical storms – about half of the storms we get in hurricane season are not even named by NHC Source: Guishard, 2007 October 2001: Subtropical Storm ⇒ Hurricane Karen 989 mb 5Source: UW-CIMSS Source: BWS In Bermuda, more gales come from marine winter storms than from hurricanes 0 10 20 30 40 50 60 1/1/2002 2/1/2002 3/1/2002 4/1/2002 5/1/2002 6/1/2002 7/1/2002 8/1/2002 9/1/2002 10/1/2002 11/1/2002 12/1/2002 1/1/2003 2/1/2003 3/1/2003 4/1/2003 5/1/2003 6/1/2003 7/1/2003 8/1/2003 9/1/2003 10/1/2003 11/1/2003 12/1/2003 1/1/2004 2/1/2004 3/1/2004 4/1/2004 5/1/2004 6/1/2004 7/1/2004 8/1/2004 9/1/2004 10/1/2004 11/1/2004 12/1/2004 1/1/2005 2/1/2005 3/1/2005 4/1/2005 5/1/2005 6/1/2005 7/1/2005 8/1/2005 9/1/2005 10/1/2005 11/1/2005 12/1/2005 1/1/2006 2/1/2006 3/1/2006 4/1/2006 5/1/2006 6/1/2006 7/1/2006 8/1/2006 9/1/2006 10/1/2006 11/1/2006 12/1/2006 1/1/2007 Fabian Florence Winter Winter Winter Winter 5 Bermuda daily average winds over 5 years Source: BWS Recap El Niño – Natural climate pattern – Reduces hurricane formation in the Atlantic… – …by changing the shear pattern in the main development region. Vulnerability to Hurricanes is compounded by social and physical aspects like – Geography, – Population and – Awareness, – in addition to the frequency and intensity of the events themselves. Recap The effects of global warming on hurricanes in the Atlantic are not as clear as they might appear at first. Unreliable database before: –Satellites and Radar –Aircraft reconnaissance –Weather Balloons Water is the biggest cause of fatalities in hurricanes, not direct wind strength. Hurricane force winds don't always come from what we classically think of as hurricanes. Any Questions? Dr. Mark Guishard Director, Bermuda Weather Service